DFT, TDDFT, UV-Vis and electrochemical investigation of chalcone sulfonate esters: Structure-property relationships and substituent effects.

Abstract

Chalcone sulfonate esters constitute an emerging class of hybrid molecules in which the conjugated chalcone framework is combined with an arylsulfonyloxy substituent, providing opportunities to modulate their electronic and electrochemical properties. In this study, three chalcone sulfonate esters were investigated using a combination of Density Functional Theory (DFT), time-dependent DFT (TDDFT), UV-Vis spectroscopy, and cyclic voltammetry (CV). Geometry optimizations show that the chalcone backbone remains largely planar, preserving effective π-conjugation, while the arylsulfonate ring adopts a tilted orientation diagonally above the chalcone scaffold, with minimal perturbation of the conjugated framework. Frontier molecular orbital and spin-density analyses reveal that the lowest unoccupied molecular orbital (LUMO), the calculated SOMO and unpaired spin density of the reduced moiety, are localized predominantly on the conjugated -CO-Cα=Cβ- enone fragment, identifying this region as the primary site associated with electrochemical reduction, consistent with the behaviour of unsubstituted chalcones. The arylsulfonyloxy substituent stabilizes the LUMO relative to the parent chalcone, resulting in reduction at less negative potentials. Among the functionals examined, CAM-B3LYP provided the best agreement with experiment, reproducing the dominant UV-Vis absorption maxima with a mean absolute deviation of only 5.5 nm, substantially outperforming B3LYP and ωB97X-D. CAM-B3LYP/6-311G(d,p) TDDFT calculations further show that the intense absorption band near 300 nm originates from a π→π* excitation localized on the conjugated chalcone chromophore. Hole-electron, electron-density difference, and natural transition orbital (NTO) analyses indicate that arylsulfonyloxy substitution of the chalcone has little influence on the nature of this excitation, explaining the similar absorption maxima observed for all derivatives. The experimental cathodic peak reduction potentials follow the expected inverse relationship with the calculated LUMO energies and are consistent with an empirical correlation observed for structurally related chalcones reported in the literature. Overall, the combined computational and experimental results for the compounds investigated here indicate that arylsulfonyloxy substitution preserves the intrinsic electronic excitation of the chalcone chromophore while modulating its frontier orbital energies and electrochemical reduction behaviour.