J O U R N A L O F C H E M I S T R Y Materials Communication Synthesis and polymerization of 3,3-di[(S)-(+)-2-methylbutyl]- 2,2¾:5¾,2-terthiophene: a new monomer precursor to chiral regioregular poly(thiophene) Franco Andreani, *† Luigi Angiolini, Daniele Caretta and Elisabetta Salatelli Dipartimento di Chimica Industriale e dei Materiali, Universita` di Bologna, V iale Risorgimento 4, 40136 Bologna, Italy oxidative methods, such as FeCl3 oxidation, which is easy to use and suitable to large scale production. 3,3-Di[(S)-(+)-2-methylbutyl]-2,2¾:5¾,2-terthiophene, having two equivalent reactive positions, allows the synthesis of the Monomer (+)-3,3-DMBTT was prepared from 3-[(S)-(+)- 2-methylbutyl]thiophene (+)-1, itself derived from optically corresponding optically active poly(thiophene) with regioregular enchainment by using an aspecific oxidative pure (S)-(-)-2-methylbutan-1-ol, according to the reported procedure3 which involves a bromination step to (S)-(+)-1- polymerization method.bromo-2-methylbutane followed by Grignard reaction and coupling with 3-bromothiophene in the presence of 1,3- diphenylphosphinopropanenickel dichloride [Ni(dppp)Cl2]. The 3-alkylthiophene (+)-1 was then submitted to iodination Much eVort has been devoted in the recent years to the using the method of Suzuki,19 thus obtaining a mixture of the investigation of optically active polythiophenes characterized 2-iodo and 2,5-diiodo derivatives.Iodination, instead of broby the presence of a chiral moiety linked to the 3-position of mination, of (+)-1 was adopted due to the expected higher the aromatic ring.In addition to their potential technological reactivity of the iodinated derivative in the subsequent prepapplications as materials for enantioselective electrodes and aration of the Grignard reagent. The pure mono-iodinated membranes,1–4 chiral poly(thiophene)s oVer the possibility of product, 2-iodo-3-[(S)-(+)-2-methylbutyl]thiophene (+)-2, studying the structural changes accompanying the transition [a]D25+3.95 (c 2.1, CHCl3), was obtained in good yield by from the disordered state in solution to the ordered micro- fractional distillation in vacuo, then allowed to react with aggregate or solid state by following the variation of their magnesium in THF, and finally coupled with 2,5-dibromothiochiroptical properties by circular dichroism (CD).5–8 A crucial phene in the presence of Ni(dppp)Cl2 and anisole to give (+)- role in obtaining remarkable chiroptical properties is however 3,3-DMBTT, according to Kobayashi’s procedure.20 played by a highly predominant or exclusive presence in the Monomer (+)-3,3-DMBTT purified by column chromatogmacromolecular backbone of regioregular head-to-tail raphy (SiO2, hexane), [a]D25+16.0 (c 1.7, CHCl3), was fully sequences of the 3-substituted thiophene repeating units, as characterized by IR and 1H NMR spectroscopy, mass specthe lack of this structural requirement strongly reduces the trometry (m/z 388; M+, 100%) and elemental analysis (Found: possibility of the existence of extended co-planar structures C, 67.8; H, 7.4; S, 24.8.Calc. for C22H28S3: C, 67.99; H, 7.26; characterized by a large extent of aromatic conjugation. S, 24.75%). In particular, it is worth noting the presence in Synthetic methods to produce highly regioregular poly(thio- the IR spectrum of two bands at 3102 and 3063 cm-1, related phene)s have been developed9–11 and McCullough’s procedure, to Ca–H and Cb–H stretching, respectively, typical of 3-alkylin particular, has been applied to the preparation of highly substituted thiophene rings,21 as well as the presence of an regioregular optically active 3-alkyl substituted poly(thio- out-of-plane bending signal at 797 cm-1, attributed to the 2,5- phene)s,5–8 exhibiting much enhanced optical activity as com- disubstituted central ring.22,23 pared to the corresponding regiorandom derivatives when the The 1H NMR spectrum in CDCl3 [Fig. 1(a)] exhibits the macromolecules are in the micro-aggregate or solid (film) state. expected pattern in the aromatic region, consisting of two A possible way to avoid the need for using regiospecific doublets at 7.17 and 6.91 ppm, related to the a and b protons, synthetic methods is given by the preparation of symmetrical respectively, and of one singlet at 7.05 ppm (c protons).monomers inherently unable to aVord regiorandom polymeric The oxidative polymerization of (+)-3,3-DMBTT has been derivatives even in the presence of an aspecific polymerization carried out following the general method of Sugimoto et al.,24 mechanism. Whereas symmetrically 3,3¾- or 4,4¾-disubstituted 2,2¾-bithiophenes give polymers containing regularly spaced head-to-head (50%) and tail-to-tail (50%) coupled repeating units, symmetrically 3,3- and 3¾,4¾-disubstituted 2,2¾:5¾,2- terthiophenes have been reported12–18 to produce the corresponding polythiophenes characterized by a regioregular enchainment free from head-to-head connections, provided that two identical ring substituents are present.However, no optically active terthiophene monomer precursor to a regioregular optically active polymer is mentioned in the literature. We report here the possibility of synthesizing poly(thiophene)s, characterized by relevant chiroptical properties, from the optically active 3,3-di[(S)-(+)-2-methylbutyl ]-2,2¾:5¾2-terthiophene monomer [(+)-3,3-DMBTT] (Scheme 1) having two identical reactive sites and hence polymerizable using chemical S CH3 CH3 S R* I S S S R* R* i ii, iii 73% 52% (+) - 1 (+) - 2 (+) - 3,3¢¢-DMBTT S S S R* R* n poly(3,3¢¢-DMBTT) iv 25% poly(3,3��-DMBTT) 3,3��-DMBTT Scheme 1 Reagents and conditions: i, I2, H5IO6; ii, Mg, THF; iii, 2,5- dibromothiophene, anisole, Ni(dppp)Cl2 (cat.); iv, FeCl3, CCl4 † E-mail: andreani@ms.fci.unibo.it J.Mater. Chem., 8(5), 1109–1111 1109Fig. 2 (a) UV and (b) CD spectra of poly(3,3-DMBTT) in (i) CHCl3 and (ii) CHCl3–CH3OH (71529) 5000 g mol-1, corresponding roughly to a polymerization degree x: n of 13, expressed as terthiophene co-units. This value appears to be in good agreement with the M9 n value of 4300 g mol-1 (x: n=11), determined by gel permeation chromatography using monodisperse polystyrene samples as standard references.Optimization of the polymerization reaction aimed at obtaining both better yields and higher values of M9 n in the soluble fraction of poly(3,3-DMBTT) is currently under study. Fig. 1 1H NMR spectra in CDCl3 of the aromatic region of (a) (+)- The UV spectrum of poly(3,3-DMBTT) in CHCl3 (Fig. 2) 3,3-DMBTT and (b) poly(3,3-DMBTT) displays an absorption maximum at 454 nm, related to the p–p* electronic transition of the conjugated aromatic system, similar or even higher than the maximum wavelength values using CCl4 in place of CHCl3 and lower monomer and FeCl3 molar concentrations (0.025 and 0.1 M, respectively), in order reported in the literature4,5,7,8 for optically active regioregular head-to-tail poly(3-alkylthiophene)s, thus suggesting an to reduce, according to recently reported18 results, the relative amount of insoluble material.Although the reaction conditions enhanced extent of conjugation, probably attributable to improved coplanarity of the thiophene rings along the main have not been yet optimized, we have observed that, indeed, no insoluble polymer fraction is produced at all, even though chain.A solvatochromic eVect is also evidenced in the UV spectrum upon addition of increasing amounts of methanol poly(3,3-DMBTT) is reasonably expected to be not very soluble, due to the lower average density of 2-methylbutyl side (poor solvent), which promotes the aggregation of the macromolecules. A progressive appreciable red-shift of the maximum chains as compared to the corresponding poly[3-(2-methylbutyl) thiophene].3 Poly(3,3-DMBTT), purified by exhaustive absorbance, accompanied by the appearance of vibronic bands and shoulders, up to a lmax of 467 nm is observed, correspond- extraction with CH3OH of any unreacted monomer and low molecular weighons, exhibits the disappearance of the ing to a CHCl3–CH3OH ratio of about 70530 (v/v).Upon further addition of CH3OH, precipitation of the solute takes band at 3102 cm-1, while the 3063 and 797 cm-1 signals remain unchanged, with respect to the IR spectrum of the place. This behaviour indicates an increase of conjugation length attributable to an increase of the conformational order monomeric precursor. Moreover, two bands at 818 and 833 cm-1, typical of the out of plane bending of the Cb–H in the micro-aggregate state.In this state, also, the polymer main chain becomes optically active, as revealed by its CD bond in 2,3,5-trisubstituted thiophene rings,23 appear in the spectrum, thus confirming that the polymerization has taken spectrum (Fig. 2), as a consequence of chirality transmitted by the alkyl side-chain to the conjugated backbone.place only at the Ca atom of the monomer, with no mislinkages through the b-positions occurring during the process. The The CD spectrum of the micro-aggregated macromolecules displays strong dichroic signals which correspond closely to 1H NMR spectrum [Fig. 1(b)] is in accordance with the proposed structure, as it displays two main resonances at 7.10 the UV absorbances, originated by the presence of chiral conformations assumed by the macromolecules when the crys- and 6.99 ppm, related to the c and d protons, respectively, located along the main chain.Less intense signals are also tallization begins to take place, with a remarkable value of the chiral anisotropy factor g (De/e) of -3×10-2 at 578 nm, close observable in the spectrum, due to the hydrogen atoms of terthiophenic end groups.Calculations based on the integrated to the values reported for regioregular poly(3-alkylthiophene) s.8 By contrast, the CD spectrum of poly(3,3-DMBTT) areas of these last resonances allow the assessment that the mean molecular weight of the macromolecular chains is around in pure CHCl3, which favours the presence of disordered 1110 J.Mater. Chem., 8(5), 1109–11118 B. M. W. Langeveld-Voss, M. M. Bouman, M. P. T. Christiaans, random coil conformations of the macromolecules, does not R. A. J. Janssen and E. W. Meijer, Polym. Prepr., 1996, 37, 499. display any optical activity (Fig. 2), due to the absence of 9 R. D. McCullough and R. D. Lowe, J. Chem. Soc., Chem. Commun., chirally ordered structures in the polymer in solution. 1992, 70. It can therefore be concluded that although poly(3,3- 10 R. D. McCullough, R. D. Lowe, M. Jayaraman and D. L. DMBTT) has a lower content of chiral alkyl substituents per Anderson, J. Org. Chem., 1993, 58, 904. 11 T. A. Chen and R. D. Rieke, J. Am. Chem. Soc., 1992, 114, 10 087. thiophene ring with respect to regioregular poly(3-alkylthio- 12 M.C. Gallazzi, L. Castellani, G. Zerbi and P. Sozzani, Synth.Met., phene)s, it behaves similarly, or better, as regards extent of 1991, 41–43, 495. conjugation and chiroptical properties. Interestingly, it could 13 M. C. Gallazzi, L. Castellani, R. A. 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