Ground-State Triple Proton Transfer in 7-Hydroxyquinoline. 4. Observation in Room-Temperature Methanol and Aqueous Solutions

Abstract

The stabilization of the ground-state keto tautomer of 7-hydroxyquinoline (7HQ) has been observed in roomtemperature methanol and aqueous solutions. Concentration studies have shown that the 7HQ cyclic dimer is formed at high 7HQ concentrations in methanol and water, and either the 7HQ-(MeOH)z or the 7HQ-water complex, presumably the cyclic 7HQ-(H20)2 structure, is formed at low 7HQ concentrations in methanol or water solvents, respectively. These species give rise to the keto-tautomer excitation band centered at 398 nm (7HQ dimer in water), 420 nm (7HQ dimer in methanol), 377 nm (7HQ-water complex), and 387 nm (7HQ-methanol complex). The emission band for all species appears at 520 nm. This is the first report of the ground-state stabilized keto tautomer spectrum in methanol at room temperature. The spectrum in water has been investigated in the past, but the interpretation differs from that of this work.

Resonance Raman Characterization of Different Forms of Ground-State 8-Bromo-7-hydroxyquinoline Caged Acetate in Aqueous Solutions

“…[8][9][10][11][12][13][14][15][16][17][18] Four prototropic species of 7-HQ are equilibrated in aqueous solution: a normal molecule (7-HQ(N)), an imine-protonated cation (7-HQ(C)), an enol-deprotonated anion (7-HQ(A)), and an enol-deprotonated imine-protonated tautomer (7-HQ(T)). 8 The pK a of the phenolic moiety of 7-HQ has been reported to be 9.0.…”

“…Several studies have focused on the ground singlet states and excited single and triplet states of 7-HQ and some derivatives. 9,[11][12][13] The mechanisms of the proton tautomerization of 7-hydroxuquinoline and the participation of solvent molecules have been explored in nonaqueous protic solvents 8,10,11,14 and in aqueous solution. 13,[15][16][17] It is conceivable that BHQ-OAc, as a derivative of 7-HQ, may have similar properties but the influence of its substituent groups on these properties is not yet clear.…”

Controlling Excited‐State H‐Atom Transfer along Hydrogen‐Bonded Wires

“…The two tautomers of 7-hydroxyquinoline as well as the anion 7Q À and the cation 7H 2 Q þ have been extensively characterized by absorption and fluorescence spectroscopy in non-polar, polar and protic solvents [59][60][61][62][63][64][65][66][67][68][69][70]. 7HQ emits in the 370-390 nm range, depending on the solvent, while 7KQ emits in the 525-580 nm range, strongly Stokes shifted relative to 7HQ.…”

Ground- and excited state proton transfer and tautomerization in 7-hydroxyquinoline⋅(NH3)n clusters: Spectroscopic and time resolved investigations

“…S 1 ←S 0 electronic excitation of 7HQ renders the -O-H group more acidic and the N atom more basic, leading to an ESPT reaction in bulk protic solvents. [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] In gasphase ammonia solvent clusters, laser excitation of 7HQ to the S 1 state can trigger the injection of the proton into the cluster, followed by a series of proton transfers along the ammonia wire until the aromatic molecule is reprotonated at the quinolinic N atom, equivalent to a cluster-mediated enol→keto tautomerization. 23,25 Ab initio calculations using Hartree-Fock 24 and density functional 25 ͑DFT͒ methods provide a picture of a step-by-step PT along ammonia wire clusters held at both ends by the 7HQ scaffold, and predict cluster-size dependent effects on the ground state PT.…”

“…[26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] Both spectroscopic and laser kinetic measurements have been interpreted in terms of models for excited state enol→keto tautomerization involving PT reactions along a hydrogen bonded chain of two or three solvent molecules between the -O-H group and the N atom. [28][29][30][31][32][33][34][35][36][37][38][39][40] The S 0 and S 1 state PT behavior is similar for 6-methyl-7HQ, but different for 8-methyl-7HQ, where the H bonded chain of solvent molecules is believed to be interrupted. 36 A recent review of the ground and excited state photoreactions of 7HQ in bulk solution has been given by Bardez.…”