Photochemical α-Cleavage Reaction of 3',5'-Dimethoxybenzoin: A Combined Time-Resolved Spectroscopy and Computational Chemistry Study

• Published in: Molecules (Basel, Switzerland) Vol. 25; no. 15; p. 3548
• Main Authors: Li, Yuanchun, Zhang, Xiting, Yan, Zhiping, Du, Lili, Tang, Wenjian, Phillips, David Lee
• Format: Journal Article
• Language: English
• Published: Switzerland MDPI AG 03.08.2020; MDPI
• Subjects: Absorption; Aldehydes; Atmospheric chemistry; Atomic energy levels; Benzoin; Benzoin - analysis; Benzoin - chemistry; Chromatography, Gas; Cleavage; Computational chemistry; Computer applications; Density Functional Theory; DFT calculation; Driving ability; Experiments; Free radical polymerization; Free radicals; Free Radicals - chemistry; Hydrogen bonds; Kinetics; Light; Photochemical reactions; Photochemicals; Photodissociation; photoinitiator; Photoinitiators; Polymerization; Reaction mechanisms; Spectrophotometry; Spectroscopy; transient absorption spectroscopy; α-cleavage; DFT calculation; α-cleavage; benzoin; photoinitiator; transient absorption spectroscopy
• ISSN: 1420-3049; 1420-3049
• DOI: 10.3390/molecules25153548

Benzoin is one of the most commonly used photoinitiators to induce free radical polymerization. Here, improved benzoin properties could be accomplished by the introduction of two methoxy substituents, leading to the formation of 3',5'-dimethoxybenzoin (DMB) which has a higher photo-cleavage quantum yield (0.54) than benzoin (0.35). To elucidate the underlying reaction mechanisms of DMB and obtain direct information of the transient species involved, femtosecond transient absorption (fs-TA) and nanosecond transient absorption (ns-TA) spectroscopic experiments in conjunction with density functional theory/time-dependent density functional theory (DFT/TD-DFT) calculations were performed. It was found that the photo-induced α-cleavage (Norrish Type I reaction) of DMB occurred from the nπ* triplet state after a rapid intersystem crossing (ISC) process (7.6 ps), leading to the generation of phenyl radicals on the picosecond time scale. Compared with Benzoin, DMB possesses two methoxy groups which are able to stabilize the alcohol radical and thus result in a stronger driving force for cleavage and a higher quantum yield of photodissociation. Two stable conformations ( -DMB and -DMB) at ground state were found via DFT calculations. The influence of the intramolecular hydrogen bond on the α-cleavage of DMB was elaborated.