Ultrafast Time-Resolved Transient Absorption and Resonance Raman Spectroscopy Study of the Photodeprotection and Rearrangement Reactions of p-Hydroxyphenacyl Caged Phosphates

Abstract

The kinetics and mechanism of the photodeprotection and rearrangement reactions for the pHP phototrigger compounds p-hydroxyphenacyl diethyl phosphate (HPDP) and diphenyl phosphate (HPPP) were studied using transient absorption (TA) and picosecond time-resolved resonance Raman (ps-TR(3)) spectroscopy. TA spectroscopy was employed to detect the dynamics of the triplet precursor decay as well as to investigate the influence of the solvent and leaving group on the triplet quenching process. Ps-TR(3) spectroscopy was used to directly monitor the formation dynamics for the photosolvolytic rearrangement product and its solvent and leaving group dependence. The TA and TR(3) spectroscopy experiments were also used to characterize the structural and electronic properties of the triplet precursor to the HPDP and HPPP deprotection reactions. The solvent effect observed in conjunction with the leaving group dependence of the triplet decay dynamics are consistent with a concerted solvent assisted triplet cleavage through a heterolytic mechanism for the HPDP and HPPP photodeprotection process. Correlation of the dynamics between the deprotection and rearrangement processes reveals there is a consecutive mechanism and the involvement of an intermediate between the two reaction steps. The reaction rate of the deprotection and rearrangement steps and the influence of the solvent and leaving group were determined and evaluated based on kinetic modeling of the dynamical data obtained experimentally for HPDP and HPPP in H(2)O/MeCN mixed solvents with varying water concentration in the solvent system. A solvation complex with a contact ion pair character was proposed to be the intermediate involved in the deprotection and rearrangement pathway. The results here combined with our previous study on the photophysical events occurring on the early picosecond time scale (Ma; et al. J. Am. Chem. Soc. 2005, 127, 1463-1472) provide a real time overall mechanistic description for the photodeprotection and rearrangement reactions of pHP caged phosphate phototrigger compounds.

Water‐ and Acid‐Mediated Excited‐State Intramolecular Proton Transfer and Decarboxylation Reactions of Ketoprofen in Water‐Rich and Acidic Aqueous Solutions

“…[33] In addition, an ultrafast time-resolved transient absorption and resonance Raman spectroscopy study revealed that varying the water concentration in the solvent system changes the reaction rate of deprotection and rearrangement steps of the reaction for para- www.chemeurj.org hydoxyphenacyl diethylphosphate (HPDP) and para-hydroxyphenacyl dipehnylphosphate (HPPP) in H 2 O/MeCN mixed solvents. [34] Givens and co-workers also have suggested that a water molecule chain can carry a proton from the acidic p-OH site to the leaving-group phosphate. [35] In the case of KP, the hydrogen atom in the carboxyl group seems too far to be captured by the n-p* excited triplet-state carbonyl group by means of intramolecular hydrogen transfer.…”

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

“…for HPDP and HPA respectively, and these spectra indicate that the early-time 3 pp* evolution is similar for both the compounds in the two solvents with $7-12 ps time constants for the triplet formation [24]. However, the 3 pp* lifetime is substantially shorter in the H 2 O/MeCN mixed solvent than in the neat MeCN solvent, and this solvent-and leaving-group-dependent quenching of the triplet suggests that subsequent reaction(s) occur in the H 2 O-containing solvent [24,25]. Figure 13.12(a) displays early-time ps-KTR 3 spectra in the 1450-1800 cm À1 spectral range for HPDP in 50% H 2 O/50% MeCN mixed solvent, acquired with 400 nm and 342 nm probe wavelengths after 267 nm photolysis.…”

“…[23], and spectroscopic-grade acetonitrile and deionized water were employed as solvents to make samples for the TR 3 measurements with pH 9.0 (acetate þ phosphate þ borate, I ¼ 0.1 M) used in the aqueous samples. The HPA, HPDP and HPPP phototriggers were synthesized using the procedures detailed in Refs [24] and [25]. The identity and purity of these compounds were determined from analysis of MS, NMR and UVabsorption spectra obtained for the compounds.…”

“…Spectroscopic-grade MeCN, CF 3 CH 2 OH and DMSO, as well as deionized water, were used as solvents for the time-resolved experiments performed on the phototrigger compounds shown later in the chapter. The femtosecond transient absorption (fs-TA), Kerr gated time-resolved fluorescence (fs-KTRF) and ps-TR 3 experiments were all performed employing a Ti:sapphire regenerative amplifier laser system that also contained a home-made OPA system to give a tunable femtosecond/picosecond light source (the details of this laser system are given in Refs [24] and [25]). An optical delay line was used to control the difference in time between the pump (267 nm) and probe pulses employed in the different femtosecond/picosecond spectroscopy experiments.…”

Excited-state hydrogen bonding effect on dynamic fluorescence of coumarin 102 chromophore in solution: A time-resolved fluorescence and theoretical study

“…Therefore, the nature of hydrogen bond in solution is of particular interest and has been investigated extensively by various experimental and theoretical methods, since solute-solvent interactions play a fundamental role in molecular nonequilibrium processes in liquids [11][12][13][14][15][16][17][18][19][20][21][22][23]. However, little is known about electronic excited-state hydrogen bond.…”