Time-Resolved Resonance Raman and Density Functional Theory Study of the Deprotonation Reaction of the Triplet State of p-Hydroxyacetophenone in Water Solution

Abstract

[reaction; see text] Picosecond and nanosecond time-resolved resonance Raman (TR(3)) spectroscopy was employed to investigate the deprotonation/ionization reaction of p-hydroxyacetophenone (HA) after ultraviolet photolysis in water solution. The TR(3) spectra in conjunction with density functional theory (DFT) calculations were used to characterize the structure and dynamics of the excited-state HA deprotonation to form HA anions in near neutral water solvent. DFT calculations based on a solute-solvent intermolecular H-bonded complex model containing up to three water molecules were used to evaluate the H-bond interactions and their influence on the deprotonation reaction and the structures of the intermediates. The deprotonation reaction was found to occur on the triplet manifold with a planar H-bonded HA triplet complex as the precursor species. The HA triplet species is generated within several picoseconds and then decays with a approximately 10 ns time constant to produce the HA triplet anion species after 267 nm photolysis of HA in water solution. The triplet anion species was observed to decay with a time constant of about 90 ns into the ground-state anion species that was found to have a lifetime of about 200 ns. The DFT calculations on the H-bonded complexes of the anion triplet and ground-states species suggest that these anion species are H-bonded complexes with planar quinonoidal structures containing two water molecules H-bonded, respectively, with oxygen lone pairs of the carbonyl and deprotonated hydroxyl moieties. A deactivation scheme of the photoexcited HA in regard to the deprotonation reaction in neutral water solutions was proposed. With the above dynamic and structural information available, we briefly discuss the possible implications of the model HA photochemistry in water solutions for the photodeprotection reactions of related p-HP phototrigger compounds in aqueous solutions.

Time‐resolved resonance Raman and density functional theory study of the deprotonation reaction of the triplet state of para‐hydroxybenzophenone in mixed acetonitrile/water solutions

“…27 But in similar experiments reported here for p-hBP, after the deactivation of the triplet state of p-hBP, only the ground-state anion can be detected. The deprotonation reactivity of the triplet state of p-hBP is so high that even in a 5% H 2 O/95% MeCN solvent the deprotonation can still occur very efficiently.…”

“…We note that purging with oxygen led to quenching of the triplet-state lifetime similar to what we found previously for the triplet state of benzophenone 16 and this provides further support for the triplet-state assignment of the TR 3 spectrum. Figure 5 shows a simple diagram of the optimized geometry and selected structural parameters obtained from 27 and this suggests that the p-hBP triplet state also has some Ł character. The change from a n Ł character triplet in benzophenone to a Ł character triplet in HA and p-hBP could be due in part to the introduction of an electron-donating substituent group, like -OH, into a para position that lowers the energy of the Ł triplet and/or mixes it substantially with the n Ł triplet.…”

Fluorinated photoremovable protecting groups: the influence of fluoro substituents on the photo-Favorskii rearrangement

“…A slower quenching diffusion rate will have a corresponding effect on the Stern-Volmer value which will consequently result in a lower estimated rate constant for release. As shown earlier by us 11 and confirmed by Phillips, 7,12 the triplet pHP ester is influenced by solvent H 2 O, primarily assisting in removal of the phenolic proton and also in the concomitant loss of the leaving group, GABA, through solvation with H 2 O.…”

“…1 H NMR analysis confirmed the disappearance of the starting ester 1 and formation of two major photoproducts, phydroxyphenylacetic acid (1f) and GABA (12) along with a minor amount of phydroxybenzyl alcohol (1g) (Fig. 2).…”

Hydrogen‐Bonding Effects on Excited States of Para ‐Hydroxyphenacyl Compounds

“…In some cases the experiments were also performed in solvents DMSO, CF 3 CH 2 OH and a mixture of these two solvents, so as to investigate specific HA-D 4 HA-13 C Scheme 13.1 Schematic structures of HA, HA-13 C and HA-D 4 . Reprinted with permission from [26]. Copyright 2005 American Chemical Society solvent effects.…”