8-Hydroxyquinoline Monomer, Water Adducts, and Dimer. Environmental Influences on Structure, Spectroscopic Properties, and Relative Stability of Cis and Trans Conformers

Abstract

The low fluorescence quantum yield of 8-hydroxyquinoline cannot be correctly interpreted without knowing the form that such a compound assumes in different environments. The commonly accepted emission-quenching excited-state proton transfer can follow different reaction paths if 8-hydroxyquinoline is dimeric or monomeric or if it exists in the form of cis and trans conformers; in this light, the knowledge of the compound form in a particular environment is basic. We have performed a spectroscopic and computational investigation aimed at the determination of the form of 8-hydroxyquinoline in different solvents. UV-vis, fluorescence, and IR spectral features have been assigned by ab initio computations based on the density functional theory and time-dependent density functional theory; the density functional theory and MP2 computations have been applied to the determination of the relative stability of the dimeric and monomeric cis and trans forms of 8-hydroxyquinoline in different solvents. Molecular dynamics computations have been used to determine the compound behavior in water solutions. According to our results, 8-hydroxyquinoline shows a clear preference for the cis conformation (as dimer or monomer), but, in water solutions, a small fraction of the trans conformation is also present.

Comparative solution equilibrium studies of anticancer gallium(III) complexes of 8-hydroxyquinoline and hydroxy(thio)pyrone ligands

“…Parallel to the second deprotonation step the intensity decreased, but surprisingly at pH > 9 a further increment was observed and pK 2 could not be calculated from this data set. According to literature, the neutral form of HQ appears to a certain extent in the zwitter ionic form at the excited state when the quinoline-N is protonated, while the phenolic O is deprotonated, which can lead to the formation of intramolecular hydrogen bonding preventing the determination of the ground state protonation constant [31,32].…”

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

“…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].…”

“…For 8-hydroxyquinoline (8HQ) [35,36], ultrafast intramolecular proton transfer would occur in most solvents due to the critically short distance between both proton-donating and -accepting groups. And the resulting ketonic form (8KQ) is supposed to be barely emissive [35].…”

“…For 8-hydroxyquinoline (8HQ) [35,36], ultrafast intramolecular proton transfer would occur in most solvents due to the critically short distance between both proton-donating and -accepting groups. 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.…”

Oxinate-Aluminum(III) Nanostructure Assemblies Formed via In-situ and Ex-situ Oxination of Gold-Self-Assembled Monolayers Characterized by Electrochemical, Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy, and X-ray Photoelectron Spectroscopy Methods

“…S3), i.e., in its intra-molecular H-bond form [16,76], preventing physical adsorption of 5A8HQ. This is supported by the disappearance of the above-mentioned specific O-H stretching bands at wave numbers higher than 3000 cm −1 (Section 3.…”