Diffusive and non-diffusive photo-induced proton coupled electron transfer from hydrogen bonded phenols to meso-tetrakis-5,10,15,20-pentafluorophenyl porphyrin

Abstract

Enhanced reductive fluorescence quenching of meso-tetrakis-5,10,15,20-pentafluorophenyl porphyrin (H2F20TPP) by two different phenols, 4-methoxy phenol (4-MeOPhOH) and 2,6-dimethoxy phenol (2,6-DiMeOPhOH) in the presence of various pyridine bases in dichloromethane solution is studied using steady state and time resolved fluorescence spectroscopic methods by employing time correlated single photon counting (TCSPC) and fluorescence up-conversion techniques. An enhanced quenching behaviour of H2F20TPP is observed when phenols are hydrogen bonded to various pyridine bases. Quenching observed in the steady state and time resolved studies in the nanosecond time domain follows second order kinetics and generates quenching rate constants and hydrogen bond equilibrium constants, the latter of which agree quite closely with those obtained from independent spectroscopic measurements. A significant kinetic deuterium isotope effect is observed, indicating the importance of proton movement in the quenching processes. This quenching effect is attributed to be due to a tri-molecular transition state involving H2F20TPP and a hydrogen bonded phenol complex, in which electron transfer from phenol to excited H2F20TPP is concerted with proton movement from the phenol to hydrogen bonded base. Observed quenching behaviours are rationalized by invoking diffusion controlled proton coupled electron transfer. Fluorescence up-conversion studies in the 100 ps time domain confirm ultrafast PCET for 4-MeOPhOH and base pairs which fall in a non-diffusive regime.

Water Molecules Gating a Photoinduced One‐Electron Two‐Protons Transfer in a Tyrosine/Histidine (Tyr/His) Model of Photosystem II

“…(4)] that is removed away to the porphyrin coordinating cavity through the intermediacyofwater molecules [Eq. (5)]. Put differently, ET [Eq.…”

“…[4] Models of Ty r Z /His 190 have been reported by Hammarstrçm and others and av ariety of PCET schemes have been unraveled. [5] In these systems,one-electron one-proton transfers were mainly under focus,w hereas one-electron multiproton transfers have only received little attention. SavØant and co-workers reported ao ne-electron two-proton coupled transfer (E2PT) in as ynthetic model featuring aH -bonded alcohol as ap roton relay from the oxidized phenol to ap ending base.…”

Deciphering the incognito role of water in a light driven proton coupled electron transfer process

“… 9 – 11 , 19 Meanwhile, reports on pure organic compounds which resemble the PCET process in photosystem II are very sparse and in all cited works intermolecular fluorescence quenching dynamics exhibited by hydrogen bonded phenol base pairs in non-aqueous media were routinely studied. 43 – 46 Organic compounds exhibiting intramolecular charge transfer (ESICT) coupled with proton transfer (ESIPT) in the excited state were designed and their excited state dynamics in an ultrafast timescale were unravelled. These systems possess strong ground state interactions between the electron donor and acceptor (π-conjugation) and also between the proton donor and acceptor (short and strong hydrogen bond).…”

Excited-State Proton-Coupled Electron Transfer: Different Avenues for Promoting Proton/Electron Movement with Solar Photons

“…Free base porphyrins have also been found to function as ES e – acceptors in PCET reactions involving phenol-benzimidazole dyads. − For example, a porphyrin covalently bound to phenol-benzimidazole ( 22-S ) has been studied as a unimolecular mimic of the chlorophyll-TyrZ-His190 complex in photosystem II . Emission quenching studies revealed that the singlet porphyrin excited state acts as an ES e – acceptor, triggering the intramolecular phenolic PT to benzimidazole.…”

“…Emission quenching studies revealed that the singlet porphyrin excited state acts as an ES e – acceptor, triggering the intramolecular phenolic PT to benzimidazole. Relatedly, Bangal and co-workers have shown nondiffusive ultrafast PCET coupled with fluorescence up-conversion via time-correlated single photon counting, in which 4-methoxyphenol is oxidized by the excited singlet state of meso -tetrakis-5,10,15,20-pentafluorophenyl porphyrin ( 23 ) with concerted motion of bound protons to associated pyridine on the scale of tens of picoseconds …”