Influence of Water in the Photogeneration and Properties of a Bifunctional Quinone Methide

Time-Resolved Spectroscopic Observation of Diphenylnitrenium Ion Reactions with Guanosine

Arylnitrenium ions have gained attention for their high reactivity toward guanosine, which in some cases has been linked to carcinogenesis. Although many studies have examined covalent addition reactions between arylnitrenium ions and guanosine, there is still some uncertainty regarding the attack position of nitrenium ions on guanosine and its derivatives. In this paper, we employ nanosecond transient absorption and nanosecond time-resolved resonance Raman spectroscopy to investigate the reaction between the N,N-di(4-bromophenyl) nitrenium ion (2) and guanosine. Our time-resolved spectroscopic results and photochemical product analysis results show that the reaction of guanosine with 2 generates an N7 intermediate that subsequently undergoes rearrangement and deprotonation to produce a C8 adduct. Comparing these results to our previous study between the 2-fluorenylnitrenium ion and guanosine indicates that the structure and properties of arylnitrenium ions are able to influence the reaction pathways and intermediate structures.

Time-Resolved Spectroscopic Study of N,N-Di(4-bromo)nitrenium Ions in Acidic Aqueous Solution

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.

Stoichiometric Nitroxyl Photorelease Using the (6-Hydroxy-2-naphthalenyl)methyl Phototrigger

The design and synthesis of a photoactivatable HNO donor incorporating the (6-hydroxynaphthalen-2-yl)methyl (6,2-HNM) photocage coupled to the trifluoromethanesulfonamidoxy analogue of the well-established HNO generator Piloty's acid is described. The photoactive HNO donor stoichiometrically generates HNO (∼98%) at neutral pH conditions, and evidence for concerted C-O and N-S bond cleavage was obtained. The methanesulfonamidoxy analogue primarily undergoes undesired N-O bond cleavage.

Time-Resolved Spectroscopic Study of N,N-Di(4-bromo)nitrenium Ions in Selected Solutions

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.

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

Binol quinone methides (BQMs) can be generated from 1,1'-(2,2'-dihydroxy-1,1'-binaphthyl-6,6'-diyl)bis(N,N,N-trimethylmethanamiuium) bromide (BQMP-b) in a 1:1 MeCN:H₂O mixed solution via a ground state intramolecular proton transfer (GSIPT), as mentioned in our previously reported studies. Here, the photoreaction of BQMP-b in neutral and basic aqueous solution (pH = 7, 10, 12) was investigated to explore the possible mechanisms and the key intermediates produced in the process of the photoreaction and to examine whether they are different from those in a neutral mild-mixed MeCN:H₂O solution. The studies were conducted using femtosecond transient absorption (fs-TA), nanosecond transient absorption (ns-TA), and nanosecond time-resolved resonance Raman spectroscopy (ns-TR³) in conjunction with results from density functional theory (DFT) computations. The results showed that BQMP-b was deprotonated initially and produced BQMs species more effectively through an E1bc elimination reaction in a strong basic aqueous condition (pH = 12), which differed from the reaction pathway that took place in the solution with pH = 7 or 10. A related single naphthol ring molecule 1-(6-hydroxynaphthalen-2-yl)-N,N,N-trimethylmethanaminium bromide (QMP-b) that did not contain a second naphthol ring was also investigated. The related reaction mechanisms are elucidated in this work, and it is briefly discussed how the mechanisms vary as a function of aqueous solution pH conditions.

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

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.

Direct Detection of the Open-Shell Singlet Phenyloxenium Ion: An Atom-Centered Diradical Reacts as an Electrophile

A new photoprecursor to the phenyloxenium ion, 4-methoxyphenoxypyridinium tetrafluoroborate, was investigated using trapping studies, product analysis, computational investigations, and laser flash photolysis experiments ranging from the femtosecond to the millisecond time scale. These experiments allowed us to trace the complete arc of the photophysics and photochemistry of this photoprecursor beginning with the initially populated excited states to its sequential formation of transient intermediates and ultimate formation of stable photoproducts. We find that the excited state of the photoprecursor undergoes heterolysis to generate the phenyloxenium ion in ∼2 ps but surprisingly generates the ion in its open-shell singlet diradical configuration (¹A₂), permitting an unexpected look at the reactivity of an atom-centered open-shell singlet diradical. The open-shell phenyloxenium ion (¹A₂) has a much shorter lifetime (τ ∼ 0.2 ns) in acetonitrile than the previously observed closed-shell singlet (¹A₁) phenyloxenium ion (τ ∼ 5 ns). Remarkably, despite possessing no empty valence orbitals, this open-shell singlet oxenium ion behaves as an even more powerful electrophile than the closed-shell singlet oxenium ion, undergoing solvent trapping by weakly nucleophilic solvents such as water and acetonitrile or externally added nucleophiles (e.g., azide) rather than engaging in typical diradical chemistry, such as H atom abstraction, which we have previously observed for a triplet oxenium ion. In acetonitrile, the open-shell singlet oxenium ion is trapped to generate ortho and para Ritter intermediates, one of which (para) is directly observed as a longer-lived species (τ ∼ 0.1 ms) in time-resolved resonance Raman experiments. The Ritter intermediates are ultimately trapped by either the 4-methoxypyridine leaving group (in the case of para addition) or trapped internally via an essentially barrierless rearrangement (in the case of ortho addition) to generate a cyclized product. The expectation that singlet diradicals react similarly to triplet or uncoupled diradicals needs to be reconsidered, as a recent study by Perrin and Reyes-Rodríguez (J. Am. Chem. Soc. 2014, 136, 15263) suggested the unsettling possibility that singlet p-benzyne could suffer nucleophilic attack to generate a naked phenyl anion. Now, this study provides direct spectroscopic observation of this phenomenon, with an atom-centered open-shell singlet diradical reacting as a powerful electrophile. To the question of whether a nucleophile can attack a singly occupied molecular orbital, the answer is apparently yes, at least if another partially occupied orbital is available to avoid violation of the rules of valence.