ONE-DIMENSIONAL TRIPLET ENERGY MIGRATION IN COLUMNAR LIQUID-CRYSTALS OF OCTASUBSTITUTED PHTHALOCYANINES

Time-resolved absorption spectroscopy has been used to study laser-induced triplet excitons of the octakis(alkoxymethyl) metal-free and zinc phthalocyanines {(C12OCH2)8PcH2, (C18OCH2)8PcH2 and (C12OCH2)8PcZn} in their crystalline and columnar liquid-crystalline phases. The triplet states have been characterized by their absorption spectra and their decay kinetics. At early times, the triplet state population varies as t-1/2, which is characteristic of diffusion-limited one-dimensional triplet-triplet annihilation. The long-time decays are well described by a model of random walk on a linear chain containing traps. The incoherent exciton path length between two successive defect sites is on the order of a micrometer, and it could be correlated with the size of the microdomains. The values of the exciton hopping time (0.4-68 ps) determined from the short-time decay kinetics are in agreement with those determined independently from the long-time behavior of the triplet decay. The exciton diffusion coefficients determined for the columnar mesophases are found to be higher than those observed in the crystalline phases of the same compounds, consistent with a more efficient energy migration in the liquid crystal than in the crystal.