Thiophene and Selenophene Copolymers Incorporating Fluorinated

materials: the optical band gaps and highest occupied molecular orbital levels are affected with the introduction of fluorine atoms as a result of...
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Chem. Mater. 2005, 17, 6567-6578

6567

Thiophene and Selenophene Copolymers Incorporating Fluorinated Phenylene Units in the Main Chain: Synthesis, Characterization, and Application in Organic Field-Effect Transistors David J. Crouch,† Peter J. Skabara,*,† Jan E. Lohr,† Joseph J. W. McDouall,† Martin Heeney,‡ Iain McCulloch,*,‡ David Sparrowe,‡ Maxim Shkunov,‡ Simon J. Coles,§ Peter N. Horton,§ and Michael B. Hursthouse§ School of Chemistry, UniVersity of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom, Merck Chemicals, Chilworth Science Park, Southampton SO16 7QD, United Kingdom, and Department of Chemistry, UniVersity of Southampton, Highfield, Southampton SO17 1BJ, United Kingdom ReceiVed July 18, 2005. ReVised Manuscript ReceiVed September 30, 2005

A series of thiophene oligomers bearing core phenylene and fluorinated phenylene units has been synthesized as potential semiconductor materials for organic field-effect transistors (OFETs). Polymerization of these compounds has been achieved using Stille and oxidative coupling methods. Functionalization of the phenylene unit with fluorine atoms has a marked effect on the self-assembly and electronic properties of the parent materials: the optical band gaps and highest occupied molecular orbital levels are affected with the introduction of fluorine atoms as a result of a combination of inductive effects and rigidification of the main chain. The design of these materials has focused on the self-assembly and solution processability of the materials. All the polymers are readily soluble in common organic solvents. Self-assembly and planarization of the fluorinated materials in the solid state are identified by a combination of X-ray diffraction studies, absorption spectroscopy, and cyclic voltammetry. The organizational behavior of the films is in contrast to the conformational freedom observed in solution (absorption spectroscopy) and in the gas phase (computational studies). Thin-film OFETs have been fabricated for the entire polymer series. Hole mobilities have been measured up to 10-3 cm2/(V‚s), with high current modulation (on/off ratios up to 105) and low turn-on voltages (down to 2 V). For the Stille coupled polymers, replacement of the bridging thiophene unit with selenophene generally increases the hole mobility of the polymers.

Introduction Organic conjugated oligomers and polymers are an important class of semiconductor, attracting great interest in applications such as light emitting diodes,1-3 photovoltaics,4-6 thin-film transistors (TFTs),7-10 electrochromics,11-14 and sensors.15-19 Optimization of device * To whom correspondence should be addressed. E-mail: peter.skabara@ strath.ac.uk. New address: WestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, U.K. G1 1XL. † University of Manchester. ‡ Merck Chemicals. § University of Southampton.

(1) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998, 37, 402-428. (2) Dini, D. Chem. Mater. 2005, 17, 1933-1945. (3) Akcelrud, L. Prog. Polym. Sci. 2003, 28, 875-962. (4) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater. 2001, 11, 15-26. (5) Shaheen, S. E.; Ginley, D. S.; Jabbour, G. E. MRS Bull. 2005, 30, 10-22. (6) Segura, J. L.; Martı´n, N.; Guldi, D. M. Chem. Soc. ReV. 2005, 34, 31-47. (7) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99-117. (8) Sun, Y.; Liu, Y.; Zhu, D. J. Mater. Chem. 2005, 15, 53-65. (9) Ling, M. M.; Bao, Z. Chem. Mater. 2004, 16, 4824-4840. (10) Tian, H.; Wang, J.; Shi, J.; Yan, D.; Wang, L.; Geng, Y.; Wang, F. J. Mater. Chem. 2005, 15, 3026-3033. (11) Mortimer, R. J. Chem. Soc. ReV. 1997, 26, 147-156. (12) Gaupp, C. L.; Zong, K.; Schottland, P.; Thompson, B. C.; Thomas, C. A.; Reynolds, J. R. Macromolecules 2000, 33, 1132-1133. (13) Cirpan, A.; Argun, A. A.; Grenier, C. R. G.; Reeves, B. D.; Reynolds, J. R. J. Mater. Chem. 2003, 13, 2422-2428. (14) Sonmez, G.; Sonmez, H. B.; Shen, C. K. F.; Jost, R. W.; Rubin, Y.; Wudl, F. Macromolecules 2005, 38, 669-675.

performance is a major challenge in this field, and there is a range of electronic and structural features that can be manipulated to improve a material’s suitability toward a particular device [e.g., desired highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels and band gap, electron or hole mobilities, crystallinity, processability, stability]. For TFTs, the drive is to design and synthesize materials that possess a low threshold voltage, a high on/off ratio, high mobility, and stability under ambient and operating conditions.20 Self-assembly of the polymer chain in the solid state can maximize charge transport within the bulk material, particularly through π-stacking.21,22 Exploiting liquid crystal properties23 or using inherently “flat” molecules, such as pentacene, are good examples in which co-facial packing is achieved. In the latter case, ribbon- or ladder-type structures can suffer from poor solubility and processability. Materials that give a planar conformation (15) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100, 2537-2574. (16) Higgins, S. J. Chem. Soc. ReV. 1997, 26, 247-257. (17) Ho, H.-A.; Be´ra-Abe´rem, M.; Leclerc, M. Chem.sEur. J. 2005, 11, 1718-1724. (18) Janata, J.; Josowicz, M. Nat. Mater. 2002, 2, 19-24. (19) Dai, L. M.; Soundarrajan, P.; Kim, T. Pure Appl. Chem. 2002, 74, 1753-1772. (20) Chabinyc, M. L.; Salleo, A. Chem. Mater. 2004, 16, 4509-4521. (21) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. ReV. 2005, 105, 1491-1546. (22) Scherlis, D. A.; Marzari, N. J. Am. Chem. Soc. 2005, 127, 32073212. (23) O’Neill, M.; Kelly, S. M. AdV. Mater. 2003, 15, 1135-1146.

10.1021/cm051563i CCC: $30.25 © 2005 American Chemical Society Published on Web 11/24/2005

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Crouch et al. Chart 1

through intrachain nonbonding interactions can provide a high degree of coplanarity in the solid state24 and can be compared to conventional ladder-type polymers.25 At the same time, the materials possess good solubility as a result of the rotational freedom of the main chain in solution. Fluorinated, π-conjugated oligomers are of recent topical interest. Examples include a novel fluoroarene-based acetylenic complex featuring novel self-assembled conjugated rigid rods.26 Highly fluorinated systems such as tetradecafluorosexithiophene27 and perfluoropentacene28 and perfluoroalkyl end-capped oligothiophenes29-31 have been synthesized as n-type semiconductors, but Marks et al. have found that intermediate structures with fewer fluorine substituents are essentially p-type.32 In a recent communication,33 we reported the synthesis of a series of oligothiophenes bearing a central tetrafluorophenylene system, the polymerization of two of these materials, and initial results on hole (24) Khan, T.; McDouall, J. J. W.; McInnes, E. J. L.; Skabara, P. J.; Fre`re, P.; Coles, S. J.; Hursthouse, M. B. J. Mater. Chem. 2003, 13, 24902498. (25) Scherf, U. J. Mater. Chem. 1999, 9, 1853-1864. (26) Watt, S. W.; Dai, C.; Scott, A. J.; Burke, J. M.; Thomas, R. L.; Collings, J. C.; Viney, C.; Clegg, W.; Marder, T. B. Angew. Chem., Int. Ed. 2004, 43, 3061-3063. (27) Sakamoto, Y.; Komatsu, S.; Suzuki, T. J. Am. Chem. Soc. 2001, 123, 4643-4644. (28) Sakamoto, Y.; Suzuki, T.; Kobayashi, M.; Gao, Y.; Sakamoto, Y.; Suzuki, T.; Kobayashi, M.; Gao, Y.; Fukai, Y.; Inoue, Y.; Sato, F.; Tokito, S. J. Am. Chem. Soc. 2004, 126, 8138-8140. (29) Facchetti, A.; Deng, Y.; Wang, A.; Koide, Y.; Sirringhaus, H.; Marks, T. J.; Friend, R. H. Angew. Chem., Int. Ed. 2000, 39, 4547-4551. (30) Fachetti, A.; Yoon, M.-H.; Stern, C. L.; Hutchison, G. R.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 13480-13501. (31) Yoon, M.-H.; DiBenedetto, S. A.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2005, 127, 1348-1349.

mobility measurements. Incorporating the fluorinated units serves to lower both the LUMO and HOMO energies of the materials and also facilitates planarization. Lowering of the HOMO energy level to below -4.9 eV, the level at which electrochemical reactions with wet oxygen can occur,34 is desirable for improving the ambient stability of polythiophenes. The noncovalent interactions of the fluorine atoms with components on adjacent thiophene rings result in highly planar structures in the solid state, together with excellent solubility of the oligomers and corresponding polymers in common organic solvents. Progressing with this theme, we present the synthesis and characterization of a new family of conjugated compounds (1-20, Chart 1, Scheme 2) and polymers (21-28, Chart 2). Within this work, we assess the contribution of the tetraphenylene units by comparison with related difluorinated and nonfluorinated derivatives. We also report that, for organic field-effect transistor (OFET) devices derived from the polymers, replacement of unsubstituted thiophenes with selenophene units within the polymer chain generally results in a significant increase in hole mobilities. Results and Discussion Synthesis. Compounds 1 (40%) and 3 (60%) were prepared from the reaction of 2-(trimethylstannyl)-4-hexyl (32) Facchetti, A.; Yoon, M.-H.; Stern, C. L.; Katz, H. E.; Marks, T. J. Angew. Chem., Int. Ed. 2003, 42, 3900-3903. (33) Crouch, D. J.; Skabara, P. J.; Heeney, M.; McCulloch, I.; Coles, S. J.; Hursthouse, M. B. Chem. Commun. 2005, 1465-1467. (34) De Leeuw, D. M.; Simenon, M. M. J.; Brown, A. R.; Einerhand, R. E. F. Synth. Met. 1997, 87, 53-59.

Thiophene and Selenophene Copolymers

Chem. Mater., Vol. 17, No. 26, 2005 6569 Chart 2

Scheme 1

thiophene35 with diiodobenzene and 1,4-dibromo-2,5-difluorobenzene, respectively (Scheme 1). The tetrafluoro-substituted phenylene-based monomers 5 and 7 were prepared from hexafluorobenzene and the corresponding lithiothiophene derivative (generated from the reaction of thiophene or (35) Barbarella, G.; Bongini, A.; Zambianchi, M. Macromolecules 1994, 27, 3039-3045.

3-hexylthiophene with n-BuLi), utilizing a nucleophilic aromatic substitution reaction via the resonance stabilized hexadienyl anion intermediate.36 Subsequent ejection of two fluorine ions completes the addition-elimination mechanism, giving the 1,4-bis[2-(thienyl)]-2,3,5,6-tetrafluorobenzene and 1,4-bis[2-(4-hexylthienyl)]-2,3,5,6-tetrafluorobenzene analogues in 65 and 66% yield, respectively. The synthesis of 1,4-bis(2-thienyl)-2,3,5,6-tetrafluorobenzene 5 has been reported elsewhere by Searson and co-workers;37 however, the conditions used are complex, involving trans-metalation reactions via zinc insertion, followed by palladium catalyzed aryl-aryl coupling to afford the desired compound. The method reported here dispenses with the need for coupling catalysts and affords the tetrafluorobenzene analogues in yields similar to those reported37 from simple inexpensive materials. Subsequent bromination of compounds 1, 3, 5, and 7 was achieved in excellent yield using N-bromosuccinimide in acetic acid38,39 (2, 94%; 4, 62%; 6, 91%; 8, 90%). (36) (a) Wheland, R. C.; Martin, E. L. J. Org. Chem. 1975, 40, 31013109. (b) Streitwiesser, A.; Heathcock, C. H.; Kosower, E. M. Introduction to Organic Chemistry, 4th ed.; MacMillan: New York, 1992; (c) March, J. AdVanced Organic Chemistry, 4th ed.; Wiley: New York, 1992, p 641. (37) Sarker, H.; Gofer, Y.; Killian, J. G.; Poehler, T. O.; Searson, P. C. Synth. Met. 1998, 97, 1-6. (38) Meng, H.; Huang, W. J. Org. Chem. 2000, 65, 3894-3901. (39) Le`re-Porte, J.-P.; Moreau, J. J. E.; Torreilles, C. Eur. J. Org. Chem. 2001, 1249-1258.

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Table 1. GPC Analysis of Polymers 21-28 Isolated by Soxhlet Extraction Methods conventional CH2Cl2 fraction 21-S 21-Se 22-S 22-Se 23-S 23-Se 24 25 26 27 28-S 28-Se a

microwave CHCl3 fraction

CH2Cl2 fraction

CHCl3 fraction

Mn

Mw

DPI

Mn

Mw

DPI

Mn

Mw

DPI

Mn

Mw

DPI

6530 2460 2860 3640 3000 3000 9300 11 000 2590 5800 3780

12 400 3790 3780 4600 4800 4600 27 000 16 000 3770 8780 5490

1.9 1.54 1.31 1.26 1.60 1.53 2.90 1.45 1.46 1.51 1.45

12 300

17 700

1.44

5080 5370 5370 4460 10 980 13 100 4310 28 700 10 500 7300

6580 6630 9499 8600 28 400 17 800 5410 51 800 21 500 15 600

1.29 1.23 1.77 1.92 2.58 1.36 1.26 1.68 2.04 2.13

6000 4914 5940a

9600 8992 7150a

1.60 1.83 1.20a

5020 5600

12200 6330

2.43 1.13

6040 5470 3360a 8030 5390 2960

9620 6947 4170a 9154 10 833 4620

1.59 1.27 1.23a 1.14 2.01 1.56

3795

5467

1.44

8074 10 100

10 932 15 251

1.35 1.51

Microwave reaction performed in fluorobenzene.

Scheme 2

The extended bithiophene analogues 9-18 were subsequently prepared from the dibromo compounds using Stille methodology.40 Reaction of the dibromo species with the corresponding organotin reagent, in the presence of tetrakis(triphenylphosphine) palladium(0),41 gave the extended bithiophene analogues in 28-71% yield. Treatment of compound 18 with N-bromosuccinimide gave the dibromo derivative 19 in 60% yield, which was then reacted with 2-(trimethylstannyl)-4-hexyl thiophene to give the extended compound 20 in 25% yield (Scheme 2). All compounds (1-20) were characterized by infrared and NMR spectroscopies (1H and 19F where applicable) and mass spectrometry; all the materials gave satisfactory elemental analyses (C, H, S ,and F where applicable; see section S1 in Supporting Information). Polymers 21-28 were prepared by Stille or oxidative coupling methodology. Electrochemical polymerization also worked cleanly to afford films on gold or indium tin oxide electrodes. An example of electrochemical growth is given in Supporting Information, but a full study of electropolymerized materials will be reported elsewhere. Polymerization of compounds 16, 17, 12 and 18 (to give polymers 24-27, respectively) was achieved using FeCl3 oxidative coupling in a chloroform/nitromethane solution. In each case, the products were precipitated into methanol and purified by (40) Hassan, J.; Sevignon, M.; Gozzi, C.; Schulz, E.; Lemaire, M. Chem. ReV. 2002, 102, 1359-1470. (41) Coulson, D. R.; Inorganic Syntheses 1971, 13, 121-124.

Soxhlet extraction using acetone, methanol, dichloromethane, and finally chloroform. Polymers 21-23 and 28 were prepared from the reaction of the dibromo compounds 2, 4, 8, and 19 with 2,5-bis(trimethylstannyl)thiophene42 or 2,5bis(trimethylstannyl)selenophene42 under Stille coupling conditions. These experiments were conducted in anhydrous chlorobenzene (reflux) and also by microwave heating (190 °C, 300 W, 10 min).43-45 The products were purified in the same manner as the ferric chloride coupling reactions. Full experimental details of the syntheses of 1-28 are given in Supporting Information, section S1. All analyses and device fabrication were performed on the dichloromethane and chloroform fractions which contained the higher molecular weight products. Gel permeation chromatography (GPC) experiments (see Table 1) indicated that the isolated materials were short to medium chain polymers with approximately 16 aryl units in the shortest chains and 190 in the longest (from Mn). The polydispersities were typically in the range 1.1-2.9. In the majority of cases, the chloroform fractions contained the highest molecular weight materials and the anomalies arose from some of the microwave reactions. Good quality matrixassisted laser desorption ionization time-of-flight (MALDITOF) mass spectra could only be obtained for some of the polymers. Figure 1 represents the spectrum of polymer 22S, in which the repeat unit can be clearly seen (mass ) 527 Da). The peak at 6327 represents the polymer where n ) 12, and subtraction of the repeat unit can be made sequentially down to 2104, which represents the hexadecaaryl molecule, where n ) 4. The most intense peak within each set has an average of 80 mass units above the value of n ) integer and is most likely the polymer terminated by bromine as an end group. The presence of a capping trimethyltin group is also a possibility, because there are regular, minor peaks occurring at about 160 (SnMe3) and 240 (SnMe3 + Br) above the base peak. Infrared stretching bands were observed in the region 1066-1035 cm-1 corresponding to Ar-Br stretching frequencies, providing further evidence that the polymers are end-capped with bromines. (42) Seitz, D. E.; Lee, S. H.; Hanson, R. N.; Bottaro, J. C. Synth. Commun. 1983, 13, 121-128. (43) Carter, K. R. Macromolecules 2002, 35, 6757-6759. (44) Nehls, B. S.; Asawapirom, U.; Fueldner, S.; Preis, E.; Farrell, T.; Scherf, U. AdV. Funct. Mater. 2004, 14, 352-356. (45) Tierney, S.; Heeney, M.; McCulloch, I. Synth. Met. 2005, 148, 195198.

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Figure 3. Packing diagram of 3 showing a slip-stack structure through aryl-aryl interactions.

Figure 1. MALDI-TOF mass spectrum of polymer 22-S.

Figure 2. X-ray crystal structure of compound 3 with atom labeling.

Single-Crystal and X-ray Powder Diffraction (XRD) Studies. The X-ray crystal structures of compounds 12 and 18 have been reported briefly in a recent communication,33 and further discussion is given in Supporting Information, section S3. We have also obtained the crystal structure of compound 3, which possesses some surprising H‚‚‚F intramolecular contacts in preference to S‚‚‚F interactions. The asymmetric unit of 3 is labeled in Figure 2, which identifies an inversion center that is centered within the benzene ring. The triaryl segment of the molecule is highly planar, with a maximum torsion angle of 1.18° between the phenylene and thiophene units. Close intramolecular interactions are governed by H bonding through S(1)‚‚‚H(3) (2.651 Å) and F(1)‚ ‚‚H(5) (2.277 Å) contacts, which are significantly shorter than the sum of the van der Waals radii for the corresponding atoms (3.00 and 2.67 Å, respectively). The favored H‚‚‚F containing conformation concludes that the electrostatic hydrogen bonding interaction is stronger than that of the twoelectron three-centered S‚‚‚F arrangement. The molecules assemble into a slip-stacked structure (Figure 3), with overlap between distinct sets of aromatic rings, namely, thiophenebenzene-thiophene trimers with center-to-center separations of 3.634 Å. This type of alternate arrangement is well-known for benzene-perfluorobenzene cocrystals,46-48 in which the fluorinated ring shows an inversed electron density distribution and, therefore, promotes electrostatic interactions with aromatic rings featuring a typical π-electron cloud.49 The crystal structure of the nonfluorinated derivative 9 demonstrates the importance of the F‚‚‚X intramolecular (46) Coates, G. W.; Dunn, A. R.; Henling, L. M.; Ziller, J. W.; Lobkovsky, E. B.; Grubbs, R. H. J. Am. Chem. Soc. 1998, 120, 3641-3649.

Figure 4. X-ray crystal structure of compound 9 with atom labeling. Hydrogens are omitted for clarity.

interactions toward rigidification. The asymmetric unit is shown in Figure 4 and also possesses an inversion center within the phenylene ring. The maximum torsion angle between the hexylthiophene and the benzene rings is 32.6°, while the terminal thiophene rings are also twisted from the adjacent thiophene units by 53.67°. There is rotational disorder in the terminal ring, with syn and anti conformers in a 50:50 distribution. Noticeably, there are no significant intermolecular interactions between molecules. From the crystal structures of compounds 3, 9, 12, and 18, it is clear that the fluorine containing derivatives form highly planar structures with close packing, whereas the nonsubstituted analogue is significantly twisted and supramolecularly inert. To investigate the self-assembly of the materials in film form, we chose to study one distinct family of molecules (16-18) by XRD (Figure 5), which includes one of the compounds examined by single-crystal studies. The films were drop cast from chloroform solution onto a silicon substrate precoated with octadecyltrichlorosilane and allowed to evaporate under ambient conditions. The fluorine containing compounds 17 and 18 give a common peak corresponding to a spacing of 13.3 and 13.1 Å, respectively, (47) Naae, D. G. Acta Crystallogr., Sect. B 1979, 35, 2765-2768. (48) Dai, C.; Nguyen, P.; Marder, T. B.; Scott, A. J.; Clegg, W.; Viney, C. Chem. Commun. 1999, 2493-2494. (49) Reichenba¨cher, K.; Su¨ss, H. I.; Hulliger, J. Chem. Soc. ReV. 2005, 34, 22-30.

6572 Chem. Mater., Vol. 17, No. 26, 2005

Crouch et al. Table 3. CV Data for Compounds and Polymers 1-28 in CH2Cl2/ CH3CN (1:1) versus Ag/AgCl as the Reference Electrode 1 3 5 7 9 10 11 12 13 14 15 16 17 18 20

Figure 5. XRD patterns of drop cast films of compounds 16-18. Table 2. Electronic Absorption Data for Compounds 1-20 and Polymers 21-28 (Chloroform Fractions from Soxhlet Extraction)

1 3 4 5 7 9 10 11 12 13 14 15 16 17 18 20 21-S 21-Se 22-S 22-Se 23-S 23-Se 24 25 26 27 28-S 28-Se a,b

λmax solution (nm [])

λmax solid (nm)

Eg solution (eV)

Eg solid (eV)

310a [2.0 × 104] 270, 340a [4.6 × 104] 357a [4.4 × 104] 339a [5.8 × 104] 357a [6.0 × 104] 389a [2.1 × 104] 394a [4.2 × 104] 394a [5.4 × 104] 378a [5.6 × 104] 394a [5.6 × 104] 385a [5.6 × 104] 353a [5.6 × 104] 388a [2.0 × 104] 374a [4.3 × 104] 380a [5.5 × 104] 405a [5.7 × 104] 455b [1.9 × 104] 443b [1.2 × 104] 475b [2.3 × 104] 473b [1.3 × 104] 449b [2.8 × 104] 462b [1.7 × 104] 412b [2.0 × 104] 421b [2.0 × 104] 423b [2.2 × 104] 421b [2.1 × 104] 420b [2.5 × 104] 445b [1.8 × 104]

409 416 418 416 418 396 410 401 390 392 433 480 481 510, 552 530, 568 509, 551 518, 561 439 440 515, 556 508, 566 508, 566 541, 592

2.20 2.22 1.98 2.23 2.05 2.06 2.50 2.44 2.35 2.48 2.30 2.20

2.16 2.02 1.88 1.91 2.02 1.97 2.29 2.13 1.91 1.94 1.89 1.82

Solution studies were performed in (a) hexane or (b) chloroform.

which is tentatively assigned to the width of the molecules taking into account the distance between the tips of the hexyl chains. Compound 17 has an additional peak at d ) 16.4 Å and is assigned as the distance between the conjugated chains, separated by the alkyl side groups.50 For compound 16, there are no discerning peaks, which is indicative of an amorphous solid with a low degree of order in the bulk solid. Crystals of 18 were ground to a powder and also analyzed by XRD. The peaks at 2θ ) 5.39° and 6.59° are coincidental with the major peaks found in the films of 17 and 18, providing good evidence that the fluorinated films possess a high level of order and crystallinity. Absorption Studies. The electronic absorption data for materials 1-28 are collated in Table 2. For the polymers, only the chloroform fractions of the Soxhlet extraction procedure are represented; compared with the corresponding (50) Yamamoto, T.; Arai, M.; Kokubo, H. Macromolecules 2003, 36, 7986-7993.

E1ox (V)

E2ox (V)

+1.19 +1.38 +1.72 +1.63 +1.09 +1.08 +1.36 +1.33 +1.40 +1.24 +1.33 +1.04 +1.12 +1.25 +1.06

+1.79 +1.74 +1.95 +1.99 +1.88 +1.16 +1.87 +1.61 +1.94 +1.55 +2.04 +1.22 +1.43 +1.43 +1.29

21-S 21-Se 22-S 22-Se 23-S 23-Se 24 25 26 27 28-S 28-Se

Eox (V)

onset (V)

HOMOa (eV)

+1.25 +1.25 +1.14 +1.44 +1.41 +1.41 +1.42 +1.35 +1.33 +1.43 +1.33 +1.30

+0.91 +1.00 +0.83 +0.93 +0.97 +1.10 +1.15 +0.88 +0.86 +1.07 +0.85 +1.02

-5.7 -5.8 -5.6 -5.7 -5.8 -5.9 -6.0 -5.7 -5.7 -5.9 -5.7 -5.8

a HOMO levels are calculated from the onset of the oxidation peak and referenced to the HOMO of ferrocene (-4.8 eV) which was used as an internal reference (E1/2 ) +0.50 V).

Figure 6. Solution and solid-state absorption spectra for polymer 28-S.

dichloromethane fractions, the absorption maxima differ in the range -20 to +30 nm. For the molecular systems, elongation of the conjugated chain within each family of compounds results in a bathochromic shift of the absorption maximum, as expected. Taking the tetrafluorophenylene series 7, 18, and 20 as an example, there is an increase in λmax of 23 and 25 nm, respectively. The absorption maximum for 15 is at a shorter wavelength compared to those of its direct analogues (353 nm, hexane), as a result of the steric effect of the head-head hexyl thiophene units. In the solid state, the peaks shift to longer wavelengths as a result of (i) a more planar conformation and (ii) interchain interactions. For the single molecules, the maximum shift in wavelength is 38 nm for compound 12. The optical band gaps for the polymers (measured from the onset of the longest wavelength absorption band) are in the range 1.98-2.50 eV (solution state) and 1.82-2.29 eV (solid state). In some cases, the absorption tail is quite long and there is a reasonably large margin of error for the estimation of the band gap (ca. (0.2 eV). However, the most informative features of the spectra are the shapes and values of the absorption peaks obtained from solution and solidstate measurements. For example, the absorption maximum for 28-S was found to be 420 nm in chloroform solution (Figure 6). However, a drop cast solution of the polymer gave an absorption maximum at 508 nm with a pronounced shoulder at 566 nm. Such a large bathochromic shift, together with the detailed structure of the absorption band, is

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Chem. Mater., Vol. 17, No. 26, 2005 6573

Figure 7. Definition of the torsion angles for the example of monomer 11.

Figure 8. Correlation of experimental and theoretical oxidation potentials. The oxidation potentials are the negative of the HOMO eigenvalues. In the case of the experimental values, the onset of the first oxidation wave was calculated relative to the HOMO eigenvalue for ferrocene (see Table 3). The theoretical values were calculated using B3LYP/Gen (same basis set as described).

indicative of high ordering in the solid state and a large variance in planarity between solution and film states. The solid-state spectra of polymers 22, 23, 26, 27, and 28-Se also exhibit similar shoulders, and it is noticeable that this feature is absent in the nonfluorinated analogues (21 and 24), thereby concurring with the X-ray data that there is less ordering in the unsubstituted phenylene systems. In the solution state spectra of some of the fluorinated polymers we observed minor shoulders (approximately 10% height of the main absorption peak), suggesting that there is some ordering for solvated polymers. Finally, it is worth noting that there is little variation in absorption characteristics in the solid state between structures with head-to-head coupling and unsubstituted thiophene spacers (e.g., comparison between 26, 27, and 28-S), indicating that the drive toward planarization is dominant over the steric effect. Electrochemistry. All compounds and polymers were studied by cyclic voltammetry (CV) in dichloromethane/ acetonitrile (1:1) versus Ag/AgCl (see Supporting Information, section S1, for experimental details). The monomers gave voltammograms with two sequential irreversible oxidation peaks in the range +1.04 to +2.04 V. The compounds readily electropolymerize by repetitive cycling over the first oxidation wave (see Supporting Information, section S2, for an example); therefore, the second oxidation peak has not been considered in our correlation analysis of electroactivity and structure. Within the series 1-20, there are several

expected trends. As the chain lengths are extended, the oxidation potentials decrease by about 100-300 mV for the triaryl systems to the corresponding pentaaryls and by 190 mV from 18 to 20. The attachment of two hexyl chains to compound 5 (to give 7) lowers the oxidation potential by 90 mV, but the insertion of additional hexyl groups to the main chain (see comparison between compounds 10 and 17, 11 and 12, and 12 and 15) does not seem to have a great effect. However, the positioning of the alkyl chains does have a noticeable variance for the values for E1ox between structural isomers (see comparison between 15 and 18 and also within the series 12-14). The successive incorporation of fluorine units raises the ionization potential. For example, within the analogous sets of compounds 16-18, the difference in E1ox between the unsubstituted phenylene and the difluoro compound is 80 mV, while that between 17 and the tetrafluoro derivative is 130 mV. Polymers 21-28 were also examined by CV. Very broad oxidation peaks were observed in the majority of cases, and the HOMO levels of the polymers were deduced from the onset of the oxidation waves by reference to the ferrrocene/ ferrocenium redox couple (-4.8 eV). Although the oxidation waves differ in broadness, certain trends can be identified from the electrochemical data when considering the values for the onset of the oxidation processes. In all cases, the onset potentials for the all-thiophene polymers are lower (by 90170 mV) than the values for the analogous polymers bearing a selenophene unit in the repeat unit. From this we can conclude that the incorporation of selenophenes lowers the HOMO level of the polymer. Incorporating more heterocyclopentadienes into the repeat unit does not follow a predictable pattern (compare, for example, 23-S, 27, and 28S), and presumably this is due to the influence of the headto-head hexyl chains in 27 (i.e., all thiophenes bear hexyl groups, cf. 23-S and 28-S). There is a striking correlation within the substitution pattern of the phenylene ring. Within identical series (21-S, 22-S, and 23-S; 21-Se, 22-Se, and 23Se; 24, 25, and 27), the HOMO level is lowered going from a difluorophenylene containing polymer to a tetrafluorophenylene analogue. This is as expected, because the electron withdrawing fluorine units are expected to influence the HOMO in this way. However, the HOMO is actually destabilized by adding two fluorine units to the phenylene

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Figure 9. Orbital representation for the HOMO of 5, calculated at the RB3LYP level. Table 4. Structural Information from Geometry Optimization Using B3LYP close contact distance (Å) 1 3 5 7 9 10 11 12 15 18

S-F

H-F

NA 2.855/2.879 2.807/2.808 2.797/2.795 NA NA/2.864 2.806/2.805 2.835/2.846 2.691/2.692 2.788/2.788

2.841/2.841 NA 2.273/2.274 2.258/2.251 2.826/2.826 2.751/NA 2.272/2.271 2.309/2.322 2.076/2.075 2.249/2.249

dihedral angle (deg) γ

R

β

δ

-128.05 -128.83 -155.57 -130.14 -133.86 -48.67

-28.93 -20.00 -19.98 -18.11 -27.34 -20.74 -20.14 -23.86 -1.40 -17.69

-29.28 24.00 -19.89 -17.60 -27.31 -21.90 -20.06 -24.94 -1.56 -17.68

-128.04 -129.48 -155.59 -130.06 -134.19 -48.68

Table 5. HOMO Energies for Analogues of 5 with Different Numbers of Fluorine Atoms on the Central Phenyl Ring orbital eigenvalue (eV) 5 (no F) 5 (two F) 5 (four F) Figure 10. ESP surface plots for (a) 5 and (b) 7. Note that despite the hexyl chains the surface plots are virtually identical. To save computational cost, the analogues without hexyl chains have been modeled. Note that the legend is in units of hartree.

structure and, from our X-ray experiments and absorption studies, we can deduce that this is occurring because of the induced planarization of the conjugated chain. As stated in the preceding section, we observe some planarization in solution, and this would in part explain the broad feature of the oxidation peaks. In our attempts to manipulate the HOMO levels of polymers 21-28 by the incorporation of fluorine groups, we observe a tradeoff between planarization and inductive effects. Because a high degree of planarity is important for efficient charge transport, we can conclude from our electrochemical experiments that the most lucrative structures for OFET applications are the tetrafluorophenylene polymers incorporating selenophene units. Computational Studies. All calculations were carried out using the Gaussian 03 suite of programs.51 The geometries of the monomer units were optimized using either B3LYP hybrid functional or Møller-Plesset perturbation theory of the second order (MP2) with a 6-31G(d) basis on carbon and hydrogen, 6-31+G(d) on fluorine, and 6-31+G(2d) on sulfur. It was found that in most cases the structures predicted (51) Frisch, M. J.; Trucks, G. W. et al.; Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.

-5.44 -5.71 -8.16

in the gas phase showed significant nonplanarity, in contrast to the X-ray structure. The dihedral angles, defined in Figure 7, are given for the optimized structures in Table 4. However, there is good correlation between experimental and theoretical results for the oxidation potentials as seen in Figure 8. We suspect that a number of factors play a role in planarization of the compounds, such as polymerization and solid-state packing. We have some indication from gas-phase single-point calculations that very little energy is needed to overcome the potential energy needed to make the structures planar. Possible sources for that energy might come from either intramolecular solid-state interactions or from close contact between sulfur and fluorine. To investigate the nature of the close contacts between sulfur and fluorine, the geometries were optimized for 18 individual monomers of 5 with an increasingly twisted torsion angle for one of its thiophenes over a total torsion angle of 180°. Second-order Moeller-Plesset Pertubation theory was used as this is known to yield more reliable results for nonbonded interactions than density functional theory (DFT). Because no other forces are contributing (solid state packing), it will be possible to investigate the close contact by means of potential energy scans as a function of the torsion angle for the thiophene with respect to the central ring. But because this level of theory is very computationally

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Figure 12. ESP surface plots for analogues of 11 with varying number of fluorine atoms.

Figure 11. ESP surface plots for analogues of 5 with varying number of fluorine atoms. Note that the legend is in units of hartree.

intensive, we resorted to using 5. This is adequate as our purpose is to investigate the role of short contacts on the energy. We are pursuing similar studies for a number of systems with a slight structural difference and will report more fully on that in a subsequent communication. The preliminary data suggest that there is no favorable interaction between sulfur and fluorine. There is an indication that the electron withdrawing nature of fluorine is more important. In 2005, Reichenba¨cher et al.49 published a review in which they report on the multiple effects that can be achieved by fluorinated organic compounds. Fluorine is ordinarily a strong hydrogen bond donor, but when attached to carbon in aliphatic or aromatic compounds, its ability to form strong interactions with hydrogen is limited. They also reviewed reports on the inversion of the electron density in aromatic compounds, thereby leading to different crystal packing arrangements to the otherwise nonfluorinated isomers. Generally, we find that the lowest energy conformation is close to 20-50° out of plane, but the planar form usually lies only 3-6 kJ mol-1 higher in energy. Given the short contact distances between fluorine and hydrogen there is

most probably an energy-lowering interaction. However, the fully planar arrangements seem to maximize the antibonding interactions in the HOMO (Figure 9). Consequently, there are two opposing requirements, and the nonbonded interactions are overcome by the orbital arrangement. By rotating the thiophene units out of the plane, the antibonding orbital interactions are ameliorated. The addition of fluorines to the central phenyl unit produces a strong stabilization of the HOMO energies (Table 5). Stabilization of the HOMO should make these systems less susceptible to oxidative degradation. Finally, to understand the slip-stacking arrangement observed in the X-ray structure we investigated the electrostatic potential (ESP) surface as a function of fluorine substitution. The ESP, defined as the work required to bring a unit charge from infinity to the point of interest, provides a useful means of understanding intermolecular arrangements. This contains not only the electronic influence (from the density) but also the influence of the positively charged nuclei. Thus, the ESP surface will contain regions of positive and negative potential. Depending on the number of fluorine substituents, the ESP surface changes dramatically; see Figure 11. It can also be seen that the hexyl chains have no influence on the ESP in Figure 10. Fluorine pulls electron density out of the central ring, creating regions of positive and negative potential associated with the phenyl and thiophene rings, respectively. On the basis of the ESP surface it is clear why the crystal adopts a

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Figure 13. Slip-stack arrangement for the dimer of 3 (a) along the z axis and (b) in a sideways view.

slip-stack arrangement, as in this form it can optimize the interaction of positive and negative regions. In Figure 12 are show the equivalent plots for 11 with varying number of fluorine atoms. The trend in ESP is completely analogous to that for 5. To further quantify this observation we performed a geometry optimization on a dimer of 5. The resultant structure shows the slip-stack arrangement of the two monomer units (Figure 13). The structure of 3 shows that the conformation adopted corresponds to the adjacent superposition of the regions of positive and negative ESP on the different monomer units. We are currently extending this investigation. We will report then on the performance of an energy decomposition based on the Ziegler-Rauk density functional formalism.52 This scheme allows the interaction energies between the two monomer units to be evaluated. We recognize the issues related to the evaluation of nonbonded interactions via DFT techniques, but in this case we have sufficient evidence from the ESP surfaces to suggest that the nonbonded interactions here are not principally due to dispersion and so the use of the DFT formalism should be quite reliable. So far we have found that at the optimum geometry of the dimer we find an interaction energy between the two monomers of -10.5 (52) Ziegler, T.; Rauk, A. Theor. Chim. Acta 1977, 46, 1-10.

kJ mol-1, which is greater than the energy required to force the structure planar. A more thorough investigation will be presented elsewhere. Transistor Fabrication and Measurement. Thin-film OFETs were fabricated on highly doped silicon substrates with a 230 nm thick thermally grown silicon oxide (SiO2) insulating layer, where the substrate served as a common gate electrode. Transistor source-drain gold electrodes were photolithographically defined on the SiO2 layer. Prior to organic semiconductor deposition, FET substrates were treated with a silylating agent hexamethyldisilazane. Thin semiconductor films were then deposited by spin-coating polymer solutions in chloroform (1 wt %) onto the FET substrates. The electrical characterization of the transistor devices was carried out in a dry nitrogen atmosphere using a computer-controlled Agilent 4155C semiconductor parameter analyzer. Field-effect mobility was calculated in the saturation regime [Vd > (Vg - V0)] using eq 1:

( ) dIdsat dVg

Vd

)

WCi sat µ (Vg - V0) L

(1)

where W is the channel width, L is the channel length, Ci is the capacitance of the insulating layer, Vg is the gate voltage,

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Figure 14. Transfer (a) and output (b) characteristics of a 22-Se transistor. Table 6. Data from OFET Measurements sat. µ [cm2/(V‚s)] compound 21-S 21-Se 22-S 22-Se 23-S 23-Se 24 25 26 27 28-S 28-Se

as-spun 10-5

1.3 × 3.5 × 10-5 5.4 × 10-5 4.2 × 10-4 4.0 × 10-4 4.0 × 10-5 6.4 × 10-7 3.3 × 10-6 1.5 × 10-7 4.0 × 10-4 3.0 × 10-4 2.6 × 10-4

annealeda

Ion/Ioff

V0

6.3 × 10-5 1.8 × 10-4 no improvement 1.1 × 10-3 no improvement no improvement no improvement 6 × 10-6 no improvement 6.0 × 10-4 2.0 × 10-3b 3.0 × 10-3b

104 102 104 105 104 104 102 104 102 105 105 105

-10 20 -2 2 6 2 -7 -17 -25 17 7 2

a Devices annealed at 100 °C for 10 min under nitrogen. b Devices annealed at 150 °C for 10 min under nitrogen.

and V0 is the turn-on voltage. The turn-on voltage (V0) was determined as the onset of source-drain current (Figure 14). The transfer and output characteristics of a typical OFET prepared from 22-Se after annealing are shown in Figure 14. The polymer has a mobility of 10-3 cm2/(V‚s) and a current modulation of more than 105, with a low turn-on voltage of 2 V. The transistor characteristics obtained for the higher molecular weight chloroform fractions of polymers 21-28 are summarized in Table 6. The polymers all exhibited typical p-type behavior under negative gate bias, although the electrical performance varied considerably. The as-spun mobilities were in the range from 10-6 to 10-4 cm2/ (V‚s). The films were all annealed at 100 °C for 10 min under nitrogen, generally resulting in an improvement in the charge carrier mobility, in some cases by up to a factor of 5 (21-S and 21-Se). Further annealing at 150 °C for 10 min resulted in a slight decrease in mobility in all cases except for polymers 28-S and 28-Se, which exhibited a further 2-fold increase. We attribute this improvement upon annealing to an increase in the structural order of the polymer at the interface, and the variations in effect are most likely due to differences in glass transition temperatures and viscosity of the polymer systems. The turn-on voltages for the polymer OFETs show a large degree of variation and cannot be readily related to the HOMO levels determined by CV. However, as expected by the difference in work function between the gold electrodes (∼5.1 eV) and the polymer HOMO levels,

there is some contact resistance detectable in the transistor output characteristics (Figure 14b), as evidenced by a slight curvature of the source-drain currents at low voltages. Certain trends are apparent upon examining the electrical performance. The mobilities for the selenophene containing polymers are higher than the analogous all-thiophene systems, with the exception of 23-Se. This may be due to the larger and more polarizable selenium atom increasing intermolecular interactions between polymer backbones. A similar increase in electron rather than hole mobility has been reported for an oligoselenophene system.53 The trend toward planarization upon increasing substitution of the phenylene ring is also reflected in the electrical performance, with charge carrier mobility increasing within identical series (24 < 25 < 27; 21-S < 22-S < 23-S; 21-Se < 22-Se) with increasing fluorine content. The one anomaly is again 23Se. Finally, it is apparent that the incorporation of a spacer between head-to-head coupled thiophenes leads to an improvement in charge carrier mobility (compare 27 and 28). Conclusion In summary, we have prepared a series of thiophene and selenophene copolymers bearing phenylene and fluorinated phenylene units in the main chain. The purpose of our work has been to assess the electronic and supramolecular contribution of the fluorine group toward desirable properties for OFET applications. Although the hole mobilities require further improvement, we have demonstrated unequivocally that copolymers containing the tetrafluorophenylene unit are superior to the unsubstituted or difluorophenylene analogues. This conclusion is based on issues concerning stability (lowering of HOMO levels), self-assembly in the solid state, and hole transport properties. The incorporation of the tetrafluophenylene unit into conjugated arrays involves relatively simple chemistry, without the use of catalysts, and is inexpensive. With the additional advantage of imparting good solubility through conformational freedom, this unit deserves to be considered as a key component in the design of new materials for OFETs. (53) Kunugi, Y.; Takimiya, K.; Toyoshima, Y.; Yamashita, K.; Aso, Y.; Otsubo, T. J. Mater. Chem. 2004, 14, 1367-1369

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Acknowledgment. The authors wish to thank MERCK Chemicals for funding D.J.C. and EPSRC for grant GR/T28379 (J.E.L.). Supporting Information Available: Full experimental data for the synthesis and characterization of compounds and polymers

Crouch et al. 1-28. X-ray crystallographic discussion on compounds 12 and 18. Cyclic voltammograms depicting the electrochemical growth of poly(18). Complete ref 51 (PDF). Crystallographic information files (CIF) for compounds 3 and 9. This material is available free of charge via the Internet at http://pubs.acs.org. CM051563I