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

Abstract

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.

Multi‐Photon Excitation in Photoredox Catalysis: Concepts, Applications, Methods

“…A very recent picosecond‐transient‐absorption study provides direct evidence for the bimolecular electron transfer between photoexcited PDI .− and a range of substrates with reduction potentials more positive than −1.7 V vs. SCE . However, for substrates with more negative potentials (and this is likely the case for several of the initially investigated aryl chlorides), a different mechanism, possibly a thermally activated pathway, seems to be operative …”

Use of Perylene Diimides in Synthetic Photochemistry

“…Conversely, Zhang, Schanze, and co-workers have recently reported the use of picosecond time-resolved transient absorption spectroscopy to monitor the formation of A * À * and its bimolecular quenching by aryl halides 1 through Stern-Volmer studies. [33] The authors found that A * À * underwent efficient PET with electron-poor halides 1 having reduction potential > À 1.7 V vs SCE. Besides, they calculated the potential for the couple A * À */A as À 1.87 V vs SCE.…”

Multiphotonen‐Anregung in der Photoredoxkatalyse: Konzepte, Anwendungen und Methoden

“…Eine aktuelle Studie zur transienten Pikosekunden‐Absorption lieferte direkte Belege für bimolekulare Elektronentransferreaktionen zwischen angeregtem PDI .− und einer Auswahl an Substraten mit Reduktionspotentialen, die positiver als −1.7 V gegen SCE sind . Für Substrate mit negativeren Potentialen (und das ist wahrscheinlich der Fall für einige der anfänglich untersuchten Arylchloride) scheint hingegen ein anderer Mechanismus abzulaufen, möglicherweise ein thermisch aktivierter Reaktionsweg …”