Acceleration of non-radiative relaxation of 7-azaindole by polyfluorophenols in 1 : 1 complexes: evidence of coupled proton-electron transfer effects.

Binary complexes of 7-azaindole (7AI) with various proton donor-acceptor species have long been used as prototypical systems to investigate double-proton exchange tautomerization as the key photochemical pathway of these complexes. Here, we demonstrate the occurrence of an alternative and faster electronic relaxation pathway of different degrees and suppression of double-proton exchange tautomerization in a series of binary complexes of 7AI with polyfluorophenols (FnPhOHs, where n = 0-5). The efficiency of the alternative excited-state relaxation channel is correlated with the quenching efficiency of the UV fluorescence from the locally excited state of 7AI. The quenching efficiency is found to be dependent on the number of fluorine substitutions on the phenolic sites, and the Stern-Volmer quenching constant (KSV) values increase by more than an order of magnitude from phenol (PhOH) to pentafluorophenol (F5PhOH). The quenching efficiency displays a linear correlation with the binding energies of the binary complexes between 7AI and FnPhOHs in the ground state. Furthermore, quenching is found to be greatly enhanced with the lowering of the temperature of the medium, which promotes complex formation, implying that quenching is static in nature. Given that the acidity of fluorophenols increases with the number of fluorine substitutions, the origin of the alternative electronic relaxation of 7AI is associated with a proton-coupled electron transfer (PCET) process. Electronic structure theory calculations indeed show the enhanced feasibility of PCET in the binary complexes of 7AI from PhOH to F5PhOH.