Excited-State Processes in 8-Hydroxyquinoline:  Photoinduced Tautomerization and Solvation Effects

8-Hydroxyquinoline (8-HQ), referred as to oxine in analytical chemistry, is a fluorogenic ligand. Its lack of fluorescence in water and alkanes, and its low quantum yield in many other organic solvents, are rationalized in the present study in terms of photoinduced formation of a nonfluorescent tautomeric form 8-HQ(T*). In water, intermolecular proton transfers with surrounding water molecules are expected, but intrinsic intramolecular proton transfer between the −OH and ⩾N functions cannot be ruled out because the presence of a weak internal H bond can be inferred from the ground-state properties of 8-HQ such as pKa values or solubility. In organic solvents, vapor pressure osmometry measurements in conjunction with infrared spectra allow us to show that (i) in alkane solvents, a very stable dimer is formed in the ground state (Kdim = 7 × 107 at 25 °C); biprotonic concerted proton transfers are then expected to occur within the dimer upon excitation, as was previously reported for 7-azaindole; (ii) in chlorinated solvents (CH2Cl2, CHCl3), hydration by residual water molecules likely leads to a nonnegligible fraction of hydrated open structures where excited-state proton transfer is impaired; a weak fluorescence can then be observed (ΦF ≈ 4 × 10-3).

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