Electromeric effect of substitution at 6th position in 2-(Furan-2-yl)-3-hydroxy-4 H-chromen-4-one (FHC) on the absorption and emission spectra

Five 3-Hydroxychromones (3HCs), namely, 2-(furan-2-yl)-3-hydroxy-4H-chromen-4-one (FHC) and its four derivatives by substitution of -CH3, -OH, -NO2 and -Cl at 6th position were synthesized from their corresponding 2’-hydroxyacetophenone and furan-2-carboxaldehyde. Various spectral transitions of all these 3-hydroxychromones (3-HCs) have been assigned by interpreting their absorption spectra in cyclohexane, acetonitrile and methanol. It has been shown that the electromeric effects of substitution at 2nd and 6th positions on the 2–3 double bond in ‘C’ ring are similar but the effect on the double bond of 4-carbonyl group is opposite. It has been found that the substitution at 2nd position changes mainly the electron density directly at the 4-carbonyl group and substitution at 6th position changes the electron density of the ‘C’ ring, changing the overall dipole moment of the molecule, which in turn changes the electron density at the 4-carbonyl group. Emission spectral studies showed that the increase and decrease in dipole moment by substitution at 6th position with electron withdrawing group like NO2 and electron donating group like -CH3 and -OH, stabilizes and destabilizes the N∗ state in the polar solvents respectively.