Synthesis and optical properties of poly(p-benzamide)s bearing oligothiophene on the amide nitrogen atom through an alkylene spacer

Abstract

Phenyl 4-((5′′-hexyl-2,2′-bithienyl)methyl)aminobenzoate (M1), phenyl 4-((5′′-hexyl-2,2′-bithienyl)propyl)aminobenzoate (M2) and phenyl 4-((5-hexyl-2,2′:5′,2′′-terthienyl)propyl)aminobenzoate (M3), each having oligothiophene on the nitrogen atom through the use of an alkylene spacer, were synthesized using a method in which the oligothiophene group was introduced by the reductive amination (M1) or the nucleophilic substitution (M2 and M3). The condensation polymerization was performed by adding the monomer and 4′-nitrophenyl 4-methylbenzoate to lithium bis(trimethylsilyl)amide and N,N,N′,N′-tetramethylethylenediamine (Method A). Poly(p-benzamide)s with number-averaged molecular weights ranging from 4400–7300 were obtained in high yields (∼80%). From the gel permeation chromatography profiles and the 1H-nuclear magnetic resonance spectra, the polymerization was found to proceed in a controlled manner. The C=O stretching vibration signal in the infrared spectra indicated the cis conformation of the amide group in the polymer backbone. However, the direct polycondensation of 4-((5′′-hexyl-2,2′-bithienyl)methyl)aminobenzoic acid using PPh3 and hexachloroethane in pyridine produced a cyclic trimer, that is, p-calix[3]amide (Method B). In contrast to polyM2 and p-calix[3]amide, a broad emission peak at ∼480 nm was observed for polyM1, indicating the π-stacked interaction between the bithiophene chromophores. As polyM3 (having the terthiophene) also exhibited a redshift of the emission maxima, the wide conjugated system was found to be susceptible to the strong π-stacked interaction at the polymer side chain. Three poly(p-benzamide)s bearing oligothiophene on the amide nitrogen atom through the alkylene spacer were prepared by the chain-growth condensation polymerization using initiator and base in the presence of chelating reagent. The polymerization was found to proceed in the controlled manner as confirmed by the GPC and NMR analyses. The UV and PL measurements indicated that the p-stacked interaction between oligothiophene chromophores depended on the structure of the repeating unit.