Photoreduction of p-Benzoquinones: Effects of Alcohols and Amines on the Intermediates and Reactivities in Solution¶

Assessing the source of the photochemical formation of hydroxylating species from dissolved organic matter using model sensitizers

Dissolved organic matter (DOM) is ubiquitous in natural waters and can facilitate the chemical transformation of many contaminants through the photochemical production of reactive intermediates, such as singlet oxygen (¹O₂), excited triplet state DOM (³DOM*), and hydroxylating species (˙OH and other intermediates of similar reaction chemistry). The formation mechanism of most reactive intermediates is well understood, but this is not the case for the formation of hydroxylating species from DOM. To investigate this chemistry, DOM model sensitizers were irradiated with two different probe compounds (benzene and benzoic acid) at two irradiation wavelengths (254 and 320 nm). The ability of DOM model sensitizers to hydroxylate these arene probes was assessed by measuring rates of formation of the hydroxylated probe compounds (phenol and salicylic acid). Multiple classes of model sensitizers were tested, including quinones, hydroxybenzoic acids, aromatic ketones, and other triplet forming species. Of these classes of model sensitizers, only quinones and hydroxybenzoic acids had a hydroxylating capacity. Methanol quenching experiments were used to assess the reactivity of hydroxylating species. These results have several implications for the systems tested. First, they suggest that the hydroxylating intermediate produced from hydroxybenzoic acid photolysis may not be hydroxyl radical, but a different hydroxylating species. Also, these data prompted investigation of whether quinone photoproducts have a hydroxylating capacity. These results confirm that hydroxybenzoic acids and quinones are important to the photochemical production of hydroxylating species from DOM, but the mechanism by which this occurs for these classes of sensitizers is still elusive.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

A general strategy for visible-light decaging based on the quinone cis-alkenyl lock

Combining the fast thermal cyclization of o-coumaric acid derivatives with the intramolecular photoreduction of quinones gives new visible-light photoremovable protecting groups absorbing well above 450 nm.

Exploiting Alkylquinone Tautomerization: Amine Benzylation

A general protocol for the synthesis of benzylic amines via side-chain amination of alkylquinones is reported. The reactions are initiated by the tautomerization of an alkylquinone to the corresponding quinone methide, which is subsequently trapped in situ by an amine nucleophile. This process is promoted by tertiary amines in protic solvents under mild conditions and is compatible with many functional groups. 1,2- and 1,4-benzoquinones, as well as naphthoquinones, participate in this reaction using a wide range of primary and secondary amines/anilines. The synthetic utility of this transformation is also explored.

A model for the water-oxidation and recovery systems of the oxygen-evolving complex

We propose a model for the water-oxidation and recovery systems of the oxygen-evolving complex (OEC) of the photosystem II (PSII) enzyme. The whole system is constructed from two catalytic cycles, conducted as a tandem reaction: (i) a water-oxidation loop uses cerium(IV) ammonium nitrate as an oxidant to activate a dimanganese complex for water-oxidation and thereby liberate a molecule of O2 and (ii) a recovery loop begins with photoinhibition of the dimanganese complex but then uses O2 to reactivate the manganese centre. The net result is a catalytic water-oxidation catalyst that can use self-generated O2 for recovery.

Optical properties of humic substances and CDOM: effects of borohydride reduction

Treatment of Suwanee River humic (SRHA) and fulvic (SRFA) acids, a commercial lignin (LAC), and a series of solid phase extracts (C18) from the Middle Atlantic Bight (MAB extracts) with sodium borohydride (NaBH(4)), a selective reductant of carbonyl-containing compounds including quinones and aromatic ketones, produces a preferential loss of visible absorption (> or = 50% for SRFA) and substantially enhanced, blue-shifted fluorescence emission (2- to 3-fold increase). Comparison of the results with those obtained from a series of model quinones and hydroquinones demonstrates that these spectral changes cannot be assigned directly to the absorption and emission of visible light by quinones/hydroquinones. Instead, these results are consistent with a charge transfer model in which the visible absorption is due primarily to charge transfer transitions arising among hydroxy- (methoxy-) aromatic donors and carbonyl-containing acceptors. Unlike most of the model hydroquinones, the changes in optical properties of the natural samples following NaBH(4) reduction were largely irreversible in the presence of air and following addition of a Cu(2+) catalyst, providing tentative evidence that aromatic ketones (or other similar carbonyl-containing structures) may play a more important role than quinones in the optical properties of these materials.

Oxygen Uptake upon Photolysis of 1,4-Benzoquinones and 1,4-Naphthoquinones in Air-Saturated Aqueous Solution in the Presence of Formate, Amines, Ascorbic Acid, and Alcohols

The effects of oxygen in the photoreduction of 1,4-benzoquinone (BQ), 1,4-naphthoquinone (NQ), and a series of derivatives were studied in aqueous solution in the presence of acetonitrile and formate, aliphatic amines, e.g., EDTA or triethylamine, ascorbic acid, and alcohols, e.g., methanol or 2-propanol. The quinone triplet state is quenched, whereby the semiquinone and donor radicals are formed which react subsequently with oxygen. The overall reaction is oxidation of the donors and conversion of oxygen via the hydroperoxyl/superoxide radical into hydrogen peroxide. The quantum yield (Phi-O2) of this oxygen uptake changes in 2-propanol-water (1:10) from <0.01 for BQ to Phi-O2 = 0.5-0.8 for NQ. Generally Phi-O2 increases with increasing donor concentration. The specific properties of quinone structure, the radical equilibria and reactivity, and the concentration dependences are discussed.

Photoreactions of p-Quinones with Dimethyl Sulfide and Dimethyl Sulfoxide in Aqueous Acetonitrile†

The effects of dimethyl sulfide (DMS) and dimethyl sulfoxide (DMSO) on the photoreactions of 1,4-benzoquinone (BQ), 1,4-naphthoquinone (NQ), 9,10-anthraquinone (AQ) and several derivatives in acetonitrile/water were studied. The observed triplet state of the quinones is quenched and the rate constant is close to the diffusion-controlled limit for reactions of most quinones with DMS and lower with DMSO. Semiquinone radical anions (Q*-) produced by electron transfer from sulfur to the triplet quinone were detected. For both DMS and DMSO the yield of Q*- is similar, being generally low for BQ and NQ, substantial for AQ and largest for chloranil. The specific quencher concentrations and the effects of quinone structure and redox potentials on the time-resolved photochemical properties are discussed.

On the origin of the optical properties of humic substances

Absorption and fluorescence spectroscopy and laser photobleaching experiments were employed to probe the origins of the optical properties of humic substances (HS). Luminescence quantum yields and the wavelengths of maximum emission were acquired for Suwannee River humic acid (SRHA) and fulvic acid (SRFA) at an extensive series of excitation wavelengths across the ultraviolet and visible. Laser irradiation at a series wavelength across the ultraviolet and visible was further employed to destroy selectively chromophores absorbing at specific wavelengths, using absorption spectroscopy to follow the absorption losses with irradiation time. The results provide unequivocal evidence that the absorption and emission spectra of these materials cannot result solely from a simple linear superposition of the spectra of numerous independent chromophores. Instead, the long wavelength absorption tail of HS (>350 nm) appears to arise from a continuum of coupled states. We propose that this behavior results from intramolecular charge-transfer interactions between hydroxy-aromatic donors and quinoid acceptors formed by the partial oxidation of lignin precursors. We further propose that these donor-acceptor interactions may be a common phenomenon, occurring within all natural hydroxy- or polyhydroxy-aromatic polymers that form appropriate acceptors upon partial oxidation. Examples of such species include lignin, polyphenols, tannins, and melanins.

Photoreactions of p-benzo-, p-naphtho- and p-anthraquinones with ascorbic acid

The photoreduction of 1,4-benzoquinone (BQ), 1,4-naphthoquinone (NQ), 9,10-anthraquinone (AQ) and several derivatives, e.g. dimethylBQ, trimethylBQ, duroquinone, bromoNQ, methoxyNQ, methylAQ and dimethylAQ in acetonitrile-water by ascorbate was studied by time-resolved UV-vis spectroscopy using 20 ns laser pulses at 308 nm and continuous 254 nm irradiation. The semiquinone radical (*QH/Q*(-)) is formed after H-atom transfer from ascorbate to the quinone triplet state. The rate constant for quenching is k(q)=(2-9) x 10(9) M(-1) s(-1). Termination of the radicals takes place in the micros-ms range. The results are compared with those initiated by electron transfer from DABCO under similar conditions, where the k(q) values are similar, but the termination of Q*(-) takes place by electron back transfer not yielding hydroquinones. Specific properties of the quinone triplet state, e.g. self-quenching, nucleophilic water addition and the effects of structure are discussed.