Photo-induced Proton-Coupled Electron Transfer Reactions of Acridine Orange: Comprehensive Spectral and Kinetics Analysis

The triplet excited state of acridine orange ((3)*AO) undergoes a proton-coupled electron transfer (PCET) reaction with tri-tert-butylphenol ((ttb)PhOH) in acetonitrile. Each of the reaction components possesses a spectroscopic signature, providing a rare opportunity to monitor the individual proton transfer, electron transfer, and H(•)-transfer components in parallel via transient absorption spectroscopy. This enhanced optical tracking, along with excited-state thermochemical analysis, facilitates assignment of the mechanism of excited-state PCET reactivity. (3)*AO is quenched via concerted proton-electron transfer (CPET) from (ttb)PhOH to form acridine radical (AOH(•)) and (ttb)PhO(•) (kCPET = 3.7 × 10(8) M(-1) s(-1), KIE = 1.3). Subsequently, AOH(•) reduces the phenoxyl radical (kET = 5.5 × 10(9) M(-1) s(-1)), forming AOH(+) and (ttb)PhO(-), followed by proton transfer (kPT = 1.0 × 10(9) M(-1) s(-1)) to regenerate the starting reactants.