TRIPHENYLENE COLUMNAR LIQUID-CRYSTALS - EXCITED-STATES AND ENERGY-TRANSFER

The present paper deals with the photophysical properties of columnar Liquid crystals formed by hexakis-(alkyloxy)triphenylenes. Absorption and fluorescence spectra of solutions are analyzed on the basis of qua ntum chemical calculations performed by the CS-INDO-CI (conformations spectra-intermediate neglect of differential overlap-configuration int eraction) method: the absorption maximum is due to the S-0-->S-4 trans ition while fluorescence originates from the weak S-0-->S-1 transition . In columnar aggregates, the former transition corresponds to delocal ized excited states while the latter corresponds to localized ones; ca lculation of intermolecular interactions shows that, at the temperatur e domain of the mesophases, all the molecules have the same excitation energy and, therefore, no spectral diffusion of the fluorescence is e xpected, in agreement with the time-resolved emission spectra. Excitat ion transfer is investigated by studying the fluorescence decays of me sophases doped with energy traps. Their analysis is made by means of M onte Carlo simulations considering both intracolumnar and intercolumna r jumps and using four different models for the distance dependence of the hopping probability. The best description is obtained with a mode l based on the extended dipole approximation and taking into account m olecular orientation.