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 epsilon(g) gap. (c) 2005 Elsevier Ltd. All rights reserved.