Water-Assisted Self-Photoredox of 3‑(Hydroxymethyl)benzophenone: An Unusual Photochemistry Reaction in Aqueous Solution

Abstract

An unusual photochemistry of water-assisted self-photoredox of 3-(hydroxymethyl) benzophenone <b>1</b> has been investigated by CASPT2//CASSCF computations. The water-assisted self-photoredox is found to proceed via three sequential reactions: an excited-state intermolecular proton transfer (ESIPT), a photoinduced deprotonation, and a self-redox reaction. Upon photoexcitation at 243 nm, the system of <b>1</b> is taken to the Franck–Condon region of a short-distance charge transfer (SCT) state of S_SCT(¹ππ*) and then undergoes ESIPT with a small barrier of ∼3.4 kcal/mol producing the intermediate <b>2</b>. Subsequently, the singlet–triplet crossing (STC) of STC (¹ππ*/³ππ*) relays <b>2</b> by intersystem crossing to the T_SCT(³ππ*) state followed by a deprotonation reaction overcoming a moderate barrier of ∼8.0 kcal/mol and finally produces the triplet biradical intermediate <b>3</b>. Another moderate barrier (∼5.8 kcal/mol) in the T_SCT(³ππ*) state has to be overcome so as to relax to a second singlet–triplet crossing STC­(T/S₀) that allows an efficient spin-forbidden decay to the ground state. The self-redox reaction aided by water molecules occurs with tiny barriers in the S₀ state via two steps, protonation of the benzhydrol carbon to produce intermediate <b>4</b> and then deprotonation from the benzylic oxygen to yield the final product 3-formylbenzhydrol <b>5</b>