PHOTOEXCITATION STUDIES IN POLY[1,6-DI(N-CARBAZOLYL)-2,4-HEXADIYNE] - CORRELATION OF SPECTRAL FEATURES WITH THE DEGREE OF ORDER IN POLYCRYSTALLINE SAMPLES

We have studied the electronic structure and the elementary excitations of the red phase of poly[1,6-di(N-carbazolyl)-2,4-hexadiyne] (polyDCHD) by electronic and Raman spectra and by photoinduced absorptions in the medium and near-infrared regions. The results are interpreted in terms of the coexistence in this polymer of both ordered and disordered regions. By comparing the present data with those previously obtained on a highly ordered blue form of the same polymer, the nature of the long-lived photoexcitations is found to be dependent on the degree of chain order. Indeed, in the blue polymer, charged bipolarons and triplet excitons were detected at 0.1, 0.8 and 1.25 eV, respectively. On the other hand, in the more disordered form, a signal at 1.5 eV represents the main component of the photoinduced spectra. A tentative assignment of this feature to triplet excitons trapped in a chain defect is proposed, although trapped polarons could similarly account for the large lifetime estimated from our experiments.