pH-dependent ultrafast photodynamics of <i>p</i> -hydroxyphenacyl: deprotonation accelerates the photo-uncaging reaction

Photolabile protecting groups (PPGs) possess broad application potential as they allow spatially and temporally controlled release of the protected group. While the photochemistry of many PPGs has been studied in detail, data under aqueous conditions or depending on the pH are extremely rare. However, for applications under biological conditions in water, knowledge about the photochemistry under these is critical. Here, we studied the pH-dependent reaction dynamics of <i>para</i>-hydroxyphenacyl (pHP), one prominent example of PPGs, by ultrafast transient absorption spectroscopy and quantum chemical simulation. At neutral pH, the <i>para</i>-hydroxyl group of pHP is protonated and photoproduct formation occurs within less than 1 ns from a triplet state. At basic pH, the main scaffold gets deprotonated leading to a bathochromic shift of the characteristic absorption. Upon deprotonation the substrate release occurs directly from a singlet state, shortcutting the rate determining step of intersystem crossing (ISC) in the neutral case, resulting in an acceleration of the photochemical reaction with photoproduct formation observed at ∼1 ps.