Enhanced solid-state photoluminescence and fluorescence spectral behaviors for an ESIPT molecule: An experimental and theoretical investigation

Abstract

3-Dimethylamino-1-(2-hydroxy-phenyl)-propenone (DHP) molecule with excited-state intramolecular proton transfer (ESIPT), was good candidate as active organic laser dyes. In this work, the enhanced solid-state photoluminescence and the structure-property relationship for photophysical properties are investigated by time-dependent density functional theory (TD-DFT) method and the experimental spectra. The calculated potential energy surface (PES) curve of the ground-state S0 indicates that the planar DHP-cis-enol (α isomer) conformer with innate intramolecular H-bond is the global energy minimum. The PES of the lowest excited-state S1 indicate that the photoexcitated DHP-cis-enol (α* isomer) and DHP-cis-keto (β isomer) conformers are achieved as well, corresponding to dual-peak fluorescence emission in solution. Especially, in addition to the planar DHP-cis-keto isomer of DHP deriving from ESIPT, a barrierless twist process of phenyl hydroxide group brings out another stable excited-state isomer (γ isomer), which dramatically influences the photophysical properties. Furthermore, the low-frequency modes of phenyl hydroxide group rotation with large Huang-Rhys factors for the twisted isomer result in a noteworthy conical intersection of the S0 and S1 energy curves, which dissipate the excited-state energy nonradiatively for DHP in solution. However, in solid-state, the fluorescence spectrum of DHP features only ESIPT emission band which demonstrates the β isomer mainly exist in S1 state to promote the radiative decay processes with suppressed population of the α* and γ isomers, which can explain the phenomenon of the enhanced solid-state photoluminescence.