J. Phys. Chem. 1996, 100, 11029-11032
11029
Electronic Structure of Push-Pull Molecules Based on Thiophene Oligomers L. E. Bolı´var-Marinez, M. C. dos Santos, and D. S. Galva˜ o* Instituto de Fı´sica, UniVersidade Estadual de Campinas, UNICAMP, 13081-970 Campinas SP, Brazil ReceiVed: October 30, 1995; In Final Form: April 18, 1996X
Thiophene oligomers (R-R′ linkages) have been intensively studied in the past few years. In particular, sexithiophenes have been synthesized with a high degree of purity and good crystallinity, allowing the fabrication of reliable field effect transistors. When donor-acceptor terminal groups are attached to thiophene oligomers, an anomalous behavior of the fluorescence as a function of the number of rings is reported. There are experimental evidences of charge transfer suppression for oligomers containing more than four rings. In this work, we have investigated the electronic structure of these systems. Geometrical optimizations and spectroscopic features of both ground and excited states were obtained through the use of sophisticated semiempirical methods. Our results have shown that charge transfer suppression can be explained in terms of the evolution of the electronic spectrum.
Introduction Polythiophene (PT) (Figure 1) belongs to the class of organic conjugated polymers that can present high electrical conductivity upon chemical doping.1 Applications of PT in technology have nevertheless been limited due to its poor solubility, processability, and mechanical properties. An important step toward processable long chains has been attained through the use of lateral saturated tails playing the role of fixed “solvent” molecules, e.g., alkylthiophenes.2 However, long-chain polythiophenes have the complexity of “real” polymers (such as broad distribution of chain lengths, conformational defects, etc.), and they are not good test models because it is difficult to estimate the relative contribution of each feature to the electronic behavior. In spite of many years of theoretical and experimental work on polythiophenes, some of its electronic aspects, especially nonlinear properties,3 are still not well understood, and short oligomers have recently received much attention as better model compounds for the understanding of some unusual properties presented by the polymer.4 Besides that, chromic phenomena,5 especially very intriguing thermochromic features,6 have been observed in various regioregular and nonregioregular thiophene oligomers. More detailed theoretical studies are required to provide a better understanding of these phenomena. The study of oligomers as model compounds has many advantages,4 such as (i) oligomers are well-defined chemical systems, (ii) the chain length can be easily controlled, (iii) size dependent properties (optical, electronic, and magnetic) can be checked against theoretical predictions, and (iv) solvent effects and end substitution can also be easily incorporated. On the basis of these advantages, it is not surprising that so much attention has been devoted to oligothiophenes in recent years, as testimonied by the number of papers in the literature, covering a large number of subjects.7 In particular an “anomalous” (not observed for solid-state samples) behavior of the absorption/emission spectra as a function of the number of rings8-10 (with and without donor-acceptor electron groups at chain ends) has been reported. Wei et al.9a observed for thiophene oligomers a maximum absorption energy for hexamers that is higher than that for tetramers, and independent of solvent. This has been interpreted as the conjugation length being reduced from six to four rings, due to conformational changes X
Abstract published in AdVance ACS Abstracts, June 1, 1996.
S0022-3654(95)03201-1 CCC: $12.00
Figure 1. Schematic representation of the structures studied in this work: (a) the basic thiophene unit, showing the R and β carbon sites; polythiophene is made by R-R′ linkages and is in the anti-conformation, as shown in (b); (b) R,R′-bithiophene in the anti (most stable) conformation; departure from planarity may occur by the relative twist of rings, as indicated; (c) oligothiophenes (structure 1) and the corresponding push-pull, substituted species (structure 2); calculations have been performed including up to six rings in the anti-conformation.
in the process. We would like to add that this kind of anomalous behavior has also been observed in alkyl-substituted oligothiophenes9b but related to different mechanisms, and they are solvent dependent. In this work we have investigated the evolution of the geometry and optical spectra of thiophene oligomers, including up to six rings, and also considering the presence of donoracceptor electron groups at chain ends. The results are consistent with an anomalous behavior that goes beyond the conformational disorder (ring twisting), and can be assessed in terms of a distribution of electronic states that are active in the absorption/emission process. Methodology The gas phase experimental geometries for the thiophene oligomer series are not available. It is then necessary to obtain them theoretically. The molecules we are considering here (up to six thiophene rings) are large enough to preclude the use of large basis ab initio methods, but treatable within the framework of sophisticated semiempirical methods, such as the ones contained in the MOPAC package11-14 that have been intensively used in the last few years for the study of organic compounds. However, as has been discussed in the literature, sulfur atoms are very difficult to parametrize.3 In general, it © 1996 American Chemical Society
11030 J. Phys. Chem., Vol. 100, No. 26, 1996
Bolı´var-Marinez
TABLE 1: Bond Lengths (Å) and Angles (deg) of Thiophene (Figure 1a) method expta PM3 ab initiob MP2c AM1 MNDO INDO CNDO
S-CR CR-Cβ Cβ-Cβ ∠SCRCR ∠SCRCβ ∠CRCβCβ 1.714 1.725 1.723 1.713 1.672 1.679 1.864 1.857
1.369 1.366 1.347 1.385 1.377 1.375 1.333 1.330
1.423 1.436 1.438 1.423 1.431 1.452 1.439 1.439
92.1 91.4 91.4 92.3 93.8 93.7 85.4 85.4
111.5 112.1 111.9 111.6 111.5 111.5 114.1 113.3
112.5 112.2 112.1 112.2 111.6 111.4 112.5 114.0
a Reference 19. b Split valence 3-21G, ref 20. c Moller-Plesset with DZP basis, ref 21.
has not been possible to obtain at the same time, and within the same methodology, a good description of properties such as geometry and torsion barrier energies.6,15 This is also true in the present case. Although there is a consensus that Austin model 1 (AM1) and parametric method 3 (PM3) are better than the original modified neglect of differential overlap (MNDO) for geometrical optimizations, the question whether PM3 is better than AM1 is an ongoing debate.16 In the present case, by reasons discussed below, we have chosen PM3 as the method for the geometrical calculations. Although PM3 gives very good results for geometries and heats of formation, its spectroscopic results are not of comparable quality. It is recommended that, when optical properties are described, the use of PM3 should be coupled to another method specifically designed to perform spectroscopic calculations. Our choice was the spectroscopic Zerner’s intermediate neglect of differential overlap17 (ZINDO/ S-CI), which is specifically parametrized to describe the ultraviolet-visible optical transitions of organic compounds. On average, our calculations included 200 singlet configurations (singles and doubles). Results and Discussions In Table 1 we show the geometry for the thiophene molecule obtained with different theoretical methods and also some experimental data. As we can see from the table, the PM3 geometry presents the best overall results compared to AM1 and MNDO. We have carried out calculations for thiophene oligomers of up to six units. Our results, when compared to the available experimental data, have shown that, as in the case for the monomer, the overall geometries are better described by PM3 compared to MNDO and AM1. Although NMR18 measurements have shown the coexistence of both anti-like (most stable) and syn-like conformations separated in energy by a small difference of 0.18 kcal/mol, the PM3 predicts, in contrast with AM1, syn as the most stable configuration. However, considering that the overall geometry is much better described by PM3 and that the energy difference of syn- and anti-conformations for bithiophene is within the numerical error, the natural choice for the present study is PM3. We stress that, during the geometry optimization, the starting conformation was the anti-like conformation that corresponds to a local minimum of the PM3 method. The final geometries obtained were also anti-like, so our analysis and spectroscopical calculations refer to this conformation. Thiophene oligomers without the terminal groups are not planar molecules, as obtained from PM3 theory. However, the calculations for the excited states (singlet) have shown that the molecules become planar under excitation. Thus, upon absorption of light the molecules undergo a conformational transition to planar configurations and emit from them. This implies a large Stokes shift, since the geometries connecting absorption and emission processes are quite different. This has been
Figure 2. Projected view of the fully optimized geometries of endsubstituted oligothiophenes (four to six rings). The angles between adjacent rings are also indicated.
experimentally observed.8,9 A similar situation is obtained for the push-pull thiophene molecules (Figure 2). The pattern of ring twisting along the molecule backbone is only slightly affected by the presence of the end polar groups in comparison with that in the pure oligomers. Wei et al.9a have reported for R,ω-aldehyde-capped thiophene oligomers that the maximum absorption wavelength for the hexamer is higher than that for the dimer, but lower than that for the tetramer. It is also mentioned that this behavior is independent of the solvent. They argue that these phenomena are related to the decrease in the conjugation length, probably due to disorder effects when the chains become longer. On the basis PM3 results for the structures indicated in Figure 2, we offer an alternative explanation for the anomalous behavior. We believe that it can also be applied to the structures studied by Wei et al.9a due to the presence of polar end groups. Our analysis for the twist angles between rings has shown that, in both series of compounds, with and without end groups, the molecules present approximately the same pattern of ring torsions, independent of the chain size. It is worth noting that the experimental gas phase geometry of bithiophene is not planar, and the conformation in solution is also expected not to be planar. Thus, the conjugation length is not decreasing as the chain length increases, though the degree of disorder is expected to increase with increasing chain length. The explanation for the change in the absorption behavior can be assessed in terms of a relative movement of electronic molecular levels involved in the first excitation when the chains become longer. We have carried out INDO/CI calculations to simulate the absorption (twisted geometry) and emission (planar geometry) spectra of the end-substituted oligomers. The results are displayed in Figure 3. The Stokes shift increases with the chain
Push-Pull Molecules Based on Thiophene Oligomers
J. Phys. Chem., Vol. 100, No. 26, 1996 11031
Figure 5. Spatial representation of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for the fully optimized end-subsituted terthiophene.
Figure 3. Simulated absorption (continuous lines) and emission (dotted lines) spectra of end-substituted oligomers as a function of wavelength. These curves were obtained assuming a Gaussian broadening at each transition obtained from ZINDO-CI calculations. The heights of the peaks are proportional to the oscillator strength of the transitions, and the Gaussian widths were arbitrarily taken as 0.5 eV.
Figure 4. First optically active transition energies (eV) of oligothiophenes and the corresponding end-substituted molecules in both planar (p) and fully optimized (f) geometries, as a function of the number of thiophene units.
length, in agreement with experiment.8 The evolution of the first optically active transition energy for both pure and substituted oligomers is shown in detail in Figure 4. In this figure we see that the planar set of molecules exhibits an energy of the first electronic transition that decreases smoothly, as expected for a system with increasing conjugation length. However, the characteristic decay observed does not follow the usual 1/n rule since the present calculation goes beyond Hartree-Fock theory. On the other hand, when the molecular geometries are fully optimized, the energy of the first transition
changes behavior from three to four rings in the substituted oligomers: the energy decreases, then increases, and decreases again. This behavior is not seen in pure oligothiophenes. Considering that no significant geometry modifications have been observed in going from three to four rings, the explanation for this effect does not simply reside on changes of conjugation lengths, and a detailed analysis of the electronic structure is necessary. We first analyzed the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) spatial distribution patterns. In all cases the HOMO orbital contains significant contributions from the amine end group (donor) and a typical HOMO state of thiophene rings. As these systems are not centrosymmetric, the ring closest to the end group contributes more. A similar behavior is seen for the LUMO orbital, but involving the contribution of the nitro group (acceptor). This characterizes the electron transfer in systems of the type donor-conjugated bridge-acceptor. The HOMO and LUMO molecular orbitals are shown in Figure 5 for substituted terthiophene.22 All the other structures present a similar behavior. In Figure 6 we show a schematic diagram of the distribution of the last six occupied molecular orbitals and the first six empty ones as a function of the number of rings, from the ZINDO calculations. It is seen in this figure that, as the number of rings increases, the relative distance (in energy) between the active molecular orbitals decreases. This has direct implications on the nature of the states involved in the first molecular excited state. An analysis of the CI coefficients of Slater’s determinant expansion (Table 2) clearly shows that for up to three rings the first singlet-singlet transition basically involves one single configuration (HOMO to LUMO excitation). However, in the case of four rings, the first transition is mainly composed of two configurations: HOMO to LUMO and HOMO to LUMO + 1. As the latter configuration has a higher energy than the former, the net result is an increase in energy of the transition. After five rings the first transition is a result of more and more configuration mixing, and, as the one-electron energies are closer, the transition energy slowly decreases. Thus, the
11032 J. Phys. Chem., Vol. 100, No. 26, 1996
Bolı´var-Marinez groups, have been investigated. Geometry optimizations were carried out in the framework of the semiempirical PM3 technique, and electronic transitions were analyzed in terms of the spectroscopic ZINDO/S-CI calculations including up to 200 single and double Slater determinants. An anomalous behavior of the maximum of the absorption peak and fluorescence as a function of oligomer size has been reported for oligothiophenes, which has been interpreted in terms of conformational disorder. Our results show that all the molecules considered here adopt a twisted configuration of thiophene rings, and the twist angles are approximately independent of oligomer size, which is not totally consistent with the interpretation given to the experimental results. We offered an alternative explanation for this based on the CI results: the first electronic transition is dominated by the HOMO-LUMO transition for oligomers containing up to three rings. Increasing the oligomer size produces the mixing of more and more higher energy configurations. This evolution explains the experimental data in terms of electronic processes only, and the conformational disorder does not seem to play the most important role in the phenomenon.
Figure 6. One-electron energies obtained by using ZINDO method for the thiophene oligomers (a) and for substituted oligothiophenes (b), as a function of the number of rings. The dashed line separates the occupied molecular orbitals from the empty ones. Two colunms for each structure are shown; the left and right ones indicate, respectively, the results for the planar (P) and fully optimized (F) molecules.
Acknowledgment. This work has been supported in part by the Brazilian Agencies CNPq, CAPES, FINEP, and FAPESP. We are indebted to Professor M. C. Zerner for making the code of the ZINDO-CI package available to us.
TABLE 2: Most Relevant CI Expansion Coefficients for Planar and Fully Optimized End-Substituted Oligothiophenesa
(1) Roncali, J. Chem. ReV. 1992, 92, 711. (2) dos Santos, D. A.; Galva˜o, D. S.; Laks, B.; dos Santos, M. C. Chem. Phys. Lett. 1991, 184, 579 and references therein. (3) Soos, Z. G.; Galva˜o, D. S. J. Phys. Chem. 1994, 98, 1029. (4) Kanemitsu, Y.; Suzuki, K.; Masumoto, Y. Phys. ReV. 1994, B50, 2301. (5) Faı¨d, K.; Fre´chette, M.; Ranger, M.; Mazerolle, L.; Le´vesque, I.; Leclerc, M.; Chen, T.-A.; Rieke, R. D. Chem. Mater. 1995, 7, 1390. (6) dos Santos, M. C.; Bohland-Filho, J. Proceedings of the International Symposium on Optical Science, Engineering, and Instrumentation SPIE’95 40th Annual Meeting, San Diego, July 1995; SPIE: Bellingham, WA, 1995; Vol. 2528, p 143. (7) See for example: Proceedings of the International Conference on Synthetic Metals, Singapore, 1994. Synth. Met. 1995, 69. (8) Garcia, P.; Pernaut, J. M.; Hapiot, P.; Wintgens, V.; Valat, P.; Garnier; Delabouglise, F. J. Phys. Chem. 1993, 97, 513. (9) (a) Wei, Y.; Wang, B.; Wang, W.; Tian, J. Tetrahedron Lett. 1995, 36, 665. (b) Henderson, P. T.; Collard, D. M. Chem. Mater. 1995, 7, 1879. (10) dos Santos, M. C.; Marinez-Bolı´var, L. E.; Galva˜o, D. S. Proceedings of the International Symposium on Optical Science, Engineering, and Instrumentation SPIE’95 40th Annual Meeting, San Diego, July 1995; SPIE: Bellingham, WA, 1995; Vol. 2528, p 135. (11) MOPAC program, version 6.0 (QCPE No. 455). (12) Dewar, M. J. S.; Thiel, W. J. Am. Chem. Soc. 1977, 99, 4899. (13) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. Soc. 1985, 107, 4433. (14) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209. (15) Belleteˆte, M.; Leclerc, M.; Durocher, G. J. Phys. Chem. 1994, 98, 9450. (16) Bucci. P.; Longeri, M.; Veracini, C. A.; Lunazzi, L. J. Am. Chem. Soc. 1974, 96, 1305. (17) Ridley, J.; Zerner, M. C. Theor. Chim. Acta 1976, 42, 223. (18) Galva˜o, D. S.; Soos, Z. G.; Ramasesha, S.; Etemad, S. J. Chem. Phys. 1993, 98, 3016 and references therein. (19) Harmony, M. D.; et al. J. Phys. Chem. Ref. Data 1979, 8, 619. (20) Yadav, V. K.; Yadav, A.; Poirrier, R. A. J. Mol. Struct. THEOCHEM 1989, 186, 101. (21) Simandiras, E. D.; Handy, N. C.; Amos, R. D. J. Phys. Chem. 1988, 92, 1739. (22) Molecular orbitals have been obtained from the SPARTAN, version 4.0, package (Wavefunction Inc., 18401 Von Karman Ave., No. 370, Irvine, CA 92715. Copyright 1995 Wavefunction, Inc.).
molecule
optimized geometry CI state
planar geometry CI state
A-T1-D
0.973 |H f L〉 -0.102 |H - 4 f L〉 0.955 |H f L〉 -0.188 |H - 1 f L + 1〉 0.893 |H f L〉 0.287 |H - 1 f L〉 0.806 |H f L〉 0.417 |H f L + 1〉 0.678 |H f L〉 -0.474 |H - 1 f L〉 -0.576 |H f L + 1〉 0.536 |H - 1 f L〉
0.979 |H f L〉 0.099 |H - 1 f L + 1〉 0.957 |H f L〉 0.192 |H - 1 f L + 1〉 0.911 |H f L〉 0.235 |H f L + 1〉 0.849 |H f L〉 0.385 |H f L + 1〉 0.765 |H f L〉 -0.425 |H f L + 1〉 0.660 |H f L〉 0.479 |H - 1 f L〉
A-T2-D A-T3-D A-T4-D A-T5-D A-T6-D
a These are the excited states for the first optically active transitions. H ( i f L ( j represents HOMO ( i orbital to LUMO ( j orbital excitation.
anomalous behavior of the absorption spectra can be explained not only in terms of conformational changes (since the geometries and the twist angles of the chains with three and four rings are basically the same) but also in terms of the nature of states involved in the electronic transitions. The interaction of the chains with the solvent modifies the ring rotational barriers and might also induce changes in the relative molecular energy positions. Thus, we can expect the phenomenon we discussed above and the Stokes shifts to be sensitive to the type of solvent, and to the nature of the end groups. This has been experimentally observed.8,9 Summary In this work, the electronic and geometrical structures of oligothiophenes, with and without donor and acceptor end
References and Notes
JP953201B
