STRUCTURE OF THIOPHENE-BASED REGIOREGULAR POLYMERS AND BLOCK-COPOLYMERS AND ITS INFLUENCE ON LUMINESCENCE SPECTRA

Two approaches toward control of the luminescence wavelength of polythiophenes have been explored: (i) block copolymers in which oligothiophene blocks alternate with oligosilanylene blocks and (ii) regioregular polythiophenes in which oligothiophene sequences are delimited by n-octyl substituents placed in a ''head-to-head'' fashion on adjacent rings. Both methods aim at restricting the pi-conjugation to the oligothiophene sequences. The block copolymer approach is very effective, whereas the (solution) luminescence spectra of the regioregular polymers are strongly red-shifted with respect to absorption and confined to a narrow range of wavelengths. This is due to the quinoid character of the excited singlet state, in which there is a strong electronic driving force toward coplanarity of adjacent thiophene rings, which offsets the steric hindrance of the octyl substituents and increases the size of the conjugating pi-system. This explanation is supported by calculations and by spectral data of substituted bithiophenes.