Optically Active Poly(benzene-1,4-diyl)s with Random and Alternating Copolymer Sequences Composed of Chiral and Achiral, Bulky Monomeric Units: A Systematic Study on Side-Chain Bulkiness Effects on Ground-State and Excited-State Chiroptical Properties and Chiral Recognition Ability

Abstract

Random copolymers comprising chiral and achiral benzene-14,-diyl units were prepared by Ni-mediated copolymerization of optically active 1,4-dibromo-2,5-bis((S)-2-methylbutoxy)benzene with bulky, achiral monomers, i.e., 1,4-dibromo-2,5diphenylbenzene, 1,4-dibromo-2,5-bis(3,5-dimethylphenyl)benzene, 1,4-dibromo-2,5-bis(3,5-bis(trifluoromethyl)phenyl)benzene, and 1,4-dibromo-2,5-bis(3,5-diphenylphenyl)-benzene, and the corresponding alternating copolymers were synthesized by Pd-catalyzed Suzuki−Miyaura cross-coupling of optically active 1,4-bis-(dihydroxyboranyl)-2,5-bis((S)-2-methyl-butoxy)benzene with bulky, achiral monomers. Poly(benzene-1,4-diyl)s with achiral 3,5-disubstituted or unsubstituted phenyl groups and chiral (S)-2-methylbutoxy groups in the side chain were thus obtained. The copolymers showed circular dichroism (CD) spectra whose shapes and intensities largely varied depending on the state of the sample (solution or suspension or film) and the chemical structure. In suspensions and films, both the random and alternating polymers with side-chain 3,5-disubstituted phenyl groups showed intense CD spectra suggestive of a chiral conformation such as preferred-handed helices, while they indicated only moderate CD intensities in solution. Both the random and alternating copolymers with side-chain 3,5disubstituted phenyl groups led to higher anisotropy (g CD ) values than those with side-chain phenyl groups, indicating that sidechain bulkiness plays a role in creating a bias on the population of one enantiomeric structure in the solid state. Among the three 3,5disubstituted phenyl groups, the 3,5-dimethylphenyl group, which is not seemingly the bulkiest one, tended to result in the greatest g CD values. On the other hand, the side-chain 3,5-bis(trifluoromethyl)phenyl group and unsubstituted phenyl groups led to the greatest anisotropy (g lum ) values in circularly polarized light (CPL) emission properties in films for the random and alternating copolymers, indicating that the significance of bulkiness of side-chain groups varies between the ground state and excited states. The alternating copolymer with the side-chain 3,5-bis(trifluoromethyl)phenyl group showed a g lum of +0.012, which arises from the strong electron-withdrawing effects of the −CF 3 group. CPL emission was more significant for the random copolymers than for the alternating copolymers. Some of the polymers exhibited chiral recognition abilities toward trans-stilbene oxide, Troger's base, and flavanone, where resolution performance varied depending on the solvent and polymer structure, and the order of separation factors was not particularly in agreement with the order of g CD values in CD spectra in spite of the fact that CD is often generally used to quantify the chirality of materials.