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J. Phys. Chem. B 2007, 111, 5794-5802
Molecular Engineering in Symmetric End-Substituted Oligothiophene Derivatives: Analysis of Condensed-Phase Photoemission Spectra Using Semiempirical Hartree-Fock Calculations Gary P. Kushto,* Neil J. Watkins,† Antti J. Ma1 kinen, and Zakya H. Kafafi Optical Sciences DiVision, U.S. NaVal Research Laboratory, 4555 OVerlook AVe. SW, Washington, DC 20375 ReceiVed: October 25, 2006; In Final Form: March 13, 2007
The electronic structures of two series of end-capped thiophene oligomers, one set containing the electrondeficient dimesitylboryl end-cap and one containing the electron-rich triaryl amine end-cap, have been modeled using semiempirical quantum chemical calculations and the results used to assign features in the photoemission spectra of the materials in the condensed phase. For the thiophene oligomers end-capped with the electrondeficient dimesitylboryl moieties, the energy of the occupied frontier orbitals is largely governed by π-type orbitals of the thiophene repeat units in the oligothiophene main chain. Conversely, in oligomers end-capped with electron-rich triarylamine moieties, the occupied frontier orbital energies are largely governed by orbital states of heavily mixed character associated with thiophene π-type systems and the low-lying nitrogen lone pairs of end capping groups.
1. Introduction Poly- and oligothiophenes have received much attention both experimentally and theoretically. Their well-defined physicochemical relationships provide excellent synthetic control of their molecular electronic structures through control of the thiophene chain length and chemical derivatization can be used to tune the molecular orbital energetics, carrier mobilities, and spectral properties (e.g., emission colors) of the individual molecular species.1-3 As a result, these materials have found widespread application as the active media in a myriad of electronic/ optoelectronic devices such as organic light emitting devices (OLEDs),3 photovoltaics (OPVs),4 and field effect transistors (OFETs).5 Polythiophenes substituted by solubilizing aliphatic chains have been used as the active light harvesting/carrier generating medium in high-efficiency organic solar cells.4,6 They have also exhibited the highest field-effect hole mobilities of any polymer system measured to date.7,8 In OLED applications, however, polythiophenes and unsubstituted oligothiophenes have been found to yield poorly performing devices despite their excellent carrier mobilities. This is largely due to the luminescence self-quenching exhibited by poly- and oligothiophenes caused by their propensity for forming π-π stacks in thin films.9 Shirota et al. have addressed this problem by end-capping thiophene oligomers with bulky substituents meant to disrupt intermolecular π-π stacking and thus reducing the polycrystallinity of the thin films.3 Oligothiophenes derivatized in this manner form amorphous thin films that are morphologically stable (with relatively high glass transition temperatures, Tgs), exhibit high photoluminescence quantum yields (ΦPLs) and can be used in high-efficiency OLEDs.3,10-16 Additionally, the specific chemical nature of the end-capping groups can be used to control the type of carrier that these materials preferentially * Address correspondence to this author. E-mail: gary.kushto@ nrl.navy.mil. † Current address: National Institute of Standards and Technology, 100 Bureau Rd., Stop 8360, Gaithersburg, MD 20899
transport. The parent oligothiophenes are good hole transporting materials; however, end capping with triarylamine (R,R′-bis{4-[bis(4-methylphenyl)amino]phenyl}oligothiophene, BMAnT for n ) 1-4 thiophene rings) or dimesitylboryl (R,R′bis(dimesitylboryl)oligothiophene, BMB-nT for n ) 1-3 thiophene rings) substituents yields materials with enhanced hole or even electron transport properties, respectively. Furthermore, the thiophene cores of these molecules maintain their individual spectral properties and the emission colors of the films can therefore be tuned by varying the number of repeat units in the conjugated thiophene backbone (this in a manner analogous to that of the parent oligomers). As an example, the BMA-nT (n ) 1-4) series exhibits emission λmax ranging from 456 nm for BMA-1T to 532 nm for BMA-4T in chloroform solutions.3 The effects that the triarylamine and dimesitylboryl endcapping groups have on the densities-of-states (DOS) of the oligothiophene films were previously investigated with ultraviolet photoemission spectroscopy (UPS).17,18 Therein, it was found that the nature of the end-capping group not only modified carrier conduction properties but also controlled the band lineup at both the hole (BMA-nT) and electron (BMB-nT) injecting contacts of organic semiconductor device structures. Introduction of the trimesitylboryl group increased the electron affinity (Ea) of the BMB-nT series of molecules, lowering the barrier to electron injection from common cathode metals. Furthermore, the triarylamine end-cap reduced the ionization potential (IP) of the BMA-nT series of molecules, which in turn reduced the barrier to hole injection into these materials from an ITO anode.18 Full analysis of the UP spectra of both the BMA-nT and BMB-nT series found that various spectral features could be assigned as being due to specific subgroups of the molecular structures (i.e., the thiophene core versus the end-capping groups). In the current work, we have chosen to examine the electronic structures of these molecular species more closely using semiempirical molecular orbital calculations to provide further insight into this novel class of organic semiconducting materials.
10.1021/jp067006g CCC: $37.00 © 2007 American Chemical Society Published on Web 05/08/2007
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2. Computational Approach and Spectral Modeling Quantum chemical calculations were performed with the MOPAC code version 5010mn.19 Molecular geometries were optimized by using the Austin Model 1 (AM1) parametrization of the MNDO semiempirical Hamiltonian.20 This parametrization was chosen because of its well-known capabilities for the reproduction of molecular properties such as molecular geometry, ionization potential, and heat of formation with comparatively little computational effort. Normal-mode analyses were conducted on each optimized molecular structure to verify that it was a true minimum on the molecular potential energy surface (PES). Theoretically derived DOS plots were generated from the eigenvalues of the one-electron states of the neutral molecules evaluated at the AM1 optimized geometries. To draw comparisons between the calculated DOS and the experimentally determined UP spectra we have invoked a “frozen orbital” approximation, i.e., Koopman’s theorem, and neglected any relaxation or correlation effects when considering removal of electrons from molecular spin states.21 Comparisons between the energies of Hartree-Fock single electron states (isolated single molecules) and the ultraviolet photoemission data of condensed phase molecular materials are qualitatively accurate and valid for interpreting UP spectra.21 This is because weak van der Waals intermolecular forces dominate the molecular interactions in most molecular solids resulting in what can be approximated as an ensemble of weakly interacting molecular units. The DOS measured via UPS do, however, differ from the theoretical molecular orbital (MO) energetics in several important ways. First, molecular energy levels are broadened in the solid state because of the small but finite nearest neighbor interactions. Second, electron removal energies are rigidly shifted from their expected values in a truly isolated molecular system due to the polarizability of the condensed phase medium. This polarizability screens the resultant photohole reducing the observed electron removal energies by what are usually termed as “final state effects”. The former two effects have been accounted for in the present simulated spectra via convolution of the single electron state “stick” spectra with Gaussian functions to account for natural spectral broadening. These spectra are then rigidly shifted in energy space to account for final state effects. In this work, an arbitrary shift was applied to the theoretically derived DOS spectra to align them with the experimental UP spectra, the magnitude of which was held constant within each group of molecules. For the BMB-nT molecules the applied shift was 2.06 eV, while for the BMA-nT series it was 1.94 eV. It was also found that no spectral compression was required to account for electron correlation effects, which is consistent with that reported previously for analogous simulations with the AM1 method.21 It must be noted here that when using semiempirical methods to predict thiophene molecular structures and related spectroscopic quantities, one must keep in mind the severe limitations of these theoretical methods particularly in light of the difficulty associated with the parametrization of the sulfur atom.22 Herein, great care has been taken to ensure that the geometric parameters and molecular orbital energetics have an internal data set consistency. This is important because semiempirical calculations are notorious for being simply wrong in certain instances. This is a direct result of the finite database of molecular systems used in the parametrization of the semiempirical Hamiltonian as well as the minimal atomic basis sets used in the calculations. For many of the molecules investigated in this work (most notably the parent oligothiophenes), the electronic and geometric
Figure 1. Effective chemical structures of the oligothiophene derivatives investigated herein. Bond numbers are listed in reference to Table 1. The letter “n” in the nomenclature refers to the number of thiophene repeat units in the molecular structure, e.g., terthiophene (n ) 3) end capped with dimesitylboryl substituents would be named BMB-3T.
structures have been well-characterized and provide a basis with which to compare our computational results and gauge their validity. 3. Results and Discussion 3.1. Molecular Geometries. In this work we have generated optimized geometries for BMB-nT and BMA-nT molecules (n ) 1-4) as well as those of the parent and diphenyl end-capped oligothiophenes (R-nT and DPR-nT, n ) 1-4, respectively). The effective chemical structures for these molecules are presented in Figure 1 while specific bond lengths are reported in Table 1. Typically, to provide support for the validity of a theoretical investigation the results of the calculations are compared to available experimentally derived molecular quantities (X-ray crystallographic data, gas-phase spectroscopies, etc.). To the best of our knowledge, there are no available data concerning the geometries of these materials in either the gas or condensed phases; however, extensive data exist for molecules that closely resemble the substituent parts of these molecules, i.e., the thiophene cores and the triaryboryl and triarylamine end-capping groups.
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TABLE 1: AM1 Optimized Bond Lengths (pm) for r-nT, Diphenyl End-Capped r-nT (DPr-nT), BMB-nT, and BMA-nTa bond n)1 r1 r2 rcap n)2 r1 r2 r3 r1-2 rcap n)3 r1 r2 r3 r1′ r2′ r3′ r1-2 rcap n)4 r1 r2 r3 r1′ r2′ r3′ r1-2 r3-4 rcap a
R-nT
DPR-nT
BMB-nT
BMA-nT
137.3 143.2
138.6 142.4 144.3
139.0 142.1 153.1
138.6 142.4 144.1
137.9 142.6 138.8 142.5
138.7 142.2 138.9 142.4 144.3
138.9 142.0 139.2 142.5 153.0
138.7 142.1 138.9 142.4 144.1
139.0 142.0 139.0 137.9 142.6 138.8 142.4
138.9 142.1 138.9 138.7 142.2 138.9 142.5 144.3
138.9 142.0 139.2 138.9 142.0 139.2 142.4 153.0
138.7 142.1 138.9 138.7 142.1 138.9 142.3 144.1
139.0 141.9 139.0 137.9 142.6 138.8 142.4 142.4
139.0 142.0 139.0 138.7 142.2 138.9 142.4 142.4 144.3
139.0 142.0 139.0 138.8 142.0 139.2 142.4 142.4 153.0
139.2 142.5 138.8 138.8 142.7 139.2 143.0 142.9 144.4
DPR-4T (B3LYP/6-31G**)
BMA-2T (B3LYP/3-21G*)
138.1 142.4 138.3 144.2 146.3
138.2 141.3 138.1 137.9 141.5 138.1 144.1 144.2 146.6
B3LYP/3-21G* and B3LYP/6-31G** bond lengths for BMA-2T41 and DPR-4T,33 respectively, have been included for comparison.
TABLE 2: Gas-Phase Bond Lengths (pm) for r-2T from Electron Diffraction Measurements,23 Hartree-Fock/6-31G*, B3LYP/6-31G** (Current Investigation), and AM1 Semiempirical (Current Investigation) Calculations bond
electron diffraction
HF (6-31G*)
B3LYP (6-31G**)
MP2 (6-31G**)
AM1
r1 r2 r3 r1-2 rC-S r′C-S
137.0 145.2 136.3 145.6 171.9 173.3
135.2 143.4 134.4 146.5 172.5 173.9
136.7 142.4 137.8 145.1 173.5 175.6
137.7 141.5 138.5 145.0 171.0 173.3
137.9 142.6 138.8 142.5 166.5 168.8
Considering the thiophene cores, only the geometry of bithiophene (R-2T) has been well characterized in the gas phase.23,24 The geometries published in these previous works are compared in Table 2 to geometries determined with use of HF/6-31G*,23 B3LYP/6-31G**, MP2/6-31G**, and AM1. In general there is reasonable agreement within the C-C distances found experimentally and theoretically. Of particular note is the fact that all of the calculations reproduce the expected asymmetry between r1 and r3 (for bond numbering see Figure 1) in bithiophene consistent with the π-conjugation between the rings. The largest deviation between the AM1 geometries and the others is found in the C-S bond lengths. The ab initio and hybrid DFT methods reproduce the experimental data while the semiempirical method substantially underestimates bond lengths. The source of this discrepancy has been determined to lie largely with the S atom parametrization22 in the AM1 semiempirical Hamiltonian. Calculations conducted at the Hartree-Fock level of theory with the STO-3G basis25 (this basis is similar in quality to that used in the AM1 calculations) predict a C-S bond length that is in much better agreement with the other data (C-S ≈ 173.7 pm), suggesting that the shortcoming in the AM1 calculations is not due to the use of the minimal basis.
Another discrepancy between the experimental data and the AM1 results presented here is that they predict the parent oligothiophenes to remain fully planar through four repeat units (R-4T). This is in complete agreement with previous semiempirical calculations as well as ab initio calculations with a minimal basis set.25-27 The twist along the thiophene backbone occurs when the stabilization energy associated with the conjugation of the thiophene π-system (i.e., the amount of quinoid character in the ground state) becomes small compared to the energy of inter-ring steric repulsions. Although the crystal structures for both R-4T and R-6T show largely planar thiophene backbones in the solid state, there is a significant deviation from ring coplanarity in bi- and terthiophene in the gas phase.23,24,28,29 The prediction of a planar structure in the bithiophene case with AM1 and HF/STO-3G25 can be ascribed to the limitations in the minimal basis sets used in these calculations. The use of the slightly larger 3-21G* set yields an inter-ring dihedral angle of 146.3° at the Hartree-Fock level, which is in much better agreement with experiment.23 For the calculations presented in Table 2, all save the AM1 method produce bithiophene structures with significant inter-ring dihedral angles: HF/631G*, 147.8°; MP2/6-31G**, 141.7°; and B3LYP/6-31G**, 157.6°. Although this is an obvious limitation in the AM1 method, it can be expected that even weak, intermolecular π-π stacking between different thiophene oligomers in the condensed phase can help to stabilize the fully planar molecular structure given that the rotatational barriers around inter-ring thiophene bonds are typically small (11) molecular orbital states. In the AM1 simulated spectrum for BMB-1T, there are 10 orbitals that contribute to the feature at
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J. Phys. Chem. B, Vol. 111, No. 21, 2007 5801
Figure 8. Experimental UPS (filled circles) and AM1 simulated (line) densities of states for (a) BMA-1T, (b) BMA-2T, (c) BMA-3T, (d) and BMA-4T oligothiophene derivatives in the energy range -5.5 to -11.0 eV (vacuum level reference).
-7.1 eV (b′) and an additional state that is associated with the feature at -7.5 eV (c′). Because of the inhomogeneous broadening of these states in the experimental spectrum, b′ and the c′ states are unresolvable, and hence both contribute to the experimentally observed feature. The MOs that occur in this energy region have been found to vary in character, but primarily arise from the π-systems of the dimesitylboryl groups or the π-type orbitals of the thiophene chains. The line shape of this b feature exhibits an interesting behavior as a function of the thiophene chain length. This occurs because of the interplay between the spectrally convoluted thiophene π-type orbitals which evolve with thiophene chain length and the π-type orbitals localized on the dimesitylboryl groups which remain constant in energy. For example, the c′ feature, seen in BMB-1T, is found to move to lower binding energies in BMB-2T and becomes convoluted with the b′ peak. This convolution of states has the effect of increasing the relative intensity of and slightly narrowing the experimental b feature (Figure 7). In BMB-3T, however, the c′ peak once again splits out to higher binding energies and the experimental b feature picks up a higher binding energy shoulder. The intensity of this shoulder is also affected by the myriad of other states that are building up from even higher binding energies (see the energy region between -9.5 and -8.0 eV in Figure 7). The number of MOs in this higher binding energy region of the spectrum increases with increasing thiophene chain length leading to a significantly greater breadth of the spectral feature centered at -10.2 eV in the experimental UP spectra. The molecular states that give rise to this feature are largely π-type orbitals, but their assignment would be dubious considering their energy and inadequacies of the current computational method in that energy regime.21 The UP spectra of the BMA-nT molecules presented in Figure 8 show the same excellent agreement as seen in the BMB-nT cogeners. In the experimental UP spectra, the unresolved doublet of bands (peaks labeled a and b, respectively) in all of the BMAnT spectra can be unambiguously assigned to the highly delocalized HOMO and the primarily N pz based HOMO-1 of each of the molecules. These features show very little dependence on the thiophene chain length apart from a rather complex intensity fluctuation. This is likely due to inhomogeneous broadening in the condensed phase convoluted with effects that
molecular packing have on the different symmetry molecular orbitals. This effect is further exacerbated by the relative spacings between these states as a function of chain length and the convolution of the HOMO-3 band (marked c′ in Figure 6) in the higher oligomers. As mentioned above, the HOMO-3 band (c′) exhibits a significant dependence on the number of thiophene rings. As the number of rings increases, the c′ feature moves to lower binding energies. In the broadened experimental spectra this results in a meshing of this spectral feature with the lowest lying features as we have noted previously.17,18 On the basis of the discussion above, it is clear that this spectral feature is largely phenyl-capped thiophene in character. Furthermore, in the BMA-3T and BMA-4T spectra there is another band that appears at higher binding energies (d) that is predicted by the AM1 calculations. This feature correlates to the theoretically derived HOMO-4 mentioned above, which is also primarily thiophene in character. Here the computational results show remarkable agreement with the experimentally derived spectra as depicted in Figure 8. As with the BMB-nT molecules, the UP spectra of the BMAnT molecules contain a higher binding energy, intense feature that exhibits little dependence on the thiophene chain. In the BMA-nT film data these are the broad intense features at -7.85 eV which can be assigned to the π-type orbitals of the 4-methylphenyl groups (>12 MOs) convoluted with several thiophene derived states. Interestingly, these features occur 0.4 eV to higher binding energies than the mesityl ring π-type orbitals of the BMB-nT oligomers. This seems counterintuitive based on the π-acid/base nature of the boryl versus amine moieties, respectively. However, the energies of these orbitals appear to be more affected by the methyl groups attached to the rings (BMA-nT has one methyl while BMB-nT has three) than by the pz orbitals of the heteroatoms. This is consistent with the fact that the 4-methylphenyl and mesityl rings lie at large twist angles with respect to the heteroatom pz orbitals in BMA-nT and BMB-nT molecules, respectively. At binding energies above 8 eV, the experimental UP spectra become more broad and nondistinct as the density of MOs in this region increases rapidly. As with the BMB-nT UP spectra noted above, the number of MOs in this higher binding energy region increases with increasing thiophene chain length leading
5802 J. Phys. Chem. B, Vol. 111, No. 21, 2007 to an effective increase in the experimental “background”, i.e., a multitude of highly mixed states that in the experimental spectrum are unresolvable. The molecular states that give rise to this signal are primarily mixed π- and σ-type orbitals, but as mentioned above, their assignment would be dubious considering their energy and inadequacies of the current computational method in that energy regime.21 4. Conclusion The electronic structures of two series of end-capped oligothiophene oligomers, one set containing the electron-deficient dimesitylboryl end-cap and one containing the electron-rich triarylamine end-cap, have been modeled by using semiempirical quantum chemical calculations and the results used to assign features in the photoemission spectra of the materials in the condensed phase. The geometries of the parent oligothiophenes (R-nT), diphenyl end-capped oligothiophenes (DPR-nT, BMB-nT, and BMA-nT molecules (for n ) 1-4)), have been optimized at the AM1 level of theory and compared to available theoretical results. Comparison of AM1 results for BMA-2T and DPR-4T with hybrid density functional results finds that the semiempirical method produces reasonable geometries considering the expenditure of computational effort. The AM1 Hamiltonian predicts geometries that exhibit slightly reduced BLA compared to the higher level calculations, indicating that the AM1 parametrization favors the quinoid resonance form in the oligothiophene systems. Furthermore, the AM1 Hamiltonian predicts that the introduction of the dimesitylboryl end-cap produces a twist in the thiophene backbone suggestive of a reduction in the thiophene π-conjugation. In the BMA-nT series, no such deviation in thiophene ring coplanarity is observed until the thiophene reaches a length of four repeat units (BMA-4T). Molecular DOS generated from the AM1 one-electron orbitals were compared to the experimentally measured photoemission spectra and were found to reproduce the experimental data set with remarkable accuracy. The near one-to-one correspondence between the theoretical and experimental spectra provide an excellent tool for the interpretation of the UP spectra of these oligothiophene derivatives. It has been shown that in the BMBnT series, the energies of the occupied frontier orbitals are largely governed by π-type orbitals of the thiophene repeat units. Conversely, in the BMA-nT oligomers the occupied frontier orbital energies are largely governed by orbital states of mixed character associated with thiophene π-type systems and the lowlying nitrogen lone pairs of the end-capping groups. Acknowledgment. The authors would like to thank Professor Y. Shirota for access to the BMB-nT and BMA-nT materials and for useful correspondences. N.J.W. would like to thank the National Academy of Science/National Research Council (NAS/ NRC) for administering the postdoctoral program at NRL. References and Notes (1) Ba¨uerle, P. In Electronic Materials: The Oligomer Approach; Wegner, G., Mu¨llen, K., Eds.; Wiley-VCH: Weinheim, Germany, 1998. (2) Tour, J. Chem. ReV. 1996, 96. (3) Shirota, Y. J. Mater. Chem. 2000, 10, 1-25. (4) Padinger, F.; Rittberger, R. S.; Sariciftci, N. S. AdV. Funct. Mater. 2003, 13, 85-88.
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