Dipole moments of 4'-aminoflavonol fluorescent probes in different solvents.

Electrooptical absorption measurements (EOAM) were used to measure the dipole moments of the normal form of 4′-(dimethylamino)-3-hydroxyflavone (FME), and 4′ N-(15-azacrown-5)-3-hydroxyflavone (FCR) in 1,4-dioxane, toluene, and cyclohexane. With these probes excited-state intramolecular proton transfer (ESIPT) takes place. For comparison, the dipole moments of 4′-(dimethylamino)-3-metoxyflavone (FME3ME), for which ESIPT is lacking, were measured, too. For all three probes the 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 (12.0–17.7) × 10−30 C m. Upon optical excitation, the dipole moments increase by (41.9–52.9) ×10−30 C m. The value of the change of the dipole moment vector Δaμ with excitation to the Franck-Condon state and the value of the vector μeFC for FCR and FME are practically independent on the solvent polarity. From this point of view and due to large values of the dipole moments FCR and FME are very promising probes for the investigation of the distribution of the local polarity in biological systems using site-selective excitation of the different sites. Our steady-state fluorescence studies on FME and FCR have demonstrated a high spectral sensitivity of the normal form to such solvent characteristics as polarity.