Excited state intramolecular proton transfer reaction in 4′,5-dimethoxy-3-hydroxyflavone: partial shielding of intermolecular H-bonding with protic molecules by 5-OCH3 group and its impact on ESIPT rate and fluorescence spectra

Continuing our investigations of intra- and intermolecular factors regulating the fastest known photochemical reaction, the excited state intramolecular proton transfer (ESIPT), on the example of 3-hydroxyflavone (3HF) series, methoxy group was introduced in position 5 of 3HF core with the aim to check its potential shielding effect for carbonyl group in position 4 from intermolecular H-bonding with protic molecules. Such an intermolecular hydrogen bond was considered as the main retarding factor for the ESIPT reaction of 3-hydroxyflavones. The effect of such substitution was studied both theoretically and experimentally for 4′,5-dimethoxy-3-hydroxyflavone (45DM3HF): fluorescence spectra and lifetimes allow us to estimate ESIPT rates in a series of protic solvents and compare them with the data of model compound without substituents in position 5. Partial inhibition of intermolecular H-bonding was revealed both theoretically and experimentally, owing to which ESIPT reaction in the title compound demonstrates less pronounced sensitivity to proton donors and/or humidity in aprotic surroundings. The additional effect of 5-methoxy group is the absence of typical to most 3HFs ground state anionic species formation, which were not registered both in the absorption and fluorescence spectra of 45DM3HF.