Structural and electronic effects induced by carboxylic acid substitution in isomeric 2,2′-bithiophenes and oligothiophenes: A computational study

The structural and electronic properties of carboxylic acid-substituted 2,2′-bithiophenes have been examined using quantum mechanical methods based on density functional theory. Calculations at the B3PW91/6-31+G(d,p) level were performed on 2,2′-bithiophene-4,4′-dicarboxylic acid, 2,2′-bithiophene-3,4′-dicarboxylic acid and 2,2′-bithiophene-3,3′-dicarboxylic acid, different arrangements being additionally considered for the carboxylic acid substituents of each isomer. The energy gap calculated for 2,2′-bithiophene-3,4′-dicarboxylic acid was about 0.15 eV smaller than that predicted for unsubstituted 2,2′-bithiophene. Additional calculations were performed on oligothiophenes containing n carboxylic acid substituted thiophene rings, n ranging from 2 to 7. The results, which allowed to estimate the band gap for the corresponding poly(thiophene carboxylic acid)s, indicated that the introduction of carboxylic acid substituents at polythiophene produces a very small increase in the εg gap.