Radiative transition from even-parity state in conjugated polymer

Summary form only given. In nonfluorescent /spl pi/ conjugated polymers, e.g. polydiacetylene (PDA), the dipole forbidden 2A/sub g/ state lies below the allowed 1B/sub u/ state. On the other hand, in the fluorescent polymers such as poly-(p-phenylenvinylene) (PPV) and polythiophene (PT), the 2A/sub g/ state is considered to be above the 1B/sub u/ state. Whether the polymer shows fluorescence or not is determined by the order of the 1B/sub u/ and the 2A/sub g/ states. However, the electron-phonon coupling relaxes the selection rule. Moreover, in the case where the 1B/sub u/ and 2A/sub g/ states lie close to each other, the radiative transition from the 2A/sub g/ state can be enhanced. In the research, we confirm that photoluminescence in a PT derivative contains the contribution from the 2A/sub g/ state, and conclude that the 2A/sub g/ state lies slightly below the 1B/sub u/ state in this material.