Experimental and theoretical study of hydroxyquinolines: hydroxyl group position dependent dipole moment and charge-separation in the photoexcited state leading to fluorescence

Abstract

Optical absorption and fluorescence (FL) spectra of 2-, 6-, 7-, 8-hydroxyquinolines (2-,6-,7- and 8-HQs) have been measured at room temperature in the wide range of solvents of different polarities, dielectric constant and refractive index. The ground state dipole moment (µ ) and excited state dipole moment (µ) of 2-, 6-, 7- and 8-HQs were obtained using solvatochromic shift (SS) methods and microscopic solvent polarity parameters (MSPP). Change in the dipole moment (Δµ) between the ground and photo-excited states was estimated from SS and MSPP methods. DFT and TDDFT based theoretical calculations were performed for the ground and excited states dipole moments, and for vertical transitions. A significant enhancement in the excited state dipole moment was observed following photo-excitation. The large value of Δµ clearly indicates to the charge-separation in the photo-excited states, which in turn depends on the position of the hydroxyl group in the ring.

Machine learning for the prediction of molecular dipole moments obtained by density functional theory

“…Faber et al [ 16 ] reported a mean absolute error of 0.10 D between B3LYP calculations and experiment using 49 molecules. Others have also reported good agreement between experimental and DFT calculated DMs for more complex molecules [ 17 , 18 ]. Experimental measurements and quantum chemistry calculations were performed in the gas phase.…”

The solvatochromism and electronic structure of(E)-2-(2-hydroxystyryl)quinolin-8-ol

“…Filip et al investigated the electronic structure of the 8-hydroxyquinoline molecule in some solvents and thus an ESIPT structure has been explained for this molecule [19]. Electronic structure and dipole moments estimated by using polar and aprotic solvents the 2-,6-,7-, 8-hydroxyquinolines were researched by Mehata et al [20]. The observed electronic transition wavelength values for both absorption and emission spectra in these works were smaller than those in the present work, because the phenoxy substituent gives rise to longer wavelengths.…”

“…In addition, these calculations were used to find dielectric and refraction index functions and Onsager cavity radius. Moreover, the statistical parameters found in the experimental dipole moment calculations are listed in [20]. The ground state dipole moment of HSQ with hydroxystyryl substituent decreases, while its excited state dipole moment increases.…”

“…In addition, they exhibit strong pharmacological activities [17]. Naik et al, Filip et al, and Mehata et al have investigated the electronic structure of 8-hydroxy quinoline and some of its derivatives [18][19][20].…”