Direct Observation of Photoinduced Ultrafast Generation of Singlet and Triplet Quinone Methides in Aqueous Solutions and Insight into the Roles of Acidic and Basic Sites in Quinone Methide Formation

Abstract

Femtosecond time-resolved transient absorption spectroscopy experiments and density functional theory computations were done for a mechanistic investigation of 3-(1-phenylvinyl)phenol (1) and 3-hydroxybenzophenone (2) in selected solvents. Both compounds went through an intersystem crossing (ISC) to form the triplet excited states Tππ* and Tnπ* in acetonitrile but behave differently in neutral aqueous solutions, in which a triplet excited state proton transfer (ESPT) induced by the ISC process is also proposed for 2 but a singlet ESPT without ISC is proposed for 1, leading to the production of the triplet quinone methide (QM) and the singlet excited QM species respectively in these two systems. The triplet QM then underwent an ISC process to form an unstable ground state intermediate which soon returned to its starting material 2. However, the singlet excited state QM went through an internal conversion process to the ground state QM followed by the formation of its final product in an irreversible manner. These differences are thought to be derived from the slow vinyl C-C rotation and the moderate basicity of the vinyl C atom in 1 as compared with the fast C-O rotation and the greater basicity of the carbonyl O atom of 2 after photoexcitation. This can account for the experimental results in the literature that the aromatic vinyl compounds undergo efficient singlet excited state photochemical reactions while the aromatic carbonyl compounds prefer triplet photochemical reactions under aqueous conditions. These results have fundamental and significant implications for understanding of the ESPT reactivity in general, as well as for the design of molecules for efficient QM formation in aqueous media with potential applications in cancer phototherapy.

Time-Resolved Spectroscopic and Density Functional Theory Investigation of the Photogeneration of a Bifunctional Quinone Methide in Neutral and Basic Aqueous Solutions

“…Time-dependent DFT (TD-DFT) calculations have been previously illustrated to be helpful in evaluating and identifying the absorption features of intermediate species [23,24]. Based on the spectral data, a simple mechanism can be deduced that the NMe 3 group in QMP-b leaves in a process to form the QM species due to an E1cb elimination reaction [25,26].…”

“…The experiments here used the same experimental setups and methods detailed in previous work [24,29].…”

Reaction Mechanisms of Photoinduced Quinone Methide Intermediates Formed via Excited-State Intramolecular Proton Transfer or Water-Assisted Excited-State Proton Transfer of 4-(2-Hydroxyphenyl)pyridine

“…We used ultrafast femtosecond transient absorption (fs-TA) and density functional theory (DFT) computations to study the photoinduced QM formation of 3-(1-phenylvinyl)phenol, where a concerted singlet ESPT was proposed. 32 Cui and Thiel utilized theoretical calculations on 2-phenylphenol and revealed that a barrierless ESIPT occurred to yield QM. 33 Significantly, a notion that PT to carbon competes with heteroatoms, with both pathways giving rise to QMs, was reported.…”

“…ESIPT or ESPT involving phenols to give rise to a QM that generally contains phenol as the acidic site and heteroatoms like an aromatic heterocyclic nitrogen atom as the basic site. , The first case for photoinduced QMs formation by ESIPT to carbon was o- hydroxystyrene. , Wan explored hydroxyl substituted biaryl , or triaryl systems − giving rise to the corresponding QM by ESIPT, where the carbon of adjacent aromatic moieties served as the basic site. We used ultrafast femtosecond transient absorption (fs-TA) and density functional theory (DFT) computations to study the photoinduced QM formation of 3-(1-phenylvinyl)­phenol, where a concerted singlet ESPT was proposed . Cui and Thiel utilized theoretical calculations on 2-phenylphenol and revealed that a barrierless ESIPT occurred to yield QM …”

Revealing Ultrafast Energy Dissipation Pathway of Nanocrystalline Sunscreens Oxybenzone and Dioxybenzone

“…To explain this unexpected phenomenon, DFT and time-dependent density function theory (TD-DFT) calculations, which previously successfully uncovered the reaction mechanism of the excited state for quinone methides, were carried out via employing the M062X method and 6-311G­(d,p) basis set to perform the potential energy surface (PES) scanning to reveal the main energy dissipation pathway of these two molecules referring to the former nonadiabatic dynamics simulations (Figure ). The result suggests that the ground-state OB will be excited to the S 2 state after irradiation, ,, because the oscillator strength of the S 1 state is approximately zero, whereas the value of the S 2 state is much larger, whose f = 0.2136.…”