DOI: 10.1021/cg100948m
Dicyanovinyl-Substituted Oligothiophenes Mamoun M. Bader,*,† Phuong-Truc T. Pham,‡ and El Hadj Elandaloussi§
2010, Vol. 10 5027–5030
†
Department of Chemistry, Pennsylvania State University, Hazleton, Pennsylvania 18202, United States, ‡Department of Chemistry, Pennsylvania State University Worthington Scranton, eriaux, Dunmore, Pennsylvania 18512, United States, and §Laboratoire de Valorisation des Mat Universit e Abdelhamid Ibn Badis, B.P. 227, 27000 Mostaganem, Algeria Received July 19, 2010; Revised Manuscript Received September 19, 2010
ABSTRACT: A series of three dicyanovinyl (DCV)-substituted oligothiophenes (referred to collectively as DCV-nT-DCV, where n = 1-3) were prepared and their electrochemical and structural properties were examined. The introduction of this group results in favorable structural features: planarity, π-stack formation, and close intermolecular interactions as revealed from X-ray single crystal analysis. Electrochemical measurements suggest that these molecules are easy to both oxidize and reduce and thus are expected to show ambipolar charge transport properties similar to those we reported earlier on a closely related series of oligthiophenes endowed with the tricyanovinyl group (TCV). Oligothiophenes and their derivatives play a central role in organic materials chemistry as they possess a wide range of interesting electrical and optical properties.1 They also serve as model compounds, with well-defined structures, for the corresponding polymers.2 One major advantage of thiophene-based materials is that unlike polyacenes the synthetic aspects of thiophene chemistry are well developed so that numerous structures are readily accessible. This synthetic flexibility makes systematic studies of series of structurally related molecules possible, leading to better understanding of molecular structure/property relationships. However, not only do molecular structural changes control the observed physical properties, but also the way in which these molecules assemble in the solid state plays a crucial role in influencing the observed properties.3 Our ability to make and selectively control these “non-covalent” syntheses is much less developed compared with synthetic organic chemistry.4 Therefore, studies directed at simultaneously understanding how both molecular and solid state structures impact properties are important for better understanding of their influence while at the same time correlating specific molecular modifications with desirable solid state packing and properties. One unfavorable aspect of oligothiophenes and derivatives, which is believed to limit their performance as organic semiconductors, is the fact that they mostly crystallize in herringbone motifs, thus decreasing the amount of charge mobility as a result of decreased overlap between the molecular orbitals of adjacent molecules.5 Theoretical studies suggest that π-stacking in these materials and in particular cofacial stack formation with syn conformations would result in a substantial increase of the overlap between the molecular orbitals and is predicted to enhance their organic semiconducting properties.6 Structures of substituent-free oligothiophenes and many substituted oligothiophenes are known.7 Examination of the Cambridge Crystal Structural Database reveals that π-stacking is observed frequently for thiophenes endowed with one or more electron withdrawing groups.8 We reported earlier9 on the electrochemical and structural features of some tricyanovinyl or (TCV)-substituted oligothiophenes. We demonstrated that the TCV group, a strong electron acceptor, provides a rather simple approach to modify the electrical properties of oligothiophenes as evident from the electrochemical data, enforces molecular planarity, promotes π-stack formation, and in some cases favors the syn conformation possibly as a result of intramolecular CN 3 3 3 S interactions. In addition, the TCV group plays a crucial role in
significantly improving the thermal stability and solubility of oligothiophenes, characteristics vital for the fabrication of thin film and single crystal devices both from the vapor phase and solution. Subsequently, TCV-substituted oligothiophenes were shown to have ambipolar transport properties.10 In this report, we describe our preliminary findings on structural, electrochemical, and calculated electronic structures of a series of closely related molecules, namely, dicyanovinyl (DCV)-substituted oligothiophenes. These molecules were prepared as part of our efforts to synthesize oligothiophene derivatives suitable for the construction of thin film and single crystal organic semiconductor devices. We anticipate that the DCV group would play a similar role to that observed for the TCV group as described above. It is worth noting here that the dicyanovinyl group was used for a variety of applications such as enhancing thermal properties of aromatic polyesters,11 studying intramolecular charge transfer processes,12 designing new nonlinear optical chromophores,13 low band gap materials,14 and in preparation of optical sensing dyes for the red and near-infrared spectral range.15 Three compounds are considered in this study. They are 2,5-bis-dicyanovinylthiophene, DCV-1T-DCV; 5,50 -bis-dicyanovinyl-2,20 -bithiophene, DCV-2T-DCV, and 5,500 -bis-dicyanovinyl-2,20 ;50 ,200 -terthiophene, DCV-3T-DCV.16 Their structures are shown below.
*To whom correspondence should be addressed. E-mail: mmb11@ psu.edu.
From a synthetic viewpoint, the DCV group is readily accessible from the corresponding dialdehyde by means of Knoevenagle condensation with malonitrile, and as such they are easier to prepare than their TCV counterparts. These molecules were prepared according to literature procedures from the corresponding dialdehydes using the Knoevenagle condensation reaction.17 The dialdehydes were either commercially available or prepared according to published literature procedures.18 Detailed synthetic procedures are given in the Supporting Information. Crystals suitable for X-ray structural analysis of DCV-1T-DCV and DCV2T-DCV 3 CH2Cl2 were grown from dichloromethane, whereas solvent-free pseudo polymorph crystals of DCV-2T-DCV were
r 2010 American Chemical Society
Published on Web 10/20/2010
pubs.acs.org/crystal
5028
Crystal Growth & Design, Vol. 10, No. 12, 2010
Bader et al.
Table 1. Crystallographic Information and Selected Structural Features for DCV-1T-DCV, and 2 Pseudo Polymorphs of DCV-2T-DCVa compound
DCV-T-DCV
DCV-2T-DCV solvate
DCV-2T-DCV sublimed
formula formula wt. crystal system space group color a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) temp (K) Z R indices all data GOF
C12H4N4S 236.25 orthorhombic P41212 orange, plate 5.6670(8) 5.6670(8) 35.136(10) 90 90 90 1128.4(4) 173(2) 4 R1 = 0.0331, wR2 = 0.0766 1.021
C17H8Cl2N4S2 403.29 triclinic P21/c red, needle 3.8010(5) 11.8343(16) 18.550(2) 90° 91.867(2)° 90° 834.0(2) 173(2) 2 R1 = 0.0673, wR2 = 0.1497 1.025
C16H6N4S2 318.37 monoclinic P21/c yellow needle 3.8159(5) 24.253(3) 8.5697(12) 90 102.209(2)° 90 775.17(18) 173(2) 2 R1 = 0.0418, wR2 = 0.0769 1.184
a
Cambridge Crystallographic Data Center deposition numbers: CCDC 793945-793947.
grown by means of physical vapor transport (PVT) in a horizontal quartz tube with a temperature gradient of ∼10 °C/cm.19 Their structural data are summarized in Table 1. We were unable to grow crystals suitable for X-ray analysis of DCV-3T-DCV despite repeated efforts and using different methods. We first note that all these molecules are nearly planar with the dihedral angle between the DCV plane and the ring plane ranging from 2.6° to 3.03° to 3.2° for DCV-1T-DCV, DCV-2T-DCV solvate, and DCV-2T-DCV sublimed, respectively. Shortest intermolecular distances observed in DCV-T-DCV are S 3 3 3 C (3.383 A˚) and CN 3 3 3 C (3.151 A˚). The DCV groups in DCVT-DCV assume the syn conformation, possibly driven by intramolecular CN 3 3 3 S interactions since there is only one sulfur atom in the molecule simultaneously shared with two nitrogens at both ends of the molecule. In comparison, the DCV groups adopt the anti conformation in both pseudo polymorphs of DCV-2TDCV. Furthermore, the sulfur atoms in both pseudo polymorphs of DCV-2T-DCV are in the anti conformation, whereas syn conformation for sulfur atoms was observed in the TCV-2T. Density functional theory calculations at the B3LYP/6-31G* level suggest that all three molecules are nearly planar and that the anti conformation of sulfur atoms is 1.0 kcal/mol lower in energy than the syn conformer. It is worth noting that in the solvate structure of DCV-2T-DCV, the dichloromethane solvent molecule is disordered over a crystallographic inversion center (50:50). The dichloromethane solvent molecules form directional chains, with a 50:50 distribution throughout the crystal. The short axis (a = 3.80 A˚) is longer than the thiophene-thiophene stacking distance (3.42 A˚), as the thiophene rings are canted with respect to the bc plane. Intramolecular C 3 3 3 S distances ranged from 3.317 to 3.366, to 3.330 A˚ for DCV-1T-DCV, DCV-2T-DCV (solvate), and DCV-2T-DCV (sublimed), respectively. These are somewhat longer than those observed for TCV oligomers (ca. 3.2 A˚). Stack formation was observed in all three structures with close intermolecular contacts ranging from 3.15 to 3.36 A˚ were observed with π-stacks separated by 3.36 A˚ in the sublimed crystals of DCV-2T-DCV (Figure 1). Closest intermolecular S 3 3 3 S distances within stacks where observed for DCV-2T-DCV solvate (where 3.801 A˚). Both C-S and C-N bonds were in the normal range 1.726-1.739 A˚ and 1.141-1.145 A˚, respectively. On the other hand, C-C ranged from 1.392 to 1.457 A˚ indicating a partial intramolecular charge transfer between the DCV group and the thiophene ring noting that the longest C-C bond is observed for the bond between the two thiophene rings. CdC bond lengths ranged from 1.346 to 1.386 with the shortest lengths observed for the vinyl group CHdC(DCV)2. When we consider the electrochemical data21 (Table 2), we first note that no oxidation was observed for DCV-1T-DCV, whereas
Figure 1. Molecular structure and unit cell packing for (a) DCVT-DCV, (b) DCV-2T-DCV 3 2CH2Cl2, (c) DCV-2T-DCV. *Solvent molecules are not shown for clarity.
2T and 3T compounds exhibited chemically irreversible oxidations at potentials more anodic than those of their corresponding oligothiophenes, revealing the electron-withdrawing character of the DCV group. The oxidation potential also decreases with an increasing number of thiophene units (from 1.842 for DCV2T-DCV, to 1.49 V for the DCV-3T-DCV), in parallel with the behavior of the unsubstituted oligothiophenes and similar to trends observed for the TCV series. The extent of this decrease (ca. 0.4 V per thiophene ring) is somewhat larger than that for oligothiophenes (ca. 0.3 V per thiophene ring). Compared to the unsubstituted oligothiophenes, however, reduction of all three DCV compounds was facile and reversible, with the DCV group shifting the potential of the first reduction anodically by nearly 1.72 and 1.26 V for the DCV-2T-DCV and DCV-3T-DCV, respectively. The reduction is easier for shorter molecules indicating that reduction of these compounds is dominated by the DCV
Communication
Crystal Growth & Design, Vol. 10, No. 12, 2010
Figure 2. DFT Calculated HOMO-LUMO levels for compounds 1-3. Table 2. Cyclic Voltammetry and Spectroscopic Data for DCV-nT-DCVa compound
Ep (ox)
Ep (red)
DCV-T-DCV not measured -0.342 (rev) -0.694 (rev) DCV-2T-DCV 1.842 -0.686 -0.776 DCV-3T-DCV 1.49 -0.81 -0.96 2T 1.25 -2.41 -3.10 3T 0.95 -2.07 -2.47 TCV-2T-TCV 2.12 -0.08 -0.37 -0.81
ΔEp λmax, nmb νCN, cm-1 415
2229.34
2.5
451
2223.56
2.3
502
2222.60
3.66
303.5
3.02
353.5
2.20
504
2235.13c 2225.49
a All potentials measured with cyclic voltammetry (1 mM solutions) in 0.1 Mn-Bu4NþClO4-/CH3CN at 100 mV/s (vs Ag/AgCl). b In CH2Cl2. c Two peaks were observed in the IR spectra of some TCV oligothiophenes one weaker at higher wavenumber as a shoulder of a major peak at lower wavenumber, see Supporting Information
group. It is worth noting that these trends are similar to those observed in TCV-substituted oligothiophenes. This renders confidence that DCV-substituted oligothiophenes may show similar behavior in the solid state. To further study the role of molecular structure, we also have carried out DFT-B3LYP/6-31G* level calculations on these molecules (gas phase isolated molecules) using Spartan 06 software.21 The main features of these calculations are as follows: (1) The evolution of LUMO-HOMO levels are as one expects with calculated band gaps of 3.0, 2.7, and 2.5 eV, and 3.24 eV for DCV-1T-DCV, DCV-2T-DCV, and DCV-3T-DCV respectively, compared with 4.22 and 3.42 eV for bithiophene and terthiophene, respectively. This reduction is mainly due to the lowering of the LUMO levels, Figure 2; (2) calculated molecular geometries and shapes are in good agreement with the X-ray experimental data, lending further confidence in this level of theoretical study; (3) the highest occupied molecular orbital (HOMO) localizes on the thiophene moiety and the lowest unoccupied molecular orbital (LUMO) localizes on the DCV group without overlapping the HOMO, which illustrates that the HOMO and LUMO are completely separated. We also like to note that none of these molecules showed field effect in thin film or in single crystal device configurations. The introduction of the DCV groups affords bathochromic shifts in the λmax values compared with the respective unsubstituted oligothiophenes (see Table 1). Like the unsubstituted versions, the λmax values of the DCV-substituted compounds increase with the number of thiophene rings. These optical transition energies therefore decrease in parallel with the ΔEp values, which may be expected because both effectively are measures of the HOMO-LUMO gap in these compounds. These data, coupled with the π-stacking observed in both pseudo polymorphs of DCV-2T-DCV and the likelihood of π-stacking
5029
in longer oligomers, as was the case for their TCV counterparts, suggest these compounds are candidates for n-type semiconductors in field effect transistor configurations. The electrochemical data also reveal that the difference between the oxidation and reduction potentials, ΔEp, is significantly smaller than the corresponding values for the unsubstituted oligothiophenes. This suggests that these compounds are more likely to exhibit ambipolar transport. The observations here suggest that this can be made even more likely by adding electronic-donating groups to the thiophene cores. Synthesis, crystal growth, and FET device performance of these and related molecules are currently in progress. One more interesting feature of this class of compounds is that they are highly fluorescent. We are currently evaluating their optical characteristics. Acknowledgment. The MRSEC Program of the National Science Foundation under Award Number DMR-0212302 partially supported this work. Funding from the University College at Pennsylvania State University is also acknowledged. P.T.P. and M.M.B. would like to acknowledge financial support from the Directors of Academic Affairs at Penn State Hazleton (Dr. Monica E. Gregory) and Penn State Worthington Scranton (Dr. Michael Mahalik) for the acquisition of a shared electrochemical characterization system. The authors also acknowledge B. E. Kucera, W. W. Brennessel, V. G. Young, Jr., and the X-ray Crystallographic Laboratory, Department of Chemistry at the University of Minnesota. Supporting Information Available: CIF files, synthetic procedures, UV-vis and IR data are available free of charge. This material is available free of charge via the Internet at http://pubs. acs.org.
References (1) (a) For a recent review, see Mishra, A.; Ma, C.-Q.; Buerle, P. Chem. Rev. 2009, 109, 1141–1276. (b) Heinze, J.; Frontana-Uribe, B. A.; Ludwigs, S. Chem. Rev. 2010, 110, 4724-4771; (c) Grimsdale, A. C.; Chan, K. L.; Martin, R. E.; Jokisz, P. G.; Holmes, A. B. Chem. Rev. 2009, 109, 897–1091. (2) Mullen, K., Wegner, G., Eds.; Electronic Materials: The Oligomer Approach; Wiley: Weinheim, 1998. (3) Fichou, D. J. Mater. Chem. 2000, 10, 571–588. (4) See for example: (a) Whitesides, G. M.; Simanek, E. E.; Mathias, J. P.; Seto, C. T.; Chin, D. C.; Mammen, M.; Gordon, D. M. Acc. Chem. Res. 1995, 28, 37–44. (b) Chang, S. K.; Engen, D. V.; Fan, E.; Hamilton, A. D. J. Am. Chem. Soc. 1991, 113, 7640–7645. (c) Zimmerman, S. C.; Duerr, B. F. J. Org. Chem. 1992, 57, 2215–2217. (d) Lehn, J.-M. Pharm. Acta Helv. 1995, 69, 205–11. (5) Azumi, R.; Goto, M.; Honda, K.; Matsumoto, M. Bull. Chem. Soc. Jpn. 2003, 76, 1561-1567 and references therein. (6) Beljonne, D.; Cornil, J.; Silbey, R.; Millie, P.; Bredas, J. L. J. Chem. Phys. 2000, 112, 4749–4758. (7) See for example: Nagamatsu, S.; Kaneto, K.; Azumi, R.; Matsumoto, M.; Yoshida, Y.; Yase, K. J. Phys. Chem. B 2005, 109, 9374–9378. (8) ConQuest 2002: Bruno, I. J.; Cole, J. C.; Edgington, P. R.; Kessler, M.; Macrae, C. F.; McCabe, P.; Pearson J. Taylor, R. Acta Crystallogr. 2002, B58, 389-397. (9) Bader, M. M.; Custelcean, R.; Ward, M. D. Chem. Mater. 2003, 15, 616–618. (10) Cai, X.; Burand, M. W.; Newman, C. R.; da Silva Filho, D. A.; Pappenfus, T. M.; Bader, M. M.; Bredas, J.-L.; Mann, K. R.; Frisbie, C. D. J. Phys. Chem. B 2006, 110, 14590–14597. (11) Cho, H. G.; Choi, S. H.; Kim, B. G.; Gong, M. S. Macromolecules 1993, 26, 6654–6656. (12) Katritzky, A. R.; Zhu, D. W.; Schanze, K. S. J. Phys. Chem. 1991, 95, 5737–5742. (13) Raposo, M. M. M.; Sousa, A. M. R. C.; Kirsch, G.; Cardoso, P.; Belsley, M.; Gomes, E. M.; Fonseca, A. M. C. Org. Lett. 2006, 8, 3681–3684. (14) Du, C.; Chen, J.; Guo, Y.; Lu, K.; Ye, S.; Zheng, J.; Liu, Y.; Shuai, Z.; Yu, G. J. Org. Chem. 2009, 74, 7322–7327. (15) Citterio, D.; Jenny, L.; Spichiger, U. E. Anal. Chem. 1998, 70, 3452– 3457.
5030
Crystal Growth & Design, Vol. 10, No. 12, 2010
(16) Preparation of higher oligomers was attempted, but due to solubility issues these seem illusive. We are currently working on synthetic strategies to overcome this obstacle. (17) Saadeh, H.; Wang, l.; Yu, L. Macromolecules 2000, 33, 1570–1576. (18) Synthetic procedures are included in the Supporting Information of this article. (19) Pham, P. T. T.; Xia, Y.; Frisbie, C. D.; Bader, M. M. J. Phys. Chem. C 2008, 112, 7968.
Bader et al. (20) While electrochemical measurements were carried out in solution, these data are indicative that these molecules are relatively easy to both reduce and oxidize, features of interest to construct ambipolar organic semiconductors. TCV-substituted oligothiophenes were shown to have such behavior in both solution (electrochemical data) and in the solid state (see ref 10). (21) Becke, A. D. J. Chem. Phys. 1993, 98, 5648; Spartan 06: Wavefunction Inc.: Irvine CA, 2006.
