In order to investigate the effect of the bulky pendant group and the spacer length between the pendant group and polythiophene main chain on the optical property of polythiophene backbone, a series of polythiophene derivatives were synthesized via copolymerization of 3-hexylthiophene with four different types of 4-((4-(phenyl)azo)phenoxy)alkyl-3thienylacetate. The alkyl spacers with different lengths, i.e., butyl, hexyl, octyl and undecyl groups were used between the bulky azobenzene group and the thiophene ring. The compositions and structures of these polythiophene derivatives were characterized. The structural dependence of thermochromic behaviour of these copolymers were systematically studied and results compared with poly(3-hexylthiophene) (P3HeT). A thermochromic temperature coefficient (C-TC) was defined for the first time in an attempt to quantitatively compare the sensitivity of the thermochromism in the copolymers. The results have shown that these copolymers with a bulky azobenzene group in side chain have a higher (C-TC) value than P3HT and the C-TC value of the copolymers increase with increasing the spacer length. (c) 2005 Springer Science + Business Media, Inc.