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Cite this: Dalton Trans., 2012, 41, 8493

www.rsc.org/dalton PERSPECTIVE

Transition metal complexes meet the rylenes

Felix N. Castellano*

Received 8th April 2012, Accepted 15th May 2012DOI: 10.1039/c2dt30765k

This Perspective highlights an emerging area of metal–organic chromophore science related to thephotochemistry and photophysics of coordination compounds and organometallic structures covalentlytethered to rylene (naphthalene-cored) imide and diimide scaffolds. This combination of moleculesrenders highly colourful structures producing an array of excited state behaviour and in some instancesstrongly aggregated self-assembled metal–organic architectures.

Introduction

Rylene dyes,1,2 particularly the naphthalene and perylene dii-mides, NDIs and PDIs respectively, continue to emerge as quin-tessential chromophores in a variety of photofunctionalmolecules, materials, and supramolecular assemblies.3–7 Inaddition to their desirable dye properties including visible-to-near-IR light-harvesting capacity and impressive fluorescenceproperties, both small and higher-order rylenes have establishedniche applications in molecular switching, dye lasers, chemicalsensing, organic electronics including field-effect transistors,light-emitting diodes, and photovoltaics, as well as biologicallabeling.1–7 While the myriad of organic materials constructedfrom these versatile building blocks is truly impressive, metal–

organic molecules and metallo-supramolecular structures bearingrylene dyes have expanded the inventory of available photo-chemistry and photophysics from these chromophores and as aresult their potential applications. It is these latter topics thatcomprise the focus of the present treatment. This Perspectivewill not consider the expansive literature related to rylene-basedmacrocyclic architectures, including metalloporphyrin- andmetallophthalocyanine-rylene conjugates, as the metal center inthese molecules is nominally a spectator with respect to excitedstate dynamics and decay. Similarly, only metal compound-rylene compositions whose photophysical and/or photochemicalreactivity have been explored will be discussed in detail.

Metal complex–naphthalimide conjugates

The first report of coordination complex–naphthalimide conju-gates using ligand 1 emerged from the author’s laboratory in2001, namely [Ru(bpy)2(1)]

2+ and [Ru(1)3]2+.8 These molecules

were originally designed to take advantage of the visible lightharvesting properties of the 4-piperidinyl-1,8-naphthalimide(PNI) subunit(s) whose intense singlet fluorescence overlaps themetal-to-ligand charge transfer (MLCT) absorption bandscharacteristic of the Ru(II)-diimine core. This strong spectraloverlap coupled with the short distances separating the tetheredchromophores provided idyllic conditions for facilitating reson-ance energy transfer,9–11 which indeed was the case as evidencedby the near quantitative quenching of the NI singlet fluorescencein these polychromophores with corresponding sensitization ofthe orange MLCT photoluminescence. This is precisely wherethe “textbook” behaviour ends and the ramifications of having

Felix N. Castellano

Felix (Phil) Castellano earneda B.A. in Chemistry from ClarkUniversity in 1991 and a Ph.D.in Chemistry from JohnsHopkins University in 1996.Following a NIH PostdoctoralFellowship at the University ofMaryland, School of Medicine,he accepted a position asAssistant Professor at BowlingGreen State University in 1998.He was promoted to AssociateProfessor in 2004, to Professorin 2006, and was appointed

Director of the Center for Photochemical Sciences in 2011. Hiscurrent research focuses on metal–organic chromophore photo-physics, photochemical upconversion phenomena, photocataly-sis, and interfacial electron transfer processes relevant tophotovoltaics.

Department of Chemistry and Center for Photochemical Sciences,Bowling Green State University, Bowling Green, Ohio 43403, USA.E-mail: castell@bgsu.edu; Fax: +1 419-372-9809; Tel: +1 419-372-7513

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closely lying triplet manifolds in the excited states of the compo-site molecules become readily apparent. If one measures theluminescence intensity decay of [Ru(bpy)2(1)]

2+ and [Ru(1)3]2+

in the region of the MLCT emission, their associated singleexponential decays ranged from 16 to 115 μs, depending uponthe number of PNI subunits and the solvent. The emission kin-etics correlated well with the absorption transients, each primar-ily reporting on the excited state decay of each compositechromophore. The combined data were consistent with excitedstate equilibrium between the higher lying triplet MLCT stateand the lower-lying PNI triplet state, much like that observed innumerous Ru(II)-pyrenyl molecules.12–17 Please note that McCle-naghan and co-workers have recently reviewed the generalphenomenon of integrating metal complexes with organic chro-mophores with the intention to generate extended excited statelifetimes.18 Several years later we applied nanosecond step-scanFourier transform infrared spectroscopy to [Ru(bpy)2(1)]

2+ and[Ru(1)3]

2+ in order to identify how the characteristic imide CvObands would shift in response to formation of the ligand-loca-lized triplet state on the PNI subunit(s).19 Much to our surprise,triplet state formation was easily detected by the fact that all thebands significantly red shifted by 60–100 cm−1 upon formationof the PNI triplet excited state and these results were correlatedto appropriate DFT calculations.

In 2011, the same PNI ligand 1 was coordinated to a Re(I) tri-carbonyl chloride complex, Re(1)(CO)3Cl, producing record-setting excited state lifetimes (650 μs) for the associated MLCTstate through the equilibrium-generating reservoir effect, a net3000-fold increase in lifetime with respect to the Re(phen)-(CO)3Cl charge transfer model compound.20 However, distinctfrom all prior studies investigating triplet state equilibria, thiswork monitored excited state decay using 3 different transientspectroscopic domains (absorption, luminescence, and infrared)with time scales ranging from ∼150 femtoseconds to hundredsof milliseconds. The MLCT excited state was monitored by tran-sient IR using CO vibrations through time intervals where thecorresponding signals obtained in conventional visible transientabsorption were completely obscured by overlap with strongtransients originating from the pendant PNI chromophore, asillustrated in Fig. 1 and 2. Following initial excitation of thesinglet ligand centered (1LC) state on the PNI chromophore,energy is transferred producing the MLCT state with a time con-stant of 45 ps, a value confirmed in all three measurementdomains within experimental error. Although transient spec-troscopy confirms the production of the 3MLCT state on ultrafasttime scales, Förster resonance energy transfer calculations usingthe spectral properties of the two chromophores support initialsinglet transfer from 1PNI* to produce the 1MLCT state by theagreement with the experimentally observed energy transfer timeconstant and efficiency. Intersystem crossing from the 1MLCT tothe 3MLCT excited state is believed to be extremely fast and wasnot resolved in this study. Finally, triplet energy was transferredfrom the 3MLCT to the PNI-centered 3LC state in less than 15ns, ultimately achieving equilibrium between the two excitedstates. Subsequent relaxation to the ground state occurred viaemission resulting from thermal population of the 3MLCT statewith a resultant lifetime of 650 μs. The Re(1)(CO)3Cl chromo-phore represents an interesting example of “ping-pong” energytransfer wherein photon excitation first migrates away from the

initially prepared 1PNI* excited state and then ultimately returnsto this moiety as a long-lived excited triplet which disposes ofits energy by equilibrating with the photoluminescent Re(I)MLCT excited state. The molecular photophysics of Re(1)-(CO)3Cl are summarized in the energy level diagram in Fig. 3.

Fig. 2 Ground-state FTIR spectrum (bottom) and time-resolved step-scan FTIR difference spectra (top) of Re(1)(CO)3Cl in THF following410 nm pulsed excitation (5–7 ns fwhm). The sample was purged withargon and pumped through a flow cell with a path length of 330 μm.The spectral resolution for the time-resolved measurements was∼8 cm−1, and the approximate experimental delay times are indicated.Reprinted with permission from ref. 20.

Fig. 1 Ground state FTIR spectrum and time-resolved infrared differ-ence spectra of Re(1)(CO)3Cl in THF following 400 nm pulsed exci-tation (90 fs fwhm). The spectral resolution for the time-resolvedmeasurements was ∼4 cm−1 and the experimental delay times arespecified. Reprinted with permission from ref. 20.

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Interested in producing phosphorescence from NI molecules, wecollaborated with the Zhao group in Dalian on the Pt(II) complex2,21 where intimate coupling of the Pt heavy atom into the aro-matic backbone was afforded by the acetylide linkage.22,23

Indeed, room temperature phosphorescence was observed from anaphthalimide species for the first time in this square planarchromophore. The combination of static and time-resolvedabsorption and photoluminescence data were uniformly consist-ent with triplet state photophysics localized on an appendedCuC–NI unit following excitation into the low energy absorp-tion bands. This molecule features rather impressive long-life-time, high quantum efficiency NI-based room temperaturephosphorescence (τ = 124 μs, Φ = 0.215) centered at 621 nm(Fig. 4), exemplifying how the Pt-acetylide linkage strongly pro-motes intersystem crossing in the NI subunit, representative of aclass of molecules whose excited states are typically dominatedby singlet fluorescence. The group in Dalian soon after produceda series of related molecules exhibiting similar photophysics.24

A recent report from the Petoud and Star groups at Pitt demon-strated that a 3rd generation PAMAN dendrimer whose peripherywas decorated with 32 individual 1,8-naphthalimide light-har-vesting chromophores could be used as an effective ligand scaf-fold for Eu(III) ions.25 In the final coordination assembly, 8Eu(III) ions were incorporated into the PAMAN interior and thecomposite material served as an effective solid state O2 sensorusing both Eu(III) photoluminescence, with the antenna effectimparted by the NI peripheral chromophores, and electrical con-ductance for signal transduction when present on the surface ofsingle walled carbon nanotubes.

Metal complex–naphthalenediimide conjugates

Transition metal complexes covalently tethered to naphthalene-diimides represent metal dye-rylene constructs largely investi-gated in the realm of intramolecular excited state electrontransfer photochemistry. The earliest studies linking Ru(II) poly-pyridine sensitizers to NDIs occurred in 1997 by workers inJapan and the US. In the former case, the Haga group reportedthe excited state electron transfer processes occurring in a Ru(II)–NDI–Os(III) triad using picosecond transient absorption spec-troscopy.26 Predominant excitation of the Ru(II) CT chromophorefirst results in proximate charge separation producing the Ru(III)–NDI−˙ ion pair followed by an additional charge shift reactionreducing Os(III) to Os(II). Ultimately, charge recombinationbetween the two distal metal centers occurred with a time con-stant of 110 ns. Dixon and co-workers investigated excited stateelectron transfer processes in the Ru(II)-NDI dyad 3 usingdynamic fluorescence experiments in the presence and absenceof calf thymus DNA to see if the donor–acceptor electroniccoupling would be modulated as a result of NDI intercalation.27

Even though the NDI subunit intercalates DNA as evidenced bycircular dichroism and a bathochromic shift in its UV-Vis spec-trum, this has negligible consequences in the photoinducedcharge transfer reaction. The average rates derived from a tri-exponential model of the data in aqueous buffer with andwithout DNA are 1.6 × 109 and 6.8 × 108 s−1, implying that theactivation energy in addition to the electronic coupling for theforward electron transfer reaction are strangely unaffected by theDNA support.

These initial electron transfer dyads were linked together byflexible chains resulting in a range of conformers being sampledusing transient spectroscopic measurements. This flexibility canalso assist in charge recombination and as a result followingselective excitation of the MLCT core, the charge separationtime constant can be largely attenuated in its magnitude. Inter-ested in circumventing these limitations, an extensive collabora-tive effort in Sweden produced seven distinct Ru(II) bipyridyl

Fig. 3 Energy level diagram describing the photophysical processes ofRe(1)(CO)3Cl in THF at room temperature. The solid lines representradiative transitions, and the dashed lines represent non-radiative tran-sitions. The presented time constants (τ) are the inverse of the observedrate constants for the associated processes. Reprinted with permissionfrom ref. 20.

Fig. 4 Absorption and photoluminescence spectra of 2 (solid blacklines) and the terminal acetylene NI ligand (dash-dotted blue lines) inMTHF at room temperature. Reprinted with permission from ref. 21.

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and terpyridyl molecules covalently tethered to a single NDI.28

The two series of conformationally rigid donor–acceptor com-plexes were primarily investigated using femtosecond transientabsorption spectrometry. The terpyridyl-based molecules did notshow any evidence of quenching whatsoever whereas the bipyri-dyl analogues each demonstrated excited state oxidative quench-ing as indicated by characteristic transient signals emanatingfrom the NDI radical anion at 474 and 605 nm. The dyad withthe closest donor–acceptor center-to-center distance exhibitedseparation and charge recombination electron transfer rate con-stants of 5.0 × 109 s−1 and 7.0 × 109 s−1, respectively. In thelonger aryl spaced donor–acceptor molecules the story was notas straightforward since biphasic forward electron transferkinetics were observed and approximately 20% of the charge-separated state was formed through the NDI triplet state.

This work was later extended to include a manganese complexplaying the role of donor in triad 4.29 In this comprehensivestudy, EPR and optical spectroscopy were used to demonstratethe light-induced formation of the long-lived electron transferproducts. Excitation of the [Ru(bpy)3]

2+ core ultimately resultsin the oxidation of the manganese dimer (formally Mn2

II,III) andconcomitant reduction of the NDI subunit (NDI−˙), the latterradical persisting for a record 600 μs at room temperature and toalmost 1 s at 140 K. This enhanced charge separation lifetimewas attributed to the significant reorganization energy (∼2.0 eV)with origins predominantly associated with the inner sphere reor-ganization of the appended Mn2 complex, thereby creating con-ditions favoring long-lived charge separation in the Marcusnormal region.

In a related study, a triad constructed from a triphenylaminedonor, an Ir(III) MLCT complex, and NDI acceptor (5) also

produced a long-lived charge-separated state of 120 μs at roomtemperature in CH3CN.

30 Due to significant spectral overlap,selective excitation of the MLCT complex is not possible and355 nm laser excitation gives rise to the observation of transientsignals from the oxidized triphenylamine and the reduced NDI,ultimately recombining with the 120 μs time constant mentionedabove.

Similar to the author’s work on the naphthalimides, in 2008Weinstein and co-workers synthesized the first example of thePt(II)-naphthalenediimide complex (6) and applied a battery ofphotophysical techniques to unravel the excited state compo-sition and dynamics associated with this chromophore.31 Exci-tation into the low energy absorption bands of 6 (355–395 nm)initiates a series of processes wherein many distinct excitedstates are involved, including 1NDI* (0.9 ps), 3MLCT* (3 ps),vibrational cooling of 3NDI* (19 ps) and finally decay of theextremely long-lived 3NDI* state (520 μs). Similar to the PDItriplet excited state, the 3NDI* state possesses ν(CO) vibrationsat 1607 and 1647 cm−1 that are shifted quite significantly red-shifted with respect to their ground state counterparts. Distinctfrom all the other NDI systems described above, there is no spec-troscopic evidence for excited state electron transfer but ratherenergy migration to the 3NDI* state dominates excited statedecay in 6.

Finally, Miller and Schanze described the dynamics of tripletexciton and negative polaron (electron) transport within a seriesof monodisperse platinum acetylide oligomers terminated withNDI end groups (7) in 2011.32 Negative polaron transport wasinvestigated using pulse radiolysis/transient absorption whereastriplet exciton migration was investigated using ultrafast transientabsorption spectroscopy. In pulse radiolysis experiments, elec-trons are attached to the oligomeric structures and the resultantabsorption changes monitored, eventually leading to the

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production of the NDI−˙ radical anion. Similarly, near visibleexcitation with ultrafast laser pulses generate triplet excitonslocalized on the Pt-acetylide chains, ultimately yielding acharge-separated state with characteristic NDI−˙ signatures. Thecombined results suggested that both triplet excitons and nega-tive polarons migrate over large distances quite rapidly in theseoligomeric structures, ∼3 nm in less than 200 ps. Analysis of thedynamics modeled using a site-to-site hopping mechanism indi-cated that diffusive transport occurred with time constants of∼27 ps for triplet excitons and <10 ps for electrons.

Metal complex–perylenediimide constructs

Coordination at the imide nodal region

Before proceeding it is worthwhile to mention that at presentthere is only one published example of metal complex/perylenemonoimide molecules from Espinet and co-workers, whosephotophysics is completely singlet state dominated.33 However,the realm of metal complex containing PDI structures continuesto develop and provided significant motivation for the compo-sition of this Perspective. The first molecules to be describedherein are those where the PDI is coordinated to the metal at theimide region that effectively decouples the electronic structuresof the PDI and metal components as a result of the node locatedat those bonding positions.3,6

Würthner and his co-workers in Germany are credited withsynthesizing the first two examples of coordination compoundsbearing PDIs in 2000, specifically tetranuclear Pd(II) and Pt(II)molecular squares featuring cis-geometry enforcing chelatingbis-phosphine corners, 8a and 8b.34 Unlike those molecularsquares that take advantage of the spin–orbit coupling propertiesof the metal ions resident in the structure,35,36 these moleculeswere designed solely to take advantage of metal–ligand coordi-nation-directed self-assembly while retaining the singlet state flu-orescence and redox properties of the PDI linkers.37 This wasfurther exploited in a pyrenyl-decorated PDI Pt(II) square, whichcontained 16 pyrenes and 4 PDIs, ultimately achieving periph-ery-to-center energy and electron transfer chemistry.38 In a struc-turally analogous molecule containing 16 ferrocene subunits,

each electroactive species could be independently addressedelectrochemically via one-electron oxidation.39 Concurrent withtheir work on the molecular squares, the Würthner group alsoconstructed linear coordination metallopolymers using a bifunc-tional PDI with phenyl-terpyridyl appended at the two terminalimide positions.40 These macromolecules also retained the redfluorescence properties of the starting monomer and were alsodeposited on solid supports through coordination chemistrydriven layer-by-layer deposition on quartz substrates. Alongsimilar lines, Müllen and Stang combined forces to producerhomboid and rectangular polynuclear structures containing PDIlinkers also driven by Pt(II) self-assembly.41

Workers in Spain have recently contributed in this area, evaluat-ing an Ir(III)–PDI dyad (9) proposed as a red-light emitting elec-trochemical cell exhibiting an external quantum efficiency of3.3%.42 In a subsequent report, these investigators elucidated themolecular photophysics of 9, illustrating intramolecular energytransfer occurring between the Ir(III) MLCT excited state and thePDI triplet.43 The 3PDI* state characteristically exhibits a majorT1 → Tn absorption band that is displaced blue with respect tothe ground state bleach and is therefore easily distinguished fromthe red-shifted PDI radical anion generated in excited state elec-tron transfer reactions.

Fukuzumi and co-workers have prepared a series of 3 donor–acceptor molecules constructed from electron donating ferro-cenes and electron accepting core-substituted perylenedi-imides.44 The combination of ultrafast and conventional transientabsorption spectroscopy revealed that excited state electron trans-fer takes place in all three molecules in benzonitrile. The lifetimeof the charge separated state in the dicyano-PDI-ferrocene triadcontaining 2 ferrocene subunits was more than twice that of theassociated dyad, suggesting that presence of the second ferrocenehelps to stabilize the CT state. Electron transfer quantitativelyquenches the singlet state of the PDI in the dipyrrolidine substi-tuted PDI-ferrocene chromophore and the subsequent chargerecombination reaction leads to the production of the long-livedPDI localized triplet state.

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A collaboration between research groups at Northwestern, Yale,and North Texas probed ultrafast intramolecular electron transferfrom an iridium-based water oxidation catalyst Cp*Ir(ppy)Cl(ppy = 2-phenylpyridine) to PDI derivatives (10a and 10b),45

the latter is an extremely strong photooxidant owing to the pres-ence of 4 CF3 electron withdrawing substituents present in thebay positions. Photoexcitation of the PDI in each dyad results inreduction of the chromophore to PDI−˙ occurs with time con-stants <10 ps, a process that outcompetes any generation of3PDI*. Biexponential charge recombination largely to the PDI–Ir(III) ground state is suggestive of multiple populations of thePDI−˙–Ir(IV) ion-pair, whose relative abundance varies withsolvent polarity. Electrochemical studies of the dyads showstrong irreversible oxidation current similar to that seen formodel catalysts, indicating that the catalytic integrity of themetal complex is maintained upon attachment to the PDIphotosensitizer.

Functionalization in the aromatic bay region

In 2007, Rybtchinski and co-workers intended to purposely alterthe electronic structure, excited state decay, and the resultantphotofunctionality of PDI chromophores by placing σ-bondedPd(II) subunits 1,7-in the aromatic bay positions.46 Both dinuc-lear molecules were synthesized through oxidative addition ofthe corresponding 1,7-Br2-PDI precursor with Pd(dba)3 in thepresence of either PPh3 or 1,2-bis(diphenylphosphino)ethane(dppe), respectively. Unfortunately, even in the presence of twoproximate heavy atoms, the PDI-based fluorescence quantumefficiencies remained large, Φ = 0.65 and 0.22, and the singletexcited state lifetimes were also on the nanosecond time scale.The combined experimental results, also corroborated byTD-DFT calculations, revealed that the Pd(II) centers were welldecoupled from the PDI aromatic π-system even though theywere directly σ-bonded.

Interested in the potential of utilizing triplet excitons inorganic photovoltaics,47,48 we designed molecules 11–13 in2008 hoping to efficiently generate the triplet state of PDI fol-lowing photoexcitation in three distinct Pt(II) mononuclearsquare planar structures.49 The platinum-acetylide motif wasselected because this combination is known to induce strongspin–orbit coupling facilitating rapid (kisc > 1011 s−1) singlet →triplet intersystem crossing in the appended organic chromo-phore(s).50 Structures 12 and 13 examined the influence ofcis- vs. trans-PDI-acetylide ligation whereas 11, inherently posses-sing a more complex electronic structure, addressed the conse-quence of positioning a 3CT excited state energetically proximate

to the PDI triplet. In all three molecules the PDI singlet fluor-escence was quantitatively quenched, 1O2 photoluminescencewas efficiently sensitized at 1270 nm, and conventional transientabsorption difference spectra illustrated production of the 3PDIexcited state with long associated lifetimes, Fig. 5. Unfortu-nately, no phosphorescence could be detected from these struc-tures at room temperature. In the following year, we evaluatedthe ultrafast transient absorption dynamics of these chromo-phores and contrasted the results with the “free” acetylene-termi-nated ligand, abbreviated as PDI-CCH.51 Upon ligation to thePtII center, the bright singlet-state fluorescence (Φ = 0.91, τ =4.53 ns) of the free PDI-CCH chromophore is quantitativelyquenched and no long wavelength photoluminescence isobserved from any of the Pt-PDI complexes in deaerated sol-utions. Ultrafast transient measurements revealed that upon lig-ation of PDI-CCH to the Pt(II) center, picosecond intersystemcrossing (τ = 2–4 ps) from the 1PDI excited-state is followed byvibrational cooling (τ = 12–19 ps) of the hot 3PDI excited state,whereas only singlet state dynamics, including stimulated emis-sion, were observed in the “free” PDI-CCH moiety. Recently, wehave extended this work to include the terpyridyl variant, [Pt(tBu3tpy)(CC-PDI)]

+, whose HOMO and LUMO orbitals calcu-lated by DFT (Gaussian ’09) in a CH2Cl2 continuum (PCM) arepresented in Fig. 6. The calculated orbitals depicted in Fig. 6 areintended to illustrate the strong ligand-localized character of thelowest energy optical transition and the minor perturbationimparted by the Pt(II) center. It should be noted that bimolecularreactions occurring between metal-containing chromophores andPDIs also represent another emerging area of investigation. Forexample, the strong singlet fluorescence associated with PDIchromophores makes them attractive molecules as acceptors/annihilators in photon upconversion schemes based on sensitizedtriplet–triplet annihilation.52,53

Fig. 5 Transient absorption difference spectra of 11 (A), 12 (B), and13 (C) measured in CH2Cl2 using 450 nm laser pulses (2.5–3 mJ perpulse) with the corresponding single exponential excited state lifetimesindicated. Reprinted with permission from ref. 49.

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Rybtchinski and co-workers recently developed a uniqueapproach for non-covalent nanoscale synthesis using a single amolecular building block containing a PEG substituted PDIchromophore and a terpyridyl ligand covalently attached to thediagonal bay position (14);54 this work has been recently high-lighted by Frauenrath as well.55 Prior to metal coordination, 14self-assembles into long fibers (at least several microns long)from water/THF mixtures as established from cryogenic trans-mission electron microscopy (cryo-TEM) images. These fiberspossessed all the characteristics of face-to-face π-stacks (Haggregates) of PDI structures stacked in a perpendicular fashionwith respect to the fiber axis. Compound 14 behaves as a disas-sembled molecular species when dissolved in CH2Cl2. Thesupramolecular self-assembly process was markedly modified by

the coordination of Pd(II), Pt(II), and Ag(I), producing tubular,vesicular, and nanoplatelet aggregates, respectively, as deter-mined by cryo-TEM. Most notably, the aggregated structurederived from Pt(II) coordination exhibited the usual optical hall-marks of Pt(II)–Pt(II) metal–metal interactions typically observedin Pt-terpyridyls in both the solid state and in solution aggre-gates.56,57 The PDI-based singlet fluorescence intensity decaysin all aggregates formed displayed pump fluence dependenceinterpreted as having its origin in exciton annihilation processeswhereas the corresponding disaggregated structures measured inchloroform did not display any power-dependent singlet fluor-escence behaviour. In 2011, the same group further elaboratedthis concept, preserving the features of the Pt(II) derivative of 14whilst integrating peptide conjugates through thiolate coordi-nation at the Pt(II) center.58 An interesting facet of the newly pro-duced structure lies in its self-assembly processes in aqueousmedia that are under kinetic control, enabling pathway-depen-dent assembly sequences wherein different resultant nanostruc-tures are elaborated in a stepwise manner. At the time of thisperspective, photophysical characterization of the various aggre-gated structures described in this report have yet to beelaborated.

Interested in the development of photocatalytic systems for solarenergy conversion that preserve as much photon energy as poss-ible, Wasielewski and co-workers recently investigated photoin-duced singlet charge transfer in a ruthenium(II)–PDI complex(15).59 Similar to that observed in the ferrocene–PDI moleculesexplored by Fukuzumi,44 selective excitation of the PDI subunitin 15 results in reductive quenching of the 1PDI* state formingthe Ru(III)–PDI−˙ charge-separated ion pair in less than 150 fs.The vibrationally hot ion pair exhibits fast relaxation with a timeconstant of 3.9 ps and eventually nearly complete charge recom-bination regenerating the ground state in 63 ps. The residual tran-sient signal that persists into microseconds time scale isconsistent with the 3PDI* state likely formed as a result ofradical pair intersystem crossing induced by the paramagneticRu(III) center.

Conclusions

This contribution highlights numerous creative approachesexplored to date integrating NI, NDI, and PDI chromophoreswith transition metal complexes. As a result, noteworthy excitedstate behaviour, such as lifetime extension based on the tripletreservoir effect, has been achieved. Fundamental explorationsinto excited state electron transfer have been accomplishedwhere the rylene dye is used either as the electron acceptor or

Fig. 6 DFT-calculated isodensity plots of the (a) HOMO and (b)LUMO of [Pt(tBu3tpy)(CC-PDI)]

+ in a CH2Cl2 continuum using theB3LYP/LANL2DZ functional.

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the selectively excited chromophore eventually leading to chargeseparation and in some instances, impressive charge separationlifetimes. Topologically pleasing polynuclear metallocyclic andlinear structures have been synthesized, exhibiting either singletand triplet state dominated photophysics, depending upon thenature of the covalent linkage to the metal complex or metalcenter. Similarly, rylene-based mononuclear complexes of Ru(II),Ir(III), and Pt(II) show promise for contributing in the generalareas of light-emitting electroluminescent devices, triplet-basedorganic photovoltaics, and photocatalytic solar fuels production.By taking advantage of metal-coordination driven self-assemblyprocesses in tandem with solvent programmable PDI aggrega-tion, new opportunities have emerged for the study of largesupramolecular metal–organic materials with fascinating nano-to-microscale structures. Although still in its infancy, mergingmetal complexes with rylene dyes represents a powerful combi-nation of structures that will continue to make fundamental con-tributions ultimately paving the way for a variety of photonics-based applications.

Acknowledgements

I thank all of the following agencies for their generous supportof my own research group’s explorations in the general area ofmetal–rylene chromophores (ACS-PRF 44138-AC, NSFCAREER CHE-0134782, NSF CHE-0719050, NSFCHE-1012487, and AFOSR FA9550-05-1-0276) as well as thearduous efforts all of my co-workers and collaborators world-wide that made our contributions to this research possible. TheDFT calculations were performed using Ohio Supercomputerresources. Dr Catherine E. McCusker is acknowledged for pro-viding Fig. 6 and Valentina Prusakova is recognized forsuggesting the topic of this Perspective.

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