Transient absorption measurements of acceptor–sensitizer–donor triad molecules in solutions and Langmuir–Blodgett films

Transient absorption measurements of an acceptor–sensitizer–donor (A–S–D) triad molecule (ASD) that contained viologen, pyrene, and ferrocene as the A, S, and D moieties, respectively, were made in benzonitrile and aqueous micellar solutions, and in a Langmuir–Blodgett (LB) monolayer assembly. Intramolecular electron transfer reactions were initiated by photo-excitation of the S moiety, which gave rise to the long-lived final charge-separated A−–S–D+ state. The rate of the backward electron transfer from A− to D+ was determined from the absorption decay of the viologen cation radical monitored at 600–700 nm. The estimated lifetime of the final charge-separated state was ca. 60 ns, based on measurements of the solutions. Extremely slow decay (much longer than 600 μs) of the transient absorption of the viologen cation radical was observed for a mixed ASD LB film with 22-tricosenoic acid (TA). This slow decay observed on a LB film at higher ASD concentrations may be explained by lateral charge migration between adjacent molecules in the monolayer plane.