Competition between charge transfer via superexchange and thermally activated energy transfer in porphyrinheterodimerquinone systems

Electron transfer is observed in semi-rigid heterodimers of free base and zinc porphyrin covalently linked to the electron acceptor p-benzoquinone (Q). The rate of transfer critically depends on the relative spatial arrangement of the zinc and free-base porphyrin unit with respect to Q and varies up to two orders of magnitude in time. Complexation of the zinc porphyrin subunit with pyridine influences the energetics and transfer dynamics of the system. Excitation energy transport among the porphyrin subunits controls the efficiency of electron transfer to a large extent. Temperature-dependent measurements reveal the superposition of two processes, one of which is interpreted as superexchange mediated electron transfer. Electronic coupling elements for such a process are estimated from the analysis of the temperature dependence of electron transfer rates.