TRIPLET DYNAMICS OF CONFORMATIONALLY DISTORTED PORPHYRINS - TIME-RESOLVED ELECTRON-PARAMAGNETIC-RESONANCE

The photoexcited triplet states of the nonplanar porphyrins, free base and zinc 2,3,7,8,12,13,17,18-octaethyl-5,10, 15,20-tetraphenylporphyrins (OETPPs), were examined by laser excitation-time resolved EPR spectroscopy. Measurements were carried out at low temperatures in glassy matrices, and over a wide temperature range in a uniaxial liquid crystal (LC). (3)H(2)OETPP exhibits mainly X,Y in-plane population of the triplet sublevels, whereas in (3)ZnOETPP, the preferred population is through the out-of-plane Z axis. Both chromophores are characterized by relatively small zero-field splitting values, D, and sizable E values (D similar to 3E), triplet parameters more akin to those of analogous TPP rather than OEP derivatives. The LC spectra depend on the specific chromophore, the temperature, and the phase of the LC matrix. Line-shape analysis of the LC spectra suggests fast exchange processes between different triplet conformers attributable to fluxional conformational excursions from the X-ray structures of the OETPPs in their ground states.