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

Two approaches toward control of the luminescence wavelength of polyth iophenes have been explored: (i) block copolymers in which oligothioph ene blocks alternate with oligosilanylene blocks and (ii) regioregular polythiophenes in which oligothiophene sequences are delimited by n-o ctyl substituents placed in a ''head-to-head'' fashion on adjacent rin gs. Both methods aim at restricting the pi-conjugation to the oligothi ophene sequences. The block copolymer approach is very effective, wher eas the (solution) luminescence spectra of the regioregular polymers a re strongly red-shifted with respect to absorption and confined to a n arrow range of wavelengths. This is due to the quinoid character of th e excited singlet state, in which there is a strong electronic driving force toward coplanarity of adjacent thiophene rings, which offsets t he steric hindrance of the octyl substituents and increases the size o f the conjugating pi-system. This explanation is supported by calculat ions and by spectral data of substituted bithiophenes.