A photobasic D-A fluorophore with a high intramolecular charge-transfer as a model strategy for designing organic proton-coupled electron-transfer modulators: An analysis based on steady-state fluorescence, isotopic effect and theoretical study

Abstract

Modulation of the photoinduced proton-coupled electron transfer (PCET) mechanism is a topic of great interest because of its pivotal role in energy-transfer processes, the development of fuel cells and activation of small molecules. Herein we show that the N1-(4-chlorophenyl)-2-(trifluoromethyl)benzo[b][1,8]naphthyridin-4(1H)-one fluorophore is a convenient platform to promote selectively diverse PCET mechanisms upon phenol binding. The key points for the PCET-probe are: (i) the inclusion of an internal basic moiety into the donor (D)-acceptor (A) chain and (ii) the occurrence of intramolecular charge transfer (ICT) upon excitation. Weak fluorescence quenching and detrimental KIEs were observed when a low-CT fluorophoric analogue was used as a PCET probe. The diverse PCET mechanisms upon phenol binding were recognized from a steady-state spectroscopic study, UV-vis spectroscopy, isotopic effect experiments and theoretical calculations. A combined photoinduced electron transfer and PT-ET was identified for nitrophenols, whereas carbonylphenols promoted an apparent combined PT-ET/CPET mechanism with a quenching response dependent on the carbonyl position in the aryl ring. Alkyl-, methoxyphenols, phenol and some aminophenols involved combined CPET and ET-PT mechanisms and halophenols a CPET mechanism, whereas strongly electron-donating hydroxyl- or N-acetamidophenols and 4-aminophenol involved a weak PCET mechanism. The present investigation opens a new perspective for the rational design of selective and effective photobasic organic modulators of PCET mechanisms based on the same principles as typical metallic complexes: (i) a weak base as a hydrogen catcher and (ii) an internal ICT to enhance the basicity of the PCET modulator.