Photoinduced Coupled Proton and Electron Transfers. 2. 7-Hydroxyquinolinium Ion

Abstract

The hydroxyl group of 7-hydroxyquinoline in acidic media (i.e., of 7-hydroxyquinolinium cation, 7-HQ(C)) undergoes photoinduced deprotonation even in strongly acidic media. It occurs from dilute perchloric acid solutions up to 8 mol L -1 concentration, which is evidence of an outstanding photoacidity. Excited-state proton ejection was shown to be reversible at acid concentrations greater than 1 mol L -1 . Below this value, the proton ejection was not balanced by the proton recombination. Concerning the rates of the excited-state reactions, (i) the proton ejection rate constants k 1 increased upon acid dilution, attaining at high dilution the limiting value of (k 1 ) 0 ) 5.5 × 10 10 s -1 . k 1 was shown to vary with the water activity according to k 1 ) (k 1 ) 0 (a H 2 O ) 4 . The number 4 is then a key number for the ability of water to accept a proton from 7-HQ(C*). (ii) The proton recombination rate constant varies from 3 × 10 9 to 4 × 10 10 s -1 when the acid concentration ranges from 1 to 5 mol L -1 . This observation can tentatively be ascribed to a nondiffusional recombination process occurring with the ejected proton in concentrated acid solutions. The results as a whole are consistent with the coupling of an intramolecular electron transfer with the proton transfer, due to the withdrawing effect of the quinolinium function gNH + on the -O -group generated by the deprotonation. The photoproduct is then most likely to be the ketonic tautomeric form of 7-hydroxyquinoline rather than a zwitterion. Thus, the excited-state behavior is fully accounted for by the photoinduced synergy of the two functional groups.

Singlet oxygen reactivity in water-rich solvent mixtures

“…27 Another reaction involving 4 water molecules in photoinduced deprotonation of 7-hydroxyquinoline in acidic media has been reported in literature. 28 The reaction of water molecules with 1 O 2 * was first described in antibody-mediated reactions in cells 26 and later confirmed by other authors. 29,30 To render evident the importance of the water molecules on the …”

Picosecond Dynamics of the Photoexcited 6-Methoxyquinoline and 6-Hydroxyquinoline Molecules in Solution

“…17 On the other hand, as in alkaline solution, no transient signal is detected after decay of the 6HQ(T*) spectrum. This observation implies that proton capture by the oxy group -O -of the tautomer to yield back the initial cationic species, 6HQ(T) f 6HQ(C), follows immediately its deactivation to the ground state.…”

Anomalous Acid-Base Equilibria in Biologically Relevant Water Nanopools

“…[5][6][7][8][9][10][11][12][13] For example, the dielectric constant of a water nanopool has been reported to be much lower than that of bulk water (ε = 78. 5) 17 but similar to that of an alcohol (ε = [30][31][32][33][34][35][36][37][38][39][40]. 18,19 On the other hand, biological processes often take place based on proton relay along a hydrogen (H)-bonded chain, [1][2][3][4][20][21][22][23][24] and the dynamics of biological proton relay is determined by the size, the structure, and the motion of a water cluster which is the prime agent in most of biological systems.…”

“…[13][14][15][16][35][36][37][38][39][40][41][42][43][44] On one hand, a 7-hydroxyquinoline molecule can form a cyclically H-bonded complex with two protic solvent molecules such as water and alcohols, so that proton relay from the enolic group to the imino group of 7-hydroxyquinoline occurs along a H-bonded solvent chain in a concerted fashion. 15,16,[42][43][44] On the other hand, those two prototropic groups of 6-hydroxyquinoline (6HQ) are too far from each other to form a cyclic complex with solvent molecules, so that proton transfer from the enolic group to the imino group of 6HQ takes place in a stepwise manner via accumulating cationic or anionic intermediate species.…”