The Low-Lying Excited States of Luminescent Conjugated Polymers

Chapter 21

Downloaded by NORTH CAROLINA STATE UNIV on October 7, 2012 | http://pubs.acs.org Publication Date: September 1, 1997 | doi: 10.1021/bk-1997-0672.ch021

The Low-Lying Excited States of Luminescent Conjugated Polymers: Theoretical Insight into the Nature of the Photogenerated Species and Intersystem Crossing Processes 1

1

2

J. Cornil , D. Beljonne , Alan J. Heeger, and J. L. Brédas

1,2

1

Service de Chimie des Matériaux Nouveaux, Centre de Recherche en Electronique et Photonique Moléculaires, Université de Mons-Hainaut, Place du Parc 20, B-7000 Mous, Belgium Institute for Polymers and Organic Solids, University of California, Santa Barbara, CA 93106-5090 2

In this contribution, we first discuss the nature of the primary photoexcited species in the lowest excited state of luminescent conjugated polymers (for instance, polyparaphenylene vinylene, PPV) on the basis of quantum-chemistry calculations including both electron– phonon coupling and electron-electron interactions. We conclude that electronic excitations in long PPV chains lead to the formation of weakly bound polaron-excitons, in agreement with a large number of recent experimental measurements. We then describe singlet and triplet excited states in oligothiophenes and point to the importance of intersystem crossing as an efficient nonradiative decay process of the singlet excitations. Luminescent conjugated polymers, such as poly(paraphenylene vinylene), PPV, polythiophene, PT, poly(paraphenylene), PPP and their derivatives (Figure 1) have attracted interest as materials for use as active layers in polymer light-emitting diodes (LEDs) (1-5). The emission process in such polymers originates from the radiative decay of weakly bound polaron-excitons that are generated in the polymer layer following recombination of injected electrons and holes. The achievement of high efficiencies requires a basic understanding of the phenomena occurring within the LEDs, both to allow forfinetuning of the electronic parameters of importance (6) and to limit the numerous competing nonradiative processes that arise for example from internal conversion, singlet fission (7), interchain processes (8), intersystem-crossing between the singlet and triplet manifolds (9), quenching of the polaron-excitons by charged or conformational defects, and quenching of the polaron-excitons by low-lying two-photon states. The best reported polymer LED exhibited an external quantum efficiency of approximately 4 % (internal quantum efficiency estimated as approximately 12 %) (10). Operating lifetimes have recently improved to the point where commercial applications are feasible (77). 308

© 1997 American Chemical Society

In Photonic and Optoelectronic Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.


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309

Luminescent Conjugated Polymers

IV

V

Figure 1. Molecular structure of: I) poly(paraphenylene vinylene), PPV; II) poly[2-methoxy-5-(2'-ethyl-hexoxy)-paraphenylene vinylene], MEH-PPV; III) poly(paraphenylene), PPP; IV) poly(2-decyloxy-paraphenylene), DO-PPP; V) polythiophene.



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PHOTONIC AND OPTOELECTRONIC P O L Y M E R S

Quantum-chemistry calculations can prove to be useful in the field of organic light-emitting diodes. In particular, such calculations clarify the nature of the low-lying excited states of conjugated systems, and hence suggest new strategies yielding enhanced efficiencies. Two examples in this contribution illustrate that valuable information is obtained through analysis of theoretical models. First, we discuss the nature of the emitting species in the lowest excited state of luminescent conjugated polymers, including both electron-phonon and electron-electron interactions. Specifically, we address the following propositions that have been reported in the literature: (i) free charge carriers are generated in the excited state (72) and emission is an interband process; (ii) emission originates from a tightly bound electron-hole pair with a binding energy larger than 1 eV (75); (iii) emission is from the radiative decay of weakly bound polaron-exciton with a binding energy of a few tenths of an eV (1416). Note that the polaron-exciton terminology implies that lattice relaxations are associated to the photogenerated electron-hole pair. Second, we refer to fluorescence measurements reported for oligothiophenes and substituted derivatives for which the main nonradiative decay channel originates in intersystem-crossing processes. We demontraste there that correlated calculations help to uncover the nature of the lowestlying singlet and triplet excited states and to provide a meaningful interpretation of the evolution in fluorescence efficiency upon substitution or increase in chain length. Discussion of the nature of the photogenerated species Vibronic structure. Any Hamiltonian used to characterize the nature of the photogenerated species has to incorporate electron-phonon contributions since these correspond to a basic feature of π-conjugated compounds. A typical manifestation of lattice relaxation in the excited state is the appearance of vibronic progressions in the optical absorption spectra. Vibronic features could not be observed if the equilibrium positions in the ground and excited states were identical. This is sketched in Figure 2a (where we assume the existence of a single active vibrational mode coupled to the electronic excitation); the orthonormality of the vibrational wavefunctions would then exclusively allow transitions between vibrational levels of the two states with the same quantum number. In contrast, a displacement of the equilibrium geometry in the excited state (Figure 2b-2c) leads to the appearance of several vibronic features, usually between the zeroth vibrational level of the ground state and various levels of the excited state; when treated within the Franck-Condon approximation, the intensities are weighted by the overlap of the vibrational wavefunctions. We emphasize that the lowest energy transition (the 0-0 transition) is from the ground state to the relaxed excited state (i.e., the excited state stabilized by lattice relaxation). In a rigid band model, the excited-state relaxations (and thus the vibronic effects) are expected to decrease linearly with the number of atoms in the chain; hence, lattice relaxation would be insignificant for long conjugated molecules. The existence of a vibronic progression in the absorption spectra of conjugated macromolecules indicates, however, that self-localization phenomena (with associated lattice relaxation) occur in the excited states.


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311

a)


S=0

w w w

b)

AQ

c)

0-0J

Y

Figure 2. Schematic representation of the vibronic transitions observed in a situation where the equilibrium geometries of the ground and excited electronic states are: (a) identical (S=0); (b) weakly displaced (S=1.5); and (c) strongly displaced (S=4). S is the Huang-Rhys factor as defined in the text.


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PHOTONIC AND OPTOELECTRONIC POLYMERS


On the other hand, given the ease of derealization and polarization of π electrons, electron correlation is another major ingredient to be incorporated in the theoretical model. We have thus to consider the contribution to the binding of photogenerated electron-hole pairs due to electron-lattice coupling and to electronelectron interactions (77). We have recently set up a theoretical strategy to simulate the vibronic progressions observed in well-resolved optical absorption (18) and photoluminescence (19) spectra of PPV oligomers; we then utilize the optimized parameters to estimate the lattice relaxations occurring in the excited states. This theoretical approach is based on the INDO/SCI formalism (semiempirical Hartree-Fock Intermediate Neglect of Differential Overlap/ Single Configuration Interaction (20)), coupled to a displaced harmonic oscillators model to treat the vibronic contributions; in this context, the Franck-Condon integrals associated to vibrational mode χ are given by:

2 v

=

~''

CTP

v!

S 5 V

(D

where S denotes the Huang-Rhys factor relative to the x-mode. The total value of this factor describes the extent to which geometry deformations take place in the excited state and actually corresponds to the average number of phonons involved in the relaxation process; summing over effective modes weighted by their associated HuangRhys factor therefore allows us to estimate the relaxation energy in this state (18). Our model further assumes that the relevant vibrational levels correspond to the so-called Η modes that induce geometry deformations going along the same line as the π-bond density modifications occurring in the excited state (27). In PPV oligomers, siteselective fluorescence spectra show that these are two effective modes with energies 0.16 eV and 0.21 eV in a 1:2 ratio (22). x

Analysis of the absorption spectra of PPV oligomers indicates that the relaxation energy in the 1B excited state is approximately 0.30 eV for the 3, 4 and 5ring oligomers; the relaxation energy is, therefore, relatively insensitive to oligomer chain length. In contrast, we have established on the basis of the experimental photoluminescence spectra reported for the same oligomers (19) that, in the case of emission, the total Huang-Rhys factor of the 1B excited state decreases as the chain length increases; the evolution is linear as a function of inverse chain length, tending to 0.2 eV at the limit of long chains. These contradictory results for absorption and emission suggest that the absorption process is affected by conformational disorder; the latter is strongly reduced in the emission spectra due to migration of the polaronexcitons towards the most ordered conjugated segments. This was demonstrated by Hagler et al for blends of MEH-PPV in polyethylene; the most ordered samples showed the smallest Stokes' shift (23). We emphasize that the relaxation energy estimates provided by the analysis of the photoluminescence spectra are in excellent U

U


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313

agreement with direct geometry optimizations of the 1B excited state performed on the 2, 3, and 4-ring oligomers (24) within the AM1/CI formalism (semiempirical Hartree-Fock Austin Model 1 method (25) coupled to a configuration interaction scheme).


U

Polaron-exciton binding energy. Recent experimental measurements have been carried out on luminescent conjugated polymers to determine the polaron-exciton binding energy (defined as the difference between the creation energy of two noninteracting polarons of opposite charge and the formation energy of a neutral polaron-exciton (17)). Internal photoemission experiments have been performed on polymers in a LED architecture; the measurements give access to the energy difference between the electron and hole injections and thus the energy gap for creation of two polarons of opposite charge; the data provide a value of 2.45 eV in the case of poly(2-methoxy-5(2'-ethyl-hexyloxy) paraphenylene vinylene) - MEH-PPV (see Figure 1), taking into account image charge effects and extrapolating to zero photon energy (26). Since the 0-0 transition of the polymer peaks at 2.25 eV (27), the binding of the electron-hole pair is estimated to be 0.2 eV (±0.1 eV). Similar experiments using internal field emission report binding energies of 0.2 ± 0.2 eV in the case of MEH-PPV and poly-2decyloxy-paraphenylene, DO-PPP (see Figure 1) (27). The recent fabrication of light-emitting electrochemical cells (LECs) has enabled independent measurements of the binding energy (28); the emission process in such devices is expected to turn on at an applied voltage equal to the energy gap. The results collected for various luminescent polymers indicate that the turn-on voltage is essentially identical to the measured optical gap; the LEC data appear thus to be consistent with the semiconductor model in which the exciton binding energy is at most a few times k T at room temperature. B

Ultraviolet Photoelectron Spectroscopy (UPS) measurements performed on a PPV sample indicate that the valence band edge is located at 1.55 ± 0.10 eV below the Fermi energy (29); assuming that the Fermi level is located in the middle of the gap, the energy gap corresponds to twice this value. Since UPS spectroscopy incorporates neither relaxation effects nor interactions between the emitted electron and the remaining hole, we then subtract from this value twice the polaron relaxation energy (2x0.15 eV from our AMI calculations (30)) and obtain a value of 2.80±0.20 eV for the creation energy of two polarons of opposite signs. Since the 0-0 transition of the same PPV sample, i.e., the formation energy of a neutral polaron-exciton, is measured at 2.45 eV, the binding energy of the polaron-excitons is estimated to be on the order of 0.35±0.20 eV. Since these estimates of the polaron lattice relaxation are lower limits (for example, they do not include ring-rotations), this estimate of the binding energy is an upper limit. Based on these direct experimental measurements, we conclude that the binding energy of the singlet polaron-excitons in conjugated polymers might be relatively small


314



(S,, transition energies is compared in Figure 3 to the experimental evolution, extracted from optical absorption measurements in solution (35,36). The agreement between the experimental and theoretical results is excellent, which shows the suitability of the INDO/MRD-CI technique for the description of the electronic excitations in these oligomers. Both the spectroscopic data and the calculated values indicate a red-shift of the first one-photon allowed electronic transition with the size of the oligomers, due to the extension of the π-delocalized system. Extrapolation to an infinite chain length of the calculated transition energies, assuming a linear evolution of S ->S, with respect to 1/n (i.e., the inverse number of thiophene rings), leads to an optical gap close to 2 eV, which again is in good agreement with the spectroscopic data on polythiophene; recent optical measurements performed on regioregular polythiophenes indeed show an absorption with an onset at -1.7-1.8 eV and a peak at -2.5 eV (37,38). 0

0

In Figure 3, we also show the dependence of the energy difference between the S ground state and the T, lowest triplet excited state on the number of thiophene rings, as calculated at the INDO/MRD-CI level on the basis of the S geometry (38). As illustrated in Figure 3, the evolution with chain length of the S -»T, energy difference is much slower than that of the S—>S, excitation: the singlet-triplet energy difference is only lowered by -0.2 eV when going from the dimer to the hexamer while the corresponding singlet-singlet absorption is characterized by a bathochromic shift that amounts to -1.4 eV. The slower evolution with the number of thiopheneringsof the triplet excitation energy (with respect to that of the singlet) reflects its more localized character, which originates in the exchange term. The localized character of the triplet agrees with Optically-Detected Magnetic Resonance (ODMR) experiments on polythiophene, indicating that the T, triplet state extends over not much more than a single thiophene ring (40). Moreover, similar ODMR measurements performed on oligomers ranging in size from two to five rings (41) indicate a very small chain-length dependence of the average spin-spin distance, r, in the triplet, as deduced within the dipolar approximation from the zero-field splitting, D (using the relation 0

0

0

0



316 3

-1

D/hc=2.6017 r , where D/hc is in cm and r in Â, we find r=3.27, 3.36, and 3.49 Â, in Th3, Th4, and Th5, respectively). Optical absorption measurements in terthiophene solution (using a heavy-atom containing solvent, i.e., 1,2-dibromomethane, to increase the spin-orbit coupling) have enabled the identification of the T, excited state at -1.71 eV, which is in very good agreement with the 1.68 eV calculated value. Recently, Janssen et al. have observed that addition of C to oligothiophenes (from Th6 to Thll) in solution results in a quenching of the thiophene oligomers triplet state via energy transfer to and produces triplet-state C (42). Therefore, the T, state of oligothiophenes ranging in size from 6 to 11 rings can be estimated to lie between the triplet energy of C (1.57 eV) (43A4) and that of Th3 (1.71 eV) (45). Finally, we note that Xu and Holdcroft have measured a phosphorescence peak at -1.5 eV in polythiophene (46), a value very close to the one we can extrapolate for an infinite chain length (1.49 eV) on the basis of the calculated oligomer excitation energies.


60

60

60

Recent time-resolved fluorescence studies on unsubstituted (47,48) and substituted (49) thiophene oligomers in solution indicate a sharp increase of the fluorescence quantum yield φ when : (i) the number of thiophene rings is increased from two to seven; or (ii) electroactive end-groups are attached to the terthiophene molecule. In both cases, no significant alteration in the radiative decay rate constant, k , was observed. The evolution of φ with chain length and substitution was related to the decrease of the nonradiative decay rate, k , dominated by the contribution arising from intersystem crossing (48-51). The importance of ISC as a nonradiative decay route of the singlet excitations has been also demonstrated by recent experimental investigations on polythiophene (9). Ρ

R

Ρ

NR

Radiationless transition theory expresses the intersystem rate constant in terms of: (i) a state density factor; (ii) the matrix element of the spin-orbit coupling Hamiltonian between the singlet and triplet wavefunctions; and (iii) an overlap factor, which accounts for the decrease in rate with an increasing energy gap between the two states involved in the crossing. Although a precise description of the intersystem crossing process would require the calculation of the spin-orbit coupling interaction as well as the overlap between the excited-state vibrational levels, we wish, at this stage, to present a qualitative picture for the ISC process in oligothiophenes. As first suggested by Ponterini and co-workers (49), we assume that the evolution of k with size of the oligomers and substitution is mainly related to the variation in the overlap factor. As pointed out by Robinson and Frosch in the case of a series of aromatic hydrocarbons (52), this factor is primarily controlled by the singlet-triplet energy separation. NR

The S,-T, energy difference that we calculate in unsubstituted oligothiophenes is much too large to give efficient singlet-triplet overlap, and hence the probability for S,-T, crossing is very weak. However, ISC can also occur through other channels involving higher-lying triplet states. The INDO/MRD-CI calculations indicate that there is one triplet excited state (T ), for which the wavefunction involves excitations 4


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CORNIL E T A L .

from deep π-levels with large weights on the sulfur atoms, that lies at an energy close to that of S,. In Figure 4, we plot the evolution of the S Q - ^ and S -»T excitation energies with respect to the inverse number of thiophene rings. In bithiophene, the triplet T lies below the singlet S,, while for longer chains, due to the stronger stabilization of the S, excited state with chain length, the state ordering is reversed, S, appearing below T (note that the calculated energy of the triplet T excited state of Th3 is overestimated by comparison to the value interpolated by considering a linear relationship between the T excitation energies and inverse chain length; as the crossing between the evolutions of the singlet S, and triplet T excited states occurs for a chain length around the trimer, we associate this discrepancy to the fact that we did not include in the calculations the spin-orbit coupling which, in the crossing region, is expected to mix efficiently the singlet and triplet wavefunctions. 0

4

4

4

4

4


4

Rossi et al. have pointed out the existence of a thermally activated decay route of S, (49); they express the total nonradiative decay rate constant as:

k

NR = * i

+

7

k

H)

= i

+

A

a

2

P

where k, is a nonactivated decay rate (including processes such as nonactivated ISC, internal conversion, singlet fission) and k is the intersystem crossing temperature-dependent decay rate. On the basis of the temperature dependence of k , they estimate the pre-exponential factor, A , as equal to l ^ x l O ' V , and an activation energy of -0.05 eV, in excellent agreement with the interpolated S,-T INDO/MRD-CI energy difference for Th3 (Table I). Assuming that k, remains constant when elongating the chain, we have calculated the k rate constants on the basis of the experimental k, and A values and of the calculated activation energies, i.e., the S,-T energy difference estimated by fitting the INDO/MRD-CI excitation energies with a linear relationship, as done in Fig. 4 (39). In view of the different assumptions considered, the calculated k decay rates can be considered to be in excellent agreement with the measured values (Table I). In bithiophene (Th2), the triplet T excited state is below the singlet S, state and the S,-T ISC process is nonactivated and very efficient, leading to a very low fluorescence yield. When the chain elongates, as the activation energy becomes larger, the probability for inter-system crossing to occur decreases and φ is raised. 2

NR

2

4

NR

2

NR

4

4

Ρ

We have calculated the influence of grafting electroactive moieties on Th3; our results indicate an increase in the S,-T energy difference, which is more pronounced in the case of substitution with electron-withdrawing groups than with electrondonating moieties (39). As a consequence, we expect a stronger increase in fluorescence quantum yield for acceptor derivatives than donor derivatives. This trend is fully consistent with the experimental evolution of k when going from Th3 to Th3-OCH3 and Th3-CHO; the large increase in the singlet-triplet energy difference in Th3-CHO gives rise to a much lower nonradiative decay rate and therefore a higher fluorescence efficiency (48). 4

NR


4

318



4.5

I

I

I

1ΓI 0

I

I

I

I

j

I

I

I I 0.1

I

I

I

I

I

j

I

I

I I 0.2

I

I

I

I

I

J

I

I I 0.3

I

I

I

I

I

I

J

I

I I 0.4

,

I

I

I I 0.5

I

.

,

I

,

"I 0.6

I

I

1/n Figure 3. Comparison between the evolution with the inverse number of thiophene rings, 1/n: (i) the INDO/MRD-CI singlet-singlet S - » S , (solid line, closed circles) and singlet-triplet SQ-VT, (dashed line, open circles) energies; and (ii) the experimental S - » S , (closed squares (33) closed triangles (34)) and S ->T, (open squares (37)) energies obtained from measurements in solution. 0

0

0

Table I: Intersystem crossing rates in oligothiophenes AE(S,-T ) 4

k™* (10V)

k

ex

"(10V)

NR

b

0.018

Th2

-0.10 (-0.10)

20.1

19.7

Th3

0.24 (0.05)

3.70

3.88 3.7 5.3 0.07 0.05 0.07

Th4

0.16 (0.13)

1.23

1.54 1.50

Th5

0.18 (0.18)

1.14

0.82 0.74

Th6

0.22 (0.21)

1.13

0.70 0.73

b

a

C

a

b

c

a

b

a

b

a

b

0.20 0.18

a

b

0.28 0.36

a

b

0.42

a

AE(S,-T ) is the S1-T4 energy difference (in eV) calculated, at the INDO/MRDCI level, for the unsubstituted oligothiophenes; the values between parentheses have been extrapolated from a linear relationship between the excitation energies and the inverse number ofrings.φ is the fluorescence yield; k denotes the nonradiative decay rate measured in solution and k the value calculated on the basis of simple assumptions (see text). 4

exp

Ρ

NR

th

NR

a: Ref.47; b: Ref.48; c: Ref.49


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1

2.5 0.1

1

1

1

Γ

I

1

1

0.4

0.2

0.6

0.5

1/n Figure 4. Evolution with the inverse number of thiophene rings, 1/n, of the INDO/MRD-CI S ->S, (solid line, closed circles) and S ->T (dashed line, open circles) energies. Note that the transition energies calculated for Th3 were not included in the linear fits. 0

0

4

Synopsis In this contribution, we have illustrated that quantum-chemical calculations can be efficiently exploited to help in the interpretation of experimental measurements on luminescent oligomers and polymers. We have first compared recent theoretical and experimental results dealing with the nature of the photogenerated species in the lowest excited state of luminescent conjugated polymers; we have concluded that these polymers sustain weakly bound polaron-excitons. We have also discussed the characteristics of the lowest-lying singlet and triplet excited states in oligothiophenes and have presented a model rationalizing fluorescence measurements; these show a huge increase in quantum efficiency whèrï increasing the chain length and upon substitution with acceptor groups. Acknowledgments The work in Mons is conducted in the framework of the Belgian Federal Government "Pôle d"Attraction Interuniversitaire en Chimie Supramoléculaire" and is partly supported by the European Commission (ESPRIT Program LEDFOS-8013 and the Human Capital and Mobility Network SELMAT), the Belgian National Fund for Scientific Research (FNRS), and an IBM Academic Joint Study. JC is Aspirant and DB Chargé de Recherches of the FNRS. The work at UCSB was supported by the Office of Naval Research under Grant No. N00014-91-J-1235


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Literature cited [1] Burroughes, J.H.; Bradley, D.D.C.; Brown, A.R.; Marks, R.N.; Mackay, K.; Friend, R.H.; Burn, P.L.; Holmes, A.B. Nature 1990, 347, 539; Greenham, N.C.; Moratti, S.C.; Bradley, D.D.C.; Friend, R.H.; Holmes, A.B. Nature 1993, 365, 628. [2] Braun, D.; Heeger, A.J., Appl. Phys. Lett. 1991, 58, 1982; Gustafsson, G.; Cao, Y.; Treacy, G.M.; Klavetter, F.; Colaneri, N.; Heeger, A.J. Nature 1992, 357, 477. [3] Grem, G.; Leditzky, G.; Ullrich, B.; Leising, G. Adv. Mater. 1992, 4, 36. [4] Gill, R.E.; Malliaras, G.G.; Wildeman, J.; Hadziioannou, G. Adv. Mater. 1994, 6, 132. [5] Ohmori, Y.; Uchida, M.; Muro, K.; Yoshino, K. Solid State Commun. 1991, 80, 605. [6] Cornil, J.; Beljonne, D.; Brédas, J.L. Synth. Met., 1996, 78, 209. [7] Klein, G.; Voltz, R.; Schott, M. Chem. Phys. Lett. 1973, 19, 391. [8] Jenekhe, S.A.; Osaheni, J.A. Science 1994, 265, 7653. [9] Kraabel, B.; Moses, D.; Heeger, A.J. J. Chem. Phys. 1995, 103, 5102. [10] Baigent, D.R.; Greenham, N.C.; Grüner, J.; Marks, R.N.; Friend, R.H.; Holmes, A.B.; Moratti, S.C.; Holmes, A.B. Synth. Met. 1994, 67, 3. [11] Yu, G.; Zhang, C.; Cao, Y. Appl. Phys. Lett., in press. [12] Pakbaz, K.; Lee, C.H.; Heeger, A.J.; Hagler, T.W.; McBranch, D. Synth. Met. 1994, 64, 295; Lee, C.H.; Yu, G.; Heeger, A.J. Phys. Rev. Β 1993, 47, 15543; Hagler, T.W.; Pakbaz, K.; Heeger, A.J. Phys. Rev. Β 1994, 49, 10968. [13] Leng, J.M.; Jeglinski, S.; Wei, X.; Benner, R.E.; Vardeny, Z.V.; Guo, F.; Mazumdar, S. Phys. Rev. Lett. 1994, 72, 156; Chandross, M.; Mazumdar, S.; Jeglinski, S; Wei, X.; Vardeny, Z.V.; Kwock, E.W.; Miller, T.M. Phys. Rev. Β 1994, 50, 14702. [14] Friend, R.H.; Bradley, D.D.C.; Townsend, P.D. J. Phys. D: Appl. Phys. 1987, 20, 1367. [15] Gomes da Costa, P.; Conwell, E.M. Phys. Rev. Β 1993, 48, 1993. [16] Kersting, R.; Lemmer, U.; Deussen, M.; Bakker, H.J.; Mahrt, R.F.; Kurz, H.; Arkhipov, V.I.; Bässler, H.; Göbel, E.O. Phys. Rev. Lett. 1994, 73, 1440. [17] Brédas, J.L.; Cornil, J.; Heeger, A.J. Adv. Mater., 1996, 8, 447. [18] Cornil, J.; Beljonne, D.; Shuai, Z.; Hagler, T.W.; Campbell, I.H.; Spangler, C.W.; Müllen, K.; Bradley, D.D.C.; Brédas, J.L. Chem. Phys. Lett., 1996, 247, 425. [19] Heller, C.M.; Campbell, I.H.; Laurich, B.K.; Smith, D.L.; Bradley, D.D.C.; Burn, P.L.; Ferraris, J.P.; Müllen, K. Phys. Rev. B, in press. [20] Zerner, M.C.; Loew, G.H.; Kichner, R.F.; Mueller-Westerhoff, U. J. Am. Chem. Soc. 1980, 102, 589. [21] Hernandez, V.; Castiglioni, C.; Del Zoppo, M.; Zerbi, G. Phys. Rev. Β 1994, 50, 9815. [22] Heun, S.; Mahrt, R.F.; Greiner, Α.; Lemmer, U.; Bässler, H.; Halliday, D.A.; Bradley, D.D.C.; Burn, P.L.; Holmes, A.B. J. Phys.: Condens. Matter 1993, 5, 247. [23] Hagler, T.W.; Pakbaz, K.; Heeger, A.J. Phys. Rev. Β 1995, 51, 14199. [24] Beljonne, D.; Shuai, Z.; Friend, R.H.; Brédas, J.L. J. Chem. Phys. 1995, 102, 2042. [25] Dewar, M.J.S.; Zoebisch, E.G.; Healy, E.F.; Stewart, J.J.P. J. Am. Chem. Soc. 1985, 107, 3902.



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[26] Campbell, I.H.; Hagler, T.W.; Smith, D.L.; Ferraris, J.P. Phys. Rev. Lett. 1996, 76, 1900. [27] Yang, Y.; Pei, Q.; Heeger, A.J. Synth. Met. 1996, 78, 263. [28] Pei, Q.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A.J. Science 1995, 269, 1086. [29] Fahlman, M.; Lögdlund, M.; Stafström, S.; Salaneck, W.R.; Friend, R.H.; Burn, P.L.; Holmes, A.B.; Kaeriyama, K.; Sonoda, Y.; Lhost, O.; Meyers, F.; Brédas, J.L. Macromolecules 1995, 28, 1959. [30] Cornil, J.; Beljonne, D.; Brédas, J.L. J. Chem. Phys. 1995, 103, 842. [31] Choi, H.Y.; Rice, M.J. Phys. Rev. Β 1991, 44, 10521. [32] Shuai, Ζ.; Pati, S.K.; Su, W.P.; Brédas, J.L.; Ramasesha, S. Phys. Rev. B, in press. [33] Kersting, R.; Lemmer, U.; Mahrt, R.F.; Leo, K.; Kurz, H.; Bässler, H.; Göbel, E.O. Phys. Rev. Lett. 1993, 70, 3820. [34] Buenker, R.J.; Peyerimhoff, S.D. Theoret. Chim. Acta 1974, 35, 33. [35] ten Hoeve, W.; Wynberg, H.; Havinga, E.E.; Meijer, E.W. J. Am. Chem. Soc. 1991, 113, 5887; Havinga, E.E.; Rotte, I.; Meijer, E.W.; ten Hoeve, W.; Wynberg, H. Synth. Met. 1991, 41-43, 473. [36] Charra, F.; Fichou, D.; Nunzi, J.M.; Pfeffer, N. Chem. Phys. Lett. 1993, 192, 566. [37] Chen, T.A.; Rieke, R.D. Synth. Met. 1993, 60, 175. [38] McCullough, R.D.; Lowe, R.D.; Jayaraman, M.; Anderson, D.L. J. Org. Chem. 1993, 58, 904. [39] Beljonne, D.; Cornil, J.; Friend, R.H.; Janssen, R.A.J.; Brédas, J.L. J. Am. Chem. Soc. 1996, 118, 6453. [40] Swanson, L.S.; Shinar, J.; Yoshino, K. Phys. Rev. Lett. 1990, 65, 1140. [41] Bennati, M.; Grupp, Α.; Bauërle, P.; Mehring, M. Mol. Cryst. Liq. Cryst. 1994, 256, 751. [42] Janssen, R.A.J.; Moses, D.; Sariciftci, N.S. J. Chem. Phys. 1994, 101, 9519; Janssen, R.A.J.; Smilowitz, L.; Sariciftci, N.S.; Moses, D. J. Chem. Phys. 1994, 101, 1787. [43] Wei, X.; Vardeny, Z.V.; Moses, D.; Srdanov, V.I.; Wudl, F. Synth. Met. 1993, 54, 273. [44] Zeng, Y.; Biczok, L.; Linschitz, H. J. Phys. Chem. 1992, 96, 5237. [45] Scaiano, J.C.; Redmond, R.W.; Mehta, B.; Arnason, J.T. Photochem. Photobiol. 1990, 52, 655. [46] Xu, B.; Holdcroft, S. J. Am. Chem. Soc. 1993, 115, 8447. [47] Chorosvian, H.; Renstch, S.; Grebner, D.; Dahm, D.U.; Birckner, E. Synth. Met. 1993, 60, 23. [48] Becker, R.S.; de Melo, J.S.; Maçanita, A.L.; Elisei, F. Pure&Appl. Chem. 1995, 67, 9. [49] Rossi, R.; Ciofalo, M.; Carpita, Α.; Ponterini, G. J. Photochem. Photobiol A: Chem. 1993, 70, 59. [50] Periasamy, N.; Danieli, R.; Ruani, G.; Zamboni, R.; Taliani, C. Phys. Rev. Lett. 1992, 68, 919. [51] Egelhaaf, H.J.; Oelkrug, D. SPIE 1994, 2362, 398. [52] Robinson, G.; Frosch, R. J. Chem. Phys. 1963, 38, 1187.


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