Photodynamics of intramolecular proton transfer in polar and nonpolar biflavonoid solutions

Abstract

Using methods of steady state luminescence and femtosecond spectroscopy, we have studied the mechanism of intramolecular proton transfer in synthesized 3,7-dihydroxy-2,8-di(4-methoxyphenyl)-4H,6H-pyrano[3,2-g]chromen-4,6-dion in polar and nonpolar solutions, films, and polycrystals at 293 and 77 K. In an excited singlet state, intramolecular proton transfer occurs in two stages. At the first stage, a tautomer with one transferred proton (OTP tautomer) is formed from the Franck-Condon state within τ₁ = 0.6 ps. At the second stage, the second proton is transferred within τ₂ = 3.1 ps and a tautomer with two transferred protons (TTP tautomer) is formed, which fluoresces in toluene at 293 K with a high quantum yield, Φ _f = 0.66, and the fluorescence spectrum of which is characterized by a large Stokes shift, 9900 cm⁻¹. At 293 K, polar solvents (dimethylformamide, dimethyl sulfoxide, ethanol, etc.) solvate the BFV molecule in the ground state, while, in the excited state, an OTP tautomer is mainly formed. In polar ethanol at 77 K, a dual fluorescence spectrum is observed, which is caused by the fluorescence emission of polysolvates with λ ^f_max = 460 nm and TTP phototautomers at λ ^f_max = 610 nm.