EPR study of oriented photoexcited triplets of zinc porphyrin in a discotic liquid crystal: Evidence for axial symmetry in the condensed phase

An EPR study of the photoexcited triplet of ZnTPP in the dicostic mesophase of p-n-hexa-hexyloxytriphenylene is reported. The results indicate that (i) the observed spectra are temperature independent (between ∼100–140 K) and typical of axially symmetric guest molecules, with zero field splitting values ‖D‖=0.0308 cm−1, and ‖E‖=0, (ii) the line intensity of the out-of-plane canonical orientation of the guest molecules exhibits anisotropic patterns with respect to the sample orientation in the external field. These results are best interpreted by the columnar structure of the host mesogen which forms rodlike structures whose directors are distributed about a plane perpendicular to the applied external magnetic field with a normal distribution and standard deviation of ∼3–8°. The ZnTPP guest molecules are located outside the columns and orient such that the porphyrin planes are parallel to the columns axes. It is also concluded that, in the temperature range studied, the porphyrin molecules are distributed in this liquid crystal as noninteracting monomers retaining their axial symmetry.