Solvent effect in randomly and partially oriented triplets of the sapphyrin dication. Optical and fast EPR-magnetophotoselection measurements

The photoexcited triplet state of the sapphyrin dication, H₅·SaP²⁺, was investigated by optical absorption and fast EPR-magnetophotoselection (MPS) spectroscopies. Results indicate that H₅·SaP²⁺ exists as a monomer in nonpolar isotropic solvents (chloroform and toluene) and nematic liquid crystals, while in a polar solvent, e.g., ethanol, the stable form exists as a face-to-face dimer via specific solvent-mediated interactions. Analysis of the MPS-triplet EPR and optical absorption results provides the following conclusions: (i) relation between the molecular and magnetic frames of references; (ii) out-of-plane location of the optical transition moment for both the monomer, (H₅·Sap²⁺, and the dimer, (H₅·Sap²⁺)₂, suggesting a possible admixture of (n,π*) and (π,π*) states; (iii) in-plane triplet spin alignment and planar molecular structure of the monomer; (iv) a negative value of the zero-field splitting parameter, D, for the monomer and the dimer.