Structural and Spectral Features of 4′-Substituted 2′-Hydroxychalcones in Solutions and Crystals: Spectroscopic and Theoretical Investigations

The article describes investigations of 2'-hydroxychalcone and its three derivatives bearing differently sized alkyloxy groups at position 4'. The compounds are investigated from the point of view of crystal structure, electronic absorption, fluorescence features in solutions and crystals using X-ray diffraction and electronic spectroscopy methods, and quantum chemistry calculations. In general, both in solutions and in the crystal phase, the influence of substituents on absorption spectra of chalcones was found to be insignificant. Exclusively in the case of 4'-(4-methoxybenzyloxy)-2'-hydroxychalcone, molecular packing influences the absorption features, which is because of the intermolecular interactions of substituent's phenyl ring and chromophore fragment of the neighboring molecules. The lack of fluorescence of the excited enol form of chalcones in solutions and crystals is mainly due to intersystem crossing and excited state intramolecular proton transfer. Fluorescent properties of the phototautomer keto species formed by the proton transfer depend on molecular conformation. In solutions, the excited keto form is twisted and effectively deactivates nonradiatively due to conical intersection. In the crystal phase, the fixed planar geometry disables the conical intersection and the fluorescence of the keto form becomes detectable.