Azobenzene-functionalized polythiophene derivatives, Poly[4-((4-(phenyl)azo)phenoxy)alkyl-3-thienylacetate] (alkyl=hexyl and octyl) (P6 and P8) and the copolymers of 3-hexylthiophene and 4-((4-(phenyl)azo)phenoxy)alkyl-3-thienylacetate (alkyl=hexyl and octyl) (P66 and P86) were synthesized. The composition, structure, and thermal property of these polythiophene derivatives were fully characterized by NMR, FTIR, GPC, MDSC, and XRD. The structural dependence of the photochromic features and thermochromic behaviors were also investigated by means of photoluminescence and UV-Vis absorption spectroscopy. The results have shown that the azobenzene substitution renders the homopolymer (P6 and P8) some interesting optical properties that can be modulated by UV light irradiation. In these azobenzene-modified polythiophenes, the intensity of photoluminescent emission associated with the conjugated polythiophene main chain was found to decrease significantly upon UV irradiation. The finding suggests that the photo-induced trans-cis isomerization of the azobenzene pendant groups has significant effect on photoluminescent emission. However, the effect becomes less prominent for copolymer P66 and P86 due to the lower content of azobenzene chromophore in the side chain of the copolymers. (c) 2005 Wiley Periodicals, Inc.