Thermochromic behavior in novel conducting polymers at the solid-liquid phase transition

The thermochromic behavior of poly(2,5-dialkoxy-p-phenylene)s, ROPPVs has been studied. The temperature dependence of electrical/optical properties and the variation of properties with the side chain length have been compared with those of poly(3-alkylthiophene)s, PATs. The bandgap 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 bandgap 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 region of this temperature dependence is not clear at this stage.