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Solubility control of regioregular 3-substituted polythiophenes bearing 2-phenylnaphthalene side chain by copolymerisation

Two types of 2,5-dibromothiophene monomers having a dodecyl chain and a 2-phenylnaphthalene one were randomly copolymerised in different feed molar ratio by Ni-catalysed chain-growth polymerisation method. Regioregularity of all polymers were sufficiently high (88–97%). Absorption λmax due to π-π* transition of polythiophene backbones in solutions were observed at 444–451 nm for all polymers, which were almost similar to each other and typical of the optical characteristics of regioregular poly(3-alkylthiophene)s. The homopolymer of the latter monomer that had the phenylnaphthalene side group showed poor solubility to common organic solvents, i.e., it was insoluble in chloroform at room temperature and could dissolve only in hot solvents above 100 °C. The copolymers, which had higher number average molecular weights (Mn), had better solubility than that of the homopolymer.


Introduction
Polythiophene and related derivatives having alkyl side chains have been studied worldwide energetically, while examples of studies about polythiophene derivatives having aromatic side groups with special functionalities such as liquid crystalline properties are limited so far. Recently, our group realised synthesising 3-substituted polythiophenes bearing a liquid crystalline and semiconducting 2phenylnaphthalene moiety [1]. Polymers provided by the same 2,5-dibromo-3-substituted thiophene monomer but different polymerisation methods exhibited quite different properties. The polymers prepared by Yamamoto dehalogenative polycondensation method showed modest head-to-tail regioregularity, good solubility in organic solvents, large liquid crystalline domains, and wide energy gap (E g ) between HOMO and LUMO levels. In contrast, the polymers prepared by Ni-catalysed chaingrowth polymerisation method showed better regioregularity, poor solubility, smaller liquid crystalline domains, and narrower energy gap. The latter polymers must have unique semiconducting properties of the regioregular poly(3-substituted thiophene) backbone and aggregated phenylnaphthalene side chain in an oriented state, however, the unexpected low solubility due to large intermolecular interaction holds up further investigations. In this study, 3-substituted thiophene bearing 2phenylnaphthalene moiety was copolymerised with 3-alkylthiophene by the Ni-catalysed chain-growth polymerisation method in order to control solubility and regioregularity of poly(3-substituted thiophene)s bearing 2-phenylnaphthalene side chains, and basic properties of these polymers were investigated.
To determine M n and dp of P 0:1 , 1 H NMR spectrum in hot chlorobenzene-d5 was used because of its poor solubility to the gel permeation chromatography (GPC) eluent (THF) at ambient temperature [1]. Figure 2 shows 1 H NMR spectra of the polymers. Average molecular weights of the polymers except P 0:1 , which were determined by GPC, were enough high and similar to those of poly(3alkylthiophene)s provided by a similar method [6]. Average formula masses considered actual composition ratios of two units were used to calculate dp. The actual composition ratios of the copolymers were calculated from integration ratio between a well-separated 1 H NMR peak at 4.0 ppm due to naphthoxy-linked methylene protons of the M2 unit (D) and overlapped peaks at 0.9 ppm due to terminal methyl groups of both monomer units (B and F). It is verified that reactivities of two monomers during polymerisation were almost same, because composition molar ratios of the polymers were quite similar with their initial feed molar ratios of the monomers. Regioregularities of the polymers were estimated similarly by the method in our previous report [1] considering integration values at methylenes A, C, and E in the 1 H NMR spectra (Figure 2), and revealed to be high and similar each other except for P 0:1 . The relatively low regioregularity observed for P 0:1 was attributed to its low molecular weight.

Solubilities of polymers
Solubilities of the polymers were evaluated using chloroform which is popular and most often-used good solvent for conjugated polymers. Figure 3 is pictures of the polymer solutions in chloroform. Weight of polymer and volume of chloroform were determined 1.0 mg and 1.0 mL. All the polymers were purple in solid state but their colour changed to light orange when they were fully dissolved. After well mixing them at respective temperatures of ambient, 40 °C, 50 °C, and 60 °C, the mixing samples were left to stand at least 5 min at each temperature to check solubility. The homopolymer of M1, P 1:0 , was soluble in chloroform at room temperature as is well known. On the other hand, the homopolymer of M2, P 0:1 , did not dissolve even if it was heated at 60 °C in spite of its low molecular weight. Solubility of the copolymers was increased as increasing the content ratio of M1 unit, that is, P 1:5 , P 1:3 , and P 1:1 , dissolved completely at above 60 °C, 40 °C, and room temperature, respectively. From these results, it was found that the copolymerisation with 3-dodecylthiophene was effective for improving solubility of regioregular poly(3-substituted thiophene) bearing 2-phenylnaphthalene side group. It is suggested that the 3-dodecylthiophene (M1) unit encouraged dissolving and disturbing aggregation of the phenylnaphthalene side group. Differential scanning calorimetry (DSC) measurement at a heating/cooling rate of 10 °C/min was used to investigate phase transition behaviours of the polymers. Because of their high intermolecular forces in condensed states, the transition temperatures from isotropic liquid to the first less mobile phase of P 1:0 , P 1:1 , P 1:3 , P 1:5 , and P 0:1 were clearly observed at 115 °C, 107 °C, 114 °C, 164 °C, and 171 °C, in the second cooling process of DSC thermograms. Although these phase trtansition temperatures are affected by molecular weights of polymers, it can be said that introduction of 3-dodecylthiophene (M1) unit was effective to improve fusibility of poly(3-substituted thiophene) bearing 2-phenylnaphthalen side group, P 0:1 .

Optical properties
UV-vis absorption spectra of the polymers in solution states and drop-cast films were shown in Figure  4 and Figure 5, and data of the UV-vis absorption λ max and photoluminescence (PL) λ em of the polymers in solution and film states due to π-π* transition were listed in Table 2. The absorption λ max and PL λ max of the polymers in solutions were observed at 444-451 nm and 579-588 nm, and they were similar each other and almost identical to the data of regioregular poly(3-dodecylthiophene) in a previous report [7]. On the other hand, the absorption λ max of P 1:1 , P 1:3 , and P 1:5 , in films were hypsochromic shifted than that of P 0:1 , which might be due to effect of steric hindrance of the bulkier phenylnaphthalene side group than the dodecyl group ( Figure 5). However, absorption edges of P 1:1 , P 1:3 , and P 1:5 , in films were little shifted from that of P 0:1 , which means that these copolymers have almost same bandgaps (E g ) as P 0:1 has. 300 400 500 600 700 Wavelength /nm Absorbance (a. u.)

Energy levels
Energy levels of highest occupied molecular orbital (E HOMO ) of the polymers were estimated from the onset oxidation potentials of the polymers measured by cyclic voltammetry (CV), and Figure 6 shows the oxidation waves of the polymers. E HOMO values of P 1:0 , P 1:1 , P 1:3 , P 1:5 , and P 0:1 estimated from each onset potential were −4.91, −5.18, −5.24, −5.25, and −5.40 eV, although these polymers had almost same E g . The E HOMO of P 0:1 was deeper than that of P 1:0 and the E HOMO values of the copolymers were located between the homopolymers. This phenomenon implies that steric hindrance and/or some specific electronic interaction exist between the polythiophene backbone and the 2-phenylnaphthalene side group, which stabilises polythiophene backbone against oxidisation.  Figure 6. Difference of onset potentials in oxidation waves of the polymers observed by CV in 0.1 M Et 4 NBF 4 /acetonitrile at a scan rate of 50 mV/s.

Conclusions
A thiophene monomer, 2,5-dibromo-3-{10-[6-(4-octylphenyl)naphthalene-2-yloxy]decyl}thiophene (M2), was randomly copolymerised with 2,5-dibromo-3-dodecylthiophene (M1) by the Ni-catalysed chain growth method in order to improve solubility of regioregular poly(3-substitutedthiophene) with 2-phenylnaphthalene side group. The copolymers, especially P 1:1 , realised much better solubility in organic solvents than P 0:1 with maintaining high regioregularity. It is proved that insertion of the 3dodecylthiophene moiety into a homopolymer structure of M2 can disturb aggregation between the 2phenylnapthalene moieties and/or polythiophene backbone, effectively. In contrast, it is assumed that presence of the 2-phenylnaphthalene moiety in the regioregular 3-substituted polythiophenes weakens π-stacking interaction between polythiophene backbones in consequence of steric effect of the bulky side chain in some degree, which lowered their E HOMO levels in spite of bearing similar regioregularity and E g .