Prototropic equilibria of electronically excited molecules. Part II. 3-, 6-, and 7-Hydroxyquinoline

Abstract

The absorption and the emission spectra of the four ionic species formed by 3-, 6-, and 7-hydroxyquinoline in aqueous solution have been measured, and the constants governing the prototropic equilibria between the ionic forms of each compound in the lowest singlet excited state have been estimated theoretically from the transition energies and experimentally from the change in fluorescence spectrum with hydrogen-ion concentration. It is found that the phenolic group of the hydroxyquinolines is more acidic, and the ring nitrogen atom more basic, in the excited than in the ground electronic state, as expected from the theoretical electronic charge redistribution on excitation derived from a corresponding-carbanion model. The overall velocity of the two-stage prototropic change in the excited state from the neutral molecule to the zwitterion of the hydroxyquinolines in ethanol or neutral aqueous solution is found to be slower than the rate of emission, but the corresponding one-stage processes in acid or alkaline solution are comparable in rate or faster.

Wagging motion of hydrogen-bonded wire in the excited-state multiple proton transfer process of 7-hydroxyquinoline·(NH3)3 cluster

“…They explained that the fluorescence is forbidden due to symmetry. However, with the same symmetry, the fluorescence from both the cationic and anionic forms of 6HQ have been observed in emission spectra [37,39]. Meanwhile, by using CASSCF and CASPT2 calculations without symmetry constraints, Serrano-Andrés and co-workers suggested that [43], ESMPT would be preferred over hydrogen-atom transfer in the excited-state tautomerization of 7HQÁ(NH 3 ) 3 .…”

“…And the resulting ketonic form (8KQ) is supposed to be barely emissive [35]. On the other hand, the hydrogen-bonded wire can be hardly formed in 6-hydroxyquinoline (6HQ) [37][38][39], which requires an extreme long bridge to connect the functional groups. We believe that, the hydrogen-bonded wire with two or three components (H 2 O, NH 3 or alcohols, etc.)…”

“…However, some key molecular processes of proton transfer via hydrogen-bonded wire in biological protein have remained poorly understand so far due to the complex environment, such as the green fluorescent protein (GFP) [32]. Hence, studying on hydrogen-bonding dynamics in the excited-state multiple proton transfer (ESMPT) via hydrogen-bonded wire becomes one of the most popular topics in nowadays, which is very useful for biological investigations [33][34][35][36][37][38][39][40][41][42][43][44][45][46].…”

Anomalous Acid-Base Equilibria in Biologically Relevant Water Nanopools

“…[5][6][7][8][9][10][11][12][13] For example, the dielectric constant of a water nanopool has been reported to be much lower than that of bulk water (ε = 78. 5) 17 but similar to that of an alcohol (ε = [30][31][32][33][34][35][36][37][38][39][40]. 18,19 On the other hand, biological processes often take place based on proton relay along a hydrogen (H)-bonded chain, [1][2][3][4][20][21][22][23][24] and the dynamics of biological proton relay is determined by the size, the structure, and the motion of a water cluster which is the prime agent in most of biological systems.…”

“…[13][14][15][16][35][36][37][38][39][40][41][42][43][44] On one hand, a 7-hydroxyquinoline molecule can form a cyclically H-bonded complex with two protic solvent molecules such as water and alcohols, so that proton relay from the enolic group to the imino group of 7-hydroxyquinoline occurs along a H-bonded solvent chain in a concerted fashion. 15,16,[42][43][44] On the other hand, those two prototropic groups of 6-hydroxyquinoline (6HQ) are too far from each other to form a cyclic complex with solvent molecules, so that proton transfer from the enolic group to the imino group of 6HQ takes place in a stepwise manner via accumulating cationic or anionic intermediate species.…”