Triplet dynamics of conformationally distorted porphyrins in isotropic liquids and liquid crystals. Time-resolved electron paramagnetic resonance study

The photoexcited tripler states of four nonplanar (distorted) porphyrins we re examined by laser excitation-time-resolved EPR spectroscopy. The compoun ds examined were 2,3,7,8,12,13,17,18-octaethyl-5,10, 15,20-tetraphanylporph yrin (H2OETPP); 2,3-diethyl-5,10, 15,20-tetraphenylporphyrin (H2DETPP); 2,3 ,12, 13-tetraethyl-,lo,10,20-tetraphenylporphyrin(H(2)tTETPP)and 2,3,7,8-te traethyl-5,10,15,20-tetraphenyporphyrin (H(2)cTETPP). Measurements were car ried out at low temperatures in glassy isotropic matrixes and over a wide t emperature range in a uniaxial LC (liquid crystal, E-7). The triplet states of H(2)tTETPP, H(2)cTETPP, and H2OETPP are characterized by relatively sma ll zero-field splitting (ZFS) values, D, compared to free base porphyrin (H 2TPP), For the H2OETPP the relation D similar to 3E between the ZFS paramet ers was observed. In the case of H2DETPP the ZFS parameter D was found to b e larger relative to H2TPP. Line shape analyses indicate that the tripler s pectra depend on the specific chromophore, the temperature, and the LC phas e. The results in LC suggest that intermolecular triplet energy transfer oc curs in all four chromophores, and in the case of H2OETPP there is evidence for the simultaneous presence of two types of triplets (different saddle c onformers of H2OETPP), which are identical in their ZFS parameters. To chec k on the sign of D and triplet spin alignment, the photoexcited tripler sta tes in isotropic solvents were investigated by fast EPR-magnetophotoselecti on through the determination of the position of the optical transition mome nt. These experiments allow determining the location of the optical transit ion moment and the triplet spin alignment.