3′,4′-methylenedioxy-3-hydroxyflavone: switchover from reversible to irreversible ESIPT along the n-alcohol series

•ESIPT rate of 3′,4′-methylenedioxy-3-hydroxyflavone (medo3HF) in n-alcohols decreases with increasing hydrocarbon chain-length.•ESIPT time-constants follow the same trend as solvation dynamics.•Solvation dynamics rate also plays a critical role deciding the solvent configuration at which ESIPT takes place.•Hence, ESIPT switches from reversible-type in low n-alcohols to irreversible-type in higher n-alcohols

Introduction of the 3′,4′-methylenedioxy substituent in 3-hydroxyflavone (3HF) strongly inhibits the rate of Excited State Intramolecular Proton Transfer (ESIPT), especially in polar protic solvents like n-alcohols, much in the same manner as the introduction of the well-known electron donating 4′-N,N-dimethylamino substituent. Thus, ESIPT time-constants of 50 ps to ∼100 ps are found, which increase along the n-alcohol series from methanol to 1-decanol, following the same trend as solvation dynamics time-constants. Due to the higher dipole moment of the excited state enol E*, solvation preferentially stabilizes it over the tautomer T* in the excited state surface of the flavonol. Hence, the relative energies of the two species change with progress of solvation. In solvents with slow solvation dynamics like 1-decanol, much of the ESIPT occurs within a solvent configuration where the E* is energetically higher than T*, promoting irreversible ESIPT. But in solvents with fast solvation dynamics like methanol, a configuration where E* and T* are energetically similar is rapidly attained, conducive to a reversible ESIPT pathway.

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