Reaction Mechanisms and Structural Characterization of the Reactive Intermediates Observed after the Photolysis of 3-(Hydroxymethyl)benzophenone in Acetonitrile, 2-Propanol, and Neutral and Acidic Aqueous Solutions

Intramolecular Photoredox Reaction Mechanism of Naphthoquinone Compounds: Combined Time-Resolved Spectroscopies and DFT Calculations

Time-resolved spectroscopies and DFT calculations were utilized to investigate the photoredox mechanisms of naphthoquinone compounds. 5-Methoxy-8-tetrahydropyrane-1,4-naphthoquinone (NQ) and 2-methyl-3-(3-methylbut-2-en-1-yl) 1,4-naphthoquinone (MNQ) were excited to singlet excited species (labeled NQ(S₁) and MNQ(S₁), respectively). NQ(S₁) underwent intersystem crossing to produce a triplet NQ, which further underwent hydrogen atom transfer to form a biradical intermediate. The biradical underwent electron transfer to form a zwitterion, followed by cyclization and proton transfer to generate a photoproduct. MNQ(S₁) underwent a 1,4-proton transfer process to produce a quinone methide intermediate (1,3-QM) with zwitterionic character, which tautomerized to 1,2-QM. Then, 1,2-QM underwent electrocyclization. The substituent on the parent naphthoquinone is the key factor leading to the different reaction processes for NQ and MNQ.

Ultrafast Time-Resolved Spectroscopic Study on the Photophysical and Photochemical Reaction Mechanisms of ortho-Methylbenzophenone in Selected Solutions

The photophysical and photochemical reaction pathways of ortho-methylbenzophenone (o-MeBP) in different solutions were investigated by employing femtosecond to nanosecond transient absorption and nanosecond time-resolved resonance Raman spectroscopy methods. In pure acetonitrile, neutral or pH 1 aqueous solutions, o-MeBP exhibit similar excited-state evolutions upon excitation in which o-MeBP will experience excitation to an excited state then undergo intersystem crossing and solvent arrangement followed by 1,5 hydrogen atom transfer processes to form the first singlet excited state, triplet state (n, π*), biradical intermediates, and enol form transients, respectively. However, in a pH 0 acidic solution, the protonation of o-MeBP will form the cation biradical intermediate that facilitates radical coupling to generate a benzocyclobutanol product, which causes a dramatic reduction of the lifetime of the enol form transients. In contrast, in sodium bicarbonate solution, the biradical intermediate may be quenched by the bicarbonate ion to construct a C-C bond and form the carboxylic acid that causes a fast decay of biradical intermediate. These results demonstrate that the photophysical and photochemical reaction pathways of o-MeBP are pH-dependent in aqueous solution which may be very useful for the capture of CO₂ capture by photoexcitation of aromatic ketones.

Time-Resolved Spectroscopic Observation and Characterization of Water-Assisted Photoredox Reactions of Selected Aromatic Carbonyl Compounds

In recent years, unusual and efficient self-photoredox reactions were detected for selected benzophenone derivatives (BPs) and anthraquinone derivatives (AQs) in aqueous environments by Wan and co-workers, where the carbonyl undergoes reduction to the corresponding alcohol and a side-chain alcohol group undergoes oxidation to the corresponding carbonyl. To unravel the photoredox reaction mechanisms of these types of BPs and AQs in aqueous environments, we have utilized a combination of time-resolved spectroscopy techniques such as femtosecond transient absorption, nanosecond transient absorption, and nanosecond time-resolved resonance Raman spectroscopy to detect and characterize the electronic absorption and vibrational spectra of the intermediates and transient species from the femtosecond to microsecond time region after they are generated in the photoredox reactions. With the assistance of density functional theory calculations to simulate the electronic absorption and Raman spectra, the structural and kinetic information on the key reactive intermediates is described. Furthermore, the reaction pathways were calculated by finding the transition states connecting with the reactant and product complexes to better understand the photoredox reaction mechanism. In this Account, we summarize some of our time-resolved spectroscopic observations and characterization of water-assisted photoredox reactions of selected BPs and AQs. In the strong hydrogen-donor solvent isopropanol, the commonly studied photoreduction reaction for aromatic carbonyls via an intermolecular hydrogen atom tranfer process was observed for BPs and AQs. The photoredox reactions for the investigated BPs and AQs in aqueous environments occur on the triplet excited-state surface. Under moderately acidic aqueous conditions, the photoredox reactions for BPs and AQs are triggered by a proton transfer (PT) pathway. In neutral aqueous solutions, AQs may also undergo proton-coupled electron transfer (PCET) leading to the photoredox reaction, while BPs generate the ketyl radical species. Both BPs and AQs prefer the photohydration reaction in high-proton-concentration aqueous solutions (pH 0). The PT and PCET processes were found to offer more possibilities for the aromatic carbonyl compounds to lead to new photochemical reactions like the unusual photoredox reactions associated with BPs and AQs described here. Clear characterization of the photophysical pathways and the photochemical reactions of representative aromatic carbonyl compounds in aqueous environments not only provides fundamental information to better understand the photochemistry of carbonyl-containing compounds but also will facilitate the development of applications of these systems, like photochemical synthesis, drugs, and photolabile protecting groups. In addition, the importance of water molecules in the photochemical reactions of interest here may also lead to further understanding of how water influences the photochemistry of related carbonyl-containing compounds in aqueous environments.

Kinetics and mechanism of the reaction of cyanocobalamin with potassium hydroxide in non-aqueous media

The reaction of cyanocobalamin (CNCbl) with potassium hydroxide (KOH) was studied in isopropyl alcohol (iPrOH) and dimethyl sulfoxide (DMSO) under anaerobic conditions.

pH Dependent Photodeprotection of Formaldehyde: Homolytic C-C Scission in Acidic Aqueous Solution versus Heterolytic C-C Scission in Basic Aqueous Solution

The photodeprotection of formaldehyde was investigated for 3-(1-hydroxypropan-2-yl)benzophenone (3-HPBP) with ultrafast time-resolved spectroscopy. The femtosecond transient absorption results indicated the singlet excited state of 3-HPBP transformed efficiently into its triplet state by a fast intersystem crossing. In acidic (pH = 0) and basic (pH = 12.5) aqueous solutions, the triplet intermediate was a key precursor for the deprotection of formaldehyde via two different pathways. However, little photodeprotection was observed in neutral (pH = 7) aqueous solution where the triplet intermediate appeared to undergo a proton coupled electron transfer process to form a ketyl radical transient. The important benzylic biradical intermediates seen in the acidic and basic aqueous solutions were identified by time-resolved resonance Raman spectra whose vibrational frequency patterns were consistent with DFT calculation results for the benzylic biradical intermediate. The results here indicate that the β-carbon alcohol group of the triplet state 3-HPBP is deprotonated in basic aqueous solutions and this leads to a heterolytic C-C bond cleavage to deprotect formaldehyde and produce the benzylic carbanion triplet state species, whereas protonation of the carbonyl moiety of the triplet state 3-HPBP leads to direct generation of a benzylic biradical intermediate and the deprotection of formaldehyde in acidic aqueous solutions via a homolytic C-C bond cleavage.

To Photoredox or Not in Neutral Aqueous Solutions for Selected Benzophenone and Anthraquinone Derivatives

The experimental and theoretical results in neutral aqueous solutions reported here indicate that a proton-coupled electron transfer (PCET) from an alcohol C-H bond to the para-carbonyl is the initial and crucial process for the photoredox reaction of 2-(1-hydroxyethyl)-anthraquinone (HEAQ) to occur while the counterpart 3-(hydroxymethyl)-benzophenone (3-BPOH) compound displays a different PCET from an alcohol O-H bond to the carbonyl as the first step, followed by an intersystem crossing process that does not lead to the analogous photoredox, which is caused by a subtle charge-radical coupled effect between HEAQ and 3-BPOH. This can account for experimental results in the literature that HEAQ can undergo efficient photoredox but 3-BPOH does not under neutral aqueous conditions. These results have implications for the pH-dependent photochemical behavior of aromatic carbonyl compounds in aqueous media.

Competition between "Meta Effect" Photochemical Reactions of Selected Benzophenone Compounds Having Two Different Substituents at Meta Positions

Recent studies conducted on some "meta effect" photochemical reactions focused on aromatic carbonyls having a substitution on one meta position of the benzophenone (BP) and anthraquinone parent compound. In this paper, two different substitutions were introduced with one at each meta position of the BP parent compound to investigate possible competition between different types of meta effect photochemistry observed in acidic solutions containing water. The photochemical pathways of 3-hydroxymethyl-3'-fluorobenzophenone (1) and 3-fluoro-3'-methylbenzophenone (2) were explored in several solvents, including acidic water-containing solutions, using time-resolved spectroscopic experiments and density functional theory computations. It is observed that 1 can undergo a photoredox reaction and 2 can undergo a meta-methyl deprotonation reaction in acidic water-containing solutions. Comparison of these results to those previously reported for the analogous BP derivatives that contain only one substituent at a meta position indicates the introduction of electron-donating (such as hydroxyl) and electron-withdrawing groups (such as F) on the meta positions of BP can influence the meta effect photochemical reactions. It was found that involvement of an electron-donating moiety facilitates the meta effect photochemical reactions by stabilizing the crucial reactive biradical intermediate associated with the meta effect photochemical reactions.

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

A novel self-photoredox reaction for 2-(1-hydroxyethyl)-9,10-anthraquinone has been theoretically rationalized to take place through two steps of triplet excited state intra-molecular proton transfer aided by water wires.

Investigation of the role of protonation of benzophenone and its derivatives in acidic aqueous solutions using time-resolved resonance Raman spectroscopy: how are ketyl radicals formed in aqueous solutions?

The formation mechanism of ketyl radicals and several other selective photoreactions of benzophenone and its derivatives are initiated by the protonation of their triplet state and have been investigated using nanosecond time-resolved resonance Raman spectroscopy (ns-TR(3)) in solutions of varying conditions. Evidence is found that the ketyl radical is generated by the combined action of a ketone protonation and a subsequent electron transfer based on the results from previous studies on the photochemistry and photophysics of benzophenone and the ns-TR(3) results reported here for benzophenone, 1,4-dibenzoylbenzene, 3-(hydroxymethyl)benzophenone, and ketoprofen in neutral and acidic solution. In order to better understand the role of the protonated ketone, results are summarized for some selective photochemical reactions of benzophenone and its derivatives induced by protonation in acidic solutions. For the parent benzophenone, the protonation of the ketone leads to the photohydration reactions at the ortho- and meta-positions of the benzene ring in acidic aqueous solutions. For 3-(hydroxymethyl)benzophenone, the protonation promotes an interesting photoredox reaction to become very efficient and the predominant reaction in a pH = 2 aqueous solution. While for ketoprofen, the protonation can initiate a solvent-mediated excited-state intramolecular proton transfer (ESIPT) from the carboxyl group to the carbonyl group that then leads to a decarboxylation reaction in a pH = 0 acidic aqueous solution. We briefly discuss the key role of the protonation of the ketone in the photochemistry of these aromatic ketones.

Time-resolved spectroscopic characterization of a novel photodecarboxylation reaction mediated by homolysis of a carbon α-bond in flurbiprofen

Flurbiprofen (Fp), a nonsteroidal anti-inflammatory drug (NSAID) currently in use for arthritis pain relief and in clinical trials for metastatic prostate cancer, can induce photosensitization and phototoxicity upon exposure to sunlight. The mechanisms responsible for Fp phototoxicity are poorly understood and deserve investigation. In this study, the photodecarboxylation reaction of Fp, which has been assumed to underpin its photoinduced side effects, was explored by femtosecond transient absorption (fs-TA), nanosecond transient absorption (ns-TA), and nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopic techniques in pure acetonitrile (MeCN) solvent. Density functional theory (DFT) calculations were also performed to facilitate the assignments of transient species. The resonance Raman and DFT calculation results reveal that the neutral form of Fp was the predominant species present in MeCN. Analysis of the ultraviolet/visible absorption spectrum and results from TD-DFT calculations indicate that the second excited singlet (S2) can be excited by 266 nm light. Due to its intrinsic instability, S2 rapidly underwent internal conversion (IC) to decay to the lowest lying excited singlet (S1), which was observed in the fs-TA spectra at very early delay times. Intriguingly, three distinct pathways for S1 decay seem to coexist. Specifically, other than fluorescence emission back to the ground state and transformation to the lowest triplet state T1 through intersystem crossing (ISC), the homolysis of the carbon α-bond decarboxylation reaction proceeded simultaneously to give rise to two radical species, one being carboxyl and another being the residual, denoted as FpR. The coexistence of the triplet Fp (T1) and FpR species was verified by means of TR(3) spectra along with ns-TA spectra. As a consequence of its apparent high reactivity, the FpR intermediate was observed to undergo oxidation under oxygen-saturated conditions to yield another radical species, denoted as FOR, which subsequently underwent intramolecular hydrogen transfer (IHT) and dehydroxylation (DHO) to form a final product, which could react with the carboxyl from the decarboxylation reaction to generate a minor final product. TD-DFT and transient state (TS) calculations for predicting the absorption bands and activation energies of the transient species produced in the photodecarboxylation reaction have provided valuable mechanistic insights for the assignment of the intermediate species observed in the time-resolved spectroscopy experiments reported here. The results of the time-resolved spectroscopy experiments and DFT calculations were used to elucidate the reaction mechanisms and intermediates involved in the photochemistry of Fp.

meta versus para substitution: how does C-H activation in a methyl group occur in 3-methylbenzophenone but does not take place in 4-methylbenzophenone?

The photophysical and photochemical reactions of 3-methylbenzophenone (3-MeBP) and 4-methylbenzophenone (4-MeBP) were investigated using femtosecond transient absorption (fs-TA) and nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopy and density functional theory (DFT) calculations. 3-MeBP and 4-MeBP were observed to behave similarly to their parent compound benzophenone (BP) in acetonitrile and isopropyl alcohol solvents. However, in acidic aqueous solutions, an unusual acid-catalyzed proton exchange reaction (denoted the m-methyl activation) of 3-MeBP (with a maximum efficiency at pH 0) is detected to compete with a photohydration reaction. In contrast, only the photohydration reaction was observed for 4-MeBP under the acidic pH conditions investigated. How the m-methyl activation takes place after photolysis of 3-MeBP in acid aqueous solutions is briefly discussed and compared to related photochemistry of other meta-substituted aromatic carbonyl compounds.

Organic aspects. Oxygen-containing functions

In this chapter, most of the reported work deals with the photochemistry of carbonyl compounds; however, the photoreactions of other functions, such as the photo-Claisen rearrangement or the photocleavage of cyclic ethers, are also included. In the present volume, time coverage is 2010–2011, and only original research articles are quoted. In general, reviews or purely theoretical calculations are not systematically included. As usually, the material is organized according to established types of reactions (e.g., Norrish I/II, hydrogen abstraction, Paternò-Büchi, photoelimination, photo-Fries/photo-Claisen, etc.). After presenting the basic photochemical aspects, more specific findings are reported. They include synthetic applications, stereoselectivity, and biological or technological implications. Next, the attention is focused on photochemical reactions in anisotropic media, including (micro)heterogeneous or supramolecular systems, solid matrixes or fully organized crystals. Finally, mechanistic studies based on direct experimental evidence are highlighted, especially when transient absorption spectroscopy or related ultrafast detection are employed.

Time-resolved resonance Raman spectroscopy: exploring reactive intermediates

The study of reaction mechanisms involves systematic investigations of the correlation between structure, reactivity, and time. The challenge is to be able to observe the chemical changes undergone by reactants as they change into products via one or several intermediates such as electronic excited states (singlet and triplet), radicals, radical ions, carbocations, carbanions, carbenes, nitrenes, nitrinium ions, etc. The vast array of intermediates and timescales means there is no single "do-it-all" technique. The simultaneous advances in contemporary time-resolved Raman spectroscopic techniques and computational methods have done much towards visualizing molecular fingerprint snapshots of the reactive intermediates in the microsecond to femtosecond time domain. Raman spectroscopy and its sensitive counterpart resonance Raman spectroscopy have been well proven as means for determining molecular structure, chemical bonding, reactivity, and dynamics of short-lived intermediates in solution phase and are advantageous in comparison to commonly used time-resolved absorption and emission spectroscopy. Today time-resolved Raman spectroscopy is a mature technique; its development owes much to the advent of pulsed tunable lasers, highly efficient spectrometers, and high speed, highly sensitive multichannel detectors able to collect a complete spectrum. This review article will provide a brief chronological development of the experimental setup and demonstrate how experimentalists have conquered numerous challenges to obtain background-free (removing fluorescence), intense, and highly spectrally resolved Raman spectra in the nanosecond to microsecond (ns-μs) and picosecond (ps) time domains and, perhaps surprisingly, laid the foundations for new techniques such as spatially offset Raman spectroscopy.