A theoretical investigation of p-hydroxyphenacyl caged phototrigger compounds: An examination of the excited state photochemistry of p-hydroxyphenacyl acetate

Ab initio and density functional theory methods were employed to study the excited states and potential energy surfaces of the p-hydoxyphenacyl acetate (HPA) phototrigger compound. Complete active space (CAS) ab initio calculations predicted adiabatic electronic transition energies for the HPA-T 1(3nπ*), HPA-T2( 3ππ*), HPA-S1(1nπ*), HPA-T3(3nπ*), HPA-S2( 1nπ*), HPA-S3(1ππ*) ← HPA-S0 transitions that were similar to and in agreement with those found experimentally for closely related aromatic ketones such as p-hydroxyacetophenone and results from similar calculations for other related aromatic carbonyl systems. The α or β bond cleavage reactions from the S1 excited state were both found to have relatively high barriers to reaction, and the S1, T1, and T2 states are close in energy with the three S1(1nπ*), T 1(3nπ*), and T2( 3ππ*) surfaces intersecting at the same region. The calculations suggest that intersystem crossing (ISC) can occur very fast from the S1 state to the nearby triplet states. This is consistent with results from ultrafast spectroscopy experiments that observe the S1 state ISC occurs within about 1-2 ps to produce a triplet state for HPA and related pHP compounds. The α and β bond cleavage reactions for the T1 state of HPA are both predicted to have fairly high barriers and compete with one another. However, this is not completely consistent with experiments that observe the photodeprotection reactions (e.g. the β bond cleavage) of HPA and some other pHP phototriggers in largely water containing solvents are predominant and occur very fast to release the leaving group. Comparison of the computational results with experimental results for HPA and related pHP compounds suggests that water molecules likely play an important part in changing the triplet state β bond cleavage so that it becomes the predominant pathway and occurs very fast to give an efficient deprotection reaction. The results reported here provide new insight into the photophysics, reaction pathways, and photochemistry of the p-hydoxyphenacyl acetate and related pHP caged phototrigger compounds and also provide a benchmark for further and more sophisticated investigations in the future.