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

The photoexcited triplet states of four nonplanar (distorted) porphyrins were examined by laser excitation-time-resolved EPR spectroscopy. The compounds examined were 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin (H₂OETPP); 2,3-diethyl-5,10,15,20-tetraphenylporphyrin (H₂DETPP); 2,3,12,13-tetraethyl-5,10,15,20-tetraphenylporphyrin (H₂tTETPP); and 2,3,7,8-tetraethyl-5,10,15,20-tetraphenylporphyrin (H₂cTETPP). Measurements were carried out at low temperatures in glassy isotropic matrixes and over a wide temperature range in a uniaxial LC (liquid crystal, E-7). The triplet states of H₂tTETPP, H2_cTETPP, and H₂OETPP are characterized by relatively small zero-field splitting (ZFS) values, D, compared to free base porphyrin (H₂-TPP). For the H₂OETPP the relation D ∼ 3E between the ZFS parameters was observed. In the case of H₂DETPP the ZFS parameter D was found to be larger relative to H₂TPP. Line shape analyses indicate that the triplet spectra depend on the specific chromophore, the temperature, and the LC phase. The results in LC suggest that intermolecular triplet energy transfer occurs in all four chromophores, and in the case of H₂OETPP there is evidence for the simultaneous presence of two types of triplets (different saddle conformers of H₂OETPP), which are identical in their ZFS parameters. To check on the sign of D and triplet spin alignment, the photoexcited triplet states in isotropic solvents were investigated by fast EPR-magnetophotoselection through the determination of the position of the optical transition moment. These experiments allow determining the location of the optical transition moment and the triplet spin alignment.