Room-Temperature Proton Switching of 7-Hydroxyquinoline Dissolved in Rigid Hydroxylic and Carboxylic Polymeric Matrices

Abstract

Room-temperature absorption and fluorescence spectra of 7-hydroxyquinoline in hydroxylic and carboxylic matrices show the coexistence of free, monobonded (bridged and unbridged), dibonded (bridged and unbridged), and keto forms of this dye in these media. In both matrices, the ground-state keto tautomer is stable. The bridged enol forms (λ em ) 380-400 nm) can undergo an excited-state (double or triple) proton-switching reaction, leading to the keto tautomer (λ em ) 500-520 nm) through an energy barrier. The reverse process does not occur upon excitation of the keto tautomer due to the greater stabilization (79 kJ mol -1 ) of this tautomer relative to that of the enol species in the excited state. Irradiation of the enol forms leads to stable keto tautomers in the ground state, and this opens the way for storing information at a molecular level.

Experimental and Theoretical Studies of the Proton-Hopping Reaction of 7-Hydroxyquinoline in Viscous Hydroxylic Media

“…The observation of a dual emission when exciting around 330 nm suggests that a fraction of the species absorbing in this region undergoes a photoreaction. It is well-known that the green fluorescence band at 530 nm of 7HQ complexed by hydroxylic solvents (liquid or matrixes media) originates from the keto phototautomer, formed by an intermolecular proton-transfer reaction in the electronically first excited state of H-bond-bridged enol species. − Observation of a similar fluorescence when exciting the keto tautomer (Figures and ) supports this assignment and is in agreement with the results reported by Itoh and co-workers 9 and those measured in polymeric matrixes. , The emission spectra suggest that not all of the species absorbing at 330 nm undergo the proton photoreaction. These complexes must have a geometry that does not allow the establishment of the H-bond bridge (HBB) required for the proton dislocation and hence the formation of the K species. , On the basis of the structure of the HBB suggested to be formed in alcohol solutions 9 and polymeric matrixes, , we suggest that the photoreaction occurs through a triple proton-transfer process in the bridged complexes, as noted in the Introduction.…”

“…Very recently, Hynes and co-workers suggested the involvement of several proton-transfer elementary steps in the mechanisms of hydrochloric ionization at the surface of stratospheric ice and hydrolysis of chlorine nitrate in the ice lattice . The fast motion of the hydrogen ion is thought to arise from successive protonation−dissociation reactions mediated by hydrogen-bonded water chains, “proton wires”. − As a result of the importance of proton hopping in chemistry and biology, much work has been devoted to elucidating this phenomenon in DNA base pairs, water clusters, and in several H-bonded species. − From the point of view of experiment, proton-transfer reactions of 7-azaindole (7AI) 5-7 and hydroxyquinoline − are of particular interest and constitute the most studied chemical systems in this field.…”

“…We have recently reported the absorption and fluorescence spectra of 7-hydroxyquinoline (7HQ) and the van der Waals complexes with water and methanol in jet-cooled free conditions 12 and in hydroxylic and carboxylic polymeric matrixes at room temperature. , The jet-cooled experiment shows the existence of cis and trans rotamers in a way similar to that of 2-naphthol . Microscopic solvation of 7HQ with one or two molecules of water or methanol in the gas phase does not yield the keto phototautomer, which is photoproduced in the condensed phase.…”

Direct-dynamics approach to catalytic effects: The tautomerization of 3-hydroxyisoquinoline as a test case

“…Complexes of 7HQ and 3HIQ with water, ammonia and AA have been studied both experimentally and theoretically in a number of papers. [4][5][6][7][8][9][10][11][12] These studies underscore the important role of external hydrogen bridges connecting oxygen and nitrogen atoms of 7HQ and 3HIQ on the rate of proton transfer. Such externally assisted processes occur also commonly in many biological processes like in bacteriorhodopsin, 13 carbonic anhydrase II, 14 formation of zwitterionic forms of amino acids in water, 15 formamidine, 16 etc.…”

Photophysical properties of substituted intramolecularly hydrogen bonded compounds: A combined experimental and theoretical study

“…It is shown that ESIPT reactions can be used efficiently as a probe to study the binding sites and conformations of proteins [9,10] explore the hydrophobic nanocavities like cyclodextrins [11,12].…”