Mercury photoreduction and photooxidation in lakes: Effects of filtration and dissolved organic carbon concentration.

Mercury is a globally distributed, environmental contaminant. Quantifying the retention and loss of mercury is integral for predicting mercury-sensitive ecosystems. There is little information on how dissolved organic carbon (DOC) concentrations and particulates affect mercury photoreaction kinetics in freshwater lakes. To address this knowledge gap, samples were collected from ten lakes in Kejimkujik National Park, Nova Scotia (DOC: 2.6-15.4mg/L). Filtered (0.2µm) and unfiltered samples were analysed for gross photoreduction, gross photooxidation, and net reduction rates of mercury using pseudo first-order curves. Unfiltered samples had higher concentrations (p=0.04) of photoreducible divalent mercury (Hg(II)RED) (mean of 754±253pg/L) than filtered samples (mean of 482±206pg/L); however, gross photoreduction and photooxidation rate constants were not significantly different in filtered or unfiltered samples in early summer. DOC was not significantly related to gross photoreduction rate constants in filtered (R2=0.43; p=0.08) and unfiltered (R2=0.02; p=0.71) samples; DOC was also not significantly related to gross photooxidation rate constants in filtered or unfiltered samples. However, DOC was significantly negatively related with Hg(II)RED in unfiltered (R2=0.53; p=0.04), but not in filtered samples (R2=0.04; p=0.60). These trends indicate that DOC is a factor in determining dissolved mercury photoreduction rates and particles partially control available Hg(II)RED in lake water. This research also demonstrates that within these lakes gross photoreduction and photooxidation processes are close to being in balance. Changes to catchment inputs of particulate matter and DOC may alter mercury retention in these lakes and could partially explain observed increases of mercury accumulation in biota.

Photocyclization and Photoaddition Reactions of Arylphenols via Intermediate Quinone Methides.

A series of five benzannelated derivatives of 2-phenylphenol were prepared, and their photochemistry was investigated. Two of these (3-phenyl-2-naphthol, 10, and 1-phenyl-2-naphthol, 11) were photoinert. For 2-(1-naphthyl)phenol (12) and 1-(1-naphthyl)-2-naphthol (13), ESPT took place to either the 2'-position or the 7'-position of the naphthalene ring to give quinone methides (QMs) that underwent either reverse proton transfer (RPT) or electrocyclic ring closure to give dihydrobenzoxanthenes. The intermediate QMs for 12 and 13 were detected and characterized by laser flash photolysis. For 2-(9-phenanthryl)phenol (14), ESPT took place either to the 5'-position to give a QM that underwent quantitative electrocyclic ring closure to give the corresponding benzoxanthene or to the 10'-position to give a QM that underwent RPT. If the solution contained methanol, the QM produced on ESPT to the 10'-position in 14 could be trapped as the photoaddition product. The compounds studied in this work demonstrate three possible reactions of QMs produced following ESPT to aromatic carbon atoms: (1) reverse proton transfer (RPT) to regenerate starting material; (2) addition of hydroxylic solvents to give the photoaddition product; and (3) electrocyclic ring closure to give benzoxanthene derivatives.

Photodecaging from 9-substituted 2,7-dihydroxy and dimethoxyfluorenes: competition between heterolytic and homolytic pathways.

2,7-Dihydroxy-9-fluorenol (9), 2,7-dimethoxy-9-fluorenol (10), and 2,7-dimethoxy-9-acetoxyfluorene (11) were prepared and their photochemistry was studied in methanol and aqueous methanol solution in the hopes of observing efficient expulsion of the substituents positioned at the 9-position. For all three compounds, the primary photoproducts were 2,7-disubstituted-9-fluorenes and 2,7-disubstituted-9-methoxyfluorenes. A mechanism of reaction is proposed for production of these products, and involves competing homolytic and heterolytic pathways that produce radical and carbocation intermediates. Reaction quantum yields (for substrate disappearance) ranged between 0.21 and 0.31.

Efficient photodecarboxylation of trifluoromethyl-substituted phenylacetic and mandelic acids.

A total of eight CF(3)-substituted phenylacetic and mandelic acids are shown to undergo efficient photodecarboxylation (PDC; Phi = 0.37-0.74) in basic aqueous solution to give the corresponding trifluoromethyltoluenes or trifluoromethylbenzyl alcohols. The products are consistent with the almost exclusive formation of benzylic carbanions that subsequently react with water, with minor amounts (< or = 5%) of radical-derived products detected. Quenching studies indicate that the reaction likely proceeds from the singlet excited state. This work demonstrates that the CF(3) group greatly facilitates the excited state ionic PDC of phenylacetic acids.

Photolabile protecting groups based on the singlet state photodecarboxylation of xanthone acetic acid.

A new photolabile protecting group (PPG) for carboxylic acids and amines has been developed based on the rapid singlet state photodecarboxylation of xanthone acetic acids with several features that are superior to many other systems. We demonstrate that the "xanthonate" PPG can photorelease carboxylic acids and amines (via the carbamates) quantitatively in neutral phosphate buffer solution with a remarkable "uncaging cross section" (Phi x epsilon = 3900 M(-1) cm(-1) with UVA irradiation). Advantageous features include a high reaction efficiency (Phi > 0.6), very clean photochemistry, subnanosecond release kinetics, good water solubility, and excellent UVA absorption. Investigations into the mechanism of release employing nanosecond laser flash photolysis (LFP), time-resolved and steady-state fluorescence spectroscopy, and product studies support an initial rapid decarboxylation from the singlet excited state to form a carbanion intermediate which undergoes a very rapid beta-elimination of the carboxylate or carbamate leaving group, all occurring on the subnanosecond time scale.

How drug photodegradation studies led to the promise of new therapies and some fundamental carbanion reaction dynamics along the way.

The photodegradation of nonsteroidal anti-inflammatory drugs (NSAIDs), a class of medications that includes aspirin and ibuprofen, has generated considerable interest since the 1990s, largely because of the phototoxic and photoallergic effects that frequently accompany their therapeutic use. Among NSAIDs, ketoprofen, which contains a benzophenone chromophore, has been extensively studied, reflecting both its notorious adverse effects and the fascination that photochemists have with benzophenone. The photochemistry of ketoprofen involves the intermediacy of an easily detectable carbanion with a remarkable lifetime of 200 ns in water; its life expectancy can in fact be extended to minutes under carefully controlled anhydrous conditions. Over the past decade, we have used some key properties of the ketoprofen carbanion to conduct mechanistic studies on carbanions under various conditions. In particular, its ease of photogeneration provides the temporal control required for kinetic studies, which, combined with its long lifetime and readily detectable visible absorption, have enabled extensive laser flash photolysis work. These studies have led to an intimate understanding of the reaction dynamics for carbanions in solution, including the determination of absolute rate constants for protonation, S(N)2, and elimination reactions. Together they provide excellent exemplars of reactivity patterns that today are part of all introductory curricula in organic chemistry and illustrate the fundamentals of nucleophilic substitution paradigms. More recently, we have begun to exploit the photochemistry of ketoprofenate and have developed the ketoprofenate photocage, a valuable tool for the photocontrolled cleavage of protecting groups and concomitant drug release. The photorelease has been illustrated with ibuprofen, among many other molecules. These photocages have been further improved with the use of the xanthone chromophore; the goal is the release of antiviral agents taking advantage of the improved UVA absorption of xanthone (xanthonate photocages). In this Account, we survey our work of the past few years on the photochemistry of ketoprofen and related chromophores. Beginning with studies on the phototoxicity of ketoprofen, we have made the journey to new prodrug candidates, unraveling mechanistic elements of aroyl-substituted benzyl carbanions along the way.

A simple and smart oxygen sensor based on the intrazeolite reactions of a substituted anthraquinone.

Inclusion of 2-(hydroxymethyl)anthraquinone in zeolite NaY leads to a solid, photoactivated, reusable oxygen sensor capable of reporting and memorizing oxygen contamination events by simple visual inspection.

Photodecarboxylation of xanthone acetic acids: C-C bond heterolysis from the singlet excited state.

[reaction: see text] Irradiation of 2- and 4-xanthone acetic acid in aqueous buffer (pH 7.4) leads to efficient (Phi = 0.67 and 0.64, respectively) photodecarboxylation to give the corresponding methyl products, consistent with an intermediate benzylic carbanion. Fluorescence and laser flash photolysis (LFP) studies suggest singlet state reactivity, which is unusual for aromatic ketones. 3-Xanthone acetic acid is photoinert under the same conditions. The results suggest that the reactive xanthone acetic acids are promising precursors for carbanion-mediated photocages.

Photogeneration and chemistry of biphenyl quinone methides from hydroxybiphenyl methanols.

The photosolvolysis of several biphenyl methanols (Ph-PhCH[Ph]OH) substituted with hydroxy or methoxy groups on the benzene ring not containing the -CH(Ph)OH moiety has been studied in aqueous solution. This work is a continuation of our studies of photosolvolysis of hydroxy-substituted arylmethanols that generate quinone methide intermediates, some of which are known to be relevant intermediates in toxicology and in biological and organic chemistry in general. In this study, we further probe the ability of the biphenyl ring system to transmit charge from the ring substituted with a potential electron-donating group (hydroxy and methoxy) to the adjacent benzene ring that contains a labile benzyl alcohol moiety. We show that in systems with a hydroxy substituent, biphenyl quinone methides (BQM) are the first formed intermediates that are detectable by nanosecond laser flash photolysis, and are responsible for the observed overall photosolvolysis reaction of these compounds. The highly conjugated BQM are found to absorb at long wavelengths (lambda(max) 580 and approximately 750 nm for the p,p' and o,p'-isomers, respectively) with relatively long lifetimes in neutral aqueous solution (500 and 30 micros, respectively). The BQM from the o,p'-isomer was found to undergo a competing intramolecular Friedel-Crafts alkylation, to give a fluorene derivative.

Formal intramolecular photoredox chemistry of meta-substituted benzophenones.

[reaction: see text]. Photolysis of 3-(hydroxymethyl)benzophenone (1) in aqueous solution (pH < 3) results in clean formation of 3-formylbenzhydrol (2) at dilute (<10(-4) M) conditions. Evidence suggests that the highly efficient (Phi approximately 0.6) reaction involves a unimolecular mechanism and an overall formal intramolecular photoredox process, which requires electronic communication between the 1,3-positions of the benzene ring, an unprecedented example of the photochemical meta effect. The photoredox reaction was not observed in organic solvents, where only photoreduction of the benzophenone moiety was observed.

Carbanion-mediated photocages: rapid and efficient photorelease with aqueous compatibility.

A new photocage is proposed, based on ketoprofen-derived compounds and mediated by carbanions. The new photocage has significant advantages over the widely used o-nitrobenzyl derivatives, including aqueous compatibility, faster photorelease, higher quantum yield, and innocuous byproducts. The photorelease of ibuprofen illustrates the properties of the new photocage.

Photoaddition of water and alcohols to the anthracene moiety of 9-(2'-hydroxyphenyl)anthracene via formal excited state intramolecular proton transfer.

The title compound undergoes efficient photoaddition of a molecule of a hydroxylic solvent (H(2)O, MeOH, (Me)(2)CHOH) across the 9- and 10-positions of the anthracene moiety to give isolable triphenylmethanol or triphenylmethyl ether type products. The reaction is believed to proceed via a mechanism involving water-mediated formal excited state intramolecular proton transfer (ESIPT) from the phenolic OH to the 10-position of the anthracene ring, generating an o-quinone methide intermediate that is observable by nanosecond laser flash photolysis, and is trappable with nucleophiles. A "water-relay" mechanism for proton transfer seems plausible but cannot be proven directly with the data available. Irradiation in deuterated solvents led to incorporation of one deuterium atom at the methylene position in the photoaddition product, and partial deuterium exchange of the 10-position of recovered starting material, consistent with the proposed formal excited state proton transfer mechanism. The deuterium exchange and photoaddition reach maximum quantum efficiency at approximately 5 M water (in CH(3)CN or CH(3)OH), with no reaction observed in the absence of a hydroxylic solvent, demonstrating the sensitivity of this type of ESIPT to solvent composition.

An antenna triplet sensitiser for 1-acyl-7-nitroindolines improves the efficiency of carboxylic acid photorelease.

Conjugates of a triplet sensitiser (a 4,4'-dialkoxybenzophenone) with 1-acetyl-7-nitroindolines show up to 20-fold enhancement for photorelease of acetate (relative to the same indolines lacking the attached sensitiser) upon irradiation at 300 nm in neutral aqueous solution. The sensitised photolysis can be carried out in the presence of dissolved oxygen and will be applicable to photorelease of other carboxylates. The enhanced efficiency is mediated by an antenna function of the sensitiser, which transfers its triplet energy to populate the reactive, short-lived triplet state of the acylnitroindoline. This energy transfer is confirmed by a large reduction of the sensitiser's triplet lifetime in the conjugates compared with that of the sensitiser alone.

Photohydration and photosolvolysis of biphenyl alkenes and alcohols via biphenyl quinone methide-type intermediates and diarylmethyl carbocations.

Evidence is presented for the photochemical generation of novel biphenyl quinone methide (BQM)-type intermediates on photolysis of hydroxybiphenyl alkenes 7 and 8 and hydroxybiphenyl alcohols 9 and 10. Mechanistic investigations utilizing product, fluorescence, and nanosecond laser flash photolysis (LFP) studies indicate two distinct pathways for the formation of these BQMs depending upon the functional groups of the progenitor. Formal excited-state intramolecular proton transfer (ESIPT) between the phenol and the alkene led to BQMs upon irradiation of the hydroxybiphenyl alkenes 7 and 8, while excited-state proton transfer (ESPT) to solvent followed by dehydroxylation was responsible for BQM formation from the hydroxybiphenyl alcohols 9 and 10. Photolysis of 7 and 8 in aqueous CH(3)CN gave photohydration products via attack of water on the respective BQMs, while photolysis of the analogous methyl ethers (of the phenolic moiety) gave only carbocation intermediates. Hydroxybiphenyl alcohols 9 and 10 yielded the corresponding photomethanolysis products in aqueous methanol, through attack of CH(3)OH on the respective BQMs. Although no evidence was found for BQM formation in LFP studies of 8 and 10, due to its suspected short lifetime, the respective diaryl carbocation (lambda(max) 420 nm, tau = 8.5 micros) has been observed upon irradiation of 8 in 2,2,2-trifluoroethanol. A BQM (lambda(max) 580 nm) was observed for 9 but not for 10, the latter having more complex chemistry on laser excitation, resulting in a transient that appears to mask any BQM absorption. Significant quenching of fluorescence from the hydroxybiphenyl alkenes at low water content implies that H(2)O is directly involved in reaction from the singlet excited state. The decrease in fluorescence intensity of 8 was found to depend on [H(2)O](3); however, the distance required for ESIPT in these systems is too large to be bridged by a water trimer. The nonlinear quenching has been attributed to deprotonation of the phenol by two water molecules, with concerted protonation at the alkene by another molecule of water. Fluorescence quenching of the hydroxybiphenyl alcohols required much higher water content, implying a different mechanism of reaction, consistent with the proposal of ESPT (to solvent water) followed by dehydroxylation.

Excited-state intramolecular proton transfer in o-hydroxybiaryls: a new route to dihydroaromatic compounds.

An excited-state intramolecular proton transfer (ESIPT) from the phenol OH to the 7'-carbon on the naphthyl ring in o-(1-naphthyl)phenol (3) and 1-(1'-naphthyl)-2-naphthol (4) leads to efficient (Phi = 0.1-0.2) formation of the corresponding dihydrobenzoxanthenes (5 and 7) via quinone methide intermediates. This new reaction represents a clean, efficient, and high-yielding route to benzoxanthenes and dihydrobenzoxanthenes. A related ESIPT of similar efficiency has been detected at the 2'-aromatic position in these systems, by deuterium labeling studies.

A new type of excited-state intramolecular proton transfer: proton transfer from phenol OH to a carbon atom of an aromatic ring observed for 2-phenylphenol.

The photochemical deuterium incorporation at the 2'- and 4'-positions of 2-phenylphenol (4) and equivalent positions of related compounds has been studied in D(2)O (CH(3)OD)-CH(3)CN solutions with varying D(2)O (CH(3)OD) content. Predominant exchange was observed at the 2'-position with an efficiency that is independent of D(2)O (MeOD) content. Exchange at the 2'-position (but not at the 4'-position) was also observed when crystalline samples of 4-OD were irradiated. Data are presented consistent with a mechanism of exchange that involves excited-state intramolecular proton transfer (ESIPT) from the phenol to the 2'-carbon position of the benzene ring not containing the phenol, to generate the corresponding keto tautomer (an o-quinone methide). This is the first explicit example of a new class of ESIPT in which an acidic phenolic proton is transferred to an sp(2)-hybridized carbon of an aromatic ring. The complete lack of exchange observed for related substrates 6-9 and for planar 4-hydroxyfluorene (10) is consistent with a mechanism of ESIPT that requires an initial hydrogen bonding interaction between the phenol proton and the benzene pi-system. Similar exchange was observed for 2,2'-biphenol (5), suggesting that this new type of ESIPT is a general reaction for unconstrained 2'-aryl-substituted phenols and other related hydroxyarenes.

Excited state intramolecular redox reaction of 2-(hydroxymethyl)anthraquinone in aqueous solution.

The title compound undergoes a novel excited state intramolecular redox reaction in which the 'distal' side chain benzylic alcohol is oxidized to the aldehyde and the carbonyl moieties of anthraquinone reduced, with evidence suggesting that the primary photochemical process is a deprotonation of the benzylic C-H proton (by water) mediated by the solvent.