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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2007-12, Vol.111 (51), p.13403-13414
• Main Authors: Amati, Mario, Belviso, Sandra, Cristinziano, Pier Luigi, Minichino, Camilla, Lelj, Francesco, Aiello, Iolinda, La Deda, Massimo, Ghedini, Mauro
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp074510s

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.