Excited-State Proton Transfer and Proton Reactions of 6-Hydroxyquinoline and 7-Hydroxyquinoline in Water and Ice

Abstract

Time-resolved and steady-state emission spectroscopies as well as absorption UV−vis spectroscopy were employed to study the photoprotolytic cycle and other protic processes of the bifunctional 6-hydroxy- and 7-hydroxyqunoline molecules in methanol-doped ice over a wide range of temperatures. In ice at high temperatures of T > 173 K, the excited-state proton transfer rate decreases as the temperature decreases. The emission band of the H+NRO−* zwitterion, where the imine nitrogen is protonated and the hydroxyl is deprotonated, is observed. At T > 173 K, the formation rate of the H+NRO−* emission band is approximately that of the decay rate of the neutral form, NROH*. Below 173 K, the rate of the photoprotolytic process is much slower than the radiative and the nonradiative rates, and the excited-state proton transfer could not be clearly observed. Addition of a small concentration of acetic acid increases the proton transfer rate significantly at temperatures below 235 K. The reaction rate in the presence of acetic acid is temperature-independent over a wide range of temperatures (80−235 K). We propose as an explanation for this observation that there exists a direct proton transfer from the hydroxyl group to water−acetic acid complexes at temperatures below 235 K.

Solvatochromism of pyranine-derived photoacids

“…41,42 These so-called ''super-photoacids'' may offer further insights into a proton transfer mechanism in aprotic environments. 41,[43][44][45][46] Further investigations were dedicated to the effects of temperature, 18,36,[47][48][49][50][51][52][53] pressure, 54 salt concentration 37,55,56 or solvent composition 17,57,58 on the ESPT process.…”

Competition between excited state proton and OH− transport via a short water wire: solvent effects open the gate

“…[16][17][18][19][20][21][22][23][24][25][26][27][28] 7-Hydroxyquinoline (7HQ), a bifunctional chromophore, provides both an acidic site with proton donating functionality as well as a basic site that can accept a proton. [29][30][31][32][33][34][35][36][37][38][39][40] In aqueous solutions, 7HQ can adopt four protropic equilibrium species, a neutral molecule (N), an enol-deprotonated anion (A), an imine protonated cation (C), and the equilibrium keto (K) form (see in Fig. 1).…”

Excited State Dynamics of Isolated 6‐ and 8‐Hydroxyquinoline Molecules

“…[4] Too many works have appeared on this question to quote them thoroughly. In all of them, as in the work by Poizat and coworkers, the n-hydroxyquinoline molecules are coupled to an environment: solvents or solutions, [5][6][7][8][9][10] polymer films, [11] cyclodextrin [12] or reverse micelles. [13] The present work aimed at investigating the intrinsic photoinduced dynamics of n-hydroxyquinoline molecules, i. e. the dynamics of these molecules when free of any environment.…”

“…Too many works have appeared on this question to quote them thoroughly. In all of them, as in the work by Poizat and coworkers, the n‐hydroxyquinoline molecules are coupled to an environment: solvents or solutions, [5–10] polymer films, [11] cyclodextrin [12] or reverse micelles [13] …”