Intramolecular energy transfer dynamics in differently linked zinc porphyrin–dithiaporphyrin dyads

Intramolecular energy transfer dynamics in two molecular dyads, in which zinc porphyrin (ZnN4) and dithiaporphyrin (N2S2) units were linked covalently by different bridges, namely phenylene (ph) and diphenylethyne (dpe), were studied employing ultrafast time-resolved transient absorption and fluorescence spectroscopic techniques. The rates of energy transfer in both these dyads are slower than in the corresponding ZnN4-N-4 dyads, in spite of the better gradient for energy flow in the case of the ZnN4-N2S2 dyads. Quantum chemical calculations reveal that the frontier orbital characteristics of the porphyrins are not significantly altered by sulphur substitution at the acceptor porphyrin core, and thus this does not modify the electronic factor in the energy transfer mechanism. However, a significant decrease in overlap between the absorption spectrum of the donor and the emission spectrum of the acceptor results in lower efficiency of the intramolecular energy transfer. The energy transfer process in dpe-linked dyads follows a through-bond super-exchange mechanism, whereas, in ph-linked dyads, the through-space multipole resonance interaction plays an important role.