A. Harriman et al., INTRAMOLECULAR ELECTRON AND ENERGY-TRANSFER WITHIN A BISPORPHYRIN IN A LOW-TEMPERATURE GLASS, Journal of physical chemistry, 98(19), 1994, pp. 4982-4989
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.