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 provide 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.