THE PHOTOEXCITED TRIPLET-STATE OF SAPPHYRIN DICATION - UNUSUAL SPIN ALIGNMENT IN MONOMERS AND SPIN DELOCALIZATION IN DIMERS

Sapphyrin (Sap) and its stable dicationic form, Sap2+, originally prepared by Woodward and Johnson, are large porphyrin-like systems which exhibit unique chemical and photochemical properties which are directly related to the primary donor ("special pair") in primary photosynthesis. We report on the photoexcited triplet detection by time-resolved cw-EPR, of the dication dissolved in toluene, a nematic liquid crystal, and ethanol. In the former two solvents of low dielectric constants, the Sap2+ exists as a monomer, whereas in ethanol the stable form is the dimer, (Sap2+)2. The triplet state of the dimer exhibits a substantial reduction of both zero-field splitting (ZFS) parameters (D and E) as compared to the monomer. This observation is interpreted in terms of triplet spin delocalization (charge-transfer character) within the dimer, which may be considered as a supermolecule. Moreover, the triplet-EPR line shape of (Sap2+)T oriented in the liquid crystal indicates that, unlike other known porphyrinoid systems, the ZFS term, D, is associated with the in-plane, head-to-tail, alignment of the triplet electron spins. These unusual triplet properties suggest a negative value of the ZFS parameter D for the monomer and dimer. © 1990.