Reversible thermochromism and luminescence in copolymers of 3-alkylthiophene

The copolymers of 3-tetradecylthiophene and 3-p-trifluoromethylphenylthiophene with various copolymerization ratios were synthesized. The electronic absorption, FL, and EL spectra showed reversible thermal behavior. Spectral changes up to ca. 70 degreesC was explained in terms of the thermal motions of the side chains leading to extension of rc-conjugation to the phenyl ring. Spectral changes above ca. 70 degreesC was interpreted in terms of interruption of pi -conjugation by the onset of the thermal motions of the main chain above Tg. The lambda (max) of the electronic absorption spectra showed blue shift with the increase in the mole fraction of TFT units. This is because the trifluoromethlyphenyl group is more electron-withdrawing than the alkyl (tetradecyl) group. The copolymers have better quantum efficiency than P3AT because the hole- and electron-injection abilities are balanced by lowering the HOMO and LUMO levels.