Dipole moments of 4′-aminoflavonols determined using electro-optical absorption measurements or molecular Stark-effect spectroscopy

The electro-optical absorption measurements (EOAM) were used to measure the dipole moments of the normal form of 4′-(methoxy)-3-hydroxyflavone (FOM), 4′-(dimethylamino)-3-hydroxyflavone (FME), and 4′-N-(15-azacrown-5)-3-hydroxyflavone (FCR). For these probes the excited state intramolecular proton transfer (ESIPT) takes place. For comparison, the dipole moments of 4′-(dimethylamino)-3-methoxyflavone (FME3ME), for which ESIPT is lacking, were measured. In the case of FCR, FME, and FME3ME the equilibrated ground (μg) and excited Franck–Condon state (μeFC) electrical dipole moments are parallel to each other and also parallel to the transition dipole moment. The electrical dipole moments of FCR, FME, and FME3ME in their ground state have values within the range (14.1–17.7)×10−30 C m. Upon optical excitation, the dipole moments increase by (47.1–50.4)×10−30 C m. Among the studied flavonols, μg and μeFC decrease in the order FME3ME>FCR>FME. This effect can be explained by the existence of an intramolecular H-bond for FCR and FME, which partly compensates a negative charge on the carbonyl oxygen in the ground state and thus diminishes the electron-donating ability of the amino-group in the excited Franck–Condon state. For FOM the equilibrated ground and excited Franck–Condon state dipole moments of the normal form have values of 9.4×10−30 C m and 26.3×10−30 C m, respectively. The smaller values of the dipole moments for FOM, in comparison with other flavonols, is caused by the absence in FOM of a strong electron-donating group, such as the amino-group. The dipole moments are not parallel and the angles between the different dipole moments are in the range 4–55°.