Time-Resolved Resonance Raman Study of the Triplet State of the p-Hydroxyphenacyl Acetate Model Phototrigger Compound

Abstract

Picosecond and nanosecond time-resolved resonance Raman spectroscopy have been used to study the dynamics and structure of the phototrigger compound p-hydroxyphenacyl acetate (HPA) in acetonitrile solution. An intermediate was observed that exhibited significant quenching by oxygen. This intermediate was identified and attributed to the ππ* triplet state of HPA. Temporal evolution at early picosecond times indicates rapid intersystem crossing conversion and subsequent relaxation of the excess energy of the initially formed triplet state. B3LYP/6-311G** density functional theory (DFT) calculations were done to determine the structures and vibrational frequencies for both the HPA triplet and ground states. The experimental and theoretical results were compared and discussed in relation to the initial pathway for the p-hydroxyphenacyl deprotection reaction and the nature of the triplet state relative to other aromatic carbonyl compounds. The present results demonstrate the utility of applying ultrafast vibrational spectroscopy to study the mechanism of the photorelease process in phototrigger compounds.

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.…”

“…7 Milligram quantities of caged compounds and caffeine or acetamidophenol were weighed out on a Fisher brand microbalance and dissolved in 4 ml of 18 MΩ ultrapure water, salt solutions of various concentrations, buffers with or without adjusted ionic strengths, or purified organic solvents were then added to a quartz tube and vortexed, resulting a homogenous solution of the caged compound and internal standard. Concentrations of the caged compounds ranged from 1-9 mM.…”

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

“…Because the leaving group has little effect on the triplet property of the pHP cage [23,43], the H-bonded effect on the triplet structure presented here for HP and MAP is also expected to be applicable generally to the pHP phototrigger compounds. The triplet that appears to be the precursor for the photodeprotection reaction of the pHP phototrigger compound is an H-bonded triplet complex with a planar and ring-localized biradical structure.…”

“…In Figure 13.12, the growth of the intensity of the triplet Raman feature tracks a frequency upshift of $40 cm À1 and a bandwidth that decreases by $40 cm À1 for HPA and HPDP in both of the H 2 O/MeCN mixed and neat MeCN solvents (see Refs [23] and [43] for details), and this kind of short-time behaviour in Raman band positions and bandwidths is typical of an excess energy relaxation process [23,43] that generally occurs on a timescale of around 10 ps, which correlates well with the $7-12 ps time constant seen in the spectra of Figure 13.12. The short-time behaviour of the 3 pp* Raman spectra appears to be due to the combined dynamics associated with the growth of the triplet population and the relaxation of the excess energy ($11 000 cm À1 ) deposited into the system by 267 nm photolysis and subsequent very fast ISC conversion to populate the triplet.…”

“…Water solvation of the hydroxy proton is also needed to account for the results of previous studies that found that photolysis of MPDP (the p-methoxy counterpart of HPDP) in a H 2 O/MeCN (1:1) solvent leads to very little if any deprotection products [14,21]. Because the hydroxy and methoxy moieties have similar electronic effects on the intrinsic properties of the triplet states [43,57], the lack of deprotection reactivity for MPDP indicates that there is not just a simple primary step involving only CÀO bond heterolysis in the HPDP triplet, and suggests that the proton-donating ability of the hydroxy group in the pHP triplet is associated with the different photochemical deprotection reactivity of MPDP from HPDP in the same water/MeCN mixed solvent.…”