ESIPT molecular probe FET: Electric field effect on spectral properties

Abstract

Employed DFT calculations to investigate the electric field effect on the structure, spectral properties, excited-state intramolecular proton transfer (ESIPT) reaction, and dipole moments of 4′-(Diethylamino)-3-hydroxyflavone (FET). FET, a well-known fluorescent 3-hydroxyflavone, exhibits efficient dual fluorescence from its normal and tautomeric forms. Our simulations revealed a significant field effect on the singlet state energies, electronic transition frequencies, and molecular orbital surfaces. FET molecules demonstrate significant polarizability, leading to a ∼50 % increase in the permanent dipole moment in the singlet states.

The changes in electronic transition frequencies induced by the applied electric field provide a basis for understanding the modulation of spectral characteristics in solvents where the solute interacts with the electric field of the reaction cavity. Furthermore, incorporating intermolecular interactions using the Onsager model allows us to elucidate how solvatochromism and significant inhomogeneous spectral broadening of vibronic spectra may arise in polar solvents due to thermal fluctuations of the electric field.

Selective fluorescence spectroscopy of FET in acetonitrile reveals a moderate red shift in the normal form's spectrum and a dependence of its intensity on the excitation energy, a phenomenon known as the red-edge excitation effect (REEE), at room temperature. Traditionally, REEE is observed when solutes are in immobilized environments, such as frozen matrices, polymers, or membranes. We attribute the observed REEE pattern to interplay of spectral solutes inhomogemeouty and the unique characteristics of the fast ESIPT reaction.