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Ambipolar Charge Transport in r-Oligofurans: A Theoretical Study† Sasmita Mohakud, Andrews P. Alex, and Swapan K. Pati* Theoretical Sciences Unit and New Chemistry Unit, Jawaharlal Nehru Center for AdVanced Scientific Research, Jakkur Campus, Bangalore 560064, India ReceiVed: May 24, 2010; ReVised Manuscript ReceiVed: August 14, 2010
The molecular scale charge transport has been investigated in a few recently synthesized molecular crystals of R-oligofuran via thermally activated hopping mechanism described by the semiclassical Marcus theory. The microscopic order parameters such as reorganization energy and hopping matrix elements, governing charge transfer phenomena, are estimated accurately using quantum chemical calculations. The dispersion corrected density functional calculations are carried out to capture the weak van der Waal interactions between the π-stacked molecules. The hopping matrix elements or charge transfer integrals are computed as the offdiagonal elements of Kohn-Sham matrix using fragment orbital approach which explicitly considers the spatial overlap between the molecular orbitals. Our study reveals that such oligofuran molecular crystals are excellent conductors for both charge carriers. However, the hole mobility is found to be slightly larger than electron mobility in smaller oligofuran molecular crystals, whereas the reverse holds true for larger molecule. Such ambipolar organic crystals with higher electron mobility show the possibility of sophisticated device fabrication in advanced electronics. In addition, we compare all our results with analogous oligothiophene crystals by performing the same level of calculations. Introduction Over past few decades, conjugated organic materials have been the subject of immense interest from both the basic science and technological perspective due to their potential applications in electronics and optoelectronic devices such as organic light emitting diodes,1-5 organic field effect transistors,6-8 solar cells,9 batteries and sensors.10,11 Among the conducting polymers, polythiophene,12-15 polyacene,16 polypyrrols,17-20 and their related derivatives13,21,22 have become the most active elements for such optoelectronic devices and have attracted wide interest. As for polymers, the discrete oligomers such as oligothiophenes (particularly sexithiophene)7,12-14,17,23,24 and oligoacenes (particularly pentacene)16,25,26 have also been intensively investigated owing to their superior electronic properties such as high conductance, flexibility, or rigidity and ease of synthetic modifications. Moreover, these oligomers are widely studied as the informative models for the organic polymers.12,13,16,17,24,27 Although some organic materials have shown tremendous progress in performance, the relatively lower carrier mobilities in such organic materials in comparison to inorganic amorphous silicon have become the stumbling block for their industrial applications. Hence, there have been many recent efforts in the search for new compounds with superior electronic properties governing the optical and charge transport phenomena. Recently, a family of new heterocyclic organic compounds known as poly- and oligoselenophenes22,27-30 have been synthesized and characterized and show many advantages over poly- and oligothiophenes. These compounds can act as an alternative to thiophene based compounds because of their narrow band gap, better planarity, and electro-chromic properties.28,29 In fact, the solubility of thiophene compounds, which is quite important for processing, can be improved by the †
Part of the “Mark A. Ratner Festschrift”. * Corresponding author. E-mail:
[email protected]. Tel.: 91 (80) 2208 2839/2575. Fax: 91 (80) 2208 2766/2767.
introduction of appropriate substituents to thiophene rings or replacement of sulfur atoms in the thiophene rings by some other atoms.21,22,27 Similarly, many other properties like conjugation length, bandwidth, molecular packing, electrical, optical, and charge transfer properties of the heterocyclic compounds are greatly influenced by the heteroatom identity.17,21,22 Recently, there have been reports on the synthesis and characterization of R-oligofuran crystals that are highly fluorescent, electron rich, and exhibit better packing, greater rigidity, and better processability than oligothiophenes.31 Moreover, the furan based compounds are biodegradable and can be obtained from renewable resources. With all such advantages over other conjugated systems, it would thus be very interesting to study the charge transport phenomena in these oligomers that are yet to be explored in detail. Although there exists a few studies on the optical properties of oligofurans and poly furans,17,21,32,33 the details of the charge carrier mobilities and the microscopic factors affecting them are poorly investigated due to the lack of characterization of its molecular crystals earlier. In this regard, here, we consider the molecular crystals of a few recently synthesized R-oligofurans and study their charge transport properties with accurate estimation of carrier mobility and the key parameters controlling it, within density functional theory formalism. Furthermore, we also have performed a comparative study of the charge transport between oligofurans and oligothiophenes within the same level of theory. Computational Details In almost all π-conjugated organic materials, the molecular scale charge transport at room temperature occurs via a thermally activated hopping type mechanism since the dynamic structural disorder strongly localizes the charge at high temperature and invalidates the band type mechanism.17,24,34-37 Such an incoherent hopping mechanism is well approximated within the semiclassical Marcus theory.38 However, there is vast experimental evidence suggesting the mobility of organic crystals to
10.1021/jp1047503 2010 American Chemical Society Published on Web 08/31/2010
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be 10-20 cm2 V-1 s-1, casting doubts on localized charge state model.39,40 Even in the case of pure crystals, the strong intermolecular interaction fails to localize the charge, revealing the nonapplicability of the hopping model. Moreover, there have been recent reports on a few organic single crystals such as pentacene and rubrene, which show a decrease in the mobility with increase in temperature, pointing out a bandlike charge transport at high temperature (170-300 K for rubrene and 225-340 K for pentacene).40,41 Hence, for the microscopic understanding of charge transport in such organic materials, quantum correction to the Marcus theory is essential, which has been reported recently to explain the band like behavior of carrier mobility within a hopping model.42 Our study suggests that in the case of oligofurans, the intermolecular interaction is very weak in comparison to charge relaxation. Thus, we use the incoherent hopping model for our study as the charges are localized. Within the semiclassical Marcus theory, the rate of charge transfer (W) dictated by both the reorganization energy (λ) and charge transfer integrals (Jeff) can be expressed as38
( ) (
2Jeff2 π3 W) h λkBT
1/2
λ exp 4kBT
)
(1)
where kB is the Boltzmann constant and T is the temperature (300 K for our calculations). It is clear from the above expression that the rate of hopping would be high if the reorganization energy is low and the charge transfer couplings are high. The reorganization energy mainly includes the structural modification of the molecules upon charging and the surrounding mediums due to polarization effects. Our calculation considers only the structural modification of the molecules since the consideration of polarization effect in the surrounding medium is computationally more expensive. The reorganization energy for both charge carriers are evaluated using the following formulas.43-45
λelectron ) (E*- - E-) + (E*anion - E)
(2)
λhole ) (E*+ - E+) + (E*cation - E)
(3)
where E is the optimized ground state energy of the neutral molecule, E- (E+) is the optimized energy of the anionic * (E*cation) is the energy of the neutral (cationic) molecule, Eanion * (E*) molecule in anionic (cationic) geometry, and E+ is the energy of the anionic (cationic) molecule in neutral geometry. All the anionic and cationic molecular geometries of each oligomer are optimized at the B3LYP level with 6-31G++(d,p) basis set using the Gaussian-0346 package. However the hydrogen atom positions for each oligomer crystal in its neutral state have been relaxed within the same level of calculations keeping the other atom coordinates fixed, since the X-ray crystallography cannot precisely determine the hydrogen positions. In our study, all the open-shell calculations are carried out using unrestricted density functional theory method. The charge transfer integrals for both carriers that explicitly depends upon the molecular packing of the crystals are calculated by density functional theory methods using the fragment orbital approach, as implemented in the Amsterdam density functional program (ADF).37,47 Within this approach, the dimer molecular levels are expressed as the linear combina-
tion of individual monomer molecular levels and the charge transfer integral can be obtained as the off-diagonal elements of the Kohn-Sham Hamiltonian matrix, which is expressed as follows.
HKS ) SCEC-1
(4)
where S is the intermolecular overlap matrix, C is the molecular orbital coefficient, and E is the molecular orbital energy. This method provides an accurate estimation of charge transfer integrals due to the consideration of the spatial overlap between the monomers. For our calculation, we use the Becke-Perdew exchange correlation functional with triple-ζ double polarization basis sets.48,49 Also, we have explicitly taken the dispersion correction factor into account, which arises due to weak van der Waal interaction between the π-stacked neighboring monomers.50 The effective charge transfer integral is calculated using the following formula.
1 Jeff ) J - S(E1 + E2) 2
(5)
where S is the spatial overlap and E1 and E2 are the site energies of two molecular orbitals where charges are localized. The charge transfer phenomena is assumed to be diffusive type in the absence of any external potential and the diffusion coefficient related to the hopping rate between pairs of molecules can be calculated as
D)
1 2d
∑ ri2Wi2 i
∑ Wi
(6)
i
where d is the dimensionality of the crystal (we consider here d ) 3), r is the stacking distance between the adjacent molecules, and D is normalized over the total probability of charge transfer (ΣiWi). Note that the diffusion coefficient calculated through eq 6 is invalid for 3-D anisotropic crystals. So, there have been recent efforts that consider the effect of crystal packing as well as anisotropy since the charge transport phenomena at the microscopic level depends upon the crystal packing rather than intermolecular packing.24,42 However, the more ordered packing of longer oligofuran crystals demonstrates the applicability of eq 6 in our case. Given D, the final drift mobility (µ) for the charge carriers at a given temperature (T) can be computed using Einstein’s relation µ ) e/kBT D. Results and Discussion Recently, a considerable number of single crystals of R-oligofurans have been synthesized by the Stille coupling method and their crystal structures are characterized by X-ray analysis.31 It has been reported that longer R-oligofurans exhibit better packing compared to their analogous oligothiophenes due to the smaller size of the oxygen atom than the sulfur atom, which minimizes the steric repulsions. For our study, we obtain the crystal structures of different oligofurans (nF; where n () 3, 4, 6) is the number of furan units) from the crystal information files found experimentally31 and they are shown in Figure 1a-c for 3F, 4F, and 6F, respectively. For a comparative study, the analogous series of oligothiophene crystals, namely, 3T,51
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Figure 1. Crystal structure and radial distribution functions of 3F (a and d), 4F (b and e), and 6F (c and f).
4T,52,53 and 6T54,55 retrieved from Cambridge Crystallographic Database are also considered, which are shown in Figure S1a-c (Supporting Information). As reported recently, 3F and 4F display more complicated and less ordered crystal packing as compared to 3 and 4 unit oligothiophenes (3T and 4T), whereas 6F exhibits a more tight herringbone arrangement than a 6 unit oligothiophene (6T). In the case of 4T and 6T, two different crystal structures exist depending upon the sublimation temperature at which crystals are grown. The crystals grown at high temperature (HT) have two molecules in one unit cell whereas, the crystals at low temperature (LT) contain four molecules. In our study, we have considered both the HT and LT crystals of 4T and 6T. However, 3T contains 8 molecules in its crystal and shows different packing. A detailed study on various oligothiophenes, describing the effect of molecular size and crystal packing on charge transport phenomena has already been discussed.24 The diversity in the molecular packing of such organic crystals provides different hopping pathways for the charge carriers. For detailed understanding of the molecular arrangements and possible charge hopping pathways in each crystal, we calculate the radial distribution function, g(r), which describes how, on average, the molecules in a given crystal are radially packed around each other. A super cell of dimension 3 × 3 × 3 Å3 of the unit cell and a cutoff distance of 20 Å are considered for the calculation of g(r) for oligofuran crystals, there are presented in Figure 1 (see Figure 1d-f). From the radial distribution functions, it is clear that the most significant contributions for each oligofuran crystal appear at short intermolecular contacts of less than 10 Å. In fact, the same remains true for oligothiophene case as well. However, to calculate the charge transfer integrals or coupling matrix elements, we consider all possible neighboring hopping routes for each oligomer taking various intermolecular distances
as well as orientations into account. Figures S2-S4 (Supporting Information) demonstrate the possible hopping routes in terms of unique stacked dimer pairs for each oligomer retrieved from their crystal structures. We use dispersion corrected BeckePerdew exchange correlations with triple-ζ double polarization basis set for our calculation. Such dispersion corrected calculations accurately capture the long-range weak dispersion forces which are primarily of π-π stacking and van der Waals in nature.52 Note that the treatment of dispersion effects in π-conjugated systems is basis set dependent as studied earlier.17,50 The effective charge transfer integral (Jeff) of electron and hole for each selected dimer of oligofurans (3F, 4F, and 6F) and oligothiophenes (3T, 4T, and 6T) crystals are estimated using eq 5 and are listed in Table 1 and Table S1 (Supporting Information), respectively. In the case of 3F, the less ordered packing results in many hopping pathways for charge carriers, as can be seen in Figure S2 (Supporting Information). Among these, the dimer with shorter intermolecular distance (3.74 Å) arranged in the slipped parallel manner has the most significant contribution since the orbital interaction is more in this case which in turn results in larger splitting of valence and conduction band widths. With change in orientations and increase in intermolecular distances, the charge transfer integrals decrease due to less interaction between the molecular orbitals leading to less splitting. In most of the dimers, the conduction bandwidth is larger in magnitude than the valence bandwidth. Similarly, in the case of 3T, the molecular contacts at short intermolecular distances of less than 10 Å have significantly large coupling matrix elements for both the hole and electron, although electron coupling is larger than that of hole (see Table S1, Supporting Information). The neighboring monomers in the case of 4F crystal are well separated compared to 3F and 6F crystals, leading to smaller charge transfer integrals for both charge carriers. The charge
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TABLE 1: Coupling Matrix Elements for Electron and Hole Carriers (eV) of Selected Dimers of 3F, 4F, and 6F Crystals oligomers
type of dimers
ra
3
1 2 3 4 5 6 7
3.74 5.31 5.37 7.05 7.33 7.87 8.72
0.052 0.025 0.009 0.009 0.011 0.021 0.0007
0.049 0.027 0.024 0.008 0.005 0.011 0.0003
4
1 2 3
5.36 6.11 11.23
0.009 0.005 0.0001
0.016 0.023 0.00003
6
1 2 3
5.2 5.3 8.9
0.076 0.031 0.0009
0.028 0.054 0.0004
a
electron Jeff
hole Jeff
r (Å) is the intermolecular separation.
transfer integral for a hole is found to be 1 order larger than that for an electron in contrast to the 4T crystal in both HT and LT phases except for the dimer with quite large intermolecular separation (11.23 Å), which has negligible contributions to the charge transfer integrals. Hence, this pathway is not included in the calculations of charge carrier mobility. However, the more ordered crystal packing of both 4T/HT and 4T/LT results in substantial larger transfer integrals for both hole and electron conductance in comparison to the 4F crystal (see Table 1 and Table S1, Supporting Information). The tight herringbone arrangement in the case of the 6F crystal as compared to the 6T crystal demonstrates significant spatial overlap between the molecular orbital. For the 6F crystal, the conduction bandwidth is larger than the valence bandwidth in the case of a dimer with an intermolecular distance of 5.2 Å, and the reverse is true for other dimers with molecular separations of 5.3 Å, which is primarily due to the change in orientations of selected dimers. We neglect the contributions of the dimer with an intermolecular separation of 8.9 Å, which is diminishingly small. On the other hand, the electronic coupling is always found to be larger than the hole coupling for both 6T/HT and 6T/LT crystals. The variation in the transfer integrals demonstrates that the molecular orientations and the intermolecular separations have a strong impact on the charge transfer integrals (see Table S1, Supporting Information). Our calculations of charge transfer integrals in the case of oligothiophenes compare fairly well with the previous theoretical studies.24 We now compute the reorganization energy for each oligofuran single molecule using eqs 2 and 3, as described earlier and compare the same with the corresponding oligothiophenes. The computed reorganization energies for hole and electron transfer demonstrate that the electron reorganization energy is found to be larger than the same for the hole in the case of 3F and 4F molecules whereas for the 6F case, the reorganization energies for both carriers are almost similar in magnitude. Although there is no significant change in the planarity of the molecular geometries during charge transfer, we find that the change in C-O bond lengths in the anionic case is more compared to the cationic structure for 3F and 4F molecules. However, such changes in C-O bond lengths in the cationic and anionic structures of 6F molecule are comparable. Moreover, we find that there is no significant effect of inter ring dihedral angle on the reorganization energies of these molecules. The optimized geometries for neutral, cation, and anion
molecules of 3F, 4F, and 6F are shown in Figure 2. In contrast, the significant change in C-C single and double bond in the cationic structures of the oligothiophenes, i.e., 3T, 4T, and 6T, enhances the hole reorganization energy in comparison to the electron. Moreover, we find that both hole and electron reorganization energies of oligofurans are smaller than their oligothiophene analogs. Similar behavior has also been reported previously.13,17 Our computed reorganization energies for both oligothiophenes and oligofurans compare fairly well with the previous studies.13,17,24 A small difference in the magnitude arises due to the consideration of double polarized basis set (631G++(d,p)) in our calculation. However, the qualitative picture remains invariant. The reorganization energies of the electron and hole for both oligofurans and oligothiophenes are summarized in Table 2. As can be seen, the reorganization energy decreases with an increase in the molecular size, i.e., the number of monomer units, due to the delocalized nature of charge carriers in longer oligomers.17,37 The computed effective charge transfer integral and reorganization energies are used to estimate the rate of charge transfer employing the Marcus rate equation given in eq 1. The diffusion coefficient dictated by the charge transfer rate and intermolecular separation is calculated for all the crystals. Finally, the room temperature mobilities for electron and hole carriers are estimated using Einstein’s equation and listed in Table 3. As can be clearly seen, the carrier mobility for the hole is found to be slightly larger than that for the electron in the case of the 3F crystal due to the smaller hole reorganization energy. In contrast, the electron mobility energy for the 3T crystal is 2 times larger than that for the hole, which is mainly governed by smaller electron reorganization energy and larger electronic coupling. Moreover, we find that the hole and electron mobilities for the 3T crystal are 2-3 times larger than those of the 3F crystal because of more ordered packing of the 3T crystal. On the other hand, the hole mobility is 1 order larger than that of the electron for the 4F crystal, which is governed by both a smaller hole reorganization energy and larger hole coupling matrix elements. Conversely, the smaller reorganization energy and larger coupling matrix elements for the electron result in a 1 order larger electron mobility in the case of 4T/ HT and 4T/LT crystals than for the hole mobility. However, we find that both carrier mobilities of the 4T/LT crystal are 1 order smaller than those of the 4T/HT crystal, which compare fairly well with the earlier studies. The detailed discussion on such findings has also been reported previously.24 In fact, the more ordered structures of 4T crystals result in the larger carrier mobilities than those of the 4F crystal. Interestingly, the larger electron coupling and tight herringbone packing in the case of the 6F crystal lead to greater carrier mobility in comparison to 6T. The hole mobility of the 6F crystal is 1 and 2 orders larger than the hole mobility of 6T/HT and 6T/LT crystals, respectively. In the 6F crystal, the electron mobility is larger than the hole mainly due to larger electronic coupling, whereas the greater electron mobility in the 6T/HT and 6T/LT crystals is governed by both smaller reorganization energy and larger transfer integral for electron. Our calculated mobility values for oligothiophenes compare quite well with previous theoretical and experimental values.24,56 However, an important inference obtained by comparing the carrier mobilities of oligofuran and oligothiophene is that while the electron mobility in oligothiophenes is 1 order larger than the hole mobility, both carrier mobilities in the case of oligofuran are almost similar in magnitude, leading to an ambipolar behavior of such compounds except for the 4F crystal. Gener-
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Figure 2. Optimized molecular geometries of neutral, cationic, and anionic forms of (a) 3F, (b) 4F, and (c) 6F molecular crystals.
TABLE 2: Reorganization Energies (eV) of Electron and Hole for Both Oligofuran and Oligothiophene Crystals λelectron
λhole
oligomers
furan
thiophene
furan
thiophene
3
0.3965
0.3387
0.3697
0.3832
4
0.2731
0.3144
0.2684
0.3596
6
0.2330
0.2752
0.2371
0.3104
molecular size, i.e., the number of monomer units except for the electron mobility of 4F crystal because of the increase in conjugation length and degree of charge delocalization in longer oligomers. Such a discrepancy of the electron mobility in 4F molecular crystal can be attributed to its less ordered molecular packing. Conclusions
TABLE 3: Hole and Electron Mobilities (cm2 V-1 s-1) for Both Oligofuran and Oligothiophene Crystals µelectron thiophene
µhole
oligomers
furan
furan
thiophene
3
0.0134
0.0539
0.015
0.0260
4
0.003
0.2300 (HT) 0.0584 (LT)
0.0243
0.0237 (HT) 0.0035 (LT)
6
0.326
0.3038 (HT) 0.0843 (LT)
0.153
0.0351 (HT) 0.0078 (LT)
ally, the ambipolar behavior in conjugated organic materials is rare and difficult to achieve experimentally as the injection of both electron and hole in the same electrode is difficult. Moreover, the injection barrier of the hole, which is defined as the energy difference between the ionization potential of the material and the work function of the electrode, is much smaller than the electron since the ionization potential of most of the organic materials and work function of the commonly used electrodes are of the same order. Hence, most commonly used device configuration favors the injection of holes than electrons. A more detailed discussion on ambipolar behavior including examples of promising materials was been reviewed earlier.57 In this regard, our theoretical calculation suggesting the ambipolar behavior of oligofurans would find huge potential applications. Now we discuss the effect of molecular size on the charge carrier mobility. Table 3 demonstrates that the overall mobility values for both charge carriers increase with an increase in
In summary, we have studied the molecular scale charge transfer in a few recently synthesized oligofuran molecular crystals via a thermally activated hopping mechanism, estimating the key parameters controlling the carrier mobilities. We also have made a comparison with analogous oligothiophenes as well as with earlier studies. The internal reorganization energy and effective charge transfer integrals are calculated within the density functional theory formalism. The treatment of frontier orbital overlap and dispersion effects has been considered explicitly in our calculations for the charge transfer integrals. The internal reorganization energy is found to decrease with an increase in the number of monomer units as the charge delocalization increases with oligomer length. The computed carrier mobilities at ambient temperature indicate that the hole mobility is larger than the electron mobility in 3F and 4F crystals, which is governed by the smaller hole reorganization energy in contrast to 3T and 4T crystals. The more ordered crystal packing in the case of the 3T and 4T crystals results in larger carrier mobility in comparison to 3F and 4F. Interestingly, we find the 6F crystal to be an n-type conductor like 6T due to large electron coupling. Moreover, the tight herringbone packing of the 6F crystal, leading to more ordered crystal structure, exhibits a higher carrier mobility as compared to 6T crystals. However, both carrier mobilities in 3F and 6F crystals are almost similar in magnitude, leading to an ambipolar behavior of such compounds, which show huge possibilities of electronic device application. We also find that the overall carrier mobilities increase with an increase in oligomer length (with an exception for the electron mobility of 4F crystal) as the reorganization energy decreases due to charge delocalization. The calculated values of carrier mobilities, reorganization energies, and transfer integrals in our study compare fairly well with the earlier studies.
Ambipolar Charge Transport in R-Oligofurans Our comparative study reveals that the oligomer length and molecular packing have a significant effect on charge carrier mobilities. Acknowledgment. We thank DST, Govternment of India, for the research support. Supporting Information Available: Figures showing the crystal structure of oligothiophene for 3T, 4T, and 6T and different charge hopping pathways for 3F, 4F, and 6F crystals. Table listing coupling matrix elements of electron and hole carriers for 3T, 4T, and 6T crystals along with the intermolecular separation are given. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Scherf, U.; List, E. J. W. AdV. Mater. 2002, 14, 477. Bredas, J. L.; Calbert, J. P.; da Silva Filho, D. A.; Cornil, J. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5804. Lin, B. C.; Cheng, C. P.; Lao, Z. P. M. J. Phys. Chem. A 2003, 107, 5241. Garnier, F.; Hajlaoui, R.; Yassar, A.; Srivastava, P. Science 1994, 265, 1684. (2) Braun, D.; Heeger, A. J. Appl. Phys. Lett. 1991, 58, 1982. (3) Burrows, P. E.; Forrest, S. R. Appl. Phys. Lett. 1994, 64, 2285. (4) Tang, C. W.; Slyke, S. A. V. Appl. Phys. Lett. 1987, 51, 913. Chen, A. C. A.; Culligan, S. W.; Geng, Y.; Chen, S. H.; Klubek, K. P.; Vaeth, K. M.; Tang, C. W. AdV. Mater. 2004, 16, 783. (5) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackey, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539. Sheats, J. R.; Antoniadis, H.; Hueschen, M.; Leonard, W.; Miller, J.; Moon, R.; Roitman, D.; Stocking, A. Science 1996, 273, 884. Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; dos Santos, D. A.; Bredas, J. L.; Logdlund, M.; Salaneck, W. R. Nature 1999, 397, 121. (6) Horowitz, G. AdV. Mater. 1998, 10, 365. Bao, Z. AdV. Mater. 2000, 12, 227. Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, R. N.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. W. Nature 1999, 401, 685. (7) Katz, H. E. J. Mat. Chem. 1997, 7, 369. Pati, S. K.; Ramasesha, S.; Shuai, Z.; Bredas, J. L. Phys. ReV. B 1999, 59, 14827. Lakshmi, S.; Dutta, S.; Pati, S. K. J. Phys. Chem. C 2008, 112, 14718. (8) Katz, H. E., Dodabalapur, A., Bao, Z., Eds. Handbook of Oligoand Polythiophenes, Wiley-VCH: Weinheim, Germany, 1999. Katz, H. E.; Lovinger, A. J.; Johnson, J.; Kloc, C.; Siegrist, T.; Li, W.; LIn, Y. Y.; Dodabalapur, A. Nature 2000, 404, 478. (9) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992, 258, 1474. Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498. Yu, G.; Wang, J.; McElvain, J.; Heeger, A. J. AdV. Mater. 1998, 17, 1431. Brabec, C. J.; Sariciftci, N. S.; Hummelen, C. J. AdV. Funct. Mater. 2001, 11, 15. Spanggard, H.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2004, 83, 125. (10) Torsi, L.; Dodabalapur, A. Anal. Chem. 2005, 77, 380. (11) Katz, H. E. J. Appl. Phys. 2002, 91, 1572. Electronic Materials: The Oligomer Approach Mullen, K., Wegnerm, G., Eds.; Wiley-VCH: Weinheim, 1998. (12) Handbook of Oligo and Polythiophenes; Fichou, D., Ed.; WileyVCH: Weinheim, 1999. Handbook Of Thiophene Based Materials; Perepichka, I. F., Perepichka, D. F., Eds.; Wiley-VCH: Weinhein, 2009. (13) Zade, S. S.; Bendikov, M. Chem.sEur. J. 2007, 13, 3688. (14) Tour, J. M. Chem. ReV. 1996, 96, 537. (15) Roncali, J. Chem. ReV. 1997, 97, 173. (16) Bredas, J. L.; Beljonne, D.; Coropceanu, V.; Cornil, J. Chem. ReV. 2004, 104, 4971. Coropceanu, V.; da Silva Filho, D. A.; Kwon, O.; SanchezCarrera, R. S.; Bredas, J. L. IPAP Conf. Ser. XXX, 6, 15. (17) Hutchinson, G. R.; Ratner, M. A.; Marks, T. J. J. Phys. Chem. B 2005, 109, 3126; J. Am. Chem. Soc. 2005, 127, 2339; J. Am. Chem. Soc. 2005, 127, 16866. (18) Seixas de Melo, J.; Elisei, F.; Becker, R. S. J. Chem. Phys. 2002, 117, 4428. (19) Groenendaal, L.; Niziurski-Mann, R. E.; Cava, M. P. AdV. Mater. 1993, 5, 547. Parakka, J. P.; Jeevarajan, J. A.; Jeevarajan, A. S.; Klispert, L. D.; Cava, M. P. AdV. Mater. 1996, 8, 54. Kozaki, M.; Parakka, J. P.; Cava, M. P. J. Org. Chem. 1996, 61, 3657. (20) Kanazawa, K. K.; Diaz, A. F.; Gill, W. D.; Grant, P. M.; Street, G. B.; Gardini, G. P.; Kwak, J. F. Synth. Met. 1980, 1, 329.
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