Role of hydroxy substitution on the conformation and excited state dynamics of chalcone

Chalcones, a class of α,β-unsaturated carbonyl compounds with rich photophysical properties, were investigated together with an ortho-hydroxy-substituted analogue (2-hydroxychalcone) using a combination of steady-state and ultrafast spectroscopy supported by quantum chemical calculations. Steady-state absorption measurements reveal that 2-hydroxychalcone exists as an equilibrium mixture of ground-state conformers in solution, giving rise to two distinct absorption bands with pronounced solvent dependence. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations provide a conformer-resolved interpretation of the electronic spectra and indicate enhanced charge-transfer character for the lower-energy absorbing conformer in polar solvents. Femtosecond transient absorption spectroscopy uncovers a striking contrast in the excited-state relaxation pathways of unsubstituted chalcone and 2-hydroxychalcone. Photoexcitation of the unsubstituted chalcone to the ππ* (S₂) state leads to the formation of a transient absorption band centered near 460 nm, which decays on a picosecond timescale to yield the triplet state. In contrast, excitation of 2-hydroxychalcone to the ππ* state reveals two distinct transient species: a short-lived absorption band centered around 600 nm, assigned to the nπ* (S₁) singlet state, and a longer-lived transient near 490 nm, attributed to a twisted S₁ singlet state formed within approximately 2 ps of optical excitation. The excited-state relaxation of 2-hydroxychalcone is dominated by barrierless dihedral twisting, which drives structural reorganization within the singlet manifold. The observation of coherent low-frequency vibrational wavepacket motion provides direct spectroscopic evidence for this torsion-driven relaxation pathway. These results highlight how ortho-hydroxy substitution fundamentally alters excited-state dynamics by suppressing triplet formation and promoting structurally driven, coherent relaxation, offering new insights into the photophysics of chalcone-based systems.