Excited-state antiaromaticity relief drives facile photoprotonation of carbons in aminobiphenyls.

A combined computational and experimental study reveals that -, - and -aminobiphenyl isomers undergo distinctly different photochemical reactions involving proton transfer. Deuterium exchange experiments show that the -isomer undergoes a facile photoprotonation at a carbon atom excited-state intramolecular proton transfer (ESIPT). The -isomer undergoes water-assisted excited-state proton transfer (ESPT) and a photoredox reaction proton-coupled electron transfer (PCET). The -isomer undergoes a water-assisted ESPT reaction. All three reactions take place in the singlet excited-state, except for the photoredox process of the -isomer, which involves a triplet excited-state. Computations illustrate the important role of excited-state antiaromaticity relief in these photoreactions.