Monitoring the Microscopic Molecular Mechanisms of Proton Transfer in Acid‐base Reactions in Aqueous Solutions

Abstract

This article provides a review of the authors’ collaborative work on acid‐base reactions under conditions allowing direct observation of mechanistic details of the proton transfer within reactive complexes of OH groups and the bases RCOO− (R=organic residues) in diffusion‐assisted neutralization reactions by using time‐resolved UV/Vis emission and IR absorption spectroscopy. We discuss the considerable insight gained by these experiments and outline the current status of the field.

Caught in the act: Observation of the solvent response triggered by excited-state proton transfer in real time

“…This is in line with Kumpulainen et al who argued that the CIP* intermediate, if present, might be extremely short-lived and hence thermodynamically unfavorable or bypassed by a Grotthuss-type proton-hopping mechanism 79 . Our experimental results are in line with a two-step mechanism, see Siwick and Bakker 46 , or to a small number of solvent (water) molecules ultimately leading to proton transfer 48 . For HPTS, OPTP spectroscopy we observe an amplitude modulation with a second, local maximum in transmission, with a small amplitude (reduced by a factor of 1000 compared to the initial 4 ps oscillation).…”

“…Transient IR absorption spectroscopy allows to follow the proton dissociation of the HPTS photoacid and the concomitant generation of the HPTS photobase, as well as to characterize the transient absorption of the aqueous proton shuttling in between acid and base by observation of the IR proton continuum. However, the exact molecular mechanism of the key role of the solvent still remains to be unraveled 48 .…”

Caught in the act: Observation of the solvent response triggered by excited-state proton transfer in real time

“…This is in line with Kumpulainen et al who argued that the CIP* intermediate, if present, might be extremely short-lived and hence thermodynamically unfavorable or bypassed by a Grotthuss-type proton-hopping mechanism 79 . Our experimental results are in line with a two-step mechanism, see Siwick and Bakker 46 , or to a small number of solvent (water) molecules ultimately leading to proton transfer 48 . For HPTS, OPTP spectroscopy we observe an amplitude modulation with a second, local maximum in transmission, with a small amplitude (reduced by a factor of 1000 compared to the initial 4 ps oscillation).…”

“…For the proton transfer of HPTS into water we find no spectroscopic evidence for the formation of a 46 , or to a small number of solvent (water) molecules ultimately leading to proton transfer 48 . For HPTS, OPTP spectroscopy we observe an amplitude modulation with a second, local maximum in transmission, with a small amplitude (reduced by a factor of 1000 compared to the initial 4 ps oscillation).…”

Tailoring the Schiff base photoswitching – a non-adiabatic molecular dynamics study of substituent effect on excited state proton transfer

“…Acting at ultrashort timescales including even the femtosecond regime, ESIPT allows for a rapid redistribution of excitation energy and, thus, enhances the photostability of the excited molecular systems, giving rise to rich applications in the organic UV photostabilizers industry [2][3][4] . ESIPT plays also a crucial role in the activity of photoacids and photobases [5][6][7] ; it occurs in photo-sensitive proteins [8][9][10][11] and metal complexes; 12 and it is even considered among primary factors enabling the evolution of life, due to its role in nucleic-bases photochemistry [13][14][15] . The photoinduced proton transfer, usually accompanied by a pronounced redistribution of the electronic density, results in large emission Stokes shifts, opening possibilities for bioluminescence color tuning and, by creation of photochromism between potential meta-stable ground state isomers [16][17][18][19] , enables selective molecular photoswitching and information storage 20 .…”