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Theoretical studies of ground and excited electronic states of OLEDmaterial bis(2-methyl-8-quinolinolato)gallium(iii) chlorine

Hong-Ze Gaoa,*, Zhong-Min Sub

aFundamental Department, Chinese Peoples Armed Police Force Academy, Langfang 065000, Hebei province, ChinabInstitute of Functional Material Chemistry, Northeast Normal University, Changchun 130024, Jilin Province, China

Received 9 October 2004; accepted 1 December 2004

Available online 19 March 2005

Abstract

By means of ab initio HF and DFT B3LYP methods, the structure of bis(2-methyl-8-quinolinolato)gallium(III) chlorine

complex(GaMq2Cl) was optimized and the electronic transition mechanism was studied in the complex . The lowest singlet excited state

(S1) of GaMq2Cl has been studied by the singles configuration interaction (CIS) method and time-dependent density functional theory (TD-

DFT). The lowest singlet electronic transition (S0/S1) of GaMq2Clis pp* electronic transitions and primarily localized on the phenol and

pyridyl ligands. The emission of GaMq2Cl is due to the electron transitions from the phenol donor to the pyridyl acceptor including C/C

and O/N transference. Two possible electron transfer pathways are presented, one by carbon, oxygen and nitrogen atoms, and the other via

metal cation Ga3C. The comparison between the CIS optimized excited-state structure and the Hartree-Fock ground-state structure indicates

that the geometric shift is mainly confined to the one quinoline and these changes can be easily understood in terms of the nodal patterns of

the highest occupied and lowest unoccupied molecular orbitals. TD-B3-LYP calculations predict an emission wavelength of 504.57 nm. This

is comparable to GaMq2Cl 492 nm observed experimentally for photoluminescence. Lending theoretical corroboration to recent

experimental observations and supposition, the nature of the electron transition mechanism was revealed.

q 2005 Elsevier B.V. All rights reserved.

Keywords: Bis(2-methyl-8-quinolinolato)gallium(III) chlorine complex; Ab initio; TD-DFT; Electronic transition mechanism

1. Introduction

Organic light-emitting diodes (OLEDs) are heterojunc-

tion devices in which layers of organic transport materials

are usually incorporated into devices as amorphous thin

solid films. These devices normally consist of at least one

hole-transport layer and one electron-transport layer form-

ing an organic/organic heterojunction. Holes from the anode

and electrons from the cathode travel through the transport

layers until they form a singlet exciton that relaxes giving

rise to electroluminescence. Research into organic materials

for use in OLEDs has been mostly focused on conjugated

polymers [1,2] or low molecular weight materials [3]. In

1987, Tang and VanSlyke, [4] reported the first efficient low

molecular weight OLED. Following the initial report,

metaloquinolates have become the focus of new electro-

luminescent materials research, [5,6] with Alq3 being the

most often used [7].

Generally, a heavy metal ion in the same group of the

periodic table gives a metal-chelate complex with low

fluorescent intensity. For example, tris(8-hydroxyquinoli-

nato)gallium (Gaq3) has a weaker fluorescence than Alq3.

Therefore, Gaq3 and other gallium complexes have seldom

been used in OLEDs.

But a gallium complex with a modified molecular

structure (GaMq2Cl) which with two parts of 2-methyl-8-

hydroxyquinoline (Mq) and a chlorine as ligands, showed

strong bluegreen fluorescence. It also exhibited high

performance as an emitting material, an electron trans-

port material, and a host material [8]. The photolumi-

nescent peak of GaMq2Cl at 492 nm was as strong in

intensity as that of tris (8-hydroxyquinolinato) aluminum

(Alq3). The OLED using GaMq2Cl as an emitting

material showed bluegreen luminance of 10,490 cd/m2.

When it was used as an electron transport material in

Journal of Molecular Structure: THEOCHEM 722 (2005) 161168

www.elsevier.com/locate/theochem

0166-1280/$ - see front matter q 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.theochem.2004.12.040

* Corresponding author. Tel.:C86 3162068419; fax:C86 3162069584.

E-mail addresses: [email protected] (H.-Z. Gao), zmsu@

nenu.edu.cn (Z.-M. Su).
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a rubrene doped cell, an OLED with a high luminance of

27,700 cd/m2 was obtained. The PL peak wavelength of

GaMq2Cl was 51 nm shorter than that of Gaq3 (PL peak

wavelength of Gaq3: 543 nm).GaMq2Cl had a steric

hindrance compared with Gaq3 because of the existence

of the methyl group in 2-methyl-8-hydroxyquinoline may

be the one reason.In order to reveal the nature of both the ground and the

excited states involved in the absorption and/or photoemis-

sion, lend theoretical corroboration to recent experimental

observations and supposition, we carried out calculations of

the geometry and electronic properties of GaMq2Cl in

neutral state, as well as the bonding characteristics in the

complex.

The main difficulties against a reliable computational

appoach are related to the system and to the presence of

strong electron correlation effects. Both properties are

difficult to treat in the framework of the quantum

mechanical methods rooted in the Hartree-Fock (HF)

theory.

As a matter of fact, the post-Hartree-Fock methods

needed to obtain reliable excited states properties have

scaling properties with the number of electrons (N6 or

worse) that prevent their application to large systems.

Recently, investigations have been carried out at the

approximate level of theory (semi-empirical models)

[9,10] or obtained by a reduction of the size of the

system [11].

On the other hand, density functional theory (DFT)

successful at providing a means to evaluate a variety of

ground-state properties with an accuracy close to that of

post-HF methods [12,13]. As a consequence, there iscurrently a great interest in extending DFT to excited

electronic states [14]. In this context, the time dependent

DFT approach (TDDFT) offers a rigorous route to the

calculation of vertical electronic excitation spectra [1517].

Furthermore, remarkable structural predictions have been

obtained especially using the hybrid density functionals

[18,19] such as B3LYP and B3PW91 combining exact

exchange with gradient-corrected density functionals. For

excited states of closed shell molecules, time-dependent

DFT methods (TDDFT) have been developed. Applications

of TDDFT approaches have recently been reported on

transition metal complexes and get a considerably good

result [2023].

Gaussian offers the Configuration Interaction

approach, modeling excited states as combinations of

single substitutions out of the Hartree-Fock ground state

and the method is thus named CIS [24]. When paired

with a basis set, it also may be used to define excited

state model chemistries whose results may be compared

across the full range of practical systems. Theoretical

investigations on excited states are uncommon but

necessary for the molecules used in organic light

emitting diode devices (OLEDs), because the calculation

of excited-state properties typically requires significantly

more computational effort than is needed for the ground

states. More over, CIS is nearly the only approach to

optimize the excited state geometries for the organic light

emitting complexes and materials which have practically

applied value.

In the present work, the CIS method is adopted to study

the first singlet excited state (S1) of GaMq2Cl. The excited-state equilibrium geometry is compared with the optimized

ground-state structure. More accurate estimates of the

excitation energies for the complex were computed using

time-dependent density functional theory with a hybrid

functional. Assisting in the interpretation of results of the

ground state and excited state, the luminescent nature of

metal Ga complex was to be understood.

2. Methods of calculation

All the results presented in this work were obtained at

the ab initio HF and DFT B3LYP levels of theory by

means of GAUSSIAN 98 program [25] Beckes three

parameters hybrid method [19] using the Lee-Yang-Parr

correlation function [18] was employed for all the

density functional calculations. The structure of GaMq2Cl

was optimized and its frontier molecular orbital charac-

teristics and energy levels have been analyzed system-

atically in order to study the electronic transition

mechanism in the complex. The calculation model are

shown in Fig. 1, A and B represent different quinoline

ligand in the compound, respectively. The structures ofthe complex were fully optimized using sequence of

basis sets of increasing flexibility including the split-

valence 6-31G, both 6-31G* basis sets. The structures of

GaMq2Cl were optimized in the first singlet excited state

(S1) using configuration interaction with all singly

excited determinants [24] (CIS) in the frozen-core

approximation and the 6-31G* basis set. On the basis

of the CIS-optimized structure of the excited state,

TD-B3-LYP calculations predict an emission wavelength.

And an absorption wavelength was predicted on the

optimization geometry of B3LYP/6-31G*.

23

4

5

1

6

78

9A

B N

H3C

O

Ga

N

CH3

O Cl

Fig. 1. The schematic structure.

H.-Z. Gao, Z.-M. Su / Journal of Molecular Structure: THEOCHEM 722 (2005) 161168162


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3. Results and discussion

3.1. The geometry of stationary points

and electronic structure

3.1.1. Ground state structure

The geometry of GaMq2Cl was optimized by means of

ab inition HF and DFT B3LYP methods. The main

optimized parameters of each stationary point are listed in

Table 1, the serial numbers of atoms are shown in Fig. 1.

The calculated results from different methods are

consistency, that is to say, the results calculated are reliable.

In the compound GaMq2Cl, the geometries from different

methods are a little different. The bond distance Ga-OA are

1.9042 A (B3LYP/6-31G). while in the Gaq3, the value are

1.9330A(B3LYP/6-31G) [26]. It is clear that the distance

between the gallium and oxygen in GaMq2Cl becomes

shorter than that in Gaq3 owing to the introduction methyl to

the 8-hydroxyquinoline at position 2. There is the same rule

in the quinoline B as quinoline A. This shows the gallium-oxygen becomes stronger, w hich maybe m ake

the interaction between gallium and nitrogen atoms become

weaker in some extent.

3.1.2. Molecular orbitals

3.1.2.1. Orbital population. For metal chelate, the structure

of the ground state and excited state, electron transition andthe energy transfer mechanism and so on have pronounced

effect on its EL efficiency. In order to explore the electron

transition property of GaMq2Cl, we made a systematic

analysis on the population of GaMq2Cl molecular orbitals. It

was based on the stable geometrical structure optimized at

the ab initio HF/6-31G* and B3LYP/6-31G* levels. Mean-

while, the square sum of all kinds of atoms or molecular

parts in GaMq2Cl indicates the contribution of each atom or

molecular moiety to one molecular orbital. All the atoms in

GaMq2Cl were divided into seven parts: (1) gallium atom;

(2) phenol ring, including oxygen atom and atoms with

numbers from 1 to 6 and mating hydrogen atoms;

(3) pyridine ring, including nitrogen atom and atoms with

numbers from 5 to 9 and mating hydrogen atoms; (4) oxygen

atom; (5) nitrogen atom; (6) chlorine atom; (7) methyl. 10

orbitals extracted from the frontier occupied orbitals

and unoccupied orbitals, respectively. The results are

summarized in Tables 2 and 3.

The result analysis demonstrates that the orbital popu-

lations coincide with the two methods, which implies that the

molecular theoretical resultsare reliable. Theelectron clouds

of the highest occupied molecular orbital(HOMO) is con-

centrated on the carbon and oxygen of phenol ring in

8-hydroxyquinoline ring A and ring B. The electronic cloud

is mainly composed of the p-orbital ingredient and includesoxygen about 13% or so, and few s orbital characters are

Table 1

Geometrical parameters optimized for GaMq2Cl

Bond distance

(A)

HF/6-31G* GaMq2Cl

B3LYP/

6-31G

B3LYP/

6-31G*

Gaq3[26]

B3LYP/

6-31G

GaNA 2.1169 2.1002 2.1016 2.1080

GaNB 2.1168 2.1002 2.1016

GaOA 1.8620 1.9040 1.9042 1.9330

GaOB 1.8620 1.9039 1.9042

Table 2

The molecular orbital components of GaMq2Cl% (B3LYP/6-31G*)

No. Orbital Orbital

energy (eV)

Ga Phenol

ring A

Phenol

ring B

Pyridine

ring A

Pyridine

ring B

OA OB NA NB Cl CH3(A) CH3(B)

117 2.4632 0.6 9.8 9.8 36.1 36.1 0.0 0.0 0.1 0.1 0.1 5.2 5.2

116 2.2907 4.9 18.8 18.8 23.3 23.3 1.3 1.3 1.6 1.6 0.4 7.0 7.0

115 2.2395 39.4 12.9 12.9 15.7 15.7 0.9 0.9 0.8 0.8 3.1 1.6 1.6

114 1.1018 60.7 7.4 7.4 7.8 7.8 4.2 4.2 3.7 3.7 9.8 0.7 0.7

113 0.7935 2.8 40.9 40.9 14.8 14.9 2.6 2.6 2.9 2.9 0.1 1.3 1.3

112 0.7361 1.0 42.8 42.7 14.9 14.9 2.5 2.5 2.4 2.4 0.2 0.6 0.6

111 K0.5159 0.2 30.4 30.5 35.1 35.3 0.0 0.0 0.0 0.1 0.0 1.1 1.1110 K0.5399 0.8 30.1 30.0 34.8 34.7 0.0 0.0 0.1 0.1 0.1 1.3 1.3

109 K1.7054 0.5 12.5 12.5 37.2 37.2 1.2 1.2 10.5 10.5 0.4 2.6 2.6

108(LUMO) K1.7837 1.9 12.6 12.6 36.1 36.0 0.7 0.7 9.5 9.5 1.4 2.3 2.3

107 (HOMO) K5.4581 0.5 42.8 42.8 9.9 9.9 13.3 13.3 2.3 2.3 0.8 0.5 0.5

106 K5.5952 2.4 42.5 42.4 9.5 9.4 11.3 11.2 1.9 1.9 1.9 0.3 0.3

105 K6.8875 0.1 32.8 33.7 29.9 30.8 0.1 0.1 0.1 0.1 0.7 2.3 2.4

104 K6.8908 0.1 34.0 33.1 30.5 29.7 0.1 0.1 0.2 0.2 0.0 2.4 2.4

103 K7.4505 1.2 1.9 1.9 5.4 5.4 0.6 0.6 3.3 3.3 84.5 0.6 0.6

102 K7.5071 1.7 8.8 8.9 1.7 1.7 6.2 6.2 0.3 0.3 77.6 0.9 0.9

101 K7.8544 10.0 21.2 21.2 24.2 24.2 12.8 12.8 15.9 15.9 3.5 2.1 2.1

100 K7.9744 3.3 32.9 32.9 14.1 14.2 20.1 20.1 2.1 2.1 9.8 0.9 0.9

99 K7.9912 2.8 21.5 21.5 28.6 28.5 12.2 12.2 6.6 6.6 3.1 0.1 0.1

98 K8.1861 3.9 26.7 26.7 20.7 20.7 15.4 15.4 3.6 3.6 10.9 0.5 0.5

H.-Z. Gao, Z.-M. Su / Journal of Molecular Structure: THEOCHEM 722 (2005) 161168 163


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observed, nitrogen about 2%(B3LYP/6-31G*). While in the

lower occupied orbitals, the contribution of pyridine ring is

primary. the electronic clouds of the lowest unoccupied

molecular orbital (LUMO) is concentrated on the pyridine

ring in 8-hydroxyquinoline ring A and ring B, which is

mostly composed of the p-P* orbitals of carbon and

nitrogen, s orbital is very little either, including nitrogen

about 10%. In the part higher unoccupied orbitals are from

phenol ring. In the lower occupied orbitals, as for orbital 101,

99 et al., the contribution of nitrogen became bigger for one

molecular orbital, but the gallium ingredient is still very little

in addition to orbital 101, occupying about 2%, which only

contributes little in deeper orbital. This shows the gallium-

nitrogen is weaker. We note that the two lowest unoccupied

orbitals are essentiallyp* orbitals of the pyridine moieties.

Whereas the substitute(-CH3) is located at the pyridine ring,

so methyl has a direct effect on LUMO. Compared with the

HOMO-LUMO bandgap of Gaq3(K3.3503 eV /B3LYP/6-

31G[26]), we observed that the electron-releasing-CH3group resulting in the increased HOMO-LUMO bandgap

(K3.6806 eV/B3LYP/6-31G), which then leads to a

decrease inlmax values.

3.1.2.2. Orbital shapes. The interpretation of observed

spectral features is greatly assisted by molecular orbital

calculations, which, in addition to providing orbital energies

for comparison with experiment, furnish a detailed descrip-

tion of orbitals, including spatial characteristics, nodal

patterns, and individual atom contributions. The frontier

orbital levels of GaMq2Cl consist of sets of closely spaced

twosome. The highest occupied molecular orbitals

(HOMOs) and the lowest unoccupied molecular orbitals

(LUMOs) of GaMq2Cl largely preserve the electronic

structure of the 8-hydroxyquinoline ligands with little

contribution from the central gallium. For intuition, the

contour plot of the orbitals for molecule are depicted in

Fig. 2, including least bound HOMO orbital, second highest

energy orbital (HOMOK1), lowest energy LUMO and

second lowest energy orbital (LUMOC1) computed using

HF/6-31G* and B3LYP/6-31G*.In the contour plots, the

HOMO localized over the phenoxide both in A and B, the

LUMO are seen to be mainly localized over the pyridine

both in A and B. Thus there must be a mixture intra

8-hydroxyquinoline and inter 8-hydroxyquinoline rings

when transition from ground state to excited state.

The ionization energy of the GaMq2Cl has been reported

to be 5.86 eV[8], within Koopmans approximation[27],this

value can be compared to the computed the least bound

HOMO orbital energy from B3LYP/6-31G*, agreeing

within ca. 0.4 eV.

3.2. S0/S1 Excitation energy and the s1

excited-state structure

Experimental investigations of the excited-state proper-

ties of GaMq2Cl have been made on the photoluminescence,

and electroluminescence of this OLED material[8]. Inspec-

tion of the emission spectras from GaMq2Cl in condensed-

phase systems shows that both the EL and PL spectrum have

a similar shape, a photoluminescent peak at 492 nm. The

results presented here support the localized nature of the

orbitals involved in the lowest energy electronic transitions

in GaMq2Cl. The lowest electronic transitions are p/p*

transitions in the different quinolate rings and intra

quinolate ring, involving partial charge transfer from the

phenoxide side to the pyridyl side. There are two ways of

Table 3

The molecular orbital components of GaMq2Cl% (HF/6-31G*)

No. Orbital Orbital

energy (eV)

Gs Phenol

ring A

Phenol

ring B

Pyridine

ring A

Pyridine

ring B

OA OB NA NB Cl CH3(A) CH3(B)

117 6.4527 28.5 10.1 10.1 24.1 24.0 0.1 0.1 0.3 0.3 3.0 2.0 2.0

116 6.3629 0.7 10.5 10.6 35.1 35.3 0.0 0.0 0.4 0.4 0.1 6.2 6.2

115 6.2364 79.9 2.6 2.6 3.0 3.0 0.3 0.3 0.6 0.6 8.5 0.9 1.0

114 5.4448 94.5 0.6 0.6 1.1 1.1 0.4 0.4 0.4 0.4 1.3 0.5 0.5

113 5.0872 22.4 31.9 31.7 11.7 11.6 1.5 1.5 2.0 2.0 0.7 1.5 1.5

112 5.0026 1.2 41.5 41.9 15.0 15.1 1.9 1.9 2.1 2.1 0.2 1.0 1.1

111 3.3013 0.4 31.9 31.4 35.5 35.0 0.0 0.0 0.1 0.1 0.1 1.2 1.2

110 3.2646 1.2 31.1 31.5 34.4 34.9 0.1 0.1 0.1 0.1 0.0 1.5 1.5

109 2.0313 0.6 12.5 12.3 36.5 36.1 0.8 0.8 9.2 9.0 0.3 3.3 3.2

108 (LUMO) 1.9375 2.3 12.5 12.7 34.6 35.1 0.5 0.5 8.3 8.4 1.4 2.7 2.8

107(HOMO) K7.6633 0.3 42.4 42.3 10.5 10.5 9.1 9.1 3.2 3.2 0.5 0.3 0.3

106 K7.7975 1.2 42.4 42.4 10.1 10.1 7.6 7.6 2.8 2.8 1.1 0.2 0.2

105 K8.9542 0.0 46.0 24.7 42.4 22.8 0.1 0.1 0.2 0.1 0.0 1.9 1.0

104 K8.9556 0.1 24.7 45.9 22.7 42.2 0.1 0.1 0.2 0.3 0.0 1.1 1.9

103 K10.9361 0.4 7.3 7.2 18.8 18.8 3.1 3.1 5.8 5.8 50.7 0.4 0.4

102 K11.0294 0.6 15.6 15.6 37.8 37.8 7.4 7.4 8.8 8.8 0.0 0.1 0.1

101 K11.1916 0.9 3.2 3.2 0.8 0.8 2.1 2.1 0.1 0.1 91.1 0.2 0.2

100K

11.2947 0.9 7.1 7.1 15.6 15.6 3.0 3.0 3.9 3.9 55.2 0.8 0.899 K11.7304 9.3 9.3 9.3 25.1 25.1 2.1 2.1 15.6 15.6 28.7 1.2 1.2

98 K12.1834 5.3 31.2 31.2 12.6 12.5 17.7 17.6 5.9 5.8 12.8 1.3 1.3



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electron transfer: one is the direct transition through carbonin quinoline ring; the other is through the metal ion, which is

not only a support but also a bridge of electron transfer.

3.2.1. S0/S1 Vertical excitation energy

In the present study, configuration interaction with all

single (CIS) excited determinants is employed to study the

lowest energy singlet excited state (S1) of GaMq2Cl. CIS

represents for excited states a general zeroth-order method,

just as Hartree-Fock is for the ground state of molecular

systems. Besides being relatively inexpensive, permitting it

to be applied to large molecules such as Alq3, analytic

derivatives are available for CIS allowing the efficientcalculation of excited-state structures and properties [24].

The vertical excitation energies of GaMq2Cl computed

using CIS are presented. The CIS/6-31G* vertical excitation

energies of GaMq2Cl varies from 3.7 to 5 eV. CIS theory is

known to overestimate electronic excitation energies, due to

the neglect of the effects of electron correlation and higher

order excitations. To investigate the effect of electron

correlation on the computed energies, calculations using

time-dependent density functional theory and the hybrid

density functional, B3-LYP, were carried out with the

6-31G* basis set for GaMq2Cl, so direct comparison with

the CIS/6-31G* results can be made. Pople and co-workers

[24] observed that with the CIS method, the use of basis sets

that included significant polarization resulted in a larger

overestimation of excitation energies; however, the excited-

state potential energy surface was found to be more

accurate. This behavior was attributed to the fact that

these functions lower the ground-state reference energies to

a greater extent than the excited-state energies do.

With the prerequisite ground-state DFT calculation in

hand, we proceed to the time-dependent calculation of

GaMq2Cl to find the characters and energies of its low-lying

singlet and triplet excited states. We begin with the

singlet/singlet spin-allowed transition. Ten singlet excited

states are calculated at the optimized structure of the groundstate(B3LYP/6-31G*) for the complex and only the singlet

excited states with the greatest oscillator strengths are listed

as analysis example in Table 4. The energy of each excited

is vertical excitation energy in electron-volts (eV) from the

ground state. There are significant oscillator strength

throughout the 3w5 eV region, No excited states or

absorption features are found below 2 eV.

The 10 lowest-energy triplet excited states were also

calculated, using analogous TD-DFT methodology. The first

five triplet excited states are listed in Table 4. LLCT excited

states are all seen, but most of them are the mixed character

excited states, as with the singlets. As expected from Hunds

rule, transitions to the triplet states tend to be lower in energy

than their corresponding singlets. For example, the first triplet

vertical transition energy is 2.28 eV lower than that of the first

singlet excited state (3.09 eV) where both represent (pre-

dominantly) a MO 107/MO 108 transition.

A commonly used model of an excited state corresponds

to excitation of an electron from an occupied to a virtual MO

(i.e., a one-electron picture). However, the excited states

calculated herein demonstrate that excited-state electronic

structures are best described in terms of multiconfigurations,

wherein a linear combination of several occupied-to-virtual

MO excitations comprises a given optical transition.

Assignment of the character of each excited state wasbased on the compositions of the occupied and virtual MOs

of the dominant configuration(s) for that excited state. For

example, for S1 excited state the dominant excitation is

107/108 and since the occupied orbital (107) is phen-

oxide-based and the virtual orbital (108) is pyridyl p*, the

transition is designated a phenoxide-to-pyridyl ligand

charge transfer (LLCT).

3.2.2. S1 Excited-state structure

Studies of the excited-state properties for a number of

molecules using the CIS method have found that despite

Fig. 2. Molecular orbital surfaces of the HOMO, second highest energy molecular orbital (HOMOK1), LUMO and second lowest energy molecular orbital

(LUMOC1) of GaMq2Cl.



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the tendency of CIS to overestimate electronic transition

energies, the excited state potential energy surface can

often be quite accurate, as evidenced by comparison of

equilibrium excited-state structures with experiment [24].

To investigate the geometry change associated with

electronic excitation to the lowest energy singlet excitedstate (S0/S1), the geometry of GaMq2Cl was optimized

at the CIS/6-31G* level of theory in the S1 state for

comparison with the HF/6-31G* ground-state structure.

Table 5 presents the optimized ground-and excited-state

bond lengths for GaMq2Cl. Note that positive and

negative values in the% difference columns indicate

bond elongation and contraction in the excited state,

respectively.

Comparison of the excited-and ground-state geometries

for A-, and B-quinolates in GaMq2Cl indicates that

the structural shift is predominantly localized on the

B-quinolate. The A-quinolates in GaMq2Cl is practically

unaffected except for changes in the AlO and AlN bond

lengths.

3.2.3. Orbital analysis

The lowest energy singlet transition for GaMq2Cl at the

CIS/6-31G* level of theory involves transitions from

the least bound HOMO orbital to the lowest and the second

lowest energy LUMO orbitals with about equal weight. The

least bound HOMO orbital of GaMq2Cl is mainly localized

on the phenoxide, but the LUMOs also have contributions

from the pyridyl, and the localized nature of the electronic

excitation is clear. The observed geometry relaxation in

GaMq2Cl can be rationalized by consideration of the nodal

patterns of the HOMO and LUMO orbitals in Fig. 2. The

lowest energy singlet excitation (S0/S1) is mainly

HOMO/LUMO in character. The LUMO has nodes

Table 4

Selected Calculated Excitation Energies (E), Wavelengths (l), Oscillator Strenghs () and Dominant excitation character for low-lying singlet (Sn) and Triplet

(Tn) States of GaMq2Cl

State Excitation E (eV) l (nm) Character

Singlet excited states

1 107O108 (0.65010) 3.0953 400.56 0.0868 3.0953 400.56 0.0868 LLCT

2 106O108 (0.33649) 3.1771 390.24 0.0033

107O109 (0.57018)

3 106O108 (0.59581) 3.2840 377.54 0.0006 LLCT

107O109(K0.37635)

4 106O109 (0.66328) 3.3342 371.85 0.0061 LLCT

107O108(K0.21343)

5 104O109 (0.27090) 4.1803 296.59 0.0022 LLCT

105O108 (0.32347)

106O111 (0.28291)

107O110 (0.47820)

6 104O108 (0.32148) 4.1873 296.09 0.0005 LLCT

105O109 (0.27630)

106O110 (0.31232)

107O111 (0.45657)

7 106O110 (0.53332) 4.5284 273.79 0.0005 LLCT

107O

111 (K

0.45270)8 105O108 (0.10123) 4.5329 273.52 0.0040 LLCT

106O111 (0.54277)

107O110 (K0.43399)

9 104O109 (K0.49002) 4.7160 262.90 0.0046 LLCT

105O108 (0.50309)

10 104O108 0.0005 4.7171 262.84 (0.49808) LLCT

105O109 (K0.49675)

Triplet excited states

1 106O109 (0.47156) 2.2774 544.40 0.0000 LLCT

107O108 (0.58786)

2 106O108 (0.51302) 2.2879 541.90 0.0000 LLCT

107O109 (0.55198)

3 106O108 (0.51811) 3.2693 379.24 0.0000 LLCT

107O109 (K0.48057)

4 106O

109 (0.55139) 3.2831 377.64 0.0000 LLCT107O108 (K0.43787)

5 104O109 (K0.25301) 3.5638 347.90 0.0000 LLCT

105O108 (K0.27272)

106O111 (0.40929)

107O110 (0.48184)



7/8

across the C1C2, C3C4 bonds in the phenol ring and there

has been bonding in the pyridyl ring, but the HOMO is

bonding in corresponding regions and has no bonding in the

pyridyl ring. Therefore one would expect elongation of

these bonds and contraction in some region of pyridyl ring;

Table 5 shows that these bonds are in fact considerably

longer and shorter in the excited state, respectively.

3.2.4. S0/S1 Emission energy

The CIS calculations for AlMq2OH also provide an

estimate of the relaxed emission energy from the optimized

CIS excited state structure to the Hartree-Fock ground state

(S1/S0). TD-B3-LYP/6-31G* calculations were carried

out for GaMq2Cl at the CIS/6-31G*-optimized excited-state

structure to obtain more accurate estimates of the emission

energy. We list the lowest four singlet excited states, which

consists of the transition from HOMO to LUMO, and thus

assigned as the localized character. With TD-B3-LYP, the

emission energy is predicted to be ca. 2.4572 eV corre-

sponding to emission at ca. 504.57 nm, which is in much

closer agreement with the energy of the experimental

photoluminescence emission observed in condensed-phase

at ca. 492 nm [8].

State ExcitationE

(eV)l

(nm) Singlet excited states

1 107O108 2.4572 504.57 0.0481

2 106O108 2.9864 415.16 0.0041

3 107O109 3.0741 403.32 0.0058

4 106O109 3.3903 365.70 0.0355

Experiment8:492 nm

State Excitation E (eV) l (nm)

Singlet Excited states

1 107O108 2.4572 504.57 0.0481

2 106O108 2.9864 415.16 0.0041

3 107O109 3.0741 403.32 0.0058

4 106O109 3.3903 365.70 0.0355

Experiment8:492 nm

4. Conclusions

The first singlet excited state (S1) of GaMq2Cl has been

studied using the CIS/6-31G* and TD-B3-LYP/6-31G*

levels of theory. The electronic excitation and the structural

relaxation in the excited state for GaMq2Cl has been

interpreted in terms of the nature and nodal characteristics

of the HOMO and LUMO. The correlation between the

electronic excitation and the structural relaxation in the

excited state for GaMq2Cl has been made. The S0/S1

excitation is found to be mainly localized on the quinoline

ligand. At the TD-B3-LYP level of theory, the calculatedwavelength for emission agree very well with the exper-

iment. the nature of the electron transition in the complex

can be interpreted from theoretical results.

Acknowledgements

Thank the professor Y.S. Ji, Fundamental Department,

Chinese Peoples Armed Police Force Academy, for

emendating English composition.

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Table 5

HF/6-31G* Ground-state and CIS/6-31G* Excited-state bond lengths for the A-and B-Quinolate ligands in GaMq 2Cl

Bond distance (A) HF/6-31G* CIS/6-31G* (RCIS-RHF)ORHF!100%

A B A B A B

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C2C3 1.4144 1.4143 1.4137 1.3587 0.00 K3.93

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NC6 1.3592 1.3592 1.3598 1.3375 0.00 K1.60

C7N 1.3057 1.3057 1.3066 1.3840 0.01 6.00

C7C8 1.4179 1.4179 1.4176 1.3642 0.00 K3.79

C8C9 1.3584 1.3584 1.3583 1.4124 0.00 3.98

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C5C9 1.4157 1.4157 1.4147 1.4000 K0.01 K1.11

C7C (CH3) 1.5039 1.5040 1.5046 1.4978 0.00 K0.41

GaCl 2.2100 2.2087 K0.01



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