A consideration of thermochromic behavior in poly(p-phenylene vinylene) derivatives

The thermochromic behavior of poly(2,5-dialkoxy-p-phenylene vinylene)s (ROPPV)s and poly(2,5-alkyl-p-phenylene vinylene)s (RPPV)s have been studied. The temperature dependence of optical properties and the variation of properties with the side chain length have been compared with those of poly(3-alkylthiophene)s (PAT)s. The difference of bandgap energy between PPV derivatives and PATs can be tentatively interpreted in terms of the main chain structure. The photoluminescence intensity of ROPPVs and RPPVs decrease monotonically with increasing temperature. To explain this mechanism, we speculate on the separation of the excited species due to the reduction of distance between polymer main chains and the enhancement of the overlap of π-electrons with the twist of polymer main chain. Furthermore, the interaction between polymer main chain and the oxygen atoms of alkoxy side chain plays an important role for the optical properties of ROPPVs.