Search

The 3′,5′-dimethoxybenzoin (DMB) system has been widely investigated as a photoremovable protecting group (PRPG) for the elimination of various functional groups and has been applied in many fields. The photolysis of DMB fluoride leads to a highly efficient photocyclization–deprotection reaction, resulting in a high yield of 3′,5′-dimethoxybenzofuran (DMBF) in a MeCN solution, while there is a competitive reaction that produces DMB in an aqueous solution. The yield of DMB increased as the volume ratio of water increased. To understand the solvent effect of the photolysis of selected DMB-based compounds, a combination of femtosecond to nanosecond transient absorption spectroscopies (fs-TA and ns-TA), nanosecond time-resolved resonance Raman spectroscopy (ns-TR3) and quantum chemical calculation was employed to study the photophysical and photochemical reaction mechanisms of DMB fluoride in different solutions. Facilitated by the bichromophoric nature of DMB fluoride with electron-donating and -withdrawing chromophores, the cyclized intermediates could be found in a pure MeCN solution. The deprotection of a cyclic biradical intermediate results in the simultaneous formation of DMBF and a cyclic cation species. On the other hand, in aqueous solution, fs-TA experiments revealed that α-keto cations could be observed after excitation directly, which could easily produce the DMB through the addition of a hydroxyl within 8.7 ps. This work provides comprehensive photo-deactivation mechanisms of DMB fluoride in MeCN and aqueous conditions and provides critical insights regarding the biomedical application of DMB-based PRPG compounds. [ABSTRACT FROM AUTHOR]

Abstract Quinone methides are important reactive intermediates in photobiology and photochemistry, with little known regarding the photo-induces generation of reactive quinone methides species from relevant precursors in aqueous solutions. Here, the time-resolved spectroscopy methods including the transient absorption experiments and time-resolved Resonance Raman were employed to directly study the photophysics and photochemical processes of 6,6′-Bis(hydroxymethyl)-1,1′-binaphthyl-2,2′-diol (BQMP-a) in neat MeCN and 1:1 MeCN:H 2 O aqueous solutions. In aqueous solution, the intramolecular excited state proton transfer (ESIPT) process was observed where the water molecules help deliver the proton on the hydroxyl group to the naphthol ring within 125 ps in the singlet excited state of BQMP-a. This solvent mediated ESIPT will then lead to the leaving of the H 2 O group in 906 ps followed by the generation of the binol quinone methide (BQM-a) intermediate with a lifetime of 1.7 μs. As a result, the BQMP-a provides a different production rate of quinone methides compared with the 1,1′-(2,2′-Dihydroxy-1,1′-binaphthyl-6,6′-diyl) bis( N,N,N -trimethylmethanaminium) bromide system, which indicates the leaving group is an important factor in the generation of the reactive DNA alkylated intermediates. [ABSTRACT FROM AUTHOR]

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 (cis-DMB and trans-DMB) at ground state were found via DFT calculations. The influence of the intramolecular hydrogen bond on the α-cleavage of DMB was elaborated. [ABSTRACT FROM AUTHOR]

Nitrenium ions are common reactive intermediates with high activities towards some biological nucleophiles. In this paper, we employed femtosecond transient absorption (fs-TA) and nanosecond transient absorption (ns-TA) as well as nanosecond time-resolved resonance Raman (ns-TR3) spectroscopy and density function theory (DFT) calculations to study the spectroscopic properties of the N(4,4′–dibromodiphenylamino)–2,4,6–trimethylpyridinium BF4− salt (1) in an acidic aqueous solution. Efficient cleavage of the N–N bond (4 ps) to form the N,N–di(4–bromophenyl)nitrenium ion (DN) was also observed in the acidic aqueous solution. As a result, the dication intermediate 4 appears more likely to be produced after abstracting a proton for the nitrenium ion DN in the acid solution first, followed by an electron abstraction to form the radical cation intermediate 3. These new and more extensive time-resolved spectroscopic data will be useful to help to develop an improved understanding of the identity, nature, and properties of nitrenium ions involved in reactions under acidic aqueous conditions. [ABSTRACT FROM AUTHOR]

Nitrenium ions are important reactive intermediates in chemistry and biology. In this work, femtosecond and nanosecond transient absorption (fs-TA and ns-TA) along with nanosecond time-resolved resonance Raman (ns-TR3) experiments were employed to examine the photochemical pathways of N-(4,4′-dibromodiphenylamino)-2,4,6-trimethylpyridinium BF4− (salt (DN) from just absorption of a photon of light to the production of the important N,N-di(4-bromophenyl)nitrenium ion 2. In acetonitrile (MeCN), the formation of halogenated diarylnitrenium ion 2 was observed within 4 ps, showing the vibrational spectra with strong intensity. The nucleophilic adduct reaction of ion 2 with H2O was also examined in aqueous solutions. The direct detection of the unique ortho adduct intermediate 3 shows that there is an efficient and exclusive reaction pathway for 2 with H2O. The results shown in this paper give new characterization of 2, which can be used to design time-resolved spectroscopy investigations of covalent addition reactions of nitrenium ions with other molecules in future studies. [ABSTRACT FROM AUTHOR]