Electroreflectance studies on the excitons in PTS and DCHD single crystals

In polydiacetylene single crystals of PTS poly-2,4-hexadiyne-1,6-diol-bis(p-toluene sulfonate)] and of DCHD poly-1,6-di(N-carbazolyl)-2, 4-hexadiyne] electric field modulated reflectance spectra in the range of the π-π* excitations of the polymer indicate considerable delocalization of the electrons along the chain. Shape and size of the excitonic electroreflectance spectra are determined by a large charge transfer component of the excitons. Evaluation of the spectra yields about 20% and 40% charge transfer with excitation to neighbouring π bonds in PTS and DCHD, respectively. Large differences occur in electroreflectance in the range of vibronic excitons pointing to pronounced coupling of the carbazole side group in DCHD to the π electrons of the chain. At higher energy, ?0.48 eV above the lowest exciton transition in both polymers a large electroreflectance signal is observed which is attributed to a band transition.