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.