Theoretical investigation on excited state intramolecular proton transfer of 5-hydroxy-6,7,4'-trimethoxyflavone: state specific solvation approach versus linear response solvation

Flavonoids are polyphenolic organic compounds with wide range of physical and biological importance. In this article, we aimed to perform detailed structural analysis to predict photophysics of 5-hydroxy-6,7,4'-trimethoxyflavone (5HTMF) in vacuum and different solvents. The ground state optimised structure (S0) computed at DFT/CAM-B3LYP/6-31G(d,p) level, accurately correlates with experimental structure. Excited state (S1) was more polar than ground state (8.20–11.16 Debye in vacuum; 9.68–12.72 Debye in cyclohexane). The computed potential energy curves along proton transfer reaction coordinate showed inoperativeness of ground state intramolecular proton transfer. In sharp contrast, barrierless potential of first excited state confirms occurrence of excited state intramolecular proton transfer phenomenon. The absorption wavelength for S0 → S1 (Frank Condon) transition was within 304–310 nm, in different solvents. The polarity dependency of fluorescence emission for S0 ← S1 transition was negligible as compared to absorption, and significant Stoke's shift was identified (Δλ = 467–515 nm). All these computational outputs support experimental findings and compared with parent 5-Hydroxy-flavone (5HF) molecule. Substituting three methoxy groups in 5HF shifts fluorescence emission maximum of 5HTMF to near-infrared region, increase enormous applications. Moreover, state-specific solvation method was found a practical, theoretical approach to corroborating experimental findings.