SPECTROSCOPICAL AND PHOTOPHYSICAL INVESTIGATIONS ON POLYDIACETYLENES WITH DIFFERENT ORDERING OF THE Ag AND Bu EXCITED STATES

Absorption, photoluminescence and photoinduced absorption spectra of polycarbazolyldiacetylenes (polyCzDAs) carrying selected acyl (polyDPCHD) or alkyl (polyDCHD-HS) groups are reported. The absorption spectrum of polyDCHD-HS films, which is almost independent of the temperature, shows an excitonic absorption followed by an inhomogeneously broadened vibronic progression. Very different is instead the case of polyDPCHD whose thin films show a very sharp excitonic transition and a vibronic progression with a lower electron-phonon (el-ph) coupling with respect to that of polyDCHD-HS. Unlike other polydiacetylenes (PDAs), the polyCzDAs which exhibit narrow spectral features and reduced el-ph coupling, such as polyDPCHD films and polyDCHD-HS in benzene solutions, are luminescent thus suggesting a different ordering of the dipole forbidden and allowed states. This interpretation is confirmed by two-photon spectroscopic data. The ordering of the Ag and Bu excited states also affects the generation of triplet excitons, which are the main long-living excited states of PDAs. The rise and decay of the triplet signal for polyDCHD-HS are accounted for through a dynamical model, based on a monomolecular decay regime including saturation effects. A more complex kinetics is instead observed for polyDPCHD. From the study of the kinetics of the PIA spectra both the triplet generation efficiency and the density of traps are evaluated