4’-Nitroflavonol fluorescence: Excited state intramolecular proton transfer reaction from the non-emissive excited state

Abstract

2-(4′-nitrophenyl)-3-hydroxy-chromone (4′-nitroflavonol, NO3 H F), which can be considered as non-typical representative of fluorescent dyes of flavonol series, was synthesized. Its spectral and photophysical properties were studied experimentally and modeled with several relevant quantum-chemical approaches. Single banded, long-wavelength highly Stokes shifted fluorescence phototautomer emission was observed for the title compound in solvents of different polarity. Absence of its short-wavelength “normal” emission band was explained by the effect of ultrafast intersystem crossing (ISC) with participation of triplet nπ* levels of 4′-nitro group. Such behavior, absence of fluorescence emission below ∼500 nm is generally expected for nitro-aromatics. Thus, a rare example of excited state intramolecular proton transfer reaction (ESIPT) originated from the non-emissive “dark” excited state at conditions of its concurrence with ultrafast ISC should be considered for the title compound. Excited state proton transfer reaction plays in this case the role of an emission regulating factor: the higher is its rate, the more intensive phototautomer fluorescence is observed. Relatively long fluorescence lifetimes of 4′-nitroflavonol in solvents of intermediate polarity (∼ 4 ns) were attributed to low efficiency of radiationless processes in its phototautomer form.