Experimental and theoretical study on the structure and optical properties of 2-acyl-1,3-indandiones – Conformational effects

A series of six 2-acyl-1,3-indandione derivatives is studied by means of experimental (single crystal X-ray diffraction, NMR, electron absorption and emission spectroscopy) and theoretical methods (HF, DFT and TD-DFT). All possible tautomers and rotamers are described quantum-chemically on the basis of their structure, energy and optical properties. Although the heterocyclic substituent may easily rotate about a C–C bond, according to the calculated energies, the conformation in the most stable gas-phase optimized structures is the same as in the crystalline form. These results suggest that the intermolecular interactions are very weak compared to the intramolecular ones, like strong intramolecular hydrogen bonding and π-electron delocalization. Therefore, a good account of the experimental results was obtained theoretically by considering only the conformational modes of the isolated molecules in ground and excited states. The experimentally observed spectra of the studied series of compounds are characterized by very strong absorption in the visible region and weak fluorescence showing moderate to strong Stokes shift.