Excited state dynamics of novel diporphyrins with dissimilar cores

Abstract

The singlet and triplet excited state properties of two diporphyrins, H₂-H and Zn-H, are described. Steady state fluorescence studies indicate that the emission of the H₂-H diporphyrin is dependent on the excitation wavelength and is dominated by the emission of the individual constituent monomers at their respective excitation. Time-resolved studies show two lifetimes ascribable to the normal and thiaporphyrin subunits. However, the emission of the Zn-H diporphyrin is dominated by the thiaporphyrin subunit irrespective of the wavelength of excitation, suggesting an energy transfer from the Zn porphyrin subunit to the thiaporphyrin subunit. Lifetime measurements in toluene show two lifetimes due to open and folded conformations, while in DMF an additional component due to axial ligation is observed. The efficiency of energy transfer is moderately higher in DMF (72%) than in toluene (68%). Triplet ESR studies on the H₂-H dimer reveal a localized triplet with ZFS parameters and ESP pattern the same as for the individual monomers. On the other hand, triplet ESR of the Zn-H diporphyrin reveals triplet characteristics of the thiaporphyrin subunit, indicating an energy transfer in the triplet state.