Solvent Effects on Photoreactivity of Valerophenone: A Combined QM and MM Study

Abstract

Norrish type reactions for valerophenone in aqueous solution have been investigated by using the combined methods of DFT, CASSCF, and CASPT2 with molecular mechanics. It was found that formation of the intermolecular hydrogen bond in the complex of valerophenone with water results in a blue shift of the n,pi* excited states, while the Coulomb interaction between valerophenone and the bulk surrounding water is mainly responsible for the red shift of the pi,pi* excited states. As a result, the (3)pi pi* state becomes the lowest triplet state and is responsible for the long triplet lifetime observed for aqueous valerophenone. The (1)n pi*, (3)pi pi*, and (3)n pi* states were found to intersect in the same structural region, which appears to be the main reason why the intersystem crossing from (1)n pi* to (3)n pi* is very efficient for aqueous valerophenone and why most aromatic ketones have unique photophysical features such as a short singlet lifetime, high phosphorescence, and weak fluorescence. The Coulomb interaction between valerophenone and the bulk surrounding water has a significant influence on the alpha-C-C cleavage and the 1,5-H shift reaction. The 1,5-H shift is predicted to have a very small barrier on the triplet pathway and the alpha-C-C bond cleavages are not in competition with the 1,5-H shift reaction. This is in good agreement with the experimental findings that Norrish type II quantum yield is close to unity upon photoexcitation of aqueous valerophenone in the wavelength region of 290-330 nm.

The effects of Lewis acid complexation on type I radical photoinitiators and implications for pulsed laser polymerization

“…[55][56][57][58][59] The 1 (nπ*), 3 (ππ*), and 3 (nπ*) exited states of these species usually intersect in a similar structural region, which leads to short singlet lifetimes, rapid ISCs and minimal fluorescence. 60 Type I photoinitiators rapidly fragment via α-cleavage from their 3 (nπ*) state, while Type II initiators abstract a hydrogen atom from a coinitiating group (usually an alcohol or amine). As the excited states of acetophenone derivatives normally intersect in a similar structural region, time dependent-density functional theory (TD-DFT) calculations were used to investigate the impacts of complexation on photoexcitation behaviour.…”

“…This reordering could potentially lead to much slower rates of photolysis and longer triplet lifetimes, as 3 (ππ*) states do not readily undergo fragmentation. 60 Recent work has demonstrated that various amine-substituted benzoin derivatives, which also possess a multitude of low lying (ππ*) type triplet states, have very long triplet lifetimes and are uncharacteristically poor photoinitiators. 24 Moreover, as Fang and co-workers outlined, analogous reordering of 3 (ππ*) and 3 (nπ*) states in valerophenone significantly lengthens triplet lifetimes in aqueous solution.…”

Ketyl Radical Formation via Proton-Coupled Electron Transfer in an Aqueous Solution versus Hydrogen Atom Transfer in Isopropanol after Photoexcitation of Aromatic Carbonyl Compounds

“…The nπ*/ππ* electronic properties of the BPs and AQs of interest here reveal the clear and definitive difference that the photoexcitation invokes only one carbonyl group in the nπ* state, while one carbonyl group and one phenyl group are photoexcited in the ππ* state, which is consistent with CASSCF results. 9 This naturally brings up the question of whether the ArPK formation would depend differently on the nπ* and ππ* species. To answer this question, it is important to evaluate the relative stability of the above triplet species for each compound ( Figure 2).…”

Water-assisted self-photoredox of 2-(1-hydroxyethyl)-9,10-anthraquinone through a triplet excited state intra-molecular proton transfer pathway

“…To further explore the mechanistic details of the water-mediated self-photoredox reaction of AQ compounds we have done a more detailed theoretical investigation of the reaction mechanism that is now reported here. In our previous studies, [24][25][26][27][28][29] high-level electronic structure calculations provided detailed information on the Frank-Condon (FC) excitation, reactive intermediates, products and the relaxation pathways of various aromatic compounds, which could be compared in a quantitative manner with experimental observations. [30][31][32][33][34][35] In this study, we employ the same CASPT2//CASSCF computational protocol to describe explicitly the properties of the initial excitation and subsequent relaxation pathways in an effort to unravel the mechanism of the self-photoredox reaction for AQ compounds.…”