INTRAMOLECULAR ELECTRON AND ENERGY-TRANSFER WITHIN A BISPORPHYRIN IN A LOW-TEMPERATURE GLASS

The photophysical properties of an oblique bisporphyrin, comprising zi nc(II) and gold(III) porphyrins separated by a 2,7-diphenyl-1,10-phena nthroline spacer moiety, have been measured in a low-temperature ethan ol glass. Comparison with the properties recorded earlier in fluid, po lar solution indicates that the energy of the intermediate charge-tran sfer state (CTS), formed by electron transfer from zinc porphyrin to a ppended gold porphyrin, is increased by 0.75 eV upon freezing. This is due to a substantial decrease in the static dielectric constant of th e solvent which occurs upon moving from a fluid solution to a frozen g lass. There is a corresponding decrease in the solvent reorganization energy upon freezing, and, at 77 K, both the reorganization energy and the reaction exergonicity are essentially zero. The rate of formation of the CTS at 77 K is much slower than that predicted for a superexch ange mechanism unless there is a change in the molecular architecture upon freezing. Formation of an intermediate CTS has been confirmed by low-temperature EPR studies. The gold porphyrin excited triplet state is unable to abstract an electron from the appended zinc porphyrin, an d, instead, quantitative intramolecular triplet energy transfer takes place.