Controlled orientation of porphyrins by molecular design in thermotropic liquid crystals: a time-resolved EPR study

The orientation of the porphyrin ring in nematic liquid-crystalline phases is shown to be controlled by modification of guest-host dynamics through molecular design. Thus, attachment of rod-like mesogenic residues or "arms" to α,α,α,α,-meso-tetrakis(o -aminophenyl)porphyrin yields the porphyrin, MesogenP, having molecular compatibility with standard liquid-crystalline compounds. Both the free-base and zinc porphyrin were photoexcited to the triplet state and detected by time-resolved EPR spectroscopy. The spectra show that the orientation, porphyrin ring parallel to the director, found for the simple tetraphenyl porphyrin, is inverted in the case of the MesogenP so that the porphyrin ring is perpendicular to the director in both frozen and fluid nematic phases.