N-ethyl substituted 2-benzimidazolyl-3-hydroxychromone: Atypical to highly fluorescent dyes of flavonol series excited state intramolecular proton transfer to nitrogen

New derivative of 3-hydroxychromone possessing N-ethyl substituted benzimidazole moiety was specially synthesized with the aim to switch the excited state intramolecular proton transfer (ESIPT) to heterocyclic nitrogen atom, this ESIPT direction is generally less typical to fluorescent compounds of flavonol series. Bulky ethyl group makes conformation with intramolecular hydrogen bond to carbonyl group oxygen atom unfavorable in the ground state. We expected the same situation remaining in the excited state. Photophysics of the title compound was studied both experimentally and theoretically. Unlike weakly fluorescent 3-hydroxychromones with 2′-pyridyl or 2′-quinolyl heterocycles in position 2, the title compound demonstrates unexpectedly high fluorescence quantum yields in solvents of low-to-intermediate polarity. We consider the higher rate of intramolecular proton transfer reaction, in comparison with concurring radiationless intersystem crossing (ISC) process with participation of singlet and triplet nπ* levels of carbonyl group, as the main reason of bright fluorescence of title compound. [Display omitted] •Only ESIPT to nitrogen is typical to specially synthesized (N–Et)Bi–3HC.•Unusually high fluorescence quantum yields as for “ESIPT to nitrogen” were observed.•Reason: ESIPT in (N–Et)Bi–3HC is faster than ISC of Sππ*-Tnπ* type.