Direct Observation of the Reduction of Aryl Halides by a Photoexcited Perylene Diimide Radical Anion

The ability of the doublet excited state of perylene diimide anion radical ²(PDI^-•)* to reduce aromatic electron acceptors was probed by picosecond time-resolved transient absorption (TA) spectroscopy. Excitation of PDI^-• produces visible TA due to ²(PDI^-•)* that decays with τ = 160 ps. Aromatic electron acceptors with varying reduction potential quench ²(PDI^-•)* and, in some cases, give a new visible region absorption that is attributed to the products of bimolecular photoinduced electron transfer, ²(PDI^-•)* + Ar-X → PDI + Ar-X^-•. Stern-Volmer quenching of ²(PDI^-•)* accomplished with a series of acceptors provides bimolecular quenching rate constants as a function of acceptor reduction potential. Rehm-Weller analysis of the electron transfer quenching data affords the potential for the (*PDI^-•/PDI) electrochemical half-reaction as -1.87 V vs SCE.