Electron Transfer Quenching of Rhodamine 6G by N-Methylpyrrole Is an Unproductive Process in the Photocatalytic Heterobiaryl Cross-Coupling Reaction

In the heterobiaryl cross-coupling reaction between aryl halides (Ar-X) and N-methylpyrrole (N-MP) catalyzed by rhodamine 6G (Rh6G(+)) under irradiation with visible light, a highly active and long-lived (millisecond time range) rhodamine 6G radical (Rh6G(center dot)) is formed upon electron transfer from N,N- diisopropylethylamine (DIPEA) to Rh6G(+). In this study, we utilized steady-state and time-resolved spectroscopy techniques to demonstrate the existence of another electron-transfer process occurring from the relatively electron-rich N-MP to photoexcited Rh6G(+) that was neglected in the previous reports. In this case, the radical Rh6G(center dot )formed is short-lived and undergoes rapid recombination (nanosecond time-range), rendering it ineffective in reducing Ar-X to aryl radicals Ar-center dot that can subsequently be trapped by N-MP. This is further demonstrated via two model reactions involving 4'-bromoacetophenone and 1,3,5-tribromobenzene with insignificant product yields after visible-light irradiation in the absence of DIPEA- The unproductive quenching of photoexcited Rh6G(-) by N-MP leads to a lower concentration of photocatalyst available for competitive charge transfer with DIPEA and hence decreases the efficiency of the cross-coupling reaction.

Automated summary

In this study, the existence of an overlooked electron transfer process from N-MP to photoexcited Rh6G(+) was demonstrated using steady-state and time-resolved spectroscopy techniques. This process led to the formation of short-lived radicals that underwent rapid recombination, inhibiting the reduction of Ar-X to aryl radicals and thereby affecting the efficiency of the cross-coupling reaction.