Excited state dynamics of novel diporphyrins with dissimilar cores

The singlet and tripler excited state properties of two diporphyrins, H-2-H and Zn-H, are described. Steady state fluorescence studies indicate that t he emission of the H2-H diporphyrin is dependent on the excitation waveleng th and is dominated by the emission of the individual constituent monomers at their respective excitation. Time-resolved studies show two Lifetimes as cribable to the normal and thiaporphyrin subunits. However, the emission of the Zn-H diporphyrin is dominated by the thiaporphyrin subunit irrespectiv e of the wavelength of excitation, suggesting an energy transfer from the Z n porphyrin subunit to the thiaporphyrin subunit. Lifetime measurements in toluene show two lifetimes due to open and folded conformations, while in D MF an additional component due to axial ligation is observed. The efficienc y 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 ZF S parameters and ESP pattern the same as for the individual monomers. On th e other hand, tripler ESR of the Zn-H diporphyrin reveals triplet character istics of the thiaporphyrin subunit, indicating an energy transfer in the t riplet state. (C) 1999 John Wiley & Sons, Ltd.