Spectroscopical and photophysical investigations on polydiacetylenes with different ordering of the A g and B u excited states

Abstract

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 A_g and B_u 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.