Time-Resolved Spectroscopic Observation and Characterization of

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Time-Resolved Spectroscopic Observation and Characterization of Water-Assisted Photoredox Reactions of Selected Aromatic Carbonyl Compounds Jiani Ma,*,† Xiting Zhang,‡ and David Lee Phillips*,‡ †

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Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an 710127, P. R. China ‡ Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. China CONSPECTUS: 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 highproton-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.

1. INTRODUCTION Organic photoredox reactions are rapidly expanding topics, and many systems are focused on photoredox catalysis and applications in the areas of water splitting, reduction of carbon dioxide, and the development of new solar cell materials.1−8 In recent years, an unusual and efficient photoredox reaction in which the carbonyl is reduced to its alcohol and the side-chain alcohol moiety undergoes oxidation to its carbonyl have been discovered for selected benzophenone derivatives (BPs) and anthraquinone derivatives (AQs) in aqueous environments.9−14 These types of reactions piqued interest because of their efficient quantum yields and potential applications in new photon-driven synthetic methods, but because of a dearth © XXXX American Chemical Society

of direct detection and characterization of the reactive intermediates generated, the photoredox mechanisms remained unclear for a time. Because a lot of reactive intermediates have very short lifetimes, a range of time-resolved spectroscopic techniques have been invented to detect and characterize these shortlifetime reactive intermediates. Time-resolved spectroscopic study is a fast-growing area,15 where the time-resolved electronic absorption techniques femtosecond transient absorption (fs-TA) spectroscopy and nanosecond transient Received: December 6, 2018

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DOI: 10.1021/acs.accounts.8b00619 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Scheme 1. Overall Photoredox Reaction and Products Generated from Photoexcitation of 3-BPOH in Acidic Aqueous Solution (pH < 3)11

Scheme 2. Tentative Mechanism Proposed by Wan and Coworkers for the Observed Water-Assisted Photoredox Reaction of 3-BPOH11

To unravel the photoredox mechanisms of the selected BPs and AQs in aqueous environments, we utilized a combination of time-resolved fs-TA, ns-TA, and ns-TR3 spectroscopies to measure the electronic absorption and vibrational spectra of the transient species generated in the photoredox processes of interest.36−38 This Account is dedicated to summarizing our time-resolved spectroscopic observations and characterizations of water-assisted photoredox reactions of selected BPs and AQs. Further information and details regarding the fs-TA, nsTA, and ns-TR3 instrumentation and methods used in our laboratories can be gained from our original publications on the photoredox reactions described in this Account and elsewhere.39 In section 2 we present several examples of our laboratory’s use of time-resolved spectroscopic observation and characterization to study the water-assisted photoredox reactions of selected BPs and AQs. After that, we will end this Account with a conclusion and outlook.

absorption (ns-TA) spectroscopy are the most commonly used methods and can differentiate molecules in different electronic states on the femtosecond and nanosecond time scales, respectively.16−19 However, in cases where the intermediates display similar-wavelength electronic absorptions because they contain similar chromophores and they also have comparable lifespans, it becomes challenging to unambiguously identify and tell different transient species apart from one another using time-resolved TA spectra alone. In addition, the changes seen in the fs-TA and ns-TA spectra may not distinguish between variations in the distributions of the electronic structure and vibrational energy. Moreover, because the electronic absorptions are typically wide and featureless, it is difficult to derive details about the chemical geometry of the reactive transients from these TA spectra obtained in solution at room temperature. The investigation of molecular structural changes during a chemical reaction can be viewed as one of the fundamental topics in chemistry.20 Vibrational spectroscopy, available from Raman or infrared, can offer “fingerprint” information because the frequencies of the vibrations are sensitive to bonding and structural arrangements of atoms in functional groups of a molecule.21−28 In addition, kinetics information for the reactions can be obtained by following the intensities of vibrational bands versus time. Time-resolved vibrational spectroscopies like nanosecond time-resolved resonance Raman (ns-TR3) spectroscopy have been demonstrated to be able to provide chemical structure information about intermediates and transient species29−34 as a result of the fairly big number of features seen for the vibrational spectrum for the transient species, the relatively thin widths of these features, and the high sensitivity of the frequencies of the vibrations to electronic distribution variations in the molecule.35 For reactions occurring in water-containing solutions, the time-resolved Raman technique stands out because of the low Raman scattering of H2O, whereas the utility of infrared spectroscopy can suffer from the strong absorbance of H2O molecules, which may leave only restricted “detection windows” for infrared spectroscopy in aqueous environments.

2. INVESTIGATIONS OF PHOTOREDOX REACTIONS OF SELECTED AROMATIC CARBONYL COMPOUNDS In this section, we present our observations and characterizations of water-assisted photoredox reactions of selected aromatic carbonyl compounds using fs-TA, ns-TA, and ns-TR3 spectroscopies. The assignment and characterization of the experimental spectra and the investigation of the reaction mechanisms are facilitated with DFT calculations to simulate the electronic absorption and normal Raman spectra as well as to calculate the reaction pathways to improve understanding of the photoredox processes. a. Investigation of Photoredox Reactions of BPs in Acidic Aqueous Solutions

BP is viewed as a benchmark compound in photochemical research, and its derivatives also draw great interest for their excellent photosensitivity.40−42 BPs are well-known for their efficient intersystem crossing (ISC) to the triplet excited state, which acts as the reactive precursor to a number of photochemical reactions of BPs.43,44 The intermolecular hydrogen atom transfer (HAT) reaction can be viewed as one of the most commonly studied processes of BPs, which B


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Figure 1. (a, b) fs-TA and (c) ns-TA spectra of 3-BPOH in MeCN obtained under 266 nm excitation. (d) Intensity dependences seen at 348 and 525 nm in MeCN in the fs-TA spectra. Adapted from ref 36. Copyright 2011 American Chemical Society.

Figure 2. (left) ns-TR3 spectra seen after 266 nm excitation of 3-BPOH in IPA solution utilizing a 319.9 nm probe at varying times denoted adjacent to the spectra. The asterisks (*) label areas influenced by stray light and/or solvent subtraction artifacts. (right) The ns-TR3 spectrum acquired at 90 ns after photolysis of 3-BPOH in IPA (top) is compared to the simulated normal Raman spectrum of the ArPK of 3-BPOH species (bottom). The schematic structure of the ArPK species of 3-BPOH is shown at the top of the figure. Adapted from ref 36. Copyright 2011 American Chemical Society.

occurs in strong hydrogen-donor solvents like isopropanol (IPA) and typically leads to an overall photoreduction reaction.45,46 The photophysical and photochemical behaviors

of BPs in aqueous solutions have not been as widely studied. In recent years, a new water-assisted photoredox process was found for BPs (e.g., 3-hydroxymethylbenzophenone, 3-BPOH; C


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found in the fs-TA spectra for 3-BPOH under more acidic conditions (pH 0, 1:1 MeCN/H2O). It is well-acknowledged that the oxygen atom of aromatic carbonyls becomes more basic upon electronic excitation. Especially, the photohydration of BP was unraveled to take place first via protonation of the carbonyl oxygen in an aqueous solution at pH 0.48,49 Therefore, the consumption of (3-BPOH)3 seen in the fs-TA results in acidic aqueous solutions (pH 0 and pH 2) likely stems from protonation of (3-BPOH)3 to generate its oxygenprotonated transient. It is reasonable to propose that the photoredox process found in the pH 2 aqueous environment is also initiated by protonation of (3-BPOH)3. The ns-TR3 results for 3-BPOH in a pH 2 acidic aqueous environment are displayed in Figure 3. A characteristic species

see Scheme 1) in acidic aqueous solutions by Wan and coworkers, in which the carbonyl undergoes reduction to its alcohol and a side-chain alcohol undergoes oxidation to its carbonyl.11 The very efficient quantum yield of this photoredox reaction for BPs has potential applications for new photochemical synthetic methods. On the basis of the final product analysis and ns-TA spectroscopy, a preliminary photoredox reaction mechanism for 3-BPOH was proposed by Wan and co-workers (see Scheme 2).11 However, because of a lack of direct transient species spectroscopy, especially for structural information, the photoredox reaction mechanism for 3-BPOH needed further verification. As acetonitrile (MeCN) is a relatively inert organic solvent, only the photophysical processes were observed for BP and its derivatives in this solvent system.40,42 Time-resolved spectroscopic experiments were done for 3-BPOH in MeCN to gain characteristic spectra to facilitate the studies under the more complicated conditions of mixed acidic aqueous solution where the photoredox reaction was observed.36 Figure 1 shows the fs-TA and ns-TA spectra acquired for 3-BPOH in MeCN after 266 nm excitation. Upon excitation, the ground-state 3-BPOH is populated to a more energetic excited singlet state (denoted as Sn) absorbing at 340 nm. The transformation in the early-time TA spectra may be attributed to the relaxation of Sn to the lowest singlet excited state (denoted as (3-BPOH)1), and the transient that arises at 580 nm at around 1 ps can be attributed to the (3BPOH)1 → Sn absorption.40 Then a transient with a broad feature appears near 530 nm.36 The transformation time obtained from the fs-TA study is similar to that for the time constant of ISC for BP in MeCN.47 Later on the spectra resemble those seen in the ns-TA spectra detected for 3-BPOH in MeCN (Figure 1c). Hence, the transformation detected for 3-BPOH in MeCN by fs-TA is due to ISC from (3-BPOH)1 to (3-BPOH)3, and the transient spectra in Figure 1b are due to the (3-BPOH)3 → Tn electronic absorption. As one may expect, the ns-TR3 spectra obtained for 3-BPOH in MeCN displayed characteristic Raman feasures for (3-BPOH)3, with the largest features seen at 967, 1229, and 1546 cm−1.36 Three species were detected for 3-BPOH in IPA by ns-TR3 spectroscopy (Figure 2).36 Because the spectrum has high similarity to that recorded in MeCN, the first peak appearing at early times is assigned to (3-BPOH)3. Considering the fact that IPA is a strong hydrogen donor, the second transient with Raman features at 989, 1176, 1481, 1584, and 1697 cm−1 can be assigned to the aromatic ketyl radical species (ArPK) of 3BPOH, and further support comes from the excellent correspondence between the DFT-computed Raman spectrum of the ArPK of 3-BPOH and the experimental spectrum (Figure 2, right). Besides the (3-BPOH)3 and ArPK transients, the third transient appearing at longer time delays in Figure 2, showing a diagnostic band at 1654 cm−1, can be assigned to the intermediate via a coupling reaction between the ArPK species and the radical species of the solvent (see ref 36 for details). Fs-TA experiments were performed for 3-BPOH in a pH 2 MeCN/H2O solution. The initial spectral profiles are very similar to those found in MeCN, and ISC was also seen in the early-time spectra.36 The difference is that after 30 ps the (3BPOH)3 peak began to decay. The quick decrease in intensity of (3-BPOH)3 indicated that it is the precursor to a new photochemical process. Similar experimental results were

Figure 3. ns-TR3 spectra acquired subsequent to 266 nm photoexcitation of ∼3 × 10−4 M 3-BPOH in an acidic aqueous solution (pH 2, 1:1 MeCN/H2O) under open air conditions utilizing a 319.9 nm probe at different times denoted adjacent to the spectra. The asterisks (*) label areas influenced by stray light and/or solvent subtraction artifacts. Adapted from ref 36. Copyright 2011 American Chemical Society.

under this solvent system was probed with its Raman features at 962 and 1518 cm−1, which could result from the photoredox reaction observed by final product analysis under analogous experimental conditions. An oxygen purging experiment observed an obviously shorter lifetime of the new transient utilizing the characteristic 962 cm−1 feature than in the open air environment, suggesting that this new transient appearing under acidic aqueous conditions is triplet in nature and probably associated with the protonated diradical transient 4 or xylylene diradical species 5 in the proposed reaction pathways suggested in Scheme 2.36 The 120 ns time delay spectrum seen in the pH 2 aqueous environment from Figure 3 was chosen as characteristic of the reactive transient related to the photoredox process and was respectively compared with the calculated spectra for species 4 and 5 (Figure 4). Obviously, the experimental spectrum D


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been intensely investigated, with the efficient ISC and HAT reactions being the most commonly studied processes. For their excellent photosensitivity, AQs have been utilized as a photolabile protecting group platform.37 Similar to their close BP relatives, some AQs such as 2-(1-hydroxyethyl)-9,10anthraquinone (2-HEAQ) were also found to undergo highly efficient photoredox reactions (see Scheme 3).9,13,14 However, the photoredox reaction mechanisms for 2-HEAQ remained uncertain at that time because of the dearth of structural information on the reactive intermediates. We performed fsTA, ns-TA, and ns-TR3 experiments to study the photoredox processes for 2-HEAQ.37 As seen for 3-BPOH in MeCN,36 the fs-TA and ns-TA spectra for 2-HEAQ obtained in aqueous solvent systems also observed the IC and ISC processes with generation of the triplet excited state of 2-HEAQ (represented here by (2HEAQ)3) having characteristic bands at 383 and 454 nm with a tail around 640 nm.37 On the basis of the studies of 3-BPOH and other AQ compounds in IPA, the species probed after (2HEAQ)3 was tentatively assumed to be the semianthraquinone radical transient of (2-HEAQ)3 obtained by the HAT reaction from IPA. The computed Raman spectrum for the ketyl radical of 2-HEAQ was compared to the 200 ns experimental spectrum (see Figure 5). The major Raman features for the

resembles well the spectrum of species 5 rather than 4, leading to the assignment of the species as the xylylene diradical species 5.

Figure 4. Comparison of the 120 ns time delay experimental spectrum found in the pH 2 aqueous solution from Figure 3 to those computed for the normal Raman spectra of transients 4 (top) and 5 (bottom). The asterisks (*) label areas influenced by stray light and/ or solvent subtraction artifacts. Adapted from ref 36. Copyright 2011 American Chemical Society.

With the above experimental facts in hand, especially the direct observation of the key reactive xylylene diradical intermediate 5 related to the photoredox process, the photoredox mechanism for 3-BPOH in acidic aqueous environments can be deduced. It appears to be initiated by protonation of the carbonyl oxygen followed by deprotonation of the side chain (Scheme 2). As stated by Wirz and coworkers,49 the protonation of an excited carbonyl oxygen leads to new pathways that may result in unusual or unfamiliar photochemistry being discovered. We followed up in this area using a similar approach to elucidate the reaction mechanisms for several other BP derivatives, like the “meta effect” photochemical reaction of 3-methyl-BP,50 the photosubstitution reaction of 3-fluoro-BP,51 and the competition effects of BPs having two different substitutions at the two meta positions, such as 3-hydroxymethyl-3′-fluoro-BP and 3-fluoro3′-methyl-BP.52

Figure 5. Comparison of (a) the ns-TR3 90 ns time delay spectrum of 2-HEAQ in IPA to (b) the computed ketyl radical transient of 2HEAQ. Adapted from ref 37. Copyright 2012 American Chemical Society.

b. Investigation of Photoredox Reactions of AQs in Aqueous Solutions

ketyl radical of 2-HEAQ come from the CO and C−C vibrational motions. It is accepted that an aromatic carbonyl nπ* T1 species exhibits a high efficiency toward the HAT process but that an aromatic carbonyl molecule with a ππ* character triplet state typically has low reactivity for HAT.54 This suggests that in IPA, nπ* is the main configuration for (2-

AQs are considered to be some of the most effective drugs for cancer therapy applications. These drugs, such as valrubicin, daunorubicin, epirubicin, pirarubicin, aclarubicin, and idarubicin, have attained clinical approval.53 For drugs like these, the photophysical and photochemical behaviors of AQs have

Scheme 3. Overall Photoredox Process of 2-HEAQ and Subsequent Oxidation of the Final Product 3-Formyl-AQ in Aqueous Solution9,13,14

E


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Figure 6. (a−c) ns-TA results for 2-HEAQ subsequent to 266 nm excitation in (a) 9:1 MeCN/H2O (pH 6−7), (b) 1:1 MeCN/H2O (pH 6−7), and (c) 1:9 MeCN/H2O (pH 6−7) mixed aqueous solutions. (d) Normalized 390 nm decays of 2-HEAQ in (black) 9:1 MeCN/H2O (pH 6−7) and (red) 1:1 MeCN/H2O (pH 6−7). Adapted from ref 37. Copyright 2012 American Chemical Society.

HEAQ)3 because of the observed efficient photoreduction reaction in this solvent. In aqueous solutions (1:1 MeCN/H2O) under both neutral (pH 6−7) and acidic (pH 2) conditions, the IC and ISC processes are seen in the fs-TA results for 2-HEAQ, leading to generation of (2-HEAQ)3.37 After that, a new transient not observed in the fs-TA results in MeCN came into view with a diagnostic feature at 510 nm accompanied by a wide feature near 650 nm within 10 ps. These new features then began to decay. On the basis of the photoredox reaction study on 3BPOH, the species is tentatively assigned to be the ketone oxygen atom-protonated species of (2-HEAQ)3.37 The ns-TA spectra recorded for 2-HEAQ in aqueous solutions with varying water content near neutral pH (Figure 6)37 are substantially different from those probed in IPA solvent, indicating the occurrence of different photochemical reaction(s) in the presence of water molecules rather than the photoreduction seen in IPA. Two distinct transients were seen at lower water content (e.g., in 9:1 MeCN/H2O). Transient A has side features at 388 and 457 nm, and transient B grows in at 392 nm at later times. At a moderate water content (e.g., in 1:1 MeCN/H2O), transient B was seen after photoexcitation, and then a new transient C grew in at around 370 and 480 nm several thousands of nanoseconds later. As the water content increased to 90%, the transient spectra looked like those observed at moderate water content, but the lifetime of transient B became shorter and transient C grew in faster, within 200 ns. The feature near 390 nm decayed faster in 1:1 MeCN/H2O than in 9:1 MeCN/H2O, and the decay at 390 nm was too quick to accurately determine its lifetime with our instrument in the 1:9 MeCN/H2O solution. It is clear that an increase in the water content of the solvent caused the decay of the 390 nm feature to become faster, indicating that water is

needed for the new photochemical process of 2-HEAQ to occur efficiently. The new photochemical processes observed are promoted in higher-water-content environments, and the species observed in the ns-TA spectra were proposed to be generated via a sequential pathway with species A generating B and then C. From the ns-TA and fs-TA data, the transient A seen under neutral aqueous conditions may be attributed to (2-HEAQ)3. Transient B that appears to react with water has a spectrum profile resembling the ketone oxygen atom-protonated species of (2-HEAQ)3 observed in the fs-TA data under very similar conditions, indicating that they are likely the same transient. The long lifetime of transient C indicates it could be a groundstate intermediate or final product formed by the photoredox reaction in an air-saturated environment. Because the TA spectra are wide and relatively featureless and thus reveal little regarding the structure of the transients detected, ns-TR3 experiments were done to assist in the identification and characterization of the transients. Consistent with the fs-TA and ns-TA data, the ns-TR3 data for 2-HEAQ under pH 2 and neutral aqueous conditions show a strong resemblance to each other and appear different from those seen in IPA.37 The identical transient with Raman features at 1140, 1177, 1212, 1318, 1570, and 1612 cm−1 was generated after photoexcitation and appeared for a while in the two solutions.37 Increased scrutiny showed that this transient had a greater intensity in the acidic aqueous solution than in the neutral aqueous solution after photoexcitation. Later (10 μs and afterward) the 1140 and 1177 cm−1 Raman features disappeared, accompanied by a shift of the 1612 cm−1 feature to 1618 cm−1 and a change in the ratio of the intensities of the 1570 and 1612 cm−1 Raman features from ∼1 to >1 in the acidic aqueous environment, indicating the formation of a new F


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Figure 7. Experimental Raman data for 2-HEAQ in 1:1 MeCN/H2O (pH 2): (left) comparison of (a) the spectrum at a time delay of 700 ns to (b) the computed Raman spectrum of singlet xylylene species with its chemical geometry given; (right) comparison of (c) the spectrum at a time delay of 50 μs to (d) the calculated result for the final product 2-acetyl-9,10-AQ. The asterisks (*) label areas influenced by stray light and/or solvent subtraction artifacts. Adapted from ref 37. Copyright 2012 American Chemical Society.

Scheme 4. Proposed Mechanism for the Photoredox Reaction of 2-HEAQ under Aqueous Conditionsa

a

The letters and numbers beneath the structures correlate with the labels utilized in the main text. Adapted from ref 37. Copyright 2012 American Chemical Society.

μs was compared to the calculated Raman spectrum of 2acetyl-9,10-AQ, the product of oxidation of the photoredox product 2-(1-hydroxyethyl)-9,10-dihydroxyanthracene (Figure 7, right). The close resemblance between the computed spectra and the respective transient experimental spectra provides evidence for the attribution of the transients and the proposed photoredox mechanism shown in Scheme 4. The function of water was also explored using ns-TR3 experiments done under neutral aqueous conditions with differing water contents. As observed in the ns-TA experiments, an increase in the efficiency of the photoredox process was seen with an increase in the water content in a certain range. ns-TR3 experiments were performed in a neutral deuterium mixed sample (1:1 MeCN/D2O), and the kH/kD = 2 measured confirmed that water is involved in the photoredox process.37 Therefore, the photoredox reaction of 2-HEAQ in aqueous environments is reasonably proposed to proceed via an intermediate with a ππ* nature. This is different from the photoreduction in IPA, which occurs via a transient with mostly an nπ* nature. With water not present, a shift of the triplet state to be more nπ* in nature that favors the HAT

transient that occurs slower in a nearly neutral aqueous environment. The Raman features for (2-HEAQ)3 were also not seen, indicating that the major transient seen in the ns-TR3 data under both nearly neutral and moderately acidic (pH 2) aqueous conditions is mostly transient B seen in the ns-TA data under the corresponding conditions.37 Ns-TR3 experiments using oxygen purging of the sample were performed, and the Raman features at 1570 and 1612 cm−1 were fit with and without oxygen purging of the sample. This work showed that the yield of this transient was lowered by the presence oxygen but that its rate of decay did not change, suggesting a species in its ground state that is produced via the triplet excited state.37 As suggested on the basis of the results from 3-BPOH that the photoredox process occurs via protonation of the carbonyl with subsequent deprotonation of the side chain, the experimental spectrum at 90 ns was compared to the computed Raman spectrum of the xylylene transient generated from the photoredox reaction (Figure 7, left). Since the ns-TR3 experiments require a long spectrum acquisition time and employ a flowing liquid sample exposed to oxygen in the air, the experimental spectrum at 20 G


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Scheme 5. Photohydration Mechanism Proposed for 2-HEAQ in Acidic Aqueous Solutionsa Based on the Investigation of BP by Wirz and Coworkers49

a

Adapted from ref 37. Copyright 2012 American Chemical Society.

Figure 8. Comparisons of the experimental and computed ns-TR3 spectra of 2-HEAQ in 1:1 MeCN/H2O (pH 0): (left) comparison of the spectrum at 5 ns time delay (top) to the computed spectrum for the triplet-state trihydroxycarbonyl (bottom); (right) comparison of the spectrum at 90 ns time delay to the computed Raman spectrum of ground-state 2-HEAQ. The asterisks (*) label areas influenced stray light and/or solvent subtraction artifacts. Adapted from ref 37. Copyright 2012 American Chemical Society.

singlet character and is formed via a triplet intermediate. The first transient detected under the pH 0 aqueous conditions employing ns-TR3 was attributed to the trihydroxyl triplet. Comparison of the experimental and computed Raman features for the two transients (Figure 8) found a good resemblance that supports the photohydration being the predominant process in pH 0 aqueous environments for 2HEAQ, and no discernible Raman features for the photoredox process were seen. To conclude, the photoredox process proceeds via protonation of the AQ group and subsequent deprotonation of the methylene C−H bond in aqueous environments over a pH region from 2 to 10. In a very strongly acidic (pH 0) aqueous environment, photohydration becomes the main process.We also carried out the combined time-resolved spectroscopic studies on the AQ derivative 2-(phydroxymethyl)phenyl-AQ, in which a phenyl ring is inserted between the two active sites. This compound undergoes the photoredox reaction via a route that is similar to that for 2HEAQ but less efficient because the phenyl group reduces the efficiency of electron transfer from the carbonyl to the carbon atom in the side chain and the large distance from the carbonyl group to the side chain decreases the efficiency of the proton transfer (PT).55

can occur, correlated with the photoreduction seen in IPA. The CO vibrational modes appear near 1600 cm−1 in both the experimental and computed Raman spectra seen in the aqueous environments, providing further pieces of evidence that the photoredox takes place from a transient with a ππ* nature. Two transients were seen for 2-HEAQ in the ns-TR3 data acquired in the aqueous solution (1:1 MeCN/H2O) at pH 0. The first transient with large Raman features at 1472, 1505, 1570, and 1602 cm−1 was monitored with excitation and then decayed within 10 ns to form a second transient having features at 1570 and 1618 cm−1 and a fairly long lifetime. The clearly different ns-TR3 results for 2-HEAQ under pH 0 aqueous conditions (1;1 MeCN/H2O) compared with those observed under neutral and moderately acidic aqueous conditions demonstrate that the photoredox reaction does not happen and that 2-HEAQ accesses a different reaction pathway under higher-acid-content conditions subsequent to the production of the triplet carbonyl-protonated (2-HEAQ)3. As stated above, the photohydration reaches its highest yield for BP and is the main process for 3-BPOH in the pH 0 aqueous environment. Therefore, the 2-HEAQ photohydration is also hypothesized to be the main process seen under the same conditions, and this photohydration mechanism, based on the investigation of BP by Wirz and co-workers,49 is displayed in Scheme 5. An oxygen quenching experiment found that the lifetime of the second transient did not change but its yield decreased, indicating that the second transient has H


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Figure 9. Mechanisms proposed for 3-BPOH and 2-HEAQ under acidic and neutral aqueous conditions. The computed free energies (in kcal mol−1) are shown in parentheses and were computed at the M06-2X/6-311++G**-SMD(MeCN)//M06-2X/6-311+G** level. The H atom in blue comes from the hydroxyl group, and the H atom in purple comes from the alkyl moiety. Adapted from ref 38. Copyright 2016 American Chemical Society.

Table 1. Photophysical and Photochemical Processes Observed for BP, 3-BPOH, and 2-HEAQ in Selected Solvent Systems and Key Intermediates Observed

c. Investigation of Photochemical Reactions of BPs and AQs in Neutral Aqueous Solutions

activation energy with BPs to form the ArPK species, the generation of the ArPK radical under neutral aqueous conditions could not occur via a direct HAT process like that observed in IPA. We recently unraveled that the ArPK is formed from the ππ* configuration of (3-BPOH)3 via a proton-coupled electron transfer (PCET) process in neutral aqueous solution.56 DFT calculations were performed under neutral and acidic aqueous conditions to compare and contrast the photochemical behaviors of 3-BPOH and 2-HEAQ (Figure 9). The results clearly show that the photoredox reactions of 3-BPOH and 2-HEAQ in acidic aqueous solution occur via protonation of the carbonyl oxygen with subsequent deprotonation of the side chain. However, the pathways for the two counterparts are very different in neutral aqueous solution. Congruent with the

The photophysical and photochemical behaviors of 3-BPOH and 2-HEAQ exhibit noticeable similarity to each other in organic solvents like MeCN and IPA. As well, in acidic aqueous solutions 3-BPOH and 2-HEAQ undergo efficient photoredox reactions. The difference between the two compounds appears under neutral water-containing conditions, where 2-HEAQ still undergoes the photoredox reaction while 3-BPOH does not show a discernible photoredox process based on final product analysis. Instead, the ns-TR3 spectra of 3-BPOH in a neutral aqueous environment resemble those found in IPA (Figure 2), with detection of the ArPK radical of 3-BPOH seen in the IPA solution. Because the O−H bond of H2O is an “inert” hydrogen donor and requires a higher I


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Accounts of Chemical Research final product and ns-TR3 experimental data, the photoredox reaction of 3-BPOH from ArPK in neutral aqueous condition is inhibited since it needs the second hydrogen to leave from the C−H bond by an energy barrier of 6.3 kcal mol−1. The preferential route is to proceed via the minimum-energy crossing point (MECP2), which then recovers to ground-state 3-BPOH. Alternatively, PCET from the alcohol C−H bond to the para-carbonyl with the formation of xylylene diradical 3b is the starting and critical step for the photoredox of 2-HEAQ to take place in neutral aqueous conditions. After the formation of 3b, an exothermic ISC process occurs with the generation of singlet xylylene 15b, which then goes over a very modest energy barrier (1.6 kcal mol−1) to the photoredox product 16b. In summary, the carbonyl reduction proceeds via PCET from the side-chain alcohol O−H bond for 3-BPOH, whereas it begins via PCET from the alcohol C−H bond to the paracarbonyl for 2-HEAQ in neutral aqueous environments. These different reaction routes appear to result from a subtle chargeradical-coupled effect of 3-BPOH and 2-HEAQ on the PCET reactions.38

to further advance this exciting technique as well as other related vibrational spectroscopy methods such as time-resolved infrared absorption for application to many chemical reactions to directly investigate the geometries and properties of excited states and/or reactive intermediates that are involved in photochemical and/or chemical reactions of interest in chemistry, biology, and the natural environment.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jiani Ma: 0000-0003-3325-0762 David Lee Phillips: 0000-0002-8606-8780 Author Contributions

The manuscript was written through contributions of all authors. All of the authors approved the final version of the manuscript.

3. CONCLUSION AND OUTLOOK In this Account, several examples of photoredox reaction mechanism studies for selected BPs and AQs in aqueous solutions using fs-TA, ns-TA, and ns-TR3 techniques have been summarized. The predominant photophysical and photochemical processes of BP, 3-BPOH, and 2-HEAQ in selected solvent systems are listed in Table 1. Clear characterizations of the photophysical pathways and the photochemical reactions of representative aromatic carbonyl compounds in photochemical research areas not only provide fundamental information for better understanding of the photochemistry of carbonyl compounds in aqueous environments but also will facilitate the development of applications of these systems in photochemical and photobiological fields of research. In addition, the results presented in this Account illustrate the importance of water molecules in the photochemical reactions associated with the photochemistry of aromatic carbonyl molecules in aqueous environments. Furthermore, it is noted that the different photochemical reactions observed, such as the photoredox, photoreduction, and photohydration reactions for the BPs and AQs investigated here under different aqueous conditions, are generally triggered by the nπ* or ππ* configurations of the triplet state related with PT reactions or HAT reactions or PCET processes. 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 containing a meta-substituted alkyl group with an alcohol attached, such as for 3-BPOH and 2-HEAQ detailed in this Account, and similar structural motifs in more complex systems may be attractive to study in the future. It is noted from the examples in this Account that timeresolved vibrational spectra play a leading role in the structural investigations of short-lived intermediates. The rapid growth in ns-TR3 applications to a wide range of chemical problems obtained by a number of research groups over the years gives confidence that the TR3 technique can serve as a powerful tool to directly probe the structural information derived from the vibrational frequencies to aid clear identification and characterization of many different kinds of intermediates in chemical reactions. Meanwhile, there is plenty of room for explorations

Funding

The research was sponsored in part by grants from the National Science Fund of China (21503167) to J.M. and the Research Grants Council of Hong Kong (HKU17301815) to D.L.P. and partial support from the Areas of Excellence Scheme (Grant AoE/P-03/08), the UGC Special Equipment Grant (SEG-HKU-07), and the University of Hong Kong Development Fund 2013−2014 project “New Ultrafast Spectroscopy Experiments for Shared Facilities”. Notes

The authors declare no competing financial interest. Biographies Jiani Ma was born in 1984. She received her doctorate in physical chemistry at the University of Hong Kong and subsequently became an associate professor at Northwest University in Xi’an, China. Her current research interests are focused on investigating the photochemical and photobiological reactions using fs-TA, ns-TA and nsTR3 spectroscopies. Xiting Zhang received his doctorate in physical chemistry at the University of Hong Kong in 2016. He is currently a research associate at the University of Hong Kong. His research interests lie in reaction mechanism studies using DFT calculations. David Lee Phillips received a B.Sc. in chemistry from Iowa State University in 1984 and his Ph.D. in physical chemistry from the University of California, Irvine in 1989. He pursued postdoctoral studies at the University of Rochester in 1989−1992. In 1993 he joined the University of Hong Kong as a lecturer and became a Chair Professor of Physical Chemistry in 2009. His current research interests involve experimental and theoretical investigations of excited states, reactive intermediates, and ultrafast kinetic processes using fsTA/emission, ns-TA/emission, and ps-TR3 and ns-TR3 spectroscopies.

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