Temperature Dependence of Photoinduced Electron Transfer within Self-Assembled Uroporphyrin−Cytochrome c Complexes

In this report the temperature dependence of photoinduced electron transfer between both singlet- and triplet-state uroporphyrin ( 1 URO and 3 URO, respectively) and the heme group of ferricytochrome c (Cc) has been examined. Analysis of the forward electron-transfer rate originating from 1 URO as a function of temperature gives a reorganizational energy (λ) of 1.0 ± 0.02 eV with an associated electronic coupling term ( H AB ) of (3.7 ± 0.3) × 10 -4 eV. In addition, the singlet lifetime data exhibit a temperature-dependent Gaussian distribution that narrows as the temperature increases. This suggests a conformational distribution of bound porphyrin with a narrow range of potential wells at lower temperatures. The corresponding temperature dependence of the electron transfer between 3 URO/URO + and the heme group of Cc gives a λ = 0.49 ± 0.05 eV/0.19 ± 0.03 eV and H AB ∼ 4 × 10 -4 eV/1.4 × 10 -4 eV, respectively. These observations are consistent with orientational dependent conformational states as well as possible gating of electron transfer within the complex.