Solvent Polarity Dependent Excited State Dynamics of 2′-Hydroxychalcone Derivatives

The excited-state properties of 4-(dimethylamino)­methoxychalcones (<b>DEAMC</b>) and its derivative 4-(dimethylamino)­hydroxychalcones (<b>DEAHC</b>) were investigated in various solvents with different polarities by using steady-state and femtosecond transient absorption spectroscopy combined with quantum chemical calculations. It is found that their photophysical parameters such as fluorescence quantum yields, lifetimes, and excited-state relaxation paths strongly depend on the solvent polarity. Quantum-chemical calculations elucidate that the geometry of <b>DEAMC</b> in the ground state is slightly torsional whereas <b>DEAHC</b> adopts a near planar conformation stabilized by O–H···O chelated hydrogen bonds. Steady state spectra show that <b>DEAHC</b> is weak fluorescent in all solvents due to nonradiative relaxation in the excited enol and keto states, whereas the fluorescence quantum yield of <b>DEAMC</b> increases with the increasing of solvent polarities, and the emission yield is as large as 0.16 in acetonitrile. Femtosecond and nanosecond transient absorption spectra further prove that in nonpolar solvent the deactivation of S₁ in <b>DEAMC</b> is strongly governed by efficient formation of triplet states, whereas in polar solvent, stronger solvation induced energetically stabilization of ICT state, limiting the intersystem crossing to triplet state. The stabilization of ICT state not only leads to a higher fluorescence quantum yield for <b>DEAMC</b> but also restricts intramolecular twisting process in the enol form of <b>DEAHC</b>, facilitating efficient excited-state intramolecular proton-transfer reaction. These results clearly illustrate the dominant role of excited state solvation in modulating the emission behavior and deactivation mechanisms of fluorophores