Long-Lived Photoinduced Charge Separation in Flexible 9,10-Bis(phenylethynyl)anthracene-Phenothiazine Dyads

Photoinduced electron transfer in linked donor-acceptor dyads with bis(phenylethynyl)anthracene as light absorber and acceptor and one or two phenothiazine units as donors was investigated. Absorption spectra of the dyads can be obtained by adding contributions due to the donor and acceptor moieties indicating that the constituents do not interact in the ground state. Fluorescence of bis(phenylethynyl)anthraccne core was efficiently quenched by the donor moieties, and this was attributed to electron transfer from the phenothiazine to bis(phenylethynyl)anthracene. Ferntosecond transient absorption studies suggested formation of a charge separated state directly from the singlet excited-state of bis(phenylethynyl)anthracene. Nanosecond flash photolysis experiments gave long-lived transient absorptions assignable to the phenothiazine radical cation and bis(phenylethynyl)anthracene radical anion. These assignments were confirmed by oxygen quenching studies and secondary electron transfer experiments. Electron transfer in these systems is analyzed in the light of Marcus theory, and the slow back electron transfer exhibited is attributed to inverted region effects.