Phenylene Co-Oligomers with

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Chem. Mater. 2004, 16, 237-241

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Crystal Structures of Thiophene/Phenylene Co-Oligomers with Different Molecular Shapes Shu Hotta,*,†,⊥ Midori Goto,‡ Reiko Azumi,§ Masamitsu Inoue,| Musubu Ichikawa,| and Yoshio Taniguchi| Photonics and Materials Research Department, Kashiwa Laboratory, Institute of Research and Innovation, 1201 Takada, Kashiwa, Chiba 277-0861, Japan, Research Facilities Department, Technical Service Center, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan, Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan, and Department of Functional Polymer Science, Faculty of Textile Science and Technology, Shinshu University, Tokita 3-15-1, Ueda 386-8567, Japan Received July 29, 2003. Revised Manuscript Received October 30, 2003

We have determined crystallographic structures of four thiophene/phenylene co-oligomers with different molecular shapes. The compounds consist of the molecules straight, bent, or zigzag. All the crystals are monoclinic with space group either P21/c or P21/n, Z ) 4, and the unique axis of b. The crystals are characterized by the presence of the molecular layered structure in which the molecules form the well-known herringbone structure laterally spreading along the ab-plane. We investigate the molecular disposition in the crystals and present its peculiarity in relation to those comprising nonstraight molecules (e.g., bent or zigzag). The specific effects upon optical characteristics produced by this peculiarity are mentioned.

Introduction A great number of molecular semiconducting materials have been proposed and developed during the past two decades. Of these, a variety of oligophenylenes and oligothiophenes constitute an important class of materials and are potentially useful as optoelectronic materials that can be applied to thin-film transistors (TFTs), lightemitting diodes, and so forth.1-3 These materials are characterized by good physical properties such as high carrier mobility and high luminescent quantum efficiencies. Crystal and molecular structures for many of these compounds have been determined and are well-documented.1,2,4 Hybridizing and blending the materials are often attempted to enhance or improve physical properties desired for the materials. Examples include the strengthened light emission and low-threshold amplified spontaneous emission (ASE) from a liquid crystalline poly* Corresponding author. † Institute of Research and Innovation. ‡ Research Facilities Department, Technical Service Center, National Institute of Advanced Industrial Science and Technology. § Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology. | Shinshu University. ⊥ Current address: Department of Polymer Science and Engineering, Faculty of Textile Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan. E-mail: [email protected]. Tel.: +81-(0)75-724-7793. (1) Handbook of Oligo- and Polythiophenes; Fichou, D., Ed.; WileyVCH: Weinheim, Germany, 1999. (2) Hotta, S. In Handbook of Organic Conductive Molecules and Polymers; Nalwa, H. S., Ed.; John Wiley & Sons: Chichester, U.K., 1997; Vol. 2, Chapter 8. (3) Yanagi, H.; Okamoto, S. Appl. Phys. Lett. 1997, 71, 2563. (4) Baker, K. N.; Fratini, A. V.; Resch, T.; Knachel, H. C.; Adams, W. W.; Socci, E. P.; Farmer, B. L. Polymer 1993, 34, 1571.

mer blend.5 As another example, Hotta et al.6 have developed thiophene/phenylene co-oligomers in which two building blocks of thiophenes and phenylenes are suitably hybridized at the molecular level. In those cooligomers the total number of the thiophenes and phenylenes and their mutual arrangement can be appropriately altered in the molecules. The results of Katz et al.7 on the charge transport experiments indicate that the thin films of such co-oligomers exhibit higher mobilities on the TFT configurations compared to those comprising the parent oligothiophenes, when those (co-)oligomers have the same number of the constituent units [i.e., the number of thiophene (and phenylene) rings]. Not only are the co-oligomers characterized by a variety of extension of π-conjugation along the backbone, but these compounds also can take various molecular shapes, e.g. straight, bent, and zigzag. These characteristics make attractive the co-oligomers that are derived from the above-mentioned simple molecular design principle. In this context the nonstraight (e.g., bent or zigzag) molecules exhibit particularly interesting features. For instance, several crystals of such molecules display the ASE at room temperature.8 Highly polarized emissions have been observed for their thin films and crystals that are epitaxially grown on top of aligned or crystalline substrates.9 Such polarized optical properties (5) Kim, Y. C.; Lee, T.-W.; Park, O O.; Kim, C. Y.; Cho, H. N. Adv. Mater. 2001, 13, 646. (6) (a) Hotta, S.; Kimura, H.; Lee, S. A.; Tamaki, T. J. Heterocycl. Chem. 2000, 37, 281. (b) Hotta, S.; Lee, S. A.; Tamaki, T. J. Heterocycl. Chem. 2000, 37, 25. (c) Hotta, S.; Katagiri, T. J. Heterocycl. Chem. 2003, 40, 845. (7) Hong, X. M.; Katz, H. E.; Lovinger, A. J.; Wang, B.-C.; Raghavachari, K. Chem. Mater. 2001, 13, 4686.

10.1021/cm030579a CCC: $27.50 © 2004 American Chemical Society Published on Web 12/20/2003

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Scheme 1. Structural Formulas of 2,5-Bis(4-biphenylyl)Thiophene (BP1T), 5,5′-Bis(4-biphenylyl)-2,2′-bithiophene (BP2T), 5,5′′-Bis(4-biphenylyl)-2,2′:5′,2′′-terthiophene (BP3T), 5,5′′′-Bis(4-biphenylyl)-2,2′:5′,2′′:5′′,2′′′quaterthiophene (BP4T), and 5,5′′′′′-Diphenyl2,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′:5′′′′,2′′′′′-sexithiophene (P6T).The Line Connecting the Two p-Carbons (marked with asterisks) Represents the Molecular Long Axis

Hotta et al.

core is capped with biphenylyls at both the molecular terminals. The results of their structural analysis are compared with those for P6T. The structure analysis was carried out using single crystals that were grown in a vapor phase encapsulated in specially designed glassware.8a Experimental Section The synthetic and purification methods6 of the materials and the technique of crystals growth8a can be seen in previously published literature. The X-ray measurements were carried out at room temperature for single crystals of a size indicated in Table 1. Intensity data were collected using the ω-scan technique on an EnrafNonius CAD-4 four-circle diffractometer with monochromated Cu KR radiation and were corrected for usual Lorentzpolarization effects. Empirical absorption corrections (Psi-scan) were applied. The structure was solved by the direct method (SIR92)12 and refined by the full-matrix least-squares method on F.13 All the non-hydrogen atoms were refined anisotropically. The quantum chemical calculations at semiempirical levels were carried out as follows: First, the geometries of all the thiophene/phenylene co-oligomers were optimized using semiempirical MM+ Hamiltonians in HyperChem Ver 5.11 Professional (Hypercube, Inc.). Then the structures were reoptimized by the precise PM3 calculation in the same version. These processes readily allowed us to achieve the geometry optimization for longer molecules. The configuration interactions (CI) to generate the electronic spectra at the optimized geometry were calculated using ZINDO/S parameters in the above version. For the CI calculations, we took account of all the electron configurations within 30 molecular orbitals (from HOMO - 14 to LUMO + 14).

Results

ensue from the uniaxial alignment of the transition dipole moments along the specific (intended) direction. Thus, the newly occurring class of molecular semiconductors of thiophene/phenylene co-oligomers supplies us with a good opportunity to survey the structureproperty relationship. It will therefore be of high interest and importance to determine the crystal structure of the co-oligomers. As initial attempts Samulski et al.10 and Hotta and Goto11 have been successful in analyzing the crystal structure of a co-oligomer, 2,5-bis(4-biphenylyl)thiophene (BP1T; see the bent molecular structure in Scheme 1). In the present studies we report characteristics of the crystal and molecular structures of cooligomers with different molecular shapes (see Scheme 1). The compounds were chosen from among the cooligomers with molecules either zigzag (BP2T and BP4T), bent (BP3T), or straight (P6T). The former three compounds, together with BP1T, constitute a specific class of co-oligomers in which the thiophene(s) (8) (a) Ichikawa, M.; Hibino, R.; Inoue, M.; Haritani, T.; Hotta, S.; Koyama, T.; Taniguchi, Y. Adv. Mater. 2003, 15, 213. (b) Nagawa, M.; Hibino, R.; Hotta, S.; Yanagi, H.; Ichikawa, M.; Koyama, T.; Taniguchi, Y. Appl. Phys. Lett. 2002, 80, 544. (9) (a) Yanagi, H.; Morikawa, T.; Hotta, S.; Yase, K. Adv. Mater. 2001, 13, 313. (b) Yoshida, Y.; Tanigaki, N.; Yase, K.; Hotta, S. Adv. Mater. 2000, 12, 1587. (10) Dingemans, T. J.; Murthy, N. S.; Samulski, E. T. J. Phys. Chem. B 2001, 105, 8845. (11) Hotta, S.; Goto, M. Adv. Mater. 2002, 14, 498.

The crystal data and structural results are collected in Table 1. The ORTEP structures of BP2T, BP3T, BP4T, and P6T are shown in Figures 1-4, respectively. All the crystals are monoclinic with space group either P21/c or P21/n, Z ) 4, and the unique axis of b. The crystals are characterized by the presence of the molecular layered structure in which the molecules form the well-known herringbone structure laterally spreading along the ab-plane. The b/a (or a/b) ratios, which determine the molecular packing, range from 1.30 to 1.33. These constant ratios indicate that the molecular packing scheme is independent of the molecular shape and symmetry as depicted in Scheme 1. These ratios are characteristic of molecular crystals that possess the herringbone structure.14,15 We have determined the herringbone angles from the dihedral angles between the least-squares planes of the nearest-neighbor molecules within the molecular array spreading along the ab-plane, which parallels the crystal faces. Table 2 summarizes those herringbone angles of the crystals as well as another important crystallographic angle (vide infra). The former angles range from 43.8 to 67.6°, being (12) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.; Giacovazzo, C.; Guagliardi, A.; Polidori, G. J. Appl. Crystallogr. 1994, 27, 435. (13) Crystal Structure Analysis Package; Molecular Structure Corporation: The Woodlands, TX, 1985 and 1999. (14) (a) Hotta, S.; Waragai, K. J. Mater. Chem. 1991, 1, 835. (b) Hotta, S.; Waragai, K. Adv. Mater. 1993, 5, 896. (15) Bernstein, J.; Sarma, J. A. R. P.; Gavezzotti, A. Chem. Phys. Lett. 1990, 174, 361.

Crystal Structures of Thiophene/Phenylene Co-Oligomers

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Table 1. Crystallographic Data Collection and Structure Refinement Information formula formula weight crystal class space group color of crystal cell constants a (Å) b (Å) c (Å) β (deg) V (Å3) Z crystal size, mm Dcalc (g/cm3) radiation 2θ range (deg) max hkl collected no. reflns measd no. unique reflns no. obsd reflns no. reflns used in refinement no. parameters Rc Rwd GOF final difference peaks, e/Å3 a

BP2T

BP3T

BP4T

P6T

C32H22S2 470.65 monoclinic P21/c (No. 14) pale yellow

C36H24S3 552.77 monoclinic P21/c (No. 14) yellow

C40H26S4 634.89 monoclinic P21/n (No. 14) orange

C36H22S6 646.93 monoclinic P21/c (No. 14) red

5.7081 (7) 7.6036 (3) 52.869 (7) 97.147 (6) 2276.8 (4) 4 0.400 × 0.400 × 0.030 1.373 Cu KR (λ ) 1.5418 Å) 140.38 -6 e h e 0 0eke9 -64 e l e 64 7853 4322 3577 I g 2.0σ(I)a 3577 I g 2.0σ(I)a 307 0.0646 0.0796 1.408 0.29,-0.35

7.5262 (9) 5.7856 (7) 59.997 (7) 92.818 (6) 2609.3 (5) 4 0.500 × 0.200 × 0.030 1.407 Cu KR (λ ) 1.5418 Å) 142.86 0ehe9 0eke7 -73 e l e 73 5545 5071 3627 I g 2.0σ(I)a 3627 I g 2.0σ(I)a 352 0.0711 0.0958 2.161 0.34,-0.69

5.6824 (7) 7.5850 (5) 67.907 (6) 91.475 (5) 2925.9 (5) 4 0.300 × 0.200 × 0.070 1.441 Cu KR (λ ) 1.5418 Å) 140.34 0ehe6 0eke9 -82 e l e 82 6388 5345 3525 I g 2.0σ(I)a 3525 I g 2.0σ(I)a 397 0.0604 0.0873 1.957 0.33,-0.58

5.980 (2) 7.828 (1) 61.88 (2) 92.35 (1) 2894 (1) 4 0.350 × 0.350 × 0.010 1.485 Cu KR (λ ) 1.5418 Å) 140.48 0ehe7 0eke9 -75 e l e 75 6178 4986 2855 I g 3.0σ(I)b 2855 I g 3.0σ(I)b 379 0.0862 0.1202 2.281 0.55,-0.55

F2 g 2.0σ(F2). b F2 g 3.0σ(F2). c R ) Σ||F0| - |Fc||/Σ|F0|.

d

Rw )(Σw(|F0| - |Fc|)2/ΣwF02, w ) 1/σ2(F0).

Figure 1. ORTEP representation of BP2T.

Figure 2. ORTEP representation of BP3T.

Figure 3. ORTEP representation of BP4T.

Figure 4. ORTEP representation of P6T.

Figure 5. Side view of the BP4T crystals. Table 2. Herringbone Angles and Tilting Angles (°)

related to those of other crystals with the related herringbone structure.14,15 Figures 5 and 6 represent and contrast the disposition of the molecules for BP4T and P6T in the unit cells. Note that in terms of the molecular design BP4T results solely from replacing the two terminal thiophenes of the sexithiophene segment in P6T with phenylenes. What is obviously seen in comparing these Figures is that the molecular long axis is perpendicular to the ab-plane in

herringbone angle tilting angle (φ)

BP2T

BP3T

BP4T

P6T

61.0 1.2

43.8 2.6

67.6 1.8

62.0 21.0

BP4T, whereas it is largely tilted with P6T (Table 2). Here we define the molecular long axis as the line connecting the two carbons located at the p-positions of the terminal phenyls (see Scheme 1). Therefore, the deposition of the molecules can be measured qualita-

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Figure 6. Side view of the P6T crystals.

tively by an angle between the molecular long axis and the normal of the ab-plane. This angle φ determines the extent of the tilting of the molecules. The said angle φ is no larger than ∼2.6° for crystals BPnT (n ) 2-4). In this respect we note the following. In the crystals, molecules BPnT (n ) 2-4) are composed of three straight segments, i.e., the thiophenes core and the two biphenylyl wings. For these segments we define the segment axes as the lines C13-C20, C1-C10, and C21-C30, respectively, for e.g., BP2T. Regarding the thiophenes core, the segment axes C13-C20, C13-C24, and C13-C28 for BPnT (n ) 2-4) cross the normal of the ab-plane at angles 11.6, 4.6, and 8.2°, respectively. Such tilting can also be recognized with the biphenylyl segments as in the previous case of BP1T.11 These tilts cancel out one another, and, as a consequence, the molecular long axes stand perpendicular to the ab-plane as a whole. The smaller angles φ for BPnT (n ) 2-4) are contrasted with a relatively large tilting angle φ of 21.0° in crystals of P6T (Table 2). The latter value is related to those of the crystals with the herringbone structure that comprise straight molecules such as conventional oligophenylenes and oligothiophenes.4,14-17 Thus, we conclude that the perpendicular disposition of the molecular long axis is peculiar to the crystals of BPnT (n ) 2-4), i.e., the compounds with the thiophenes core and biphenylyls attached to both the molecular terminals. Using these three types of co-oligomer crystals in the present studies, we have re-confirmed this outstanding feature that was originally revealed for crystals of bent co-oligomer molecules of BP1T.10,11 Figure 7 displays the face-to-face molecular stacks of BP2T that develop along the a-axis. In this figure we schematically depict the perpendicular disposition of the (16) Horowitz, G.; Bachet, B.; Yassar, A.; Lang, P.; Demanze, F.; Fave, J.-L.; Garnier, F. Chem. Mater. 1995, 7, 1337.

Figure 7. Face-to-face molecular stacks of BP2T viewed along the a-axis. Sulfur atoms are marked red. The upper inset schematically represents in a cross section the perpendicular disposition of the molecules (depicted as thick zigzags) in a flake crystal. Its crystal faces and the molecular long axis are indicated with two horizontal lines and a vertical arrow, respectively. The arrow and lines cross each other nearly at a right angle.

molecular long axes. The thiophenes core and the biphenylyl wings individually form columns. Discussion Figures 1-4 show that all the molecules take the S-antisymmetry in the crystals. In Figures 5 and 6 two molecules are depicted so that they can be found to slightly sag and to deviate from the highest possible symmetry C2h or C2v. The deviation, however, is practically small. Regarding the individual adjacent thiophenes and phenylenes within the molecule, in other words, their least-squares planes intersect each other at an angle no larger than 7.3°, this angle being noticed for the phenylene and its adjacent thiophene in BP3T. In this situation the molecular long axis (defined in Scheme 1) is in pretty good accordance with the direction of the transition dipole moment of the molecule. We have confirmed this on the basis of the results that are obtained from the molecular orbital calculations performed at a semiempirical level. The departure of the molecular long axis from the transition dipole moment (17) Fichou, D.; Bachet, B.; Demanze, F.; Billy, I.; Horowitz, G.; Garnier, F. Adv. Mater. 1996, 8, 500.

Crystal Structures of Thiophene/Phenylene Co-Oligomers

was at most 5.8° (for BP4T). In the case of BP3T, the said departure was less than 0.01°, reflecting the optimized molecular symmetry of C2v.18 The very small values of φ (Table 2) as well as the aforementioned parallelism between the molecular long axis and the transition dipole (within an error of ∼6°) produce specific effects on the crystals of nonstraight molecules BPnT (n ) 2-4). This causes the transition dipoles to stand perpendicular to the ab-plane. In this configuration the direction in which the intensity of light emitted from the molecules is maximum is parallel to the ab-plane (i.e., the crystal faces of the thin flake crystals). This direction vertically penetrates an array of molecules that spread face-to-face along the ab-plane. Note that Figure 7 indicates as an example the faceto-face molecular stacks along the a-axis. What we expect from this molecular arrangement is that selfwaveguided lights propagating in the said direction can be readily amplified, producing a large optical gain along the ab-plane. This explains well why the ASE is apt to take place as edge emissions from the flake crystals of various types of thiophene/phenylene co-oligomers, in particular with the nonstraight molecules.8 The symmetry of the space group and the Z number play a decisive role in discussing the energy levels of the excited states of molecular crystals. In the present studies we have shown that all the crystals are monoclinic with Z ) 4. Because all the symmetry operations except the unit element commute the lattice points on which the molecules reside, the site group is C1.19,20 Therefore, the representation of the excited state is reduced to Ag + Bg + Au + Bu for all the cases, and, hence, its energy level is split into four.19,20 Of these symmetric species, only Au and Bu are associated with the optical transition21 as upper and lower Davydov levels.22 Notice that this feature is independent of the molecular symmetry of the individual isolated molecules. (18) Cotton, F. A. Chemical Applications of Group Theory, 2nd ed.; John Wiley & Sons: New York, 1971; Chapter 7 and Appendix III A. (19) Muccini, M.; Lunedei, E.; Taliani, C.; Beljonne, D.; Cornil, J.; Bre´das, J. L. J. Chem. Phys. 1998, 109, 10513. (20) Inui, T.; Tanabe, Y.; Onodera, Y. Group Theory and Its Applications in Physics; Syokabo: Tokyo, 1976; pp 303-309.

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Conclusion We have determined crystallographic structure of four thiophene/phenylene co-oligomers with different molecular shapes. The compounds consist of the molecules either straight, bent, or zigzag. The crystals are characterized by the presence of the molecular layered structure in which the molecules form the well-known herringbone structure laterally spreading along the abplane that parallels the crystal faces of the flake crystals. One of the most outstanding features lies in that the nonstraight molecules (e.g., bent or zigzag) crystallize with their molecular long axis perpendicular to the ab-plane. Thus, the present studies provide a model example regarding the crystal structure analysis of the thiophene/phenylene co-oligomers and the investigations into the structure-property relationship on the molecular crystals more widely. In particular, their crystallographic features are expected to be responsible for the unique optical properties such as the ASE observed for various types of thiophene/phenylene cooligomers. Acknowledgment. We thank Dr. Makoto Yamaguchi, Institute of Research and Innovation, for his helpful discussions and suggestions on the quantum chemical calculations. This work was supported by New Energy and Industrial Technology Development Organization (NEDO) both for the Harmonized Molecular Materials theme funded through the project on Technology for Novel High-Functional Materials (AIST), and for Organic Materials Technology for a Solid-State Injection Laser theme. Supporting Information Available: X-ray crystallographic data for structural determinations of BP2T, BP3T, BP4T, and P6T (CIF). This material is available free of charge via the Internet at http://pubs.acs.org. CM030579A (21) Heine, V. Group Theory in Quantum Mechanics: An Introduction to Its Present Usage; Pergamon Press: New York, 1960; Appendix K. (22) Hotta, S.; Ichino, Y.; Yoshida, Y.; Yoshida, M. J. Phys. Chem. B 2000, 104, 10316.