Density functional theory studies of selected benzophenones and anthraquinones in neutral aqueous solutions

Abstract

Density functional theory combined with time-resolved transient absorption and resonance Raman spectrums, were employed to investigate the lowest triplet state and the photochemical mechanisms for selected aromatic carbonyl compounds in different solutions. The benzophenone (BP) and 2-(1-hydroxyethyl)-anthraquinone (HEAQ) have been found to favor triplet nπ* state while p-phenyl-benzophenone (p-PhBP) and 2-(p-hydroxymethyl)-phenylanthraquinone (PPAQ) prefer triplet ππ* as the lowest excited state in acetonitrile, which demostates the para-phenyl group can stabilize the triplet ππ* state much more than the triplet nπ*. Futhermore, AQs are more conjugated which also favor stabilization of the triplet ππ* state than BPs. The polar acetonitrile can lead the triplet ππ* state to mix with the triplet nπ* state while the nonpolar cyclohexane just can tolerate the triplet nπ* state act as the lowest excited state for m-hydroxyl-benzophenone (m-HBP). Our DFT calculations also showed that the para substituent group played more significant effect on the triplet ππ* state than the triplet nπ* state as compared with the meta- one. All the above results can be explained by the spin density analysis of the triplet nπ* state for BPs and AQs exhibit that two radicals resides on one carbonyl C and O atom, respectively. The other carbonyl or phenyls still remain inert so that the substituent electronic and position effects have little influence on stabilization of the triplet nπ* state. In contrast, the spins are mainly distributed on three atoms, the carbonyl O atom, the phenyl C atom which is adjacent to the carbonyl group, and the para-C atom on the aromatic ring for the triplet ππ* state. The radical carbonyl group and aromatic ring are produced after photoexcitation and the other parts still remain inert. The substituent group can exert its effect on the stabilization of the triplet ππ* state with the help of the radical aromatic ring. The triplet state nπ* or ππ* has decisive effect on the subsequent photoreaction promoted by the. The triplet nπ* induced a hydrogen atom transfer (HAT) process from isopropanol C-H bond to carbonyl O atom and then a ketyl radical intermediate was produced. The same ketyl radical intermediate was formed through another pathway, a proton-coupled electron transfer (PCET) promoted by the triplet ππ* in neutral aqueous solutions, an experimental phenomenon reported by several groups over the past several decades. The PCET process involves concerted but highly asynchronous proton transfer from solvent O-H bond to carbonyl group and electron transfer from solvent O-H bond to phenyl C atom. The PCET from a side-chain alcohol C-H bond to the para-carbonyl is also the initial and crucial process for the triplet ππ* HEAQ promoted photoredox to occur because the excited phenyl ring would favor proton extrusion from C-H bond. m-BPOH displays a different PCET mediated by triplet ππ* from the side-chain alcohol O-H bond to the carbonyl as the first step followed by an intersystem crossing process that does not lead to the analogous photoredox. It seems that para relationship between side-chain alcohol C-H bond and carbonyl group favors photoredox in neutral aqueous solution, which is significantly different from the meta-effect in acidic condition.