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

Abstract

Light induced multisite electron proton transfer in two different phenols (simple and phenol carrying intramolecularly hydrogen bonded base) pendant on the acridinedione dye (ADD), an NADH analogue was studied by following the fluorescence quenching dynamics in ultrafast timescale. In simple phenol derivative (ADDOH), photo-excited acridinedione acquires electron from phenol intramolecularly, coupled with the transfer of proton to the solvent water. But in phenol carrying hydrogen bonded base (ADDDP), both electron and proton transfer occurs completely intramolecular. The sequence of this electron and proton transfer process was validated by discerning the pH dependency of the reaction kinetics. Since photo-excited ADDs are stronger oxidants, the sequential electron first proton transfer mechanism (ETPT) was intimidated in ADDOH and hence there is no change in the PCET reaction kinetics kETPT ∼ 6.57 x 109 s-1 in the entire pH range (pH 2–12). But phenol carrying hydrogen bonded base (ADDDP) unleashes concerted electron proton transfer where the PCET reaction rate decreases on decreasing the pH below its pKa. Noticeably, the concerted EPT process in ADDDP mimics the donor side of photosystem II and it occurs by two distinct pathways, (i) through the direct intramolecular hydrogen bonding between the phenol and amine, kDEPT ∼ 12.5 x 1010 s-1 and (ii) through the bidirectional hydrogen bond extended by the water molecule trapped in between the proton donor and acceptor, which mediates the proton transfer and serves as a proton wire, kWMEPT ∼ 2.85 x 1010 s-1. These results unravel the incognito role played by the water in mediating the proton transfer process when the structural elements does not favor the direct hydrogen bonding between the proton donor and acceptor in a concerted PCET reactions.

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Publication details

The article was received on 19 Jul 2017, accepted on 10 Nov 2017 and first published on 10 Nov 2017


Article type: Edge Article
DOI: 10.1039/C7SC03161K
Citation: Chem. Sci., 2017, Accepted Manuscript
  • Open access: Creative Commons BY-NC license
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    Deciphering the incognito role of water in a light driven proton coupled electron transfer process.

    S. K. Thiyagarajan, S. Raghpathy, R. Vadivel and P. Ramamurthy, Chem. Sci., 2017, Accepted Manuscript , DOI: 10.1039/C7SC03161K

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