Thermochromic Behavior in Poly(p-phenylene vinylene) Derivatives

The thermochromic behaviors of poly(2,5-dialkoxy- p -phenylene vinylene)s (ROPPVs) have been studied. The temperature dependence of electrical and optical properties and the variation of properties with the side chain length have been compared with those of poly(3-alkylthiophene)s (PATs). The band-gap energy of ROPPV16 increases gradually from about 2.2 eV at room temperature to about 2.4 eV at 200°C. This temperature dependence of band-gap energy is different from those of PATs, and the difference can be tentatively interpreted in terms of the main chain structure. The photoluminescence intensities of ROPPVs 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 chains. The ionization potentials of ROPPVs decrease slightly with increasing temperature. However, the origin of this temperature dependence is not clear at this stage.