Proton-Coupled Electron Transfer Reactions with Photometric Bases Reveal Free Energy Relationships for Proton Transfer

• Published in: The journal of physical chemistry. B 2016-08, Vol.120 (32), p.7896-7905
• Main Authors: Eisenhart, Thomas T, Howland, William C, Dempsey, Jillian L
• Format: Article
• Language: English
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/acs.jpcb.6b04011

The proton-coupled electron transfer (PCET) oxidation of p-aminophenol in acetonitrile was initiated via stopped-flow rapid-mixing and spectroscopically monitored. For oxidation by ferrocenium in the presence of 7-(dimethylamino)­quinoline proton acceptors, both the electron transfer and proton transfer components could be optically monitored in the visible region; the decay of the ferrocenium absorbance is readily monitored (λmax = 620 nm), and the absorbance of the 2,4-substituted 7-(dimethylamino)­quinoline derivatives (λmax = 370–392 nm) red-shifts substantially (ca. 70 nm) upon protonation. Spectral analysis revealed the reaction proceeds via a stepwise electron transfer-proton transfer process, and modeling of the kinetics traces monitoring the ferrocenium and quinolinium signals provided rate constants for elementary proton and electron transfer steps. As the pK a values of the conjugate acids of the 2,4-R-7-(dimethylamino)­quinoline derivatives employed were readily tuned by varying the substituents at the 2- and 4-positions of the quinoline backbone, the driving force for proton transfer was systematically varied. Proton transfer rate constants (k PT,2 = (1.5–7.5) × 108 M–1 s–1, k PT,4 = (0.55–3.0) × 107 M–1 s–1) were found to correlate with the pK a of the conjugate acid of the proton acceptor, in agreement with anticipated free energy relationships for proton transfer processes in PCET reactions.