Dynamics of photoinduced electron transfer in an amphiphilic A²⁺-S-D triad molecule

An amphiphilic A²⁺-S-D triad molecule and its reference compounds, S-D, A²⁺-S, and S type molecules, were synthesized and studied using time-resolved transient absorption spectroscopy. The three moieties in the triad, i.e., an electron acceptor moiety (A²⁺, viologen), a sensitizer moiety (S, perylene), and an electron donor moiety (D, ferrocene), were linearly combined by sigma-bonded tunneling bridges. Intramolecular electron transfer reactions were initiated by photoexcitation of the S moiety to ¹S*, and the long-lived final charge separated state, A^.+-S-D^.+, was formed. Although the yield of the initial charge separated state, A^.+-S^.+-D, was very high (0.93), the overall yield of the final charge separated state was ca. 0.2. The rate of the backward electron transfer from A^.+ to S^.+ was observed to be two times lower than that of the forward electron transfer from ¹S* to A²⁺, suggesting that with suitable molecular engineering, the yield of long-lived charge separation in such triads could be improved.