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Fluorescent molecular probes for detection of one-electron oxidants photochemically generated by dissolved organic matter Vivian S. Lin, Matthew Grandbois, and Kristopher McNeill Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02138 • Publication Date (Web): 19 Jul 2017 Downloaded from http://pubs.acs.org on July 21, 2017
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Fluorescent molecular probes for detection of one-electron oxidants
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photochemically generated by dissolved organic matter
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Vivian S. Lin1, Matthew Grandbois,2 Kristopher McNeill1*
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1
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Switzerland
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2
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*Corresponding author:
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Tel: +41 (0)44 632 47 55; Fax: +41 (0)44 632 14 38;
[email protected] Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, CH-8092, Zurich,
The Dow Chemical Company, 455 Forest St., Marlborough, MA 01568
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TOC:
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Abstract We report a dual probe system based on 4’-substituted biphenyl-2-carboxylic acids
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(BPAs) for analysis of photooxidants generated by dissolved organic matter. The BPA probes
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are converted to the corresponding benzocoumarins (BZCs) at different rates depending on
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the mechanism of oxidation; thus, two probes used simultaneously can differentiate strong
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triplet excited state sensitizers from hydroxylating species such as hydroxyl radical (•OH)
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present in dissolved organic matter (DOM). Comparison of the ratios of BZC-CH3 and BZC-
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CF3 product formation using model photooxidants such as NaNO2, a •OH precursor, and
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model triplet sensitizer lumichrome gave a range of 2 to 250. Application of these probes to
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DOM isolates and whole natural waters afforded intermediate ratios. Although the oxidation
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potential of BPAs (> ca. 1.80 V SHE) is significantly higher than the estimated average
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reduction potential of typical 3CDOM* samples, these results have demonstrated the presence
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of a small pool of oxidants in the selected DOM isolates and whole water samples that is
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capable of oxidizing aromatic carboxylates. As an analytical tool, this probe pair can be used
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between pH 4–6 without affecting the product formation ratio and may find applications in
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various systems involving complex mixtures of photochemically produced oxidants of
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differing natures.
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Introduction Dissolved organic matter (DOM) plays a fundamental role in the global carbon cycle,
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and its photochemical and biological transformation in terrestrial and aquatic systems is
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critical to understanding the exchange of carbon between the land, water, and atmosphere.1,2,3
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The degradation of DOM results in the formation of diverse organic molecules, altering its
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spectroscopic and chemical characteristics as well as affecting the bioavailability of this
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carbon in food webs.4 From a climate perspective, organic matter can act as both a carbon
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source and a carbon sink. Organic matter in the water column can undergo sedimentation and
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burial, leading to significant carbon storage in freshwater and marine reservoirs,5 while
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microbial metabolism and ultraviolet irradiation of DOM results in the release of CO2 into
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the atmosphere.6,7 In addition to its importance in carbon cycling, DOM has complex
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interactions with naturally occurring species such as iron, trace metals like mercury,8 and
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environmental contaminants such as organic pollutants.9 Thus, an understanding of the
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processes by which DOM is transformed will improve our ability to model and predict the
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effects of changing environmental conditions on the carbon cycle, food webs, and pollutant
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dynamics.
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The abiotic transformation of DOM in aquatic systems is largely governed by redox
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reactions, many of which are driven by interactions with sunlight. Chromophoric dissolved
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organic matter (CDOM), the light-absorbing fraction of DOM, is a complex mixture of
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sensitizing compounds with a range of oxidizing abilities.10,11 However, as DOM is
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composed of many thousands of unique chemical compounds, determining the identities of
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the sensitizers in DOM remains a significant challenge.
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In order to explore the role of triplet sensitizers in aquatic photochemical processes,
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we set out to develop molecular probes that would react with photooxidants in DOM to form
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readily distinguishable products. A number of recent advances in photoredox catalysis have
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been reported in the synthetic methodology literature,12,13 inspiring us to explore ortho-
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phenylbenzoates as fluorogenic probes for photooxidation processes. 9-Mesityl-10-
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methylacridinium (Acr+-Mes) perchlorate (Ered* = 2.30 VSHE)14,15 has been employed to
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convert various biphenyl-2-carboxylic acid derivatives to the corresponding benzocoumarins
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in high yield via oxidation of the carboxylate.13 The resulting carboxyl radical (RCOO•)
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intermediate undergoes intramolecular cyclization and oxidation by oxygen in air or another
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secondary oxidant, such as persulfate. Given that the oxidation potential of triplet excited
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state Suwanee River Fulvic Acid (3SRFA*) has been estimated in the range of 1.6-1.8 VSHE16,
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which should be capable of oxidizing an aromatic carboxylate,17 we anticipated that BPAs
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would be suitable probes for studying the reactivity of the more strongly oxidizing fraction of
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photochemically generated 3CDOM*. Eberson estimated the oxidation potential of benzoate
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with a thermochemical cycle analysis to be 1.66 VSHE.17 We have recalculated this value to be
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1.80 VSHE based on newer thermochemical data (see Supporting Information), and take this to
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be the approximate value for biphenyl-2-carboxylate. Incidentally, the oxidation potential of
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neutral biphenyl-2-carboxylic acid has been experimentally determined to be 1.95 VSHE (in
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HOAc).18 Hydroxyl radical (•OH), a potent oxidizing species that can hydroxylate arenes such
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as benzene and benzoic acid,19,20,21 is also produced by DOM via photochemical reactions as
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well as through exposure of reduced DOM to oxygen.22,23,24,25 Other hydroxylating species
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besides free •OH have also been observed to generate phenols from aromatic compounds.26,27
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The biphenyl-2-carboxylic acid probes are most susceptible to hydroxylation by •OH and
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•
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rings; hydroxylation at 2 positions will give rise to the identical benzocoumarin product
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formed by the carboxylate oxidation mechanism. In order to differentiate between these
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competing pathways, we employed a dual probe strategy based on differing sensitivities of
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the two reaction mechanisms to substrate substituent effects (Fig. 1). Previous studies have
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observed that electron rich arenes undergo reaction with carboxyl radicals more readily than
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electron poor rings.28,29 In contrast, studies on aromatic ring hydroxylation by •OH and
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similarly electrophilic oxygen radical species have shown that highly electron-donating
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substituents such as methoxy (OMe) groups favor ortho or para hydroxylated arenes, while
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highly electron-withdrawing substituents such as nitro (NO2) and cyano (CN) groups more
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readily produce the meta phenol.30,31 Under the conditions of these studies, more moderately
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directing substituents, such as Cl and Me, showed poor regioselectivity for hydroxylation,
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and similar yields of the ortho, para, and meta-substituted phenols were obtained.
OH-like species at the 8 unsubstituted sites (6 unique sites by symmetry) on the aromatic
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Figure 1. Scheme for formation of benzocoumarin products from biphenyl-2-carboxylic
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acids via two possible mechanistic pathways (R = OMe, Me, tBu, H, OH, Cl, CF3, CN). The
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top pathway involves one-electron oxidation of the carboxylate to give a carboxyl radical,
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which undergoes cyclization with the neighboring aryl ring. Subsequent oxidation gives the
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benzocoumarin product. The bottom pathway proceeds by hydroxylation of the pendant
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arene by hydroxyl radical (•OH) to give a mixture of hydroxy-substituted isomers.
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Condensation esterification of the 2’-hydroxy isomer leads to the benzocoumarin.
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When utilizing a pair of probes—one possessing an electron-donating group (EDG)
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and the other an electron-withdrawing group (EWG)— simultaneously in a sample, we
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hypothesized only compounds with very strongly directing groups would display any
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regioselectivity toward •OH. A strongly EWG was anticipated to favor formation of the
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desired benzocoumarin product, while a strongly EDG would more likely favor formation of
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the unproductive ortho-substituted regioisomer. Compounds with moderate EWG and EDG
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were expected to display low regioselectivity for hydroxylation and yield similar reaction rate
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constants for formation of benzocoumarin product. Thus, for systems in which the •OH
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EDG EWG pathway is dominant, we expected values for ⋅OH /⋅OH ≤ 1. More disparate rates for the
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EWG reaction driven by triplet-sensitized oxidation would give rise to a value for 3EDG sens* /3sens* >
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1. The ratio of the reaction rates for a pair of probes could therefore serve as an indicator for
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the major oxidation pathways that are active in the transformation of benzoic acids in a given
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DOM sample and the relative contributions of each pathway in the photooxidation of these
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probe molecules. Analysis of the specific benzocoumarins produced from these two oxidation
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pathways would be an advantage over monitoring probe loss, which may be confounded by
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other decay processes such as direct photolysis or degradation by other photochemically
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produced reactive intermediates. Furthermore, since the probes would be exposed to the same
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concentrations of oxidants, the ratio of the rate constants could be obtained directly from the
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rates, without the need to calculate oxidant steady-state concentrations.
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Experimental
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General considerations
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Chemicals.
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All chemicals and solvents, unless otherwise noted, were purchased from commercial
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suppliers and were of reagent grade or higher. Biphenyl-2-carboxylic acid was purchased
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from Alfa Aesar and used without further purification. 4’-(trifluoromethyl)-[1,1’-biphenyl]-2-
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carboxylic acid was purchased from Combi-Blocks and used without further purification.
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Suwanee River Fulvic Acid (SRFA Standard II, 2S101F), Suwanee River Humic Acid
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(SRHA II Standard, 2S101H), Pony Lake Fulvic Acid (PLFA Reference, 1R109F), and
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Suwanee River Natural Organic Matter (SRNOM, 2R101N) samples were purchased from
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the International Humic Substances Society (IHSS), reconstituted with ultrapure water, and
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filtered through a 0.22 µm syringe filter. Great Dismal Swamp water (pH 3.7) was collected
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by Prof. Kenneth Mopper in summer 2014 in Suffolk, Virginia, USA. A second sample of
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Great Dismal Swamp water collected from the Jericho Ditch in 2016 contained 8.2 µM
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ferrous iron and 25 µM total iron. Lake Bradford water was collected by Dr. Paul R. Erickson
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in January 2015 from Tallahassee, Florida, USA. Water was filtered through a sterile 0.2 µm
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Whatman Polycap TC 75 encapsulated filter (part number: 6714-7502), and stored at 4 °C in
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acid-rinsed plastic Nalgene bottles, protected from light. Solid phase extractions (SPE) of
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natural waters were conducted using C18 Empore standard density cartridges (10 mm/6 mL,
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3M, 4315SD) according to manufacturer’s instructions. SPE cartridges were conditioned with
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0.5 mL MeOH followed by 3 mL water, and then 2 x 5 mL whole water was passed through
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the cartridge. The SPE retentate was washed with 1 mL water and then eluted from the
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cartridges using 0.8 mL MeOH. Samples were dried by rotoevaporation and flushed under a
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stream of N2 overnight to ensure full removal of organic solvent. Samples were then
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reconstituted in MilliQ water and handled in the same manner as the DOM isolates.
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Stock solutions of terephthalic acid (TPA) and hydroxyterephthalic acid (hTPA) and
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benzocoumarins were stored in the dark at 4 °C and used over the course of several weeks.
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BPA solutions were stored at room temperature in the dark and used over the course of
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several weeks. Nitrite solutions were freshly prepared before each experiment. All solutions
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were prepared at room temperature in air, unless otherwise noted.
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Synthesis. Biphenyl-2-carboxylic acid derivatives were synthesized according to previously
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described Suzuki coupling conditions.32,33,34 Benzocoumarin products were synthesized
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according to oxidative cyclization conditions previously described by Wang et al.33 Detailed
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synthetic methods and structural characterization data are included in the Supporting
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Information.
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Instrumental Methods.
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Total organic carbon (TOC) analyses were performed using a Shimadzu TOC-L
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analyzer. All water used for photochemical experiments was obtained from a Barnstead
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Nanopure water purification system. Fluorescence measurements were collected using a
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Horiba Jobin Yvon Fluoromax-3 Spectrofluorometer. UV-visible light absorption spectra
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were acquired on a Varian Cary 50 Bio Spectrophotometer. pH was measured using an Orion
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Ross Ultra Semi-Micro pH electrode.
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Steady-State Photolysis. Photochemical experiments were performed using a Rayonet merry-go-round
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photoreactor outfitted with 10 x 365 nm UVA bulbs. The temperature during photolysis
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experiments was approximately 30-31 °C. Samples were irradiated in 12 x 75 mm Pyrex
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culture tubes (VWR, 99445-12). Reaction volumes were 2-4 mL and 100-200 µL aliquots
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were sampled for HPLC analysis.
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Hydroxyl radical experiments were conducted with NaNO2 solutions (200 µM). TPA
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(10 µM) was used as an internal probe for monitoring hydroxyl radical concentrations in
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BPA reactivity experiments as described by Page et al.35 Steady-state hydroxyl radical
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concentrations in NaNO2 photolyses ranged from 3.2 × 10-15 to 1.2 × 10-14 M in the presence
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of the BPA probes in MilliQ water containing 0.2 to 2.5% (v/v) MeCN. Yields were
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calculated as the fraction of BZC formed over the BPA consumed during the reaction, based
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on averaged rate constants for BZC formation and BPA degradation as monitored by HPLC.
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Addition of 1% iPrOH to NaNO2 photolysis samples was confirmed to quench all hydroxyl
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radical that was detectable by TPA. Concentrations of model sensitizers were as follows:
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Acr+-Mes (2.5 µM), lumichrome (10 µM), and 4-carboxybenzophenone (CBBP, 100 µM).
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Aliquots (50-200 µL) for HPLC analysis were taken at 5-10 min intervals, with BZC-CH3
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data typically linear between 0-20 min and BZC-CF3 typically linear between 0-60 min. For
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paired probe experiments involving model oxidants and DOM samples, BPA probe stock
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solutions were prepared in water with addition of stoichiometric NaOH. BPA-CH3 and BPA-
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CF3 were used simultaneously in the reaction mixture, each at 100 µM concentration. Paired
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probe experiments with DOM were MeCN-free and sampled at 5-10 min intervals; aliquots
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(50 µL) were diluted with 20 µL MeCN prior to HPLC analysis. Hydroxyl radical quenching
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experiments with DOM samples contained 1% v/v iPrOH and 2% v/v MeCN. Experiments
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involving CDOM as the source of oxidants were conducted with CDOM absorbance values
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of 0.1 at 365 nm. The pH of samples was adjusted using HCl or NaOH (aq); DOM samples
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were measured between pH 4-6.
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HPLC Methods HPLC detection of all compounds was performed on an Agilent 1100 Series HPLC
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with G1313A quaternary pump, G1321C 1260 FLD, and a G1314A UV standard cell.
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Samples were separated using a ZORBAX Eclipse XDB C18 column (4.6 x 150 mm, 5 µm,
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Agilent 993967-902) and a C18 guard column (4.6 x 12.5 mm, 5 µm, Agilent 820950-926),
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with a flow rate of 1 mL/min and UV-visible detection at 219 nm. TPA (rt 3.6 min) and
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hydroxy-TPA (hTPA, rt 2.5 min) analyses were carried out using an HPLC method with 70%
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pH 3 sodium phosphate buffer (with 10% MeCN) and 30% MeOH, 20 µL injections, with
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fluorescence excitation at 230 nm and emission at 460 nm. Analysis of BPA-CH3 (rt 3.7 min)
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was conducted with 64 % pH 5 ammonium acetate buffer and 36% MeCN, 20 µL injections,
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and fluorescence excitation at 250 nm and emission at 420 nm. Analysis of BPA-CF3 (rt 3.8
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min) was conducted with 60% pH 5 ammonium acetate buffer and 40% MeCN, 20 µL
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injections, and fluorescence excitation at 240 nm and emission at 370 nm. Analysis of BZC-
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CH3 (rt 4.9 min) and BZC-CF3 (rt 5.6 min) was conducted with 32% pH 5 ammonium acetate
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buffer and 68% MeCN, 50 µL injections, and fluorescence excitation at 313 nm and emission
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at 370 nm. Due to weak fluorescence of the BZC-CF3 product, UV-visible detection was used
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for quantification of BZC-CF3. Mobile phase composition and detector parameters for other
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probes are described in the supporting information. Data analysis was performed using
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Agilent OpenLAB CDS ChemStation Edition C.01.05 software.
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Results and Discussion
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Probe Pair Optimization
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To identify the best candidates amongst the 8 probes, we tested individual probe
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reactivity toward model oxidants under aqueous conditions. Using NaNO2 photolysis to
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generate •OH, we were able to eliminate several probes based on both reactivity and physical
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properties, such as solubility. For example, BPA-tBu suffered from poor water solubility and
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was therefore eliminated from consideration.
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Table 1. Summary of characterization of BPA probes and BZC products. Absorbance
Fluorescence
Acidity
Reactivity with •OH
λabs max, ε/M-1 cm-1
λex max, λem max
ΦF
pKa
krxn(•OH)/M-1 s-1
Yield BZC
BPA-OMe
254 nm, 13 300
254, 420
0.005
3.7
(2.3 ± 0.1) × 1010
0.0011
BPA-CH3
248 nm, 12 900
250, 420
0.0055
4.0
(8.8 ± 0.1) × 109
0.13
BPA-tBu
248 nm, 13 700
250, 410
0.0008
4.0
(9.5 ± 1.1) × 109
0.13
BPA-H
245 nm, 11 600
229, 391
0.0012
4.2
(1.2 ± 0.1) × 1010
0.18
BPA-Cl
248 nm, 14 400
235, 395
0.0004
3.8
(9.1 ± 0.7) × 109
0.25
BPA-CF3
245 nm, 12 300
239, 369
0.0005
3.9
(7.2 ± 0.8) × 109
0.14
BPA-CN
262 nm, 17 900
265, 355
0.0004
3.4
(5.8 ± 0.6) × 109
0.074
BPA-OH
254 nm, 12 800
257, 421
0.0002
3.8
(1.6 ± 0.9) × 109
b
BZC-OMe
332 nm, 7 100
333, 434
0.40
NDa
ND
ND
BZC-CH3
323 nm, 6 500
324, 393
0.12
ND
ND
ND
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BZC-tBu
322 nm, 5 700
323, 390
0.086
ND
ND
ND
BZC-H
318 nm, 5 800
318, 380
0.024
ND
(1.1 ± 0.1) × 109
ND
BZC-Cl
318 nm, 7 800
320, 382
0.11
ND
ND
ND
BZC-CF3
311 nm, 6 400
313, 370
0.011
ND
ND
ND
BZC-CN
328 nm, 1 100
325, 364
0.026
ND
ND
ND
BZC-OH
333 nm, 7 300
335, 440/570
0.003
ND
b
ND
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a
ND = not determined
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b
Significant direct photodegradation observed
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Direct photodegradation of some products, such as BZC-CH3 and BZC-OH, was
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observed, although direct photodegradation was substantially decreased in the presence of
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light-screening reaction constituents such as model sensitizers and DOM. Yields and rate
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constants for •OH were calculated using TPA to determine [•OH]ss (Table 1). Degradation of
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BZCs via reaction with •OH was neglected due to the low concentrations of BZCs formed
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upon photolysis (< 5 µM BZC after 60 min in nitrite samples and < 1 µM BZC after 30 min
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in DOM samples) relative to BPAs. Hydroxyl radical rapidly degraded BPA-OH while both
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BPA-OH and the expected BZC-OH product underwent significant direct photolysis.
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Benzocoumarin formation via the •OH pathway relies upon hydroxylation of the arene at a
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position meta to the substituent; consequently, benzocoumarin yields from BPA-OMe were
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very low (0.1%) as a result of the enhanced ortho-directing ability of the methoxy group.30
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Interestingly, derivatives with highly electron-withdrawing substituents such as BPA-CN also
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showed lower product yields and rate constants. While electron-withdrawing substituents
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may enhance formation of the meta substituted phenol in single aromatic ring systems, the
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presence of the second aromatic ring in the biphenyl probes may result in a more complicated
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product distribution. Overall, the rate constants for formation of benzocoumarin product
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(⋅OH ) due to oxidation by •OH spanned a wider range than expected (Fig. 2a). Nonetheless,
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empirical determination of the rate constants and yields for BPAs with moderately electron-
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donating or electron-withdrawing substituents allowed for identification of appropriate
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derivatives for further exploration.
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Initial experiments with Acr+-Mes as a model photooxidant successfully generated
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benzocoumarin products from biphenyl-2-carboxylates upon irradiation with UVA light,13
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although the photocatalyst was unstable to reaction conditions and not suitable for extensive
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kinetic studies.36 For comparison of probe reactivity via the triplet sensitization reaction
249
pathway, we opted for lumichrome due to its suitably strong oxidation potential (1.91 VSHE)11
250
and good photochemical stability. Prior studies have shown strong Hammett correlations
251
using substituent constant sigma σp+ values for radical substitution reactions involving
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intermolecular addition of benzoate radicals to arenes.37,38 We therefore also used a Hammett
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plot to look for relationships between the electronic nature of the substituents and probe
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reactivity. A strong linear correlation was apparent with the Hammett equation sensitivity
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constant ρ = -2.59 (Fig. 2b, r2 = 0.98), suggesting the reaction is highly sensitive to
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substituents relative to the model reaction, and the reaction rate is suppressed by electron-
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withdrawing groups. BPAs with electron-donating substituents such as methyl, t-butyl, and
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methoxy (R = Me, tBu, OMe) displayed relatively fast formation of the corresponding
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benzocoumarin products, while electron-withdrawing substituents (R = Cl, CF3, CN) resulted
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in much slower product formation (Fig. 2b), consistent with previous observations involving
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radical addition to arenes.28
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Figure 2. Hammett plot showing the log of the ratio of the rate constants for BZC formation
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from the corresponding 4’-substituted BPAs via (a) hydroxyl radical (•OH) and (b) single
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electron transfer by lumichrome (LC), where R refers to the substituent at the 4’-position of
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biphenyl-2-carboxylic acid. The slope of the fitted line for (b) represents the sensitivity
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constant rho (ρ). Hydroxyl radical was generated by irradiation of NaNO2 (200 µM) in M
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illiQ water at 365 nm and quantified by formation of hTPA from TPA (10 µM) in the
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presence of the BPA probes. Lumichrome reactions were performed in MilliQ water + 2%
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acetonitrile, pH 7 and sensitized by lumichrome (10 µM) with irradiation at 365 nm. Sigma
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(σp+) values from Hansch, Leo, and Taft.39 Error bars represent ± 2 s.d.
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We selected BPA-CH3 and BPA-CF3 for further study due to their similar reactivities
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and product yields upon oxidation by •OH, in contrast with their highly disparate rates of
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benzocoumarin formation by lumichrome (Fig. 2a). This strong electronic effect and the high
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ratios of BZC-CH3 to BZC-CF3 formation were also observed with other strong triplet
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photooxidants such as 4-carboxybenzophenone (CBBP, E°* = 1.84 VSHE10). Reaction of the
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BPA probes (oxidation potential > ca. 1.80 V SHE) with weaker oxidants such as triplet
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excited state 3-methoxyacetophenone (1.64 V) and perinaphthenone (1.03 V) was slow, and
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formation of BZC-CF3 was below the detection limit of the employed method. Given that
283
both 3-methoxyacetophenone and perinaphthenone are known to generate singlet oxygen
284
(1O2) photochemically, the slow reactivity of the BPA probes in the presence of these model
285
oxidants indicates that the BPA probes have very low reactivity toward 1O2. While 1O2 does
286
not appear react with the BPA probes, molecular oxygen is likely essential for product
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formation as it can act as the secondary oxidant for subsequent steps in both mechanistic
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pathways leading to BZC formation (Fig. 1). Direct photolysis of BPA-CH3 showed no
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detectable formation of BZC-CH3 in the initial 20 min, although approximately 10 nM
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product was observed in solution after 1 hr (Fig. S5). No appreciable BZC-CF3 formation or
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BPA-CF3 degradation could be detected after irradiating BPA-CF3 solutions for 1 hr.
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Yields from steady-state reactions with lumichrome were quantitative for the early
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timepoints in the linear range which were used for initial rate calculations. Laser flash
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photolysis (LFP) experiments revealed that the rate constant for quenching of lumichrome by
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BPAs showed no trend with the electron-donating or electron-withdrawing ability of the
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substituent at the 4’-position (Fig. S6). Given that electron-donating substituents accelerated
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the rate of benzocoumarin formation in steady-state experiments, these LFP experiments
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support previous observations that initial electron transfer from the carboxylic acid to triplet
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excited state sensitizers is not the rate-limiting step in formation of the benzocoumarin.13
300 301 302
Probe Application to DOM Samples Application of the BPA-CH3/BPA-CF3 probe pair to photochemical conditions in
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natural water samples and organic matter isolate solutions revealed differences in the
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production of hydroxyl radical-like oxidants compared to triplet sensitizers (Fig. 3b). All
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ratios obtained for DOM samples were intermediate compared to oxidation by either •OH
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CH
CF
306
(⋅OH3 /⋅OH3 ≈ 2.1 ± 0.3), represented by nitrite photolysis, or by excited state triplet
307
lumichrome as a model sensitizer (LC 3 /LC 3 > 251 ± 57). Adding NaNO2 to Dismal Swamp
308
water to enhance •OH production shifted the ratio toward 2, showing that the probe pair
309
remained responsive in the presence of DOM if the relative levels of •OH and excited state
310
triplet oxidants (3sens*) were artificially adjusted (Fig. S7).
CH
CF
311 NaNO2
LC 1.0
0.3 0.2
0
0.3 0.2 0.1
10 20 30 40 50 60
0
10
Time (min)
0.6 0.4 CF3
20
30
40
0
50
0
10
Time (min)
20
30
40
Time (min)
LC
NaNO2
+ iPrOH
LB
+ iPrOH
DS SPE
+ iPrOH
DS
+ iPrOH
PLFA
+ iPrOH
SRNOM
+ iPrOH
SRHA
+ iPrOH
7 6 5 4 3 2 1 0 SRFA
d BZC-CH3 /dt ln d BZC-CF3 /dt
b)
0.8
0.2 CF3
0 0
CH3
CH3
CF3
0.4
0.1
312
DS
0.4
CH3
[BZC] ( µM)
0.5
[BZC] ( µM)
[BZC] ( µM)
a)
313
Figure 3. (a) Representative graphs of BZC production during photolysis of BPA-CH3/BPA-
314
CF3 probes in the presence of 200 µM NaNO2, 10 µM lumichrome (LC), or Dismal Swamp
315
water (DS). (b) Box and whisker plot of the natural log of the ratio of formation rates of
316
BZC-CH3:BZC-CF3 in the presence of model photooxidants and organic matter samples,
317
with and without 1% isopropanol quenching.
318 319
Isopropanol quenching of •OH formed during photolysis of DOM consistently
320
resulted in a higher ratio of BZC-CH3/BZC-CF3 formation, demonstrating the ability of the
321
probe pair to detect changes in production of •OH (and hydroxyl radical-like) oxidants.
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Additionally, filtered whole water samples from Lake Bradford (LB) and the Dismal Swamp
323
(DS) showed comparatively lower ratios of BZC-CH3 to BZC-CF3 formation, suggesting
324
hydroxyl radical-like oxidants are generated at higher levels in filtered whole waters
325
compared to organic matter isolate solutions.
326
In order to compare these ratios from the BPA-CH3/BPA-CF3 probe pair in a
327
quantitative manner, equations 1 and 2 were used to describe formation of benzocoumarin
328
product, where Y represents the yield of BZC formed and 3sens* represents the overall
329
reaction rate constant for observed formation of BZC product via the triplet sensitized
330
pathway.
331 332
BZC-CH3
CH
CH
CH
CH
3 3 = ⋅OH3 ⋅OH3 ⋅OHBPA-CH3 + 3sens* 3sens* 3sens*BPA-CH3 (eq. 1)
333 334
BZC-CF3
CF
CF
CF
CF
3 3 = ⋅OH3 ⋅OH3 ⋅OHBPA-CF3 + 3sens* 3sens* 3sens*BPA-CF3
(eq. 2)
335 336
Rate constants for the CH3 and CF3 derivatives can be related according to equations (3) and
337
(4), where !⋅"# and !3sens* are the ratios derived from reaction of the probe pair with hydroxyl
338
radical or triplet sensitizer, respectively.
339 340
CH
CF
⋅OH3 =!⋅OH ⋅OH3
(eq. 3)
341 342
CH
CF
3 3 3sens* =!3sens* 3sens* (eq. 4)
343 344 345
The results from reaction of the probe pair with different model oxidants gave an !⋅"# value of 2.1 ± 0.3 and an !3sens* value of 251 ± 57 for lumichrome. Examination of other
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triplet sensitizers showed that !3sens* varied greatly amongst different sensitizers, with no
347
obvious trend (Table S1). Thus, it is not possible to predict the expected !3sens* for a given
348
selection of triplet sensitizers. Since the concentrations and oxidation potentials of discrete
349
triplet sensitizers in CDOM are poorly defined, the •OH quenching experiments with 1%
350
isopropanol were used to define the BZC-CH3 : BZC-CF3 ratio obtained from only the
351
excited state triplet photooxidants in each different organic matter sample, providing a
352
distinct, empirically determined !3sens* value for each DOM sample.
353 354
Relative Contributions of OH• and 3CDOM* to Probe Oxidation
355
By substituting equations (3) and (4) into equations (1) and (2), the relative
356
contributions of hydroxyl radical and triplet sensitizing species to the observed
357
photochemical reactivity of DOM toward the biphenyl-2-carboxylates could be assessed. The
358
total rates of formation of benzocoumarin products BZC-CH3 and BZC-CF3 were
359
experimentally determined, and then the oxidation by •OH and 3CDOM* was calculated as a
360
percentage of the total oxidation (eq. S9). DOM samples generated similar relative amounts
361
of photooxidants upon irradiation with UV-A light, with 75-96% of BPA-CH3 oxidation
362
attributable to 3CDOM*, compared to 20-67% of BPA-CF3 oxidation attributable to
363
3
CDOM* across all samples (Fig. 4).
364
Processing of Great Dismal water samples via solid phase extraction appeared to
365
change the ability of the DOM to oxidize the BPA probes. Whole Dismal Swamp water
366
displayed the highest •OH production amongst all the samples (Table S2); in contrast, Dismal
367
Swamp SPE retentate •OH steady-state concentrations were 4-fold lower than the whole
368
water sample. Water from the Great Dismal Swamp has been noted for its low pH and high
369
iron content; water collected from the Jericho Ditch contained 25 µM total iron, which is
370
consistent with previous measurements of 20-29 µM for Dismal Swamp water40,41,42 and
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comparatively higher than the dissolved iron concentrations reported for other natural waters
372
and isolates.43 The presence of iron species in Dismal Swamp water could therefore be a
373
major source of •OH produced via photo-Fenton chemistry.44 Changes in DOM composition
374
following SPE treatment with C18 resins has been previously reported, generally resulting in
375
increased average molecular weight of the retentate.45,46 Given that recent work on size
376
fractionation of DOM has noted decreasing formation of reactive intermediates such as
377
3
378
hydroxyl radical production in the Dismal Swamp water SPE retentate compared to the whole
379
water sample could be related to removal of hydroxyl radical sources such as dissolved
380
iron50,51 and/or removal of photochemically inactive, strongly absorbing compounds by SPE.
381
Further examination of the compositional changes in these samples is required to determine
382
the basis for the observed changes.
383
CDOM*, 1O2, and •OH with increasing molecular weight,47,48,49 the observed decrease in
Notably, while these probes show differences in the oxidant type—either a
384
hydroxylating species or triplet excited state sensitizer—based on the ratio of the
385
benzocoumarin products, these probes cannot differentiate free hydroxyl radical from other
386
hydroxylating species.26,27 Previous evidence for hydroxylation of benzoic acid by
387
hydroxylating species other than free •OH radical likely also applies to the BPA probes,
388
which are structurally similar. This approach treats all •OH (free and •OH-like) as quenchable
389
by isopropanol. If some of the lower energy •OH-like hydroxylating species are not quenched
390
by isopropanol, then the triplet organic matter contribution will be overestimated.
391
In this study, the BPA probes were applied solely to freshwater samples. The effects
392
of inorganic species have not been explored for this system, but given that the primary
393
measurement gained from the probes is taken as a ratio, undesired side reactions involving
394
these probes and other species present in environmental samples would have to affect BPA-
395
CH3 differently from BPA-CF3 to impact the proposed method. While the reactivity of other
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3
397
pH,52,53 since the BPA probes are structurally similar and rely upon product formation rather
398
than probe loss, we speculate that this approach would be relatively resistant to side
399
reactions; confirmation of this hypothesis requires further study.
CDOM* probe molecules such as TMP have been found to be sensitive to ionic strength and
400
401 402
Figure 4. Contribution of hydroxyl radical-like (% ·OH) or triplet sensitizer (% 3CDOM*)
403
photooxidants to conversion of BPA derivatives to BZC products for whole natural waters or
404
organic matter isolates. The percent conversions to (a) BZC-CH3 and (b) BZC-CF3 were
405
based on the probe ratios for •OH from nitrite photolysis and 3CDOM* obtained from
406
photolysis of DOM samples + 1% iPrOH. Error bars represent ± 2 s.d.
407 408
Environmental Implications
409
Overall, the BPA-CH3/BPA-CF3 probe pair can be utilized for studying
410
photochemical production of both hydroxyl radical-like and triplet excited state
411
photooxidants in natural waters, complementing existing probe molecules for
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photochemically produced reactive intermediates.22,54 Both BPA-CH3 and BPA-CF3 are
413
commercially available, easily handled solids which can be dissolved in aqueous solutions
414
with little to no organic cosolvent, depending on pH. Relying upon detection of the
415
benzocoumarin products rather than probe loss provides selectivity for highly oxidizing
416
3
417
CH3:BZC-CF3 formation is relatively pH insensitive, allowing for use of these probes
418
between pH 4-6 without the need for pH matching among samples. Analysis of different
419
DOM samples using the BPA-CH3/BPA-CF3 probe pair showed that oxidation by 3CDOM*
420
dominated over oxidation by •OH in most organic matter isolates and natural water samples.
421
CDOM* and •OH over other degradation processes. Additionally, the ratio of BZC-
This work is a strong indication that there exists a strongly oxidizing fraction of
422
3
423
photochemical reaction conditions. The results from these probes may also contribute to the
424
design of related molecules that can serve as model compounds for the naturally occurring
425
carboxylic acids present in DOM. Photooxidation of DOM has been widely observed for
426
marine and freshwater systems, although the mechanisms by which dissolved inorganic
427
carbon (DIC) is formed remain unclear. Previous studies have noted loss of aromatic groups
428
and oxygen-containing functionalities such as aldehydes, ketones, and acids upon
429
photooxidation,55 and photodecarboxylation has been proposed as a major pathway for
430
abiotic DOM mineralization.56,57 Photodecarboxylation of carboxylates, including acid
431
functionalities present in DOM, has been hypothesized to proceed via carboxyl radical
432
(RCOO•) species.58,59,60 However, based on previous kinetic studies, decarboxylation of
433
aromatic carboxyl radicals (ArCOO•) is very slow (~1–102 s-1) and does not occur to an
434
appreciable extent when other pathways, such as radical addition, are viable.29,61 Preliminary
435
experiments involving photolysis of biphenyl 4-carboxylic acid showed no loss of starting
436
material, and no decarboxylation products were detected by HPLC (Fig. S10). We therefore
CDOM* which can successfully oxidize simple aromatic carboxylic acids under
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expect that irradiation of DOM is unlikely to produce CO2 from decarboxylation of aromatic
438
carboxylates via an oxygen-centered carboxyl radical. Notably, alkyl carboxyl radicals are
439
known to decarboxylate at much more rapid rates (4 ×108 s-1),62 and DOM has been shown to
440
facilitate photodecarboxylation reactions of pharmaceuticals containing benzylic carboxylic
441
acids, such as ibuprofen and naproxen.63,64,65 Consequently, the role of alkyl carboxylates in
442
the photochemical formation of CO2 from DOM may be of greater importance for future
443
studies.
444 445
Acknowledgments
446
We would like to acknowledge Markus Schmitt for assistance with laser experiments, Prof.
447
Martin Schroth for determination of iron concentrations in Dismal Swamp water, and
448
Caroline Davis, Leandro Portenier, and Nicolas Walpen for TOC analyses. We also thank
449
Caroline Davis, Paul Erickson, Elisabeth Janssen, Kimberly Parker, and Markus Schmitt for
450
helpful discussions. We are grateful to Prof. Kenneth Mopper for the Dismal Swamp water
451
samples and Paul Erickson for collection of Lake Bradford water. V. S. L. was supported in
452
part by a Swiss Government Excellence Scholarship in conjunction with the U.S. Fulbright
453
Program.
454 455
Supporting Information
456
Synthesis and characterization of BPA probes and BZC products, laser flash photolysis
457
(LFP), and photophysical characterization. Figures showing rate constant calculations from
458
LFP plots for BPA-CF3, BPA-CH3, BPA-H, and BPA-tBu, BZC-CH3/BZC-CF3 ratios for
459
Dismal Swamp water spiked with nitrite, pH dependence of BZC-CH3 and BZC-CF3
460
formation rates in the presence of hydroxyl radical and triplet excited state sensitizers, and
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sensitization of biphenyl-4-carboxylic acid with lumichrome compared to biphenyl-2-
462
carboxylic acid. Tables with summary of BZC-CH3/BZC-CF3 ratios for different model
463
triplet sensitizers and thermochemical calculations for the redox potential of benzoic acid.
464 465
This material is available free of charge via the Internet at http://pubs.acs.org.
466 467 468 469
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