probing the effect of coordination...the effect of a coupling agent on the formation of...

Kristina Kisel

DissertationsDepartment of ChemistryUniversity of Eastern Finland

No. 152 (2019)

122/2014 BAZHENOV Andrey: Towards deeper atomic-level understanding of the structure of magnesium dichloride and its performance as a support in the Ziegler-Natta catalytic system123/2014 PIRINEN Sami: Studies on MgCl2/ether supports in Ziegler–Natta catalysts for ethylene polymerization124/2014 KORPELA Tarmo: Friction and wear of micro-structured polymer surfaces125/2014 HUOVINEN Eero: Fabrication of hierarchically structured polymer surfaces126/2014 EROLA Markus: Synthesis of colloidal gold and polymer particles and use of the particles in preparation of hierarchical structures with self-assembly127/2015 KOSKINEN Laura: Structural and computational studies on the coordinative nature of halogen bonding128/2015 TUIKKA Matti: Crystal engineering studies of barium bisphosphonates, iodine bridged ruthenium complexes, and copper chlorides129/2015JIANGYu:Modificationandapplicationsofmicro-structuredpolymersurfaces130/2015 TABERMAN Helena: Structure and function of carbohydrate-modifying enzymes 131/2015KUKLINMikhailS.:Towardsoptimizationofmetaloceneolefinpolymerizationcatalystsvia structuralmodifications:acomputationalapproach132/2015SALSTELAJanne:Influenceofsurfacestructuringonphysicalandmechanicalpropertiesof polymer-cellulosefibercompositesandmetal-polymercompositejoints133/2015 CHAUDRI Adil Maqsood: Tribological behavior of the polymers used in drug delivery devices134/2015 HILLI Yulia: The structure-activity relationship of Pd-Ni three-way catalysts for H2S suppression135/2016 SUN Linlin: The effects of structural and environmental factors on the swelling behavior of Montmorillonite-Beidellite smectics: a molecular dynamics approach136/2016 OFORI Albert: Inter- and intramolecular interactions in the stabilization and coordination of palladium and silver complexes: DFT and QTAIM studies137/2016 LAVIKAINEN Lasse: The structure and surfaces of 2:1 phyllosilicate clay minerals138/2016 MYLLER Antti T.: The effect of a coupling agent on the formation of area-selective monolayers of iron a-octabutoxy phthalocyanine on a nano-patterned titanium dioxide carrier139/2016KIRVESLAHTIAnna:Polymerwettabilityproperties:theirmodificationandinfluencesupon water movement140/2016 LAITAOJA Mikko: Structure-function studies of zinc proteins141/2017 NISSINEN Ville: The roles of multidentate ether and amine electron donors in the crystal structure formation of magnesium chloride supports 142/2017 SAFDAR Muhammad: Manganese oxide based catalyzed micromotors: synthesis, characterization and applications143/2017 DAU Thuy Minh: Luminescent coinage metal complexes based on multidentate phosphine ligands144/2017AMMOSOVALena:Selectivemodificationandcontrolleddepositiononpolymersurfaces145/2017 PHILIP Anish: PEI-mediated synthesis of gold nanoparticles and their deposition on silicon oxide supports for SERS and catalysis applications146/2017 RAHMAN Muhammad Rubinur: Structure and function of iron-sulfur cluster containing pentonate dehydratases147/2018 ANKUDZE Bright: Syntheses of gold and silver nanoparticles on support materials for trace analyses using surface-enhanced Raman spectroscopy148/2018 PENTTINEN Leena: Structure determination of enzymes with potential in processing of cell wall compounds149/2018 SIVCHIK Vasily: Tuning the photoluminescence of cyclometalated platinum(II) compounds via axial and non-axial interactions150/2019 MIELONEN Kati: Hierarchically structured polymer surfaces: Curved surfaces and sliding behavior on ice151/2019 CHAKKARADHARI Gomathy: Tuning the emission properties of oligophosphine copper and silver complexes with ancillary ligands

Probing the effect of coordination environment on the photophysical behavior of rhenium(I) luminophores

Kristina K

Isel: Probing the effect of coordination environm

ent on the photophysical behavior of rhenium(I) lum

inophores

152

Probing the effect of coordination environment on the

photophysical behavior of rhenium(I) luminophores

Kristina Kisel

Department of Chemistry

University of Eastern Finland

Joensuu 2019

31098697_UEF_Vaitoskirja_NO_152_Kristina_Kisel_LUMET_inside_19_04_24.indd 1 24.4.2019 13.08.31

2

Kristina Kisel

Department of Chemistry, University of Eastern Finland

P.O. Box 111, FI-80101 Joensuu, Finland

Supervisors

Prof. Igor Koshevoy, University of Eastern Finland

Assoc. Prof. Elena Grachova, Saint Petersburg State University, Saint Petersburg,

Russia

Referees

Prof. Risto Laitinen, University of Oulu, Oulu, Finland

Prof. Andreas Steffen, Technical University of Dortmund, Dortmund, Germany

Opponent

Kari Rissanen, University of Jyväskylä, Jyväskylä, Finland

To be presented with the permission of the Faculty of Science and Forestry of the

University of Eastern Finland for public criticism in Auditorium N100, Yliopistokatu

7, Joensuu, on May 16th at 12 noon.

Copyright © 2019 Kristina Kisel

ISBN: 978-952-61-3079-8

ISSN: 2242-1033

Grano Oy

Jyväskylä 2019


3

ABSTRACT The development of versatile light-emitting compounds and materials is crucial in

various technological fields including optoelectronic devices, photodynamic therapy,

sensing, photocatalysis, bio-imaging, and high-performance solar cells. As a result, a

large number of luminescent dyes were created over the years. Much of the research to

date has focused on transition metal complexes, due to the variety of their molecular

configurations that allows significant freedom in molecular design to attain unique

photophysical characteristics. Among the transition and rare earth metal

photofunctional compounds, the rhenium(I) luminophores have remarkable stability,

low toxicity, and appealing physical properties. Thus, they have attracted ever-

increasing interest from both academic and industrial researchers.

This work explores the synthetic approaches to new rhenium(I)-based luminescent

compounds with enriched photophysical behavior. Taking the archetypal emitter

[Re(CO)3(NN)Cl] (NN = diimine) as a starting point, I attempted to alter the optical

characteristics through two approaches: (i) modification of the coordination sphere of

Re(I) center by means of the diimine and auxiliary ligands, and (ii) introduction of

secondary chromophore units to the parent rhenium motif.

The first strategy, meant to improve the photophysical properties, is based on

incorporation of the peripheral ancillary ligands into the {Re(CO)2(NN)} core, which

bears a common chelating diimine (NN = phenanthroline). A series of

alkynylphosphines served as auxiliary axial ligands with different donor ability and

stereochemistry, which were accessibly modulated by changing the number and length

of alkynyl-phenylene substituents.

The second route of structural modification aims at extending the range of

luminescence colors to the whole visible spectrum. Such wide color-tuning of

[Re(CO)3(NN)Cl] emissions was achieved by integrating secondary chromophore

moieties into the pyridyl-phenanthroimidazole ligand.

Another feasible way to manipulate the photophysical behavior relies on the

construction of homo- and heterometallic rhenium-containing coordination assemblies

through merging two or more metal chromophores. The utilization of flexible

connecting bridges diminishes the communication between the constituting

components, while noncovalent intermolecular interactions substantially diversify the

optical properties of the bimetallic assemblies. On the other hand, coordination-driven

self-assembly using rigid cyanide linkers illustrates a facile way to attain

supramolecular aggregates decorated with multiple {Re(CO)3(NN)} chromophores.

Results from the present project should contribute to a more comprehensive

understanding of the structure-property relationships for the family of rhenium(I)

chromophores. This, in turn, should assist in elaborating novel molecular systems with

advanced photophysical functionality.


4

LIST OF ORIGINAL PUBLICATIONS This dissertation is a summary of original publications I−IV.

(1) Kondrasenko, I.; Kisel, K. S.; Karttunen, A. J.; Jänis, J.; Grachova, E. V.;

Tunik, S. P.; Koshevoy, I. O. Rhenium(I) Complexes with Alkynylphosphane

Ligands: Structural, Photophysical, and Theoretical Studies. Eur. J. Inorg.

Chem. 2015, 2015 (5), 864–875.

(2) Kisel, K. S.; Eskelinen, T.; Zafar, W.; Solomatina, A. I.; Hirva, P.; Grachova, E.

V.; Tunik, S. P.; Koshevoy, I. O. Chromophore-Functionalized Phenanthro-

Diimine Ligands and Their Re(I) Complexes. Inorg. Chem. 2018, 57 (11),

6349–6361.

(3) Kisel, K. S.; Melnikov, A. S.; Grachova, E. V.; Hirva, P.; Tunik, S. P.;

Koshevoy, I. O. Linking ReI and Pt

II Chromophores with Aminopyridines: A

Simple Route to Achieve a Complicated Photophysical Behavior. Chem. - A

Eur. J. 2017, 23 (47), 11301–11311.

(4) Kisel, K. S.; Melnikov, A. S.; Grachova, E. V.; Karttunen, A. J.; Doménech-

Carbó, A.; Monakhov, K. Y.; Semenov, V. G.; Tunik, S. P.; Koshevoy, I. O.

Supramolecular Construction of Cyanide-Bridged ReI Diimine

Multichromophores. Inorg. Chem. 2019, 58 (3), 1988–2000.

Author’s contribution The research material for the listed publications was obtained by the author and the

collaborating groups, who performed theoretical, magnetic and electrochemical

investigations. The author was responsible for the synthesis of all the compounds in

publications I-IV (except compounds L9, 7, 9, 11 in publication I). The author carried

out NMR, IR and structural characterization of all novel compounds. The author also

carried out the photophysical measurements for all the complexes discussed in the

thesis. The author participated in analyzing the computational results and in writing the

original publications.


5

CONTENTS ABSTRACT .................................................................................................................. 3

LIST OF ORIGINAL PUBLICATIONS ................................................................... 4

ABBREVIATIONS ....................................................................................................... 6

1 INTRODUCTION .............................................................................................. 7

1.1 Modulation of the photophysical behavior of rhenium(I) luminophores through

variation of the ligand environment ............................................................................. 8

1.2 Diimine rhenium(I) complexes: Diimine variation ................................................ 9

1.3 Diimine rhenium(I) complexes: Variation of ancillary ligands ........................... 13

1.3.1 Phosphine derivatives ..................................................................................... 13

1.3.2 Nitrogen-donor derivatives ............................................................................. 16

1.4 Polynuclear complexes built from rhenium(I) diimine blocks ............................ 18

1.4.1 Homometallic rhenium(I) complexes ............................................................. 18

1.4.2 Heterometallic rhenium(I) complexes ............................................................ 20

1.5 Applications ......................................................................................................... 23

1.5.1 Organic light emitting diodes (OLEDs) ......................................................... 23

1.5.2 Sensors............................................................................................................ 23

1.5.3 Photo-activated carbon monoxide releasing molecules (PhotoCORMs) ....... 24

1.5.4 Electrocatalysts ............................................................................................... 25

1.5.5 Bio-imaging .................................................................................................... 26

1.6 Aims of the study ................................................................................................. 27

2 EXPERIMENTAL ........................................................................................... 29

2.1 Syntheses ........................................................................................................... 29

2.1.1 Ligands ........................................................................................................... 29

2.1.2 Metal complexes ............................................................................................. 29

2.2 Characterization ................................................................................................... 33

3 RESULTS AND DISCUSSION ....................................................................... 34

3.1 Alkynylphosphine rhenium(I) complexes ............................................................ 34

3.2 Rhenium(I) complexes based on the extended diimine ligands ........................... 38

3.3 Bichromophore rhenium(I)–platinum(II) complexes based on the rhenium(I)

aminopyridine predecessors ....................................................................................... 44

3.4 Multichromophore rhenium(I) complexes of different nuclearity ....................... 48

4 CONCLUSIONS............................................................................................... 54

ACKNOWLEDGEMENTS

REFERENCES


6

ABBREVIATIONS CIE coordinates Chromaticity coordinates

D-A Donor-acceptor

DFT Density functional theory

DMF Dimethylformamide

DMSO Dimethyl sulfoxide

dppz Dipyridophenazine

dpyb Dipyridinebenzene

EL Electroluminescence

ESI-MS Electrospray ionization mass spectrometry

HOMO Highest occupied molecular orbital

ILCT Intraligand charge transfer

IR Infrared

ISC Intersystem crossing

LC Ligand-centered

LLCT Ligand-to-ligand charge transfer

LUMO Lowest unoccupied molecular orbital

MLCT Metal-to-ligand charge transfer

NMR Nuclear magnetic resonance

OLED Organic light emitting diode

PhotoCORM Photo-activated carbon monoxide releasing molecule

PNI 4-(1-Piperidinyl)naphthalene-1,8-dicarboximide

ppy 2-Phenylpyridine

S0 Singlet ground state

SOC Spin-orbit coupling

T1 Lowest triplet excited state

TD-DFT Time-dependent density functional theory

THF Tetrahydrofuran

UV/vis Ultraviolet/visible

XRD X-ray diffraction


7

1 INTRODUCTION The chemistry of rhenium began in 1925, when the vacant slot below manganese in the

periodic table was filled by Walter Noddack, Ida Tacke and Otto Berg, who were the

first to isolate this new element from the mineral gadolinite. Occupying the slot below

manganese in the periodic table, rhenium was the last-discovered element that has a

stable isotope,1 namely

185Re with a cosmic abundance of 37.07%. Meanwhile,

187Re

(mole fraction: 62.93%) is radioactive with a half-life of 41 billion years. Rhenium is

among the least abundant elements both in the Earth’s crust and in the solar system.

Natural sources that have a relatively high Re content for its economic extraction as the

primary commodity are extremely scarce.2,3

Currently, Re is commercially obtained as

a by-product in the process of molybdenum refining.

Although rhenium is the congener of manganese, these two elements have very

different chemical properties due to the phenomenon of secondary (supplementary)

periodicity. An important chemical characteristic of rhenium is the presence of several

oxidation states that can readily interconvert under mild conditions. The lower

oxidation states (1, 0, and -1) are typically found in a large number of carbonyl

derivatives and related organometallic compounds, which is similar to the family of

manganese relatives.4

The first coordination compounds of rhenium were discovered in the 1930s. In 1939,

Walter Hieber and colleagues reported the synthetic route to the distorted octahedral

rhenium(I) complexes [Re(CO)5Hal] by treating the corresponding

hexahalogenorhenate(IV) with carbon monoxide.5 This work laid the foundation of a

successful research direction for rhenium(I) carbonyl compounds. In 1958, G.

Wilkinson described the substitution reactions that convert [Re(CO)5I] into the di-

pyridine derivative fac-[Re(CO)3(py)2I].6 Further, R. Pietropaolo confirmed the

preferential fac-configuration of the rhenium(I) tricarbonyl compounds (Figure 1) by

IR spectroscopic measurements, and rationalized the result by analysis of the π backdonation between the rhenium atom and the carbonyl ligands.

7

Figure 1. fac-Configuration of the complex [Re(CO)3(py)2I].

6

In the 1970s, Wrighton and coworkers prepared the tricarbonyl diimine rhenium(I)

complexes [Re(NN)(CO)3L] (L = axial ligand) and performed the first systematic study


8

of their photophysical behaviors. As a result, they discovered a diverse class of Re-

based organometallic luminescent materials.8

1.1 Modulation of the photophysical behavior of rhenium(I) luminophores through variation of the ligand environment

Upon photoexcitation, most rhenium(I) carbonyl diimine complexes can undergo

several types of electronic transitions, such as ligand-centered9,10

(LC) transitions,

metal-to-ligand,8,11,12

intraligand,13,14

and ligand-to-ligand15,16

charge transfers (MLCT,

ILCT, and LLCT, respectively).17

The intense high-energy absorption bands in the near-UV region (200–300 nm) for

these compounds are ascribed to the π→π* LC transition, while the lower energy

bands correspond to the dπ(Re)→π*(diimine) MLCT transitions. The 1MLCT and

1LC

excited states can be populated simultaneously. The presence of a heavy atom like

rhenium in organometallic compounds increases spin-orbit coupling (SOC) and

thereby favors intersystem crossing (ISC) from the singlet to the lowest triplet state

(T1). The T1 state is responsible for the radiative de-excitation process of T1→S0

(Scheme 1).18

As a rule, the 3MLCT emitters exhibit broad, structureless emission bands, whereas an

appreciable 3LC contribution leads to the appearance of a vibronic progression in the

emission profiles. The 3MLCT and

3LC states often have comparable energies and

therefore their mixture accounts for the photophysical behavior of rhenium

luminophores.

Scheme 1. The state energy diagrams.

1MLCT and

1LC are the lowest excited singlet

states, 3MLCT and

3LC are the lowest excited triplet states, kISC is the intersystem-

crossing rate constant, kr and knr are the radiative and non-radiative rate constants,

respectively.19

Strong ILCT electronic transitions often originate in complexes that contain extended

donor-acceptor (D-A) diimines as chelating ligands. The charge transfer characteristics


9

depend on the energy levels of both donor and acceptor units, and can be fine-tuned by

altering one or both of them or by modulating the donor-acceptor communication.20

The co-existence of reducing- and oxidizing ligands in rhenium(I) complexes gives rise

to a low-lying LLCT excited state, by virtue of charge transfer from the donor

(ancillary ligand that predominantly accommodates the highest occupied molecular

orbital, HOMO) to the acceptor (diimine, where the lowest unoccupied molecular

orbital, LUMO, is localized). The large structural distortion of the excited state and its

low energy typically lead to a fast non-radiative deactivation process, making the 3LLCT states very short-lived. The LLCT states, which are “dark” in general, are

mainly studied by transient spectroscopy or indirectly by analyzing their influence on

the MLCT excited state lifetimes of the emissive species.17,21

In general, the electronic properties and the physical behavior of rhenium(I)

coordination compounds can be substantially influenced by changing the nature of the

ligand environment. In other words, variations of the non-carbonyl bidentate as well as

the ancillary ligands considerably affect the photoluminescence energies, lifetimes, and

quantum efficiencies; thereby allowing delicate tuning of the photophysical properties.

1.2 Diimine rhenium(I) complexes: Diimine variation

Many studies have tried to establish the relationships between the

structure/composition and the electronic/photophysical characteristics of luminescent

rhenium(I) species. A majority of these studies focus on the photophysics and

photochemistry of rhenium(I) diimine complexes.

Compounds of the general formula [Re(NN)(CO)3X] are conventionally prepared by

heating [Re(CO)5X] in the presence of the corresponding diimine. These carbonyl

derivatives are probably the most extensively investigated family of Re complexes

during the last 40 years, due to their facile synthesis, high stability, and tunability of

the photophysical characteristics by accessible modification of the diimine ligand.

In 1974, Wrighton and Morse published a pioneering report on the photophysics of

rhenium(I) systems based on 1,10-phenanthroline and related ligands. They

demonstrated that those chromophores undergo efficient radiative relaxation in their

lowest electronically excited state. The observed luminescence characteristics

suggested that the emission is associated with the Re → π*(diimine) charge transfer

state. In addition, the luminescence lifetimes and the quenching experiments were

largely consistent with the triplet → singlet character of the radiative transition. The

authors also described the unusual phenomenon of luminescence rigidochromism—a

strong dependence of the emission energy on the rigidity of the environment.8

In the mid-1980s, Kalyanasundaram presented a thorough account on the absorption

and emission properties of a series of polypyridyl Re(I) complexes, and studied the

sensitivity of their photophysical characteristics to changes in the polypyridyl ligand


10

and solvent. Namely, electron-donating substituents in the diimine structure induced

moderate hypsochromic shifts of the low-energy absorption and emission bands,

compared with complexes based on the polypyridyl ligands bearing electron-

withdrawing substituents (Figure 2).

Figure 2. Shift of the emission to changes in the polypyridyl ligands in complexes 1−5

in CH2Cl2.22

Furthermore, increasing the solvent polarity led to an obvious blue-shift in the

absorption. The shorter emission lifetimes and reduced quantum yields in more polar

solvents were ascribed to an increased rate of non-radiative decay to the ground state.

In the early 1990s, Demas and coworkers embarked on a detailed study of the isonitrile

rhenium(I) photosensitizers [Re(CO)3(NN)(NC(CH2)nCH3)]+, obtained by substituting

the chloride ion in the [Re(CO)3(NN)Cl] species with the isonitrile ligand. The

isonitrile complexes showed extremely high luminescence quantum yields up to 80%

and long lifetimes of greater than 130 µs at room temperature, illustrating the

possibility to adjust the photophysical properties of rhenium(I) complexes by varying

the ancillary ligands. The given synthetic approach demonstrated that accessible

structural variations of these types of compounds permit the design of systems with

remarkable optical behavior.23

Functionalization of the diimine ligands remains a popular strategy in the molecular

design of novel rhenium(I) luminophores. The rhenium-based phosphors exhibiting

circularly polarized luminescence are notable examples of this methodology (Figure 3).

The incorporation of rhenium ion into an extended helical π-conjugated bipyridine has

a profound effect on the chiroptical and photophysical properties of the organic motif,

leading to unprecedented circularly polarized phosphorescence.


11

Figure 3. Synthesis of rhenium(I) complexes 6 and enantio-enriched 7: (i) Re(CO)5Cl,

toluene, reflux; (ii) AgCF3SO3, EtOH/THF, then 2,6-dimethylphenyl isocyanide, THF,

NH4PF6, 75–80%; (iii) AgBF4, CH3CN, reflux, then pyridine, THF, 80%.24

Although the dRe orbitals do not strongly interact with the electronic π-systems of the

helical polyaromatic fragments, coordination of the metal increases the π-conjugation

area and promotes charge transfer excitations within the extended diimine ligand.24

An interesting case of dramatic alteration in the emission energy was observed for two

isomeric triazole-based rhenium(I) complexes, which contain two forms of the C2N3

heterocycle and consequently different dispositions of the 2-phenylbenzoxazole

substituents. As a result of these stereochemical alterations, the emission band of 8 in

solution is bathochromically shifted by 2390 cm-1

(82 nm) with respect to 9 (Figure 4).

Theoretical calculations pointed out that 8 has a smaller HOMO-LUMO gap than 9, in

line with the experimental observations.25

Figure 4. Absorption (black dotted lines), excitation (blue lines), and emission (red

and green lines) spectra of complexes 8 (top) and 9 (bottom) in dichloromethane;

photographs show the solutions under UV excitation, λ = 365 nm.25


12

An impressive case of prolonged excited state lifetime was found in the rhenium(I)

complex bearing the modified 1,10-phenanthroline with the naphthalimide moiety

(Figure 5). The equilibrium between the lower 3LC state of the phen-based diimine and

the 3MLCT emitting states greatly extended the excited state lifetime from 197 ns for

the parent Re(phen)(CO)3Cl to 651 µs for complex 10, but retained the same quantum

yield (1.3%).26

Figure 5. Proposed energy level diagram describing the photophysical processes of 10

in solution at room temperature. Empty arrow: absorption, filled arrow: radiative

transition, and wavy arrow: non-radiative transition.26

Rhenium complexes with bipolar chromophore scaffolds are photophysically diverse

species featuring intraligand charge transfer. The use of donor-functionalized diimines

promoted population in the low-lying ILCT states that are lower in energy than the

MLCT states.

In a series of compounds based on dppz-type ligands (Figure 6), the spectroscopic

properties were dominated by strong ILCT transitions from the electron-donating

amino groups to the electron deficient dppz moiety, while the MLCT transitions were

not perceptible. Unlike the non-coordinated functionalized diimines, these complexes

were weakly emissive.14,20

Figure 6. Schematic representation of complexes 11, 1214

and 13, 1420

featuring ILCT

processes.

The luminescent complexes 15−18 with substituted 1,10-phenanthroline retained the

dominating ligand-centered nature of the low-energy electronic transitions. However, it

was noted that enhancing the electron withdrawing properties of the auxiliary ligand

leads to a significant red-shift of the emission (Figure 7).27


13

Figure 7. Schematic representation of complexes 15−18 and their emission maxima in

CH2Cl2 at 293 K.27

Evidently, careful design of diimine ligands, which themselves act as organic dyes,

provides precious means for efficient modulation of the physical behavior of

rhenium(I)-containing luminophores.

Thus, the majority of reported rhenium complexes are represented by either well-

studied 3MLCT chromophores or weakly luminescent emitters with diverse nature in

their excited state (MLCT/ILCT, MLCT/LC, ILCT, and LC).

1.3 Diimine rhenium(I) complexes: Variation of ancillary ligands

In addition to the versatile variations of the diimine framework, changing the ancillary

ligands also proved to be another feasible way to influence the physical and chemical

properties of rhenium diimine species. As a consequence, a large number of these

derivatives bearing carbenes,28,29

alkynyl-,11,30,31

cyano/isocyano-,15,32

and thiolate33,34

groups, phosphines,35–37

and N-heterocycles38–40

have been synthesized. In particular, a

relatively simple modification of the neutral P- and N-donor ligands along with

straightforward coordination chemistry offers a convenient pathway to tune the steric

and electronic properties of the target rhenium(I) chromophores.

1.3.1 Phosphine derivatives

Three practical synthetic methods have been described for preparing the mono- and

disubstituted phosphine rhenium(I) complexes:

1) photochemical substitution;

2) thermal substitution;

3) carbonyl removal using trimethylamine N-oxide.

Originally, Wrighton discovered that 436-nm irradiation of the deoxygenated

acetonitrile solution of [Re(CO)3(phen)(CH3CN)]+ containing PPh3 and [n-

Bu4N]PF6/[n-Bu4N]ClO4 led to formation of the tricarbonyl species

[Re(CO)3(phen)(PPh3)]+(PF6/ClO4)

−.41


14

In 2000, the group of Ishitani presented a series of dicarbonyl compounds

[Re(NN)(CO)2(PR3)(L)]+ (L = NCCH3, py, PR´3), which were generated

photochemically from the monosubstituted precursor [Re(NN)(CO)3(PR3)]+ using

higher-energy excitation (λ > 330 nm). This protocol has been particularly suitable for

the synthesis of dicarbonyl complexes that contain two different phosphine ligands.35

Importantly, the reaction can be performed in a two-step manner, first producing the

labile acetonitrile intermediates [Re(NN)(CO)2(PR3)(CH3CN)]+, which can be further

thermally converted into a range of heteroleptic derivatives.

A decade later, the same group used photochemical substitution to synthesize a family

of diimine rhenium(I) complexes bearing two phosphine ligands with a variable

number of phenyl groups, and studied the related photophysical processes.

Figure 8. Rhenium(I) complexes 19−25 with variable number of alkyl and/or phenyl

groups on the phosphine ligands and their emission spectra.37

Upon increasing the number of phenyl substituents NPh from 0 to 6, the intensity of the

π–π* absorption band at around 260 nm was enhanced, and a blue-shift from 427 to

402 nm was observed for the MLCT absorption. Simultaneously, this structural

alteration resulted in visible growth in both the emission energies (Figure 8) and

quantum yields to 600 nm and 9%, respectively.37

An alternative synthetic route was suggested by Sullivan, who synthesized dicarbonyl

rhenium(I) complexes with monodentate or chelating phosphines via a thermal

substitution reaction. A new class of long-lived luminescent compounds was prepared

by refluxing either neutral [Re(CO)5Cl] or cationic [Re(CO)3(NN)(CF3SO3)] starting


15

materials with a stoichiometric amount of the appropriative phosphine (Scheme 2). All

the obtained complexes were emissive in the range of 600–640 nm.42

Scheme 2. Routes to the phosphine-diimine dicarbonyl rhenium complexes (i: o-

dichlorobenzene, reflux; ii: AgOTf, dichloromethane).42

Another pathway towards the phosphine-containing dicarbonyl halide complexes of

rhenium(I) was presented in 2015 by Felton and co-authors (Figure 9). It involves

treatment of [Re(CO)3(NN)PR3](PF6) species with 15 equiv. of tetraethylammonium

chloride (NEt4Cl) in the presence of 1 equiv. of trimethylamine N-oxide. This non-

photochemical ligand substitution should be generally applicable for preparing light-

sensitive Re(I) diimine carbonyl compounds.43

Figure 9. Synthesis of rhenium(I) complexes 26−33 using trimethylamine N-oxide.43

The influence of axial ligands on the electronic structures and optical properties of

related rhenium(I) systems showed that, in complexes 34−37, substitution of the

acetonitrile ligand by chloride and that of the carbonyl ligand by a phosphine led to

significant bathochromic shifts of the absorption and emission bands (Figure 10).44


16

Figure 10. Left: synthesis of rhenium(I) compounds 34−37 (i: AgOTf, CH3CN, 295 K,

then [NH4][PF6]; ii: AgOTf, acetone, reflux, then PMe3, acetone, reflux, after

[NH4][PF6]; iii: Me3NO, CH3CN, reflux; iv: [BnMe3N]Cl, CH2Cl2, reflux). Right: their

absorption (solid lines) and emission (dashed lines) spectra in CH3CN.44

1.3.2 Nitrogen-donor derivatives

At the end of the 1980s, Zipp, Demas, and DeGraff described various rhenium diimine

complexes decorated with substituted pyridines. These compounds of the general

composition [Re(NN)(CO)3L](ClO4), where L = substituted pyridine, were synthesized

by refluxing the starting chloride precursors [Re(NN)(CO)3Cl] with excess amount of

the appropriate pyridine in the presence of AgClO4. A detailed study of the

photophysical properties showed that the auxiliary pyridine derivatives exerted some

noticeable influence on the luminescence behavior, although the effect was less

pronounced than that caused by modification of the diimine ligands. For example,

small shifts of the emission bands as well as minor changes in the luminescence

lifetimes were observed.38

Variation of the ancillary ligands in the coordination sphere of rhenium complexes can

be realized by modification of the pyridine ligands after their coordination to the

metallocenter. For instance, the weakly emissive diamine parent species

[Re(CO)3(NN)(3,4-diaminopyridine)](CF3SO3) was converted into the triazole

counterparts by reacting with gaseous NO (Figure 11). As a result, an substantial

emission enhancement was observed, meaning that rhenium(I) complexes 38−41 could

be considered as sensors for nitric oxide (NO).40


17

Figure 11. Conversion of 3,4-diaminopyridine complexes 38–41 into their

triazolopyridine derivatives 42–45.40

Thus, the introduction of an additional chromophore center or a functional group into

the axial position of rhenium(I) diimine unit may be an easy way to produce emissive

materials with particular activities, e.g. luminescent probes.

In this respect, there is a keen interest in rhenium compounds containing ligands

capable of reversible photoisomerization (e.g. trans→cis) due to their potential uses as

photoinduced molecular sensors and switches. For example, the cis-isomers of

complexes with 4-styrylpyridine are luminescent showing broad and poorly structured

emission profiles, while the majority of the trans−forms undergo nonradiative

relaxation of the excited state.39,45

Upon excitation, the photochemical reaction could

proceed through the population of 1,3

MLCT Re→diimine state and the subsequent

intramolecular energy transfer to 3IL trans−L (photoswitchable ancillary ligand) state

(triplet pathway), or population of the 1IL trans−L state (singlet pathway). Highly

efficient trans→cis isomerization (81%) was obtained for rhenium compound 46 by

irradiation at 365 nm (Figure 12).45

Figure 12. Spectral changes of complex 46 in CH3CN under 356-nm irradiation.45


18

1.4 Polynuclear complexes built from rhenium(I) diimine blocks

The multimetallic compounds constitute an interesting class of efficient luminophores.

The construction of such assemblies is commonly based on a “complexes-as-ligands”

concept. In this approach, a di- or polydentate bridging ligand binds the metal cores in

a stepwise fashion. The nature of the linker and the coordination mode strongly

influence the electronic communication between the constituting fragments. For weak

interactions, the photophysical behavior combines properties of the two independent

chromophore centers. In the case of strong interactions, the compounds can be

considered as delocalized electronic systems with characteristics completely different

from their monomeric analogs. The vast majority of reported multi-chromophoric

rhenium metal complexes are based on {Re(NN)(CO)3} units, because of their unique

electronic and steric properties. By changing the linked metal components together

with the stereochemistry of bridging ligands, it is possible to rationally tune the

photophysical and photochemical characteristics.

1.4.1 Homometallic rhenium(I) complexes

The first work devoted to the synthesis of dinuclear homometallic complexes of

rhenium(I) dated from the beginning of 1990s. Kalyanasundaram described the

homonuclear cyano-bridged polypyridyl complex [(CO)3(bpy)Re–CN–Re(bpy)

(CO)3]+, obtained by reaction between the triflate precursor [Re(bpy)(CO)3(CF3SO3)]

and the cyanide complex [Re(bpy)(CO)3(CN)], which served as a metalloligand. It was

found that the charge transfer absorption band in the dinuclear complex was blue-

shifted and considerably broadened, with molar absorptivities being nearly two times

larger than those of the parent mononuclear species. Both the mono- and dinuclear

complexes displayed luminescence with single exponential decay, indicating that there

was no separate emissions from two different chromophores.32

Later, Yam presented a dinuclear rhenium(I) compound containing a bridging

butadiynyl ligand. The target complex 47 (Figure 13), synthesized by cross-coupling

two [Re(tBu2bpy)(CO)3(C≡C)2H] units in the presence of Cu(OAc)2, demonstrated

long-lived red emission from the 3MLCT (dπ(Re)→π*(

tBu2-bpy)) excited state.

Interestingly, upon excitation at λ ≥ 450 nm, an emission band appeared in the NIR

region and was assigned to 3MLCT (d(Re)→π*(C≡C)2) or

3LLCT (π(C≡C)2→π*(

tBu2-

bpy)) transitions.46


19

Figure 13. Dirhenium(I) complex 47 with the acetylenic bridging ligand.46

Complexes 48 and 49 (Figure 14) exemplify a dinuclear architecture, where the

rhenium fragments are connected through a flexible bridging bis-diimine ligand. In

low‐polarity solvents, these neutral compounds exhibited an unusual folded

conformation due to the interaction between the amide and one rhenium-carbonyl

group.

Figure 14. Synthesis of the flexible bridging ligands and the corresponding

dirhenium(I) complexes 48 and 49: (i) 1,2-bis(2-aminoethoxy)ethane, NEt3, dry THF,

stirring, 16 h; (ii) [Re(CO)5Br], toluene, reflux, 16 h.47

It was observed that the quantum yields of the complexes were dramatically affected

by the substitution pattern within the diimine ligand. Introducing two electron-

withdrawing groups into the 2,2’-bipyridine motifs turned the weakly luminescent

complex 48 into the non-emissive system 49.47

A substantial contribution to the development of supramolecular chemistry of rhenium

complexes was made by the group of Ishitani, who developed a strategy to synthesize

series of linear- and ring-shaped molecular assemblies.48–50

Polynuclear structures were

constructed by linking the rhenium fragments {Re(CO)2(NN)} with the bidentate

phosphine (PP) ligands. Depending on the flexibility of the PP diphosphines, two main

aggregation scenarios were realized: oligomerization of the linear rhenium(I) systems

and oligonuclear cycle formation. For instance, a multistep synthesis of the trinuclear

Re(I) ring with a rigid bridging PP ligand is illustrated in Scheme 3.


20

Scheme 3. Synthesis of the trinuclear Re(I) ring (PP = rigid bidentate phosphine

ligands; i: hν (λ > 330 nm), acetone/H2O; ii: CH3CN; iii: acetone, reflux).49

It is essential that these phosphine-bridged molecular assemblies are triplet emitters,

whose photophysical characteristics are remarkably improved by the strong interligand

π-π interactions. Specifically, in the Re-rings with ethynylene-containing PP ligands,

interactions between the unsaturated chains and the bpy ligands resulted in a 3-fold

enhancement of the emission intensity compared with analogues with alkyl

fragments.48–50

1.4.2 Heterometallic rhenium(I) complexes

The appealing physical properties and rather facile coordination chemistry of

rhenium(I) diimine blocks determined their popularity in the construction of

heterometallic assemblies. A variety of d- and f-metals were connected to the Re(I)

chromophore by means of diverse ligand systems to extend the optical functionality of

the target supramolecular compounds.

In the beginning of the 2000s, Lee and co-workers utilized a coordination-driven

assembly approach for the synthesis of luminescent heterometallic palladium(II)

(50−52) and platinum(II) (53, 54) tetranuclear square-like complexes (Figure 15).


21

Figure 15. Synthesis of heterometallic rhenium(I)–platinum/–palladium complexes

50−54.51

Their photophysical properties were dominated by the conjugated bridging ligands. It

was found that squares 50−54 comprising rigid ligands demonstrated weaker

luminescence than that predicted by the energy gap law. The small quantum yields

were interpreted in terms of quenching via Pt(II)- or Pd(II) oxidation, and energy

transfer from the emitting 3MLCT state to lower-lying non-emissive states localized on

the ferrocene groups.51

Yam described bichromophoric polypyridine alkynyl-bridged molecular rods

consisting of rhenium and platinum fragments (Figure 16). These complexes were

prepared by coupling [Re(NN)(CO)3(C≡C-(C6H4)n-C≡CH)] and [Pt(NNN)Cl](CF3SO3)

compounds, where NNN = terpyridine derivatives, in the presence of CuI and the base

Et3N.

Figure 16. The chemical structures of complexes 55−58.52

All dyads displayed orange to red photoluminescence both in fluid media and in the

solid state. The emission energies of complexes 55−58 were similar irrespectively of

the nature of the substituents on the diimine ligand on the rhenium(I) metal. It was

assumed that the observed emissions occur from the 3MLCT excited states, d

(Pt)→π*(NNN), perturbed by the rhenium diimine fragment and perhaps mixed with


22

the LLCT π(C≡C-R)→π*(NNN) or the metalloligand-to-ligand charge transfer

π(C≡C–C6H4-C≡C)→π*(NNN).52

Unlike compounds 55–58, the mixed-metal rhenium(I)–gold(I) families 59–62 (Figure

17) displayed luminescence assigned to the 3MLCT d(Re)→π*(NN) excited state.

Figure 17. Chemical structures of gold(I)–rhenium(I) systems with metallocenters

linked through the ancillary (59−62)53

and diimine (63 and 64)54

ligands.

The lack of emission from the alkynylgold(I) unit was explained by the efficient

intramolecular energy transfer from the gold(I) to the rhenium(I) moiety.53,54

Similar energy transfer processes arose in the dinuclear [Re(phen)(CO)3(µ-

CN)Ir(F2ppy)2(CN)] and trinuclear [{Re(phen)(CO)3(µ-CN)}2Ir(F2ppy)2](PF6)

rhenium(I)-iridium(III) systems, in which the chromophore units were linked by the

cyanide bridges.55

A different situation was described for the linear multinuclear rhenium systems

connected to a ruthenium fragment by a bis-diimine ligand (Figure 18). In this case,

energy transfer occurred from the rhenium unit (a photon absorber) to the ruthenium

motif (energy acceptor). Therefore, the emission behavior of these heterometallic

oligomers was mostly determined by the Ru(II) motif.56

Figure 18. The Ru(II)-Re(I) multinuclear complexes 65−67.

56


23

1.5 Applications

Feasible modulation of the photophysical properties along with the functionalities and

reactivity of rhenium(I) complexes has made them candidates for a range of practical

applications.

1.5.1 Organic light emitting diodes (OLEDs)

One of the strategies to achieve high-performance OLEDs is to utilize phosphorescent

compounds that have the intrinsic advantage of maximum (100%) internal quantum

efficiency of electroluminescence.57–59

Thermally stable mononuclear rhenium(I) compounds with efficient luminescence

have been used as transition metal complexes in the fabrication of phosphorescent

OLEDs.60–62

Li and co-workers designed neutral Re(I) systems that showed

significantly improved electroluminescent properties63–66

and exhibited strong

electrogenerated emission (Figure 19). Complex 70 has a maximum current, power,

external quantum efficiency, and CIE coordinates of 36.5 cd A−1

, 31.7 lm W−1

, 12.0%,

and (0.46, 0.52), respectively. An EL device based on it was among the best orange

OLEDs with rhenium(I) dopant emitters reported so far.63

Figure 19. Structures of complexes 68−70 used as dopant emitters in

electroluminescent devices.63

1.5.2 Sensors

Luminescent rhenium(I) complexes containing recognition units that interact with

analytes have emerged as a versatile class of molecular sensors.67–70

The examples of rhenium-based probes are designed for selective determination of

metal cations. The chelating coordination function in the ancillary (Figure 20A) or

diimine ligand (Figure 20B) of the rhenium chromophores makes them responsive to

the presence of selected divalent metal cations. The complexation of Cd(II) or Zn(II)

ions by the tetrazolato-quinoline ligands blue-shifts the emission of the rhenium(I)

compound 71, and causes a substantial increase of phosphorescence intensity and

elongation of the emission lifetimes.70

Similarly, the addition of Cu(II) ion to the

chemosensor 72 leads to a significant enhancement in the emission intensity and


24

excited state lifetime of the Re(I) complex due to the suppression of C=N

isomerization.67

Figure 20. A: Complexation of complex 71 with Zn(II), and the corresponding

photographs of CH2Cl2 solutions of 71 before and after the addition of Zn(II) salt.70

B:

Complexation of complex 72 with Cu(II).67

1.5.3 Photo-activated carbon monoxide releasing molecules (PhotoCORMs)

Recently, a promising approach to the controlled delivery of CO molecules for

therapeutic applications is using carbonyl organometallic compounds, which are stable

in the dark in aqueous solution for an extended period of time to afford sufficient

accumulation in the target biological structures.

The low toxicity and tunable properties of Re(I)-based carbonyls ensured their growing

usage as PhotoCORMs.71–74

For example, the photolytic liberation of carbon monoxide

from complex 73 (Figure 21) is accompanied by a change of the orange

phosphorescence (605 nm) to blue fluorescence (400 nm). This allows one to track the

entry and delivery of this CO donor within the target localization.72


25

Figure 21. Time-resolved luminescence traces at 2 min intervals of 73 in CH3CN upon

UV excitation (λ < 310 nm).72

1.5.4 Electrocatalysis

High-performance catalysts can facilitate reduction of CO2 emission into the

environment. The advantages of utilizing rhenium(I) complexes for electrocatalytic and

photocatalytic reductions are first and foremost determined by the availability of at

least two oxidation states, the presence of accessible coordination site(s), as well as

high selectivity in the conversion of CO2 to CO.75–77

Water solubility played a

substantial role in the development of compound 74 as an electrocatalytic system for

the reduction of CO2, where aqueous solutions acted as the electron source. The titled

complex was used as a molecular catalyst in the electrochemical cell for CO2-to-CO

conversion (Figure 22), which proceeded with high total faradaic efficiency (81%) and

high selectivity of CO2 reduction (98%) even in acidic solution.75

Figure 22. Schematic illustration of the full electrochemical cell for CO2-to-CO

conversion.75


26

1.5.5 Bio-imaging

The major reason of exploiting triplet emissive rhenium(I) compounds with the general

formula [Re(diimine)(CO)3(L)]0/1+

as cellular imaging probes is that their

photophysical properties are mainly guided by the bidentate diimine ligand. Thus, the

cellular uptake and localization properties of these complexes can be conveniently

adjusted via functionalization of the monodentate auxiliary ligand L without affecting

the luminescence behavior.78–81

Rhenium(I) complex 75 containing a fructose pendant group displayed the

appropriative photoluminescent characteristics (λems = 553 nm, τ = 2.9 µs, Φ = 18% in

phosphate-buffered saline) as well as high cell permeability and organelle selectivity. It

was successfully applied for imaging breast adenocarcinoma cell lines (Figure 23).

Additionally, the given compound exhibited photocytotoxic activity caused by 1O2

generation upon irradiation at λ>365 nm, making it an efficient photodynamic therapy

agent.82

Figure 23. Confocal images of four different types of cancer cells upon incubation

with complex 75 (50 μM, 1 h) at 310 K.82


27

1.6 Aims of the study The rational design and synthesis of rhenium(I) carbonyl species with adjustable

electronic parameters and tailored functional groups are very important for prospective

applications that require molecular materials with certain photophysical and chemical

behavior. The foregoing survey of literature data shows that the diimine ligands

generally play a key role in the photophysics of the archetypal rhenium-based systems

of [Re(CO)3(NN)(L)]. Therefore, electronic changes of the NN constituents are

primarily responsible for variation in the optical characteristics of rhenium derivatives.

However, it is clear that their luminescence can also be substantially influenced by

altering the ancillary ligand(s) or by coupling the emissive Re(I) motifs with additional

chromophore units.

My research focused on the development of rhenium-based luminescent compounds,

with an aim of establishing the relationship between their structures and photophysical

properties. The chosen molecular design relied on the electronic and stereochemical

modifications of the constituting fragments, following the strategy summarized in

Scheme 4.

Scheme 4. Illustration of the research objectives.


28

(I) The first objective deals with the methodological approach to efficiently

luminescent rhenium dicarbonyl compounds that are axially substituted with ancillary

phosphine ligands having variable donor ability and the conjugated substituents.

(II) In the second task, I considered systematic alteration of the new type of diimine

ligands tailored to the secondary chromophore system for constructing bichromophore

rhenium dyes containing donor−acceptor organic motifs.

(III) In the third direction, the rhenium diimine tricarbonyl block was utilized to design

series of homo- and heterometallic assemblies through merging two or more metal-

based chromophores. Here, I intended to investigate two approaches, in which the

target polynuclear compounds would differ in the rigidity and degree of conjugation

between luminescent units.


29

2 EXPERIMENTAL All reactions (except the heterometallic systems described in publication IV) were

performed under an inert atmosphere of nitrogen using standard Schlenk techniques.

2.1 Syntheses

2.1.1 Ligands

Commercially available diimines (1,10-phenanthroline (phen) and 2,2’-bipyridine

(bpy)) and aminopyridines (4-aminopyridine (pyN1), 4-(aminomethyl)pyridine

(pyN2), 4-(2-aminoethyl)pyridine (pyN3)) were used as received. Ancillary alkynyl-

phosphine ligands (P1,83

P4,84

PP1,85

and PP285

) were prepared according to the

published procedures, which involve lithiation of the terminal alkynes and subsequent

treatment with diphenylchlorophosphine (Scheme 5). The novel phosphines P2, P3,

P5, and P6 were obtained following this general protocol as described in publication I.

Scheme 5. Synthesis of alkynyl-phosphine ligands: (i) n-BuLi dropwise, Et2O/hexane,

195 K, stirring, 5 min, then 243 K, 1.5 h; (ii) PPh2Cl dropwise, 195 K, stirring, then

298 K, stirring, 12 h; (iii) PPhCl2 dropwise, 195 K, stirring, then 298 K, stirring, 2 h.

2.1.2 Metal complexes

Neutral rhenium(I) complexes [Re(NN)(CO)3Cl] (NN = bipyridine, phenanthroline)

were prepared according to the literature procedure8,86

by reacting the corresponding

diimine with the commercially available pentacarbonyl [Re(CO)5Cl]. The cationic

starting materials [Re(NN)(CO)3(NCMe)](CF3SO3)87

and

[Re(NN)(CO)3(H2O)](CF3SO3)88

were obtained by treating their rhenium(I) chloride

precursors with AgCF3SO3 in acetonitrile and wet acetone solvents, respectively.

To prepare the heterobimetallic Re–Pt species presented in publication III, the labile

platinum compounds Pt(ppy)Cl(dmso)89

and [Pt(dpyb)(acetone)](CF3SO3)90

were

obtained as described in the corresponding literature.


30

All novel homo- and heterometallic rhenium(I) complexes summarized in Table 1 were

synthesized using the general approaches of coordination chemistry. The details are

given in the original publications I−IV and in the following chapters.


31

Tab

le 1. R

hen

ium

(I) com

plex

es obtain

ed in

this stu

dy

N

ame

Com

plex

D

iimin

e

ligan

d

Ancillary

ligan

d

Publica-

tion

Cry

stal

structu

re

1.

R

e(P1)

2 [R

e(phen

)(CO

)2 (P

1)

2 ](CF

3 SO

3 ) phen

P

1

I

2.

R

e(P2)

2 [R

e(phen

)(CO

)2 (P

2)

2 ](CF

3 SO

3 ) phen

P

2

I

3.

R

e(P3)

2 [R

e(phen

)(CO

)2 (P

3)

2 ](CF

3 SO

3 ) phen

P

3

I

4.

R

e(P4)

2 [R

e(phen

)(CO

)2 (P

4)

2 ](PF

6 ) phen

P

4

I

5.

R

e(P5)

2 [R

e(phen

)(CO

)2 (P

5)

2 ](PF

6 ) phen

P

5

I

6.

R

e(P6)

2 [R

e(phen

)(CO

)2 (P

6)

2 ](PF

6 ) phen

P

6

I

7.

(R

ePP

1)

2 [{

Re(p

hen

)(CO

)2 }

2 (PP

1)

2 ](CF

3 SO

3 )2

phen

P

P1

I

8.

(R

ePP

2)

2 [{

Re(p

hen

)(CO

)2 }

2 (PP

2)

2 ](CF

3 SO

3 )2

phen

P

P2

I

9.

R

eNN

1

[Re(C

O)

3 (NN

1)C

l] N

N1

Cl

II

10.

ReN

N1-C

N

[Re(C

O)

3 (NN

1)C

N]

NN

1

CN

II

11.

ReN

N2

[Re(C

O)

3 (NN

2)C

l] N

N2

C

l II

12.

ReN

N3

[Re(C

O)

3 (NN

3)C

l] N

N3

Cl

II

13.

ReN

N4

[Re(C

O)

3 (NN

4)C

l] N

N4

C

l II

14.

ReN

N5

[Re(C

O)

3 (NN

5)C

l] N

N5

C

l II

15.

Re(p

yN

1)

[Re(p

hen

)(CO

)3 (p

yN

1)](C

F3 S

O3 )

phen

pyN

1

III

16.

Re(p

yN

2)

[Re(p

hen

)(CO

)3 (p

yN

2)](C

F3 S

O3 )

phen

pyN

2

III

17.

Re(p

yN

3)

[Re(p

hen

)(CO

)3 (p

yN

3)](C

F3 S

O3 )

phen

pyN

3

III

18.

Re(p

yN

2)P

t(pp

y)

[Re(p

hen

)(CO

)3 (p

yN

2)P

t(ppy)C

l]

(CF

3 SO

3 )

phen

pyN

2

III

19.

Re(p

yN

2)P

t(dp

yb

) [R

e(phen

)(CO

)3 (p

yN

2)P

t(dpyb)]

(CF

3 SO

3 )2

phen

pyN

2

III

20.

Re(p

yN

3)P

t(pp

y)

[Re(p

hen

)(CO

)3 (p

yN

3)P

t(ppy)C

l]

(CF

3 SO

3 )

phen

pyN

3

III

21.

Re(p

yN

3)P

t(dp

yb

) [R

e(phen

)(CO

)3 (p

yN

3)P

t(dpyb)]

(CF

3 SO

3 )2

phen

pyN

3

III


32

22.

Re

2 −b

py

[{R

e(bpy)(C

O)

3 }2 C

N](C

F3 S

O3 )

bpy

CN

IV

23.

Re

2 −p

hen

[{

Re(p

hen

)(CO

)3 }

2 CN

](CF

3 SO

3 ) phen

C

N

IV

24.

Ph

3 PR

e2 −

ph

en

[{R

e(phen

)(CO

)2 (P

Ph

3 )}2 C

N]

(CF

3 SO

3 )

phen

C

N

IV

25.

ReA

uP

Ph

3 −p

hen

[{

Re(p

hen

)(CO

)3 }

NC

Au(P

Ph

3 )]

(CF

3 SO

3 )

phen

C

N

IV

26.

Re

2 Au

−b

py

[{R

e(bpy)(C

O)

3 NC

}2 A

u](C

F3 S

O3 )

bpy

CN

IV

27.

Re

2 Au

−p

hen

[{

Re(p

hen

)(CO

)3 N

C}

2 Au](C

F3 S

O3 )

phen

C

N

IV

28.

Re

4 Pt−

bp

y

[{R

e(bpy)(C

O)

3 NC

}4 P

t](CF

3 SO

3 )2

bpy

CN

IV

29.

Re

4 Pt−

ph

en

[{R

e(phen

)(CO

)3 N

C}

4 Pt](C

F3 S

O3 )

2 phen

C

N

IV

30.

Re

6 Fe

2−b

py

[{R

e(bpy)(C

O)

3 NC

}6 F

e](CF

3 SO

3 )2

bpy

CN

IV

31.

Re

6 Fe

3−b

py

[{R

e(bpy)(C

O)

3 NC

}6 F

e](CF

3 SO

3 )3

bpy

CN

IV


33

2.2 Characterization

All novel complexes were studied in solution by NMR and FT-IR spectroscopies and

ionization electrospray mass spectrometry (ESI-MS). Most of the rhenium compounds

in Table 1 were isolated as crystalline materials suitable for single-crystal X-ray

diffraction analysis. Their molecular structures in the solid state were in agreement

with spectroscopic data obtained in the fluid medium. The only exception was the

complex ReAuPPh3−phen (publication IV), which demonstrated reversible

dissociation and rearrangement in solution as confirmed by NMR and ESI-MS

measurements. Phase identification of the ReAuPPh3−phen crystalline material was

carried out by X-ray powder diffraction analysis. The purity of the studied species was

confirmed by microanalyses carried out at the analytical laboratory of the University of

Eastern Finland.

The photophysical measurements (UV/vis absorption, excitation and emission spectra,

excited state lifetimes, and emission quantum yields) for all titled compounds were

carried out using the core facilities of St. Petersburg State University and the scientific

equipment of Peter the Great St. Petersburg Polytechnic University with the support of

Dr. Alexei Melnikov.

Supplementary magnetic and electrochemical studies were performed by collaborating

groups at RWTH Aachen University, Germany (Prof. Kirill Monakhov, now at Leibniz

Institute of Surface Engineering) and University of Valencia, Spain (Prof. Antonio

Doménech-Carbó). The computational analysis was done at the University of Eastern

Finland (Dr. Pipsa Hirva, MSc. Toni Eskelinen) and Aalto University, Finland (Prof.

Antti J. Karttunen).


34

3 RESULTS AND DISCUSSION 3.1 Alkynylphosphine rhenium(I) complexes

The prototype moiety {Re(CO)3(phen)}, which generally offers high luminescence

efficiency15,91

but often suffers from limited photostability,92–94

was utilized for

constructing robust rhenium phosphine-disubstituted platform. An expeditious route to

the target complexes relied on the displacement of carbonyl ligands in the metal

coordination sphere by thermally stable phosphines under extreme temperature

conditions. Luminescence behavior of the final Re(I) compounds was modulated

through the extension of conjugated substituents in the alkynylphosphine ligands.

The family of monodentate P1–P6 and bidentate PP1 and PP2 phosphine ligands were

readily obtained following a conventional protocol shown in Scheme 6. Alkynyl-

phosphines were incorporated into the coordination sphere of rhenium(I) species by

one-step thermal substitution, reacting [Re(CO)3(phen)(NCMe)](CF3SO3) compound

with excess amount of the corresponding P-donor ligand. Following this strategy,

phosphine dicarbonyl rhenium(I) complexes Re(P1)2−Re(P6)2, (RePP1)2, and

(RePP2)2 were generated using an autoclave technique under a nitrogen pressure of 30

atm. For selective isolation of the disubstituted species, the synthetic route was

performed under optimized conditions, where the mononuclear complexes were

obtained at 175−200 °C, while dinuclear congeners required elevated temperatures

(220 °C). Note that in contrast to the titled complexes with mono- and

dialkynylphosphines, no target compounds bearing the trialkynylphosphines P(C≡CR)3

(R = Ph, biphenyl) were obtained due to degradation of the reagents.

Scheme 6. Synthesis of the alkynylphosphine dicarbonyl rhenium(I) complexes.

All rhenium(I) complexes are air- and moisture-stable compounds. Single-crystal XRD

analysis was carried out for Re(P1)2−Re(P3)2 and (RePP1)2 species for unequivocal

identification of their molecular structures. These dicarbonyl compounds revealed

octahedral coordination geometry of the metal centers. The phosphine ligands are

located at the axial positions, i.e. above and below the equatorial plane defined by the

phenanthroline fragment and the CO groups (Figure 24), in line with the previous

studies.35–37

A visible deviation of the P–C≡C fragments from linear structure (with

angles of 162−179°) is eventually induced by intermolecular interactions in

Re(P1)2−Re(P3)2 and (RePP1)2, and is not exceptional.36,37


35

Figure 24. Molecular views of complexes Re(P1)2 and (RePP1)2. Hydrogens and

CF3SO3− counter ions were omitted for clarity.

In solution, the synthesized phosphine ligands and the related complexes were

unambiguously characterized by NMR, FT-IR, and ESI-MS spectroscopic methods.

For all complexes, two intense IR bands at 1880–1896 and 1950–1963 cm−1

were

attributed to symmetric and asymmetric stretching vibrations of the two adjacent CO

groups. The C≡O stretches in Re(P4)2−Re(P6)2 occurred at higher frequencies in

comparison with those in Re(P1)2−Re(P3)2, due to decreased electron density on the

Re(I) centers caused by the more π–accepting phosphines. The 1H and

31P NMR

spectroscopic patterns reflected the presence of a symmetrical stereochemical

environment around the central rhenium atom in the final compounds.

All rhenium(I) species were intensely luminescent, with the emission mainly

determined by 3MLCT rhenium→diimine electronic transitions (Table 2), as is typical

for the rhenium(I) chromophores.8,95,96

In dichloromethane solutions at ambient temperature, the positions of the absorption

and emission maxima as well as the luminescence intensity of rhenium(I) complexes

(Re(P1)2−Re(P6)2, (RePP1)2, and (RePP2)2) were virtually identical, pointing to the

innocent character of the phosphines with respect to the energies of the electronic

transitions. Referenced to the previous spectroscopic investigations of related Re(I)

systems,36,37,97,98

the phosphorescence quantum yields of the titled family are among the

highest (up to 44%) for reported phosphine-containing emitters.

In contrast to the solution medium, the solid-state photophysical characteristics of the

described rhenium-based systems were affected by the ancillary alkynylphosphines.


36

Figure 25. Normalized emission spectra of the crystalline complexes Re(P1)2−Re(P3)2

and Re(P1)2 in solution (CH2Cl2) at 298 K.

For instance, the emission bands were gradually red-shifted upon extending the

phenylene chain in the phosphine ligands (Figure 25). Owing to the phenomenon of

luminescence rigidochromism,99–101

a blue-shift in the solid state relative to the solution

was observed for this series of rhenium(I) emitters.

Table 2. Photophysical properties of complexes Re(P1)2−Re(P6)2, (RePP1)2, and

(RePP2)2 in solution (CH2Cl2) and in the solid state at 298 K.

λabs, nm

(ε, .104 M

−1 cm

−1)

a λem,

nm

b λem,

nm

a Φ, %

(aer/degas) a τ, μs

Re(P1)2 271 (2.9), 298 (1.8),

384 (0.4) 608 533 15/34 1.04

Re(P2)2 295 (4.0), 383 (0.3) 608 562 17/34 1.03

Re(P3)2 274 (2.7), 313 (5.3),

383 (0.3) 607 577 15/30 1.03

Re(P4)2 277 (3.4), 300 (1.9),

382 (0.4) 601 581 17/42 0.99

Re(P5)2 297 (6.3), 310 (6.6),

379 (0.6), 415 (0.4) 601 588 16/44 0.97

Re(P6)2 275 (4.6), 325 (8.2),

410 (0.4) 601 619 10/27 0.98

(RePP1)2 277 (6.4), 295 (7.4),

410 (0.7) 600 598 11/30

1.08 (90 %),

0.30 (10 %)

(RePP2)2 274 (5.2), 316 (9.0),

408 (0.7) 610 608 7/13

0.23 (26 %)

0.90 (74 %)

aCH2Cl2 solution (λext 385 nm).

bSolid state.

To provide further insight on the optical properties, quantum chemical analysis was

carried out for the electronic structures. The calculation results for complexes


37

Re(P1)2−Re(P6)2 show some overestimation of the absorption and emission

wavelengths, while the simulated T1→S0 emission wavelengths for complexes

(RePP1)2 and (RePP2)2 are rather close to the experimental data (Table 3).

Table 3. Computational estimation of the photophysical properties for complexes

Re(P1)2−Re(P6)2, (RePP1)2, and (RePP2)2.

a λabs S0→S1,

nm

λem T1→S0,

nm

Re(P1)2 438 (0.01) 661

Re(P2)2 446 (0.01) 669

Re(P3)2 477 (<0.01) 623

Re(P4)2 435 (0.04) 680

Re(P5)2 448 (0.02) 689

Re(P6)2 470 (0.01) 687

(RePP1)2 459 (0.23) 615

(RePP2)2 467 (0.25) 628

a Oscillator strengths are given in parentheses.

For all rhenium(I) species, the corresponding transitions are mainly of dπ(Re)–

π*(phen) 1,3

MLCT character, with negligible contributions from the alkynyl fragments

of the phosphines to the emissive excited states (Figure 26).


38

Figure 26. Electron density difference plots for the lowest-energy singlet excitation

(S0→S1) of systems Re(P1)2, Re(P4)2, and (RePP2)2. During the electronic transition,

the electron density increases in the blue areas and decreases in the red areas.

Research discussed in this section focused on the new phosphine-substituted rhenium

complexes to improve their photoluminescence properties. Although the ancillary

phosphine ligands showed little influence on the emission energy in solution, they

clearly affect the behavior in the solid state. Moreover, the developed synthetic

approach demonstrates a quite facile route to the rhenium compounds with high

quantum efficiency and remarkable stability, which are the prominent advantages of

these systems over most of other rhenium emitters.

3.2 Rhenium(I) complexes based on the extended diimine ligands

One of the approaches to alter the photophysical properties of rhenium(I)-based

molecular architectures relied on the coordination of the presynthesized chromophore

diimines to the metal center.20,25,102

For this purpose, the pyridyl-phenanthroimidazole

core was selected as the ligating unit due to its straightforward synthesis and

functionalization, as well as efficient binding toward the transition metal ions.103

Integration of secondary chromophore moieties into the aforementioned bidentate

platform was anticipated to tune the optical properties of the resulting

[Re(CO)3(NN)Cl] species.

Modular platforms NN1 and NN2, conventionally prepared by Debus-Radziszewski

condensation reaction, were converted into the bichromophore compounds NN3−NN5


39

containing electron-rich anthracene-derived functional groups via Sonogashira cross-

coupling reaction (Scheme 7).

Scheme 7. Synthesis of NN1−NN5 and schematic structures of rhenium complexes

ReNN1 to ReNN5 and ReNN1-CN.

The target tricarbonyl complexes ReNN1−ReNN5 were obtained following the

standard procedure.8 Exchange of the chloride ligand by cyanide was accomplished by

chloride abstraction with silver cyanide to give the compound ReNN1-CN.

In accordance with the X-ray crystallographic studies of NN1 and NN5, the nearly flat

phenanthro-imidazole fragment was almost perpendicular to the N-bounded phenyl

ring, which probably diminished the conjugation between the polyaromatic units in the

bichromophore ligands NN3−NN5. The coordination of the bulky diimines to the

rhenium(I) metallocenter through the pyridyl-imidazole chelating moiety caused steric

hindrance between the equatorial carbonyl ligand and the adjacent H−C group of

phenanthrene. The result was substantial geometry distortion that shifted the metal

atom out of the plane of the diimine. In all these complexes (ReNN1 to ReNN5 and

ReNN1-CN), the rhenium ions had the distorted octahedral environment typical for the

tricarbonyl diimine compounds (Figure 27).104–106


40

Figure 27. Molecular views of the complex ReNN5.

Spectroscopic measurements confirmed that the given diimine ligands and the

corresponding complexes retained their structures in solution. Particularly, the 1H

NMR spectra demonstrated a number of clearly resolved multiplets in the aromatic

region that pointed to the stereochemical rigidity of the titled compounds (Figure 28).

Figure 28. 1H NMR spectra of the ligand NN2 and complexes ReNN2 (400 MHz,

DMSO-d6, 298 K).


41

The photophysical studies of these species mainly concerned their solution behavior

(Table 4, Figures 29–31). The extended π-conjugated phenanthrene skeleton endowed

NN1 and NN2 with 1ππ*phenanthrene LC fluorescence, which was virtually invariant to the

para-substituent in N-bounded phenyl ring. Both ligands produced similar vibronically

structured deep blue emission with a maximum at around 380 nm and the quantum

yields of 37% and 22%, respectively. Tailoring various anthracene-containing units to

the phenanthro-imidazole core differentiated NN3, NN4, and NN5 chromophores

according to their optical properties.

Figure 29. Normalized emission spectra of NN1−NN5 (solid lines) and ReNN5

(dotted line) (CH2Cl2, 298 K).

Compound NN3 displayed luminescence nearly identical to that of reported anthracene

derivatives,107,108

with a quantum efficiency of 62%. Expansion of the π-system and

introduction of the electron-donating groups in dyes NN4 and NN5 changed the

character of the electronic transitions and significantly decreased the optical band

gap.20,109,110

In CH2Cl2 solutions, NN4 and NN5 showed highly intense structureless 1ILCT luminescence (Φem up to 92%) in the yellow (566 nm) to orange (602 nm)

region. Moreover, diimines NN4 and NN5 exhibited distinct fluorescence

solvatochromism attributed to the intramolecular charge transfer due to their donor-

acceptor (D-A) architecture.20,111–113

The emission bands of the latter displayed

substantial red shifts with increasing solvent polarity from cyclohexane to DMF

(Figure 30A).

The N-donor functions in NN1−NN5 enabled reversible switching of the photophysical

properties by protonation, which was also confirmed by NMR spectroscopy. For

instance, upon treating NN1 with CF3COOH, the luminescence spectrum showed two

new broad bands at ca. 413 and 517 nm that were bathochromically shifted with

respect to the initial signal (380 nm). This observation was attributed to the protonation

of the pyridyl-imidazole site, which presumably activated the emissive 1ILCT

phen→py excited state.114


42

Figure 30. (A) Normalized emission spectra of NN5 at 298 K in various solvents, with

the inset showing their visual appearance under UV light (λext = 365 nm). (B) Effect of

protonation on the photoemission spectrum of NN5 (c = 10−5

M, CH2Cl2, 298 K).

The protonation site, which mainly affected the optical characteristics in NN5, was the

Namino

donor. The addition of CF3COOH to a solution of NN5 (Figure 30B) converted

the broad 1ILCT emission band centered at 602 nm to the structured

1ππ*anthracene LC

fluorescence band at around 490 nm. The original luminescence of the titled

compounds was restored upon neutralizing the acid with a stronger base, e.g. 1,8-

diazabicyclo[5.4.0]undec-7-ene (DBU).

Table 4. Photophysical properties of ligands NN1 to NN5 and related complexes

ReNN1 to ReNN5 and ReNN1-CN in solution (CH2Cl2, 298 K).

a λem,

nm

a Φ,

% τ, ns a λem, nm a Φ, % τ, ns

NN1 370,

388 37 1.9

ReNN1 369, 392,

408, 616 2 (616 nm)

2.6 (392 nm)

186 (616 nm)

ReNN1-CN 582 20 3550

NN2 370,

388 22 1.9 ReNN2

371, 392,

407, 624 2 (624 nm)

2.1 (405 nm)

142 (624 nm)

NN3

433,

459,

486

65 3.3 ReNN3 436, 460,

486(sh) 3 4.1

NN4

446w,

475w,

566

52 2.1 (446 nm)

1.4 (566 nm) ReNN4 572 10 4.5

NN5 484w,

602 92

2.6 (484 nm)

3.8 (602 nm) ReNN5 614 22 2.7

aλext = 260 nm (NN1, NN2), 360 nm (NN3), 365 nm (NN4, NN5, ReNN1 to ReNN5,

and ReNN1-CN).


43

In the fluid medium, coordination compounds ReNN1 to ReNN5 and ReNN1-CN

demonstrated moderate photoluminescence with Φem up to 22%. Remarkably, the

excitation of ReNN1 and ReNN2 gave rise to dual luminescence, with two bands at ca.

400 and 620 nm corresponding to 1LC and

3ML’LCT origins, respectively (Figure 31).

The proposed assignment was based on the strikingly different dependence of the

intensity of these bands on the presence of molecular oxygen and the shapes of the

emission profiles. Notably, the phenomenon of dual emission is scarce among

rhenium(I) diimine systems.115–118

Figure 31. Normalized emission spectra of ReNN1-CN and ReNN2, the

corresponding spectrum of NN2 (filled) is shown for comparison (degassed CH2Cl2,

298 K).

In the case of ReNN1-CN, the dual luminescence disappeared, apparently as a result of

a change in the nature of the emissive excited state from mainly 3ML’LCT to the

mixture 3(LC+ML´LCT). Compared to ReNN1, ReNN1-CN revealed a blue-shifted

(by 34 nm, 950 cm-1

) and poorly structured emission profile (Figure 31) together with a

10-fold increase in quantum efficiency (from 2% to 20%).

Interestingly, complexes ReNN3−ReNN5 demonstrated optical characteristics that are

very similar to those of the parent ligands NN3−NN5 (Figure 29). This observation

implied that the emissions of ReNN4 and ReNN5 were dominated by the 1ILCT

transitions, which was also confirmed by the sensitivity to protonation and to the

solvent polarity. In contrast, most of previously reported rhenium(I) luminophores

principally displayed 3MLCT/

3IL emission

105,119 or demonstrated the formation of dark

3ILCT state,

14,120 which fundamentally distinguish them from ReNN3−ReNN5

complexes showing relatively efficient intra-ligand fluorescence.

These results illustrated that the pyridyl-imidazole core combined with tricarbonyl

rhenium(I) unit can produce complexes with versatile photophysical properties.

Compounds ReNN1 and ReNN2 displayed excitation-dependent singlet/triplet dual

emission, while changing the ancillary X ligand switched the luminescence of ReNN1-

CN to exclusively triplet in origin. The 1ILCT emissive states, which are

unconventional for rhenium luminophores, were realized for ReNN4 and ReNN5


44

species by using the extended ligands with secondary chromophore fragments of a

donor-acceptor nature. Furthermore, this family of chromophore-functionalized

diimines can potentially serve as useful building blocks to generate complexes of many

other transition metals.

3.3 Bichromophore rhenium(I)–platinum(II) complexes based on the rhenium(I) aminopyridine predecessors

Another direction in the development of di-component emitters was compounds

comprising both the rhenium(I) and platinum(II) phosphorescent units. Tethering the

emissive cycloplatinated fragments {Pt(ppy)Cl}121,122

and {Pt(dpyb)}+ 90,123

to the

{Re(phen)(CO)3} chromophore was achieved by utilizing the bifunctional

aminopyridine ligands with different lengths in the spacers. This approach assumed

that the photophysical features of each component are preserved in the bichromophore

complex.

The preparation of rhenium(I) complexes Re(pyN1)−Re(pyN3) relied on site-selective

coordination of the heterodentate aminopyridines to the

[Re(CO)3(phen)(NCMe)](CF3SO3) precursor (Scheme 8).

Scheme 8. Synthesis of the aminopyridine rhenium complexes and the rhenium(I)–

platinum(II) complexes.

The 1H NMR spectra showed a distinct difference in chemical shifts of the low-field

signals of the diimine protons for Re(pyN1) and the pair Re(pyN2), Re(pyN3),

consistent with two types of coordination of aminopyridines to the Re(I) center. As


45

indicated by X-ray crystallography data (Figure 32), linking pyN1 to rhenium diimine

unit occurred through the pyridine N-donor, while pyN2 and pyN3 were bonded to the

metal through the primary amine group. It should be noted that the Namine

coordination

mode has been found in only a few cases of Re(I) complexes.124,125

The mononuclear rhenium complexes Re(pyN1)−Re(pyN3) were 3MLCT-type

emitters (Figure 33 and Table 5), similar to other rhenium(I) species with the ancillary

N-heterocycles.38,40,126

Re(pyN1) displayed a bathochromic shift of the emission

maximum (by 10 nm) and reduced quantum efficiency (around 2 times) with respect to

Re(pyN2) and Re(pyN3), while the photophysical characteristics of the latter two

complexes were almost identical. However, in the solid phase, the different rigidity of

the crystal packing led to the dissimilar photophysical behaviors of these rhenium

congeners (Table 5).

Figure 32. Molecular views of complexes Re(pyN2)Pt(dpyb) and Re(pyN3)Pt(ppy).

CF3SO3− counter ions were omitted for clarity.

Further construction of bimetallic complexes Re(pyN2)Pt(ppy), Re(pyN2)Pt(dpyb),

Re(pyN3)Pt(ppy), and Re(pyN3)Pt(dpyb) was achieved by merging

Re(pyN2)/Re(pyN3) predecessors with the platinum(II) building blocks via Npy

vacant

coordination function of the aminopyridine ligands (Scheme 8). The compositions and

structures of the dinuclear assemblies were established by means of IR and NMR

spectroscopies, ESI-MS, and X-ray crystallography. The single-crystal structures

confirmed the assembly of the heterometallic cations Re(pyN2)Pt(ppy),

Re(pyN2)Pt(dpyb), Re(pyN3)Pt(ppy), and Re(pyN3)Pt(dpyb). These assemblies

consisted of the distorted octahedral rhenium moieties and the square-planar platinum

fragments bridged by pyN2 and pyN3 (Figure 32). Thus, spatially separated metal ions

appeared in their typical geometries.122,126–128

The solution 1H NMR spectroscopic data

of the rhenium(I)–platinum(II) architectures clearly indicated retention of the

molecular structures in solution.


46

Table 5. Photophysical properties of complexes Re(pyN1)−Re(pyN3),

Re(pyN2)Pt(ppy), Re(pyN2)Pt(dpyb), Re(pyN3)Pt(ppy), and Re(pyN3)Pt(dpyb) in

solution (CH2Cl2) and in the solid state at 298 K.

λabs, nm

(ε, .104 M

−1 cm

−1)

a λem,

nm

a Φ, %

(aer/

degas)

a τ, μs b λem, nm

b Φ,

%

b c τ, μs

Re(pyN1) 360 (0.4), 271 (4.6),

262 (4.4) 580 6/14 0.7 550 9 1.4

Re(pyN2) 362 (0.3), 324 (0.5),

276 (2.1), 258 (2.0) 570 15/29 1.5 517 20 2.5

Re(pyN3) 360 (0.3), 322 (0.5),

275 (2.0), 257 (2.0) 570 14/25 1.4 571 4 1.2

Re(pyN2)

Pt(ppy)

369 (0.5), 340 (0.9),

324 (1.1), 275 (3.5),

258 (4.2), 247 (3.8)

566 6/10 0.5 (85%)

1.4 (15%)

479, 502,

516, 537 2

d 1.1

e 1.3

Re(pyN2)

Pt(dpyb)

399 (0.5), 377 (0.8),

361 (0.7), 329 (1.0),

278 (3.4), 256 (3.6)

491,

524,

559

f 8/17 2.7

490, 525,

565, 611 2

d 0.6

e 1.4

Re(pyN3)

Pt(ppy)

371 (0.5), 340 (0.8),

324 (1.0), 274 (3.6),

257 (4.5), 249 (4.2)

570 4/8 0.4 (88%)

1.3 (12%)

492, 526,

555 1

d 0.7

e 1.3

Re(pyN3)

Pt(dpyb)

400 (0.6), 378 (0.8),

363 (0.7), 330 (1.0),

287 (2.6), 276 (3.1)

491,

527,

561

f 8/28 1.6 493, 600 5

d 1.2

e 1.3


bSolid state.

cAverage emission lifetimes for the two

exponential decays determined by the equation: τav = (A1τ12 + A2τ22

)/(A1τ1 + A2τ2); Ai is

the weight of the i-th exponent. dRhenium fragment.

ePlatinum fragment.

fcmax =

1.7×10−5

M for Re(pyN2)Pt(dpyb) and 1.2×10−5

M for Re(pyN3)Pt(dpyb).

Different from reported heterometallic species with "antenna"-type constituting

fragments,129–131

these complexes (Re(pyN2)Pt(ppy), Re(pyN2)Pt(dpyb),

Re(pyN3)Pt(ppy), and Re(pyN3)Pt(dpyb)) demonstrated reasonably efficient 3MLCT

photoluminescence with quantum yields up to 28%. The luminescence behavior of

bimetallic assemblies Re(pyN2)Pt(ppy), Re(pyN2)Pt(dpyb), Re(pyN3)Pt(ppy), and

Re(pyN3)Pt(dpyb) was altered by means of the platinum-containing units, whilst

contribution of the rhenium chromophores remained invariable (Table 5).

Re(pyN2)Pt(ppy) and Re(pyN3)Pt(ppy) in fluid media demonstrated

phosphorescence, which included featureless rhenium-dominated emission with high-

energy shoulders at ca. 490 nm of the weakly emissive platinum motif132

(Figure 33).


47

Figure 33. Normalized emission spectra of complexes Re(pyN1), Re(pyN2),

Re(pyN2)Pt(ppy), and Re(pyN2)Pt(dpyb) in solution (CH2Cl2) at 298 K.

The other pair of Re(pyN2)Pt(dpyb)/Re(pyN3)Pt(dpyb) species revealed a

substantially increased contribution of the platinum chromophore to the observed

emission. The key feature in both the absorption and emission spectra of Pt(dpyb)-

containing assemblies was their concentration dependence. In dilute solutions, the

emission profiles of Re(pyN2)Pt(dpyb) and Re(pyN3)Pt(dpyb) showed an intense

high-energy band with clear vibronic progression that corresponds to the {Pt(dpyb)}

unit, and a weaker red-shifted broad band of the {Re(phen)} chromophore (Figure

34A). For Re(pyN3)Pt(dpyb), apparent changes in the intensities of high- and low-

energy emission bands upon concentration increase were accompanied by the

appearance of a subtle shoulder at around 650 nm (Figure 34B). This new long-

wavelength phosphorescence band was assumed to originate from intermolecular

interactions.

Figure 34. A: Concentration dependence of the emission spectra of

Re(pyN3)Pt(dpyb) in solution (CH2Cl2) at 298 K. B: Deconvolution of the emission

spectrum of Re(pyN3)Pt(dpyb) at c = 6×10−5

M into three independent bands.

In order to rationalize the possible aggregates, TD-DFT studies were performed to

simulate the observed concentration-dependent alterations in the UV-visible spectra. In


48

contrast to the homoleptic {Re}−{Re} and {Pt}−{Pt} aggregates, the predicted

absorption spectra for {Re(phen)}·· ·{Pt(dpyb)} stacks reasonably agree with the

experimental data, as evidenced from Figure 35. When the aromatic systems of

{Re(phen)} and {Pt(dpyb)} fragments were separated by 3.4 Å (mimicking the

association via π-stacking at higher concentrations), there is an intensity reduction for

the lower energy absorption peak, in line with experimental data. This stacking model

portrays an unprecedented case of heteroleptic π-π interaction involving platinum

square planar complexes.

Figure 35. A: UV-vis absorption spectra of Re(pyN3)Pt(dpyb) at variable

concentrations (CH2Cl2, 298 K). B: TD-DFT simulated absorption spectra of the {Re}–

{Pt} intermolecular stacking motif to model the aggregation of complex

Re(pyN3)Pt(dpyb).

The solid-state photophysical behavior of the rhenium(I)-platinum(II) architectures

generally resembled the luminescent behavior in solution, expressed by a combination

of emissions from both chromophoric metallocenters.

In this study, I investigated the photophysical behavior of phosphorescent

bichromophores by implementing the Re(I)−Pt(II) combination. The variation of the

platinum cyclometalated fragments allowed tuning of the contributions of metal

constituents to the observed emission. Importantly, the cationic complexes

Re(pyN2)Pt(dpyb) and Re(pyN3)Pt(dpyb) demonstrated a concentration-dependent

tendency to aggregate through unconventional π–π stacking heteroleptic interactions

{Re(phen)}·· ·{Pt(dpyb)}, which occur in the ground state and result in the appearance

of panchromatic luminescence.

3.4 Multichromophore rhenium(I) complexes of different nuclearity

The synthesis of multichromophore Re(I) supramolecular structures was performed via

a coordination-driven self-assembly route. The chosen molecular design featured

terminal {Re(CO)3(phen/bpy)} units around the central [M(CN)y]n−

scaffold (M = Au,


49

Pt, Fe; y = 2−6). The nature of the assembling cyanometallates determined the

nuclearity of the resulting coordination architectures.

The construction of the multinuclear structures was based on the "metalloligand"

strategy,133–135

which consists of decorating the cyanide or cyanometallates by two or

more rhenium(I) fragments [Re(CO)3−x(phen/bpy)(PPh3)x(H2O)]+ (x = 0, 1) under mild

conditions (Scheme 9).

Scheme 9. Synthetic approach to homo- and heterometallic rhenium(I) complexes. The

detailed synthetic procedures are described in publication IV.

The stoichiometry of all compounds was confirmed by ESI-MS measurements.

However, due to reduction of Fe3+

to Fe2+

under the ESI-MS conditions, the signal of

triply charged cation Re6Fe3−bpy could not be detected.

136 Nevertheless, the cyanide

stretching frequencies of Re6Fe2−bpy and Re6Fe

3−bpy in their IR spectra (at 2089 and

2154 cm−1

, respectively) provided conclusive support for the existence of two different

Fe centers. The different CN band frequencies could be explained by the different π-basicities of the iron centers. In the

1H NMR spectra of Re2−NN and Ph3PRe2−phen

(where NN = bpy, phen), the presence of two sets of diimine signals with 1:1 ratio

suggests the formation of dyads with non-equivalent rhenium units. The 1H NMR

spectroscopic profiles of Re2Au−NN, Re4Pt−NN, and Re6Fem

−bpy (m = 2, 3)

correspond to only one type of Re(I) fragments, indicating high symmetry of the

architectures.


50

The obtained multinuclear complexes were analyzed by single-crystal XRD

techniques. In all these assemblies, the metal atoms in the diimine rhenium units had a

slightly distorted octahedral surrounding (Figure 36).137–139

The diimine and carbonyl

ligands supplied five donor atoms, while the sixth coordination vacancy was filled with

the cyanide group.

Two {Re(phen)(CO)3-x(PPh3)x} fragments connected through the cyanide bridge in a

nearly linear fashion afforded dimeric Re2−phen and Ph3PRe2−phen complexes. They

adopt nearly eclipsed conformations along the Re-CN-Re axis (torsion angles < 3°),

resembling that of previously reported Re2−bpy.32,139

The structure of ReAuPPh3−phen revealed a heterobimetallic architecture, which

consisted of neutral [Au(PPh3)(CN)] and cationic {Re(phen)(CO)3}+ subunits held

together by the cyanide.

In aggregates Re2Au−NN, Re4Pt−NN, and Re6Fem

−bpy, the central [M(CN)y]n−

scaffolds (M = Au, Pt, Fe; y = 2−6) acted as anionic polydentate ligands toward the

pendant {Re(NN)(CO)3} blocks (Figure 36).

Figure 36. Molecular views of complexes Re2Au−phen, Re4Pt−bpy, and

Re6Fe2−bpy. CF3SO3

− counter ions were omitted for clarity.

Two {Re(NN)(CO)3} units were linked to one [Au(CN)2]− core to form trinuclear

species Re2Au−bpy and Re2Au−phen with eclipsed and staggered conformations,

respectively. The resulting Re–NCAuCN–Re arrays deviated from linearity, with the

Re(I)–Au(I)–Re(I) angles of 165.4–175.4°. Assemblies involving the anionic building

block [Pt(CN)4]2−

formed square-like pentanuclear Re4Pt−bpy and Re4Pt−phen

architectures. The decoration of [Fe(CN)]4−/3−

cores by six {Re(CO)3(bpy)} fragments


51

produced the almost spherical complexes Re6Fe

2−bpy and Re6Fe

3−bpy. Each iron

center adopted the expected octahedral geometry consisting of six C atoms from

bridging cyanide ligands. Despite equivalence of the rhenium fragments in solution, in

the crystal steric requirements of the diimine ligands caused certain deformations of the

ideal arrangements of Re4Pt−NN and Re6Fem

−bpy compounds.137,140,141

All these homo- and heterometallic rhenium(I) complexes were good absorbers of

visible light. With the exception of Re−Fe assemblies, they also featured relatively

intense and long-lived luminescence associated with the triplet nature of the excited

state (Table 6). In fluid media, compounds Re2−NN, Re2Au−NN, and Re4Pt−NN

containing tricarbonyl rhenium moieties demonstrated green to yellow emission with

quantum yields up to 48%. A hypsochromic shift of the emission maxima and

systematic enhancement of the luminescence intensity were detected after replacing the

bpy ligand for the related phen. This observation was in concordance with previously

published data for [Re(NN)(CO)3L] emitters.22,142,143

In addition, phosphorescence

bands of the phen-based chromophores showed stronger sensitivity to the presence of

O2 compared to the bpy-analogues. This might be due to the longer excited state

lifetimes and larger size of the phenanathroline systems, making them more susceptible

to collisional quenching.

Table 6. Photophysical properties of complexes Re2−NN, Ph3PRe2−phen,

ReAuPPh3−phen, Re2Au−NN, Re4Pt−NN, and Re6Fem

−bpy (m = 2, 3) in solution

(CH2Cl2) and in the solid state at 298 K.

λabs, nm (ε, .10

4 M

−1

cm−1

)

a λem,

nm

a Φ, %

(aer/

degas)

a τ, μs

b

λem,

nm

b Φ,

%

b τ, μs

Re2−bpy 287 (4.6), 315 (2.4),

368 (1.0) 571 7/10 0.4 523 19 1.0

Re2−phen 260 (8.8), 287 (4.4),

355 (1.2) 564 12/30 1.6 528 22 2.5

Ph3PRe2−

phen

267 (9.8), 321 (2.5),

456 (1.2) 737 –/<1 0.023 637 2 0.1

ReAuPPh3−

phen − − − − 534 25 2.4

Re2Au−

bpy

246 (8.4), 281 (4.7),

317 (2.9), 359 (1.1) 554 11/19 0.6 522 31 1.0

Re2Au−

phen

249 (9.6), 254 (9.5),

273 (7.7), 330 (1.6) 543 15/48 1.9 562 3 0.6

Re4Pt−bpy

246 (10.3), 281

(9.0), 305 (5.7), 318

(5.1), 355 (1.8)

560 10/21 0.5 547 14 0.5

Re4Pt−phen 257 (12.5), 274

(12.5), 338 (2.2) 549 12/42 2.2 549 19 1.5


bSolid state (λext 340 nm).


52

The [Au(CN)2]

− and [Pt(CN)4]

2− cyanometallate linkers did not significantly affect the

photophysical behavior of the polynuclear rhenium(I) systems, which was clearly

supported by DFT calculations (Figure 37).

The conversion of tricarbonyl rhenium complex Re2−phen into dicarbonyl phosphine

derivative Ph3PRe2−phen shifted the emission to the near-IR region and decreased the

quantum efficiency. In comparison to the literature precedents, Ph3PRe2−phen is an

uncommon example of structurally simple rhenium(I) NIR emitter.13,144,145

Photophysical characteristics of the crystalline samples were very similar to those

found in solution, indicating that the emissive excited state had the same origin in both

phases. The DFT analysis underscored the nature of the low-energy electronic

transitions, which are localized on the {Re(NN)(CO)3} fragments and principally

correspond to the dRe→π*NN 3MLCT process (Figure 37).

Figure 37. Electron density difference plots of complexes Re2Au−phen (S0→S1 and

T1→S0) and Re4Pt−bpy (S0→S1). During the electronic transition, the electron density

increases in the blue areas and decreases in the red areas.

None of the Re−Fe complexes showed appreciable luminescence. In contrast to other

aggregates where the [M(CN)y]n−

bridges were electronically innocent, the

[Fe(CN)6]4−/3−

fragments played a major role in the lowest-energy excitation and

therefore evidently provided non-radiative relaxation pathways, which account for the

lack of emission.


53

The electrochemical and magnetic properties of Re6Fe

m−bpy were investigated. The

complexes exhibited irreversible ReI to Re

II oxidation along with a quasi-reversible

reduction of [FeII/III

(CN)6]4−/3−

moieties. The magnetic data for compound Re6Fe3−bpy

clearly indicated a low-spin octahedral configuration of the FeIII

ion, which is virtually

unaffected by the coordination of terminal {Re(bpy)} blocks.

The described coordination-driven self-assembly of the Re(I) aggregates demonstrated

an efficient synthetic route to supramolecular systems comprising several chromophore

fragments. Depending on the electronic features of the connecting cyanometallate,

optical properties of these polynuclear species are defined either by the rhenium

diimine components, or by the central heterometal ion and the energy transfer between

the constituents. Notably, despite their multichromophore nature that is often

detrimental to luminescence, most of the studied compounds retain intense emission,

which can be extended into the NIR region upon a simple modification of the ligand

sphere.


54

4 CONCLUSIONS In this work, I have investigated the structure-property relationships for a series of

organometallic luminescent complexes based on the rhenium(I) diimine-carbonyl

chromophore units. The prototype motif [Re(CO)3(diimine)L] (where L = auxiliary

ligand) offers substantial synthetic freedom along with appealing optical

characteristics. Therefore, it has been used to design several new families of Re(I)-

containing emitters. In order to manipulate the behavior of the excited state, I

implemented a number of stereochemical modifications that involve changes of the

ligand coordination sphere and the combination of two or more chromophore units in

one molecular entity.

As the first step, I studied the phenanthroline rhenium complexes, which incorporated

alkynylphosphine ligands with different electron-donor strength and stereochemistry.

In fluid medium, varying the alkynyl-phenylene substituents had negligible effect on

the electronic absorption and emission energies of the final Re(I) systems. However,

the incorporation of two phosphines into the diimine-dicarbonyl fragment led to high

quantum yields and remarkable stability in comparison with conventional tricarbonyl

systems. In contrast to the solution behavior, the solid-state emission spectra featured a

gradual bathochromic shift upon extension of the phenylene chain in the phosphine

substituents, which presumably influences rigidity of the environment of the

chromophore motif.

In the second direction, I have employed phenanthrene imidazole-pyridyl moiety as a

chelating diimine. The neutral rhenium(I) tricarbonyl derivatives displayed dual

fluorescence/phosphorescence, which can be controlled by the excitation wavelength.

The straightforward functionalization of this diimine with an additional fluorophore

fragment changed the emission origin to the singlet intra-ligand character, which is not

typical for Re(I) emitters. The energy of the corresponding electronic transitions was

readily influenced by the donor-acceptor nature of the extended ligands, which resulted

in the charge transfer emissive states. Consequently, the fluorescence of the parent

dyes and their Re(I) compounds with electron-donating pendant groups exhibited

pronounced red-shifts with increasing solvent polarity, indicating positive

solvatochromic properties. Furthermore, optical properties of these emitters can be

switched in the presence of a protonating agent in a reversible manner.

The third part of my research considered cationic dyads constructed from the Re(I) and

Pt(II) chromophores and linked through the bifunctional aminopyridine ligands. These

bimetallic systems might help clarify the delicate balance between preserving the

independent performance of each component (i.e. maintaining dual emission) and the

energy transfer between two different metal fragments. The cycloplatinated motifs

were used to regulate the phosphorescent properties, whereas the emission

characteristics of the rhenium centers remained unaltered in these assemblies.

Remarkably, the heterometallic species based on the {Pt(NCN)} moiety displayed


55

concentration dependence in both the absorption and emission spectra, which was

assigned to unusual π-stacking interactions of the heteroleptic type

{Re(NN)}·· ·{Pt(NCN)}.

In the fourth chapter of the thesis, I designed multicomponent self-assemblies

constructed from the Re(I) building blocks. The supramolecular architectures

comprised two or more rhenium(I) fragments in tandem with the cyanometallates

[M(CN)y]n−

(M = Au, Pt, Fe; y = 2−6) as bridging ligands. The photophysical

properties of the gold and platinum-centered aggregates were governed by the nature of

the diimine-rhenium platforms, while the nuclearity of the metal complexes had a

minor influence on the absorption and emission behaviors. Interestingly, the

polynuclear rhenium(I) emitters attained high luminescence with quantum efficiencies

up to 48%, despite the presence of several photofunctional components.

In total, the judicious design of multichromophore structures offers wide opportunities

for elegantly tuning of the photophysical properties of rhenium(I) diimine-based

complexes. These complexes can be used to develop versatile luminescent molecular

materials.


56

ACKNOWLEDGEMENT This work was carried out at the Department of Chemistry, University of Eastern

Finland during the years 2015-2019. Financial support provided by the Finnish

National Agency for Education (CIMO Fellowship), the Academy of Finland and the

University of Eastern Finland (SCITECO grant) is gratefully acknowledged.

Primarily, I would like to express my profound appreciation to my supervisor, Prof.

Igor Koshevoy. His solid expertise, professional guidance and unwavering support

during this period have been invaluable. I am indebted to my second supervisor, Assoc.

Prof. Elena Grachova, for giving me opportunity to work on this project.

Secondly, I am most grateful to the group of Prof. Sergey Tunik (St. Petersburg State

University), to Dr. Pipsa Hirva (UEF), to Prof. Antti Karttunen (Aalto University), to

Dr. Alexei Melnikov (Peter the Great St.-Petersburg Polytechnic University) and to

Ilya Kondrasenko for their resourceful and productive collaboration.

I would also like to extend my gratitude to all personnel at Department of Chemistry,

especially to Prof. Tapani Pakkanen, Prof. Mika Suvanto, Dr. Sari Suvanto, Ms. Mari

Heiskanen, Ms. Taina Nivajarivi, Ms. Paivi Inkinen and Ms. Eija Faari-Kapanen for

their friendly cooperation and assistance during my doctoral studies.

Furthermore, I would like to acknowledge Prof. Risto Laitinen (University of Oulu)

and Prof. Andreas Steffen (Technical University of Dortmund) for critically reading

this thesis and for valuable comments.

Additionally, I sincerely thank my colleagues and friends, especially, Andrey Belyaev,

Anastasia Solomatina and Julia Shakirova for their advice regarding my scientific work

and for their immense encouragement over the years.

At last but not the least, I would like to extend the heartfelt thanks to my family, who

are always by my side and give a precious counterbalance to the studies.

Joensuu, 2019 Kristina Kisel


57

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Kristina Kisel

DissertationsDepartment of ChemistryUniversity of Eastern Finland

No. 152 (2019)

122/2014 BAZHENOV Andrey: Towards deeper atomic-level understanding of the structure of magnesium dichloride and its performance as a support in the Ziegler-Natta catalytic system123/2014 PIRINEN Sami: Studies on MgCl2/ether supports in Ziegler–Natta catalysts for ethylene polymerization124/2014 KORPELA Tarmo: Friction and wear of micro-structured polymer surfaces125/2014 HUOVINEN Eero: Fabrication of hierarchically structured polymer surfaces126/2014 EROLA Markus: Synthesis of colloidal gold and polymer particles and use of the particles in preparation of hierarchical structures with self-assembly127/2015 KOSKINEN Laura: Structural and computational studies on the coordinative nature of halogen bonding128/2015 TUIKKA Matti: Crystal engineering studies of barium bisphosphonates, iodine bridged ruthenium complexes, and copper chlorides129/2015JIANGYu:Modificationandapplicationsofmicro-structuredpolymersurfaces130/2015 TABERMAN Helena: Structure and function of carbohydrate-modifying enzymes 131/2015KUKLINMikhailS.:Towardsoptimizationofmetaloceneolefinpolymerizationcatalystsvia structuralmodifications:acomputationalapproach132/2015SALSTELAJanne:Influenceofsurfacestructuringonphysicalandmechanicalpropertiesof polymer-cellulosefibercompositesandmetal-polymercompositejoints133/2015 CHAUDRI Adil Maqsood: Tribological behavior of the polymers used in drug delivery devices134/2015 HILLI Yulia: The structure-activity relationship of Pd-Ni three-way catalysts for H2S suppression135/2016 SUN Linlin: The effects of structural and environmental factors on the swelling behavior of Montmorillonite-Beidellite smectics: a molecular dynamics approach136/2016 OFORI Albert: Inter- and intramolecular interactions in the stabilization and coordination of palladium and silver complexes: DFT and QTAIM studies137/2016 LAVIKAINEN Lasse: The structure and surfaces of 2:1 phyllosilicate clay minerals138/2016 MYLLER Antti T.: The effect of a coupling agent on the formation of area-selective monolayers of iron a-octabutoxy phthalocyanine on a nano-patterned titanium dioxide carrier139/2016KIRVESLAHTIAnna:Polymerwettabilityproperties:theirmodificationandinfluencesupon water movement140/2016 LAITAOJA Mikko: Structure-function studies of zinc proteins141/2017 NISSINEN Ville: The roles of multidentate ether and amine electron donors in the crystal structure formation of magnesium chloride supports 142/2017 SAFDAR Muhammad: Manganese oxide based catalyzed micromotors: synthesis, characterization and applications143/2017 DAU Thuy Minh: Luminescent coinage metal complexes based on multidentate phosphine ligands144/2017AMMOSOVALena:Selectivemodificationandcontrolleddepositiononpolymersurfaces145/2017 PHILIP Anish: PEI-mediated synthesis of gold nanoparticles and their deposition on silicon oxide supports for SERS and catalysis applications146/2017 RAHMAN Muhammad Rubinur: Structure and function of iron-sulfur cluster containing pentonate dehydratases147/2018 ANKUDZE Bright: Syntheses of gold and silver nanoparticles on support materials for trace analyses using surface-enhanced Raman spectroscopy148/2018 PENTTINEN Leena: Structure determination of enzymes with potential in processing of cell wall compounds149/2018 SIVCHIK Vasily: Tuning the photoluminescence of cyclometalated platinum(II) compounds via axial and non-axial interactions150/2019 MIELONEN Kati: Hierarchically structured polymer surfaces: Curved surfaces and sliding behavior on ice151/2019 CHAKKARADHARI Gomathy: Tuning the emission properties of oligophosphine copper and silver complexes with ancillary ligands

Probing the effect of coordination environment on the photophysical behavior of rhenium(I) luminophores

Kristina K

Isel: Probing the effect of coordination environm

ent on the photophysical behavior of rhenium(I) lum

inophores

152

probing the effect of coordination...the effect of a coupling agent on the formation of...

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