Several fluorene copolymers containing isothianaphthene units (P1, P2, and P3) or similar derivatives (P4 and P5) have been synthesized by Pd-catalyzed Suzuki polymerization. The monomers containing isothianaphthene were prepared by a ring-closure reaction with Lawesson's reagent. Strong photoluminescence (PL) quenching in the film state was observed for P1 and P2, which was mainly due to the enhanced quinoid character formed by introducing the isothianaphthene unit. Their energy levels of the compounds were determined using cyclic voltammetry. Among the polymers tested, the polymer containing both an isothianaphthene and a selenophene unit, P2, showed the smallest band gap of 1.85 eV. The influence of structural variation on the band gap of the polymer chains was further investigated by optimizing the geometries of several model monomers. Our results based on the optical and electrochemical properties combined with theoretical calculations showed that polymers containing isothianaphthene have small band gaps, rigid conformations, and strong tendencies to aggregation. (C) 2008 Wiley Periodicals, Inc.