Fluorescent Molecular Probes for Detection of One-Electron Oxidants

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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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Environmental Science & Technology

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Fluorescent molecular probes for detection of one-electron oxidants

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photochemically generated by dissolved organic matter

3

Vivian S. Lin1, Matthew Grandbois,2 Kristopher McNeill1*

4

1

5

Switzerland

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2

7

*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

9 10

TOC:

11 12 13

Abstract We report a dual probe system based on 4’-substituted biphenyl-2-carboxylic acids

14

(BPAs) for analysis of photooxidants generated by dissolved organic matter. The BPA probes

15

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

17

triplet excited state sensitizers from hydroxylating species such as hydroxyl radical (•OH)

ACS Paragon Plus Environment


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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.

29 30 31

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

76

•

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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

89


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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.

98 99

When utilizing a pair of probes—one possessing an electron-donating group (EDG)

100

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



108

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

115

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.

118 119

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-

125

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

141

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

143

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

147

several weeks. Nitrite solutions were freshly prepared before each experiment. All solutions

148

were prepared at room temperature in air, unless otherwise noted.

149 150 151

Synthesis. Biphenyl-2-carboxylic acid derivatives were synthesized according to previously

152

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

155

Information.

156 157

Instrumental Methods.



158

Total organic carbon (TOC) analyses were performed using a Shimadzu TOC-L

159

analyzer. All water used for photochemical experiments was obtained from a Barnstead

160

Nanopure water purification system. Fluorescence measurements were collected using a

161

Horiba Jobin Yvon Fluoromax-3 Spectrofluorometer. UV-visible light absorption spectra

162

were acquired on a Varian Cary 50 Bio Spectrophotometer. pH was measured using an Orion

163

Ross Ultra Semi-Micro pH electrode.

164 165 166

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

168

experiments was approximately 30-31 °C. Samples were irradiated in 12 x 75 mm Pyrex

169

culture tubes (VWR, 99445-12). Reaction volumes were 2-4 mL and 100-200 µL aliquots

170

were sampled for HPLC analysis.

171

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

175

of the BPA probes in MilliQ water containing 0.2 to 2.5% (v/v) MeCN. Yields were

176

calculated as the fraction of BZC formed over the BPA consumed during the reaction, based

177

on averaged rate constants for BZC formation and BPA degradation as monitored by HPLC.

178

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

182

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-

185

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

189

involving CDOM as the source of oxidants were conducted with CDOM absorbance values

190

of 0.1 at 365 nm. The pH of samples was adjusted using HCl or NaOH (aq); DOM samples

191

were measured between pH 4-6.

192 193 194

HPLC Methods HPLC detection of all compounds was performed on an Agilent 1100 Series HPLC

195

with G1313A quaternary pump, G1321C 1260 FLD, and a G1314A UV standard cell.

196

Samples were separated using a ZORBAX Eclipse XDB C18 column (4.6 x 150 mm, 5 µm,

197

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

199

hydroxy-TPA (hTPA, rt 2.5 min) analyses were carried out using an HPLC method with 70%

200

pH 3 sodium phosphate buffer (with 10% MeCN) and 30% MeOH, 20 µL injections, with

201

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,

203

and fluorescence excitation at 250 nm and emission at 420 nm. Analysis of BPA-CF3 (rt 3.8

204

min) was conducted with 60% pH 5 ammonium acetate buffer and 40% MeCN, 20 µL

205

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

207

buffer and 68% MeCN, 50 µL injections, and fluorescence excitation at 313 nm and emission



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208

at 370 nm. Due to weak fluorescence of the BZC-CF3 product, UV-visible detection was used

209

for quantification of BZC-CF3. Mobile phase composition and detector parameters for other

210

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.

212 213

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

218

properties, such as solubility. For example, BPA-tBu suffered from poor water solubility and

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was therefore eliminated from consideration.

220 221

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

222

a

ND = not determined

223

b

Significant direct photodegradation observed

224 225

Direct photodegradation of some products, such as BZC-CH3 and BZC-OH, was

226

observed, although direct photodegradation was substantially decreased in the presence of

227

light-screening reaction constituents such as model sensitizers and DOM. Yields and rate

228

constants for •OH were calculated using TPA to determine [•OH]ss (Table 1). Degradation of

229

BZCs via reaction with •OH was neglected due to the low concentrations of BZCs formed

230

upon photolysis (< 5 µM BZC after 60 min in nitrite samples and < 1 µM BZC after 30 min

231

in DOM samples) relative to BPAs. Hydroxyl radical rapidly degraded BPA-OH while both

232

BPA-OH and the expected BZC-OH product underwent significant direct photolysis.

233

Benzocoumarin formation via the •OH pathway relies upon hydroxylation of the arene at a

234

position meta to the substituent; consequently, benzocoumarin yields from BPA-OMe were

235

very low (0.1%) as a result of the enhanced ortho-directing ability of the methoxy group.30

236

Interestingly, derivatives with highly electron-withdrawing substituents such as BPA-CN also

237

showed lower product yields and rate constants. While electron-withdrawing substituents

238

may enhance formation of the meta substituted phenol in single aromatic ring systems, the

239

presence of the second aromatic ring in the biphenyl probes may result in a more complicated

240

product distribution. Overall, the rate constants for formation of benzocoumarin product

241

(⋅OH ) due to oxidation by •OH spanned a wider range than expected (Fig. 2a). Nonetheless,

242

empirical determination of the rate constants and yields for BPAs with moderately electron-



243

donating or electron-withdrawing substituents allowed for identification of appropriate

244

derivatives for further exploration.

245

Initial experiments with Acr+-Mes as a model photooxidant successfully generated

246

benzocoumarin products from biphenyl-2-carboxylates upon irradiation with UVA light,13

247

although the photocatalyst was unstable to reaction conditions and not suitable for extensive

248

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

252

intermolecular addition of benzoate radicals to arenes.37,38 We therefore also used a Hammett

253

plot to look for relationships between the electronic nature of the substituents and probe

254

reactivity. A strong linear correlation was apparent with the Hammett equation sensitivity

255

constant ρ = -2.59 (Fig. 2b, r2 = 0.98), suggesting the reaction is highly sensitive to

256

substituents relative to the model reaction, and the reaction rate is suppressed by electron-

257

withdrawing groups. BPAs with electron-donating substituents such as methyl, t-butyl, and

258

methoxy (R = Me, tBu, OMe) displayed relatively fast formation of the corresponding

259

benzocoumarin products, while electron-withdrawing substituents (R = Cl, CF3, CN) resulted

260

in much slower product formation (Fig. 2b), consistent with previous observations involving

261

radical addition to arenes.28


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263 264 265

Figure 2. Hammett plot showing the log of the ratio of the rate constants for BZC formation

266

from the corresponding 4’-substituted BPAs via (a) hydroxyl radical (•OH) and (b) single

267

electron transfer by lumichrome (LC), where R refers to the substituent at the 4’-position of

268

biphenyl-2-carboxylic acid. The slope of the fitted line for (b) represents the sensitivity

269

constant rho (ρ). Hydroxyl radical was generated by irradiation of NaNO2 (200 µM) in M

270

illiQ water at 365 nm and quantified by formation of hTPA from TPA (10 µM) in the

271

presence of the BPA probes. Lumichrome reactions were performed in MilliQ water + 2%

272

acetonitrile, pH 7 and sensitized by lumichrome (10 µM) with irradiation at 365 nm. Sigma

273

(σp+) values from Hansch, Leo, and Taft.39 Error bars represent ± 2 s.d.

274 275

We selected BPA-CH3 and BPA-CF3 for further study due to their similar reactivities

276

and product yields upon oxidation by •OH, in contrast with their highly disparate rates of

277

benzocoumarin formation by lumichrome (Fig. 2a). This strong electronic effect and the high

278

ratios of BZC-CH3 to BZC-CF3 formation were also observed with other strong triplet

279

photooxidants such as 4-carboxybenzophenone (CBBP, E°* = 1.84 VSHE10). Reaction of the

280

BPA probes (oxidation potential > ca. 1.80 V SHE) with weaker oxidants such as triplet



281

excited state 3-methoxyacetophenone (1.64 V) and perinaphthenone (1.03 V) was slow, and

282

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

287

formation as it can act as the secondary oxidant for subsequent steps in both mechanistic

288

pathways leading to BZC formation (Fig. 1). Direct photolysis of BPA-CH3 showed no

289

detectable formation of BZC-CH3 in the initial 20 min, although approximately 10 nM

290

product was observed in solution after 1 hr (Fig. S5). No appreciable BZC-CF3 formation or

291

BPA-CF3 degradation could be detected after irradiating BPA-CF3 solutions for 1 hr.

292

Yields from steady-state reactions with lumichrome were quantitative for the early

293

timepoints in the linear range which were used for initial rate calculations. Laser flash

294

photolysis (LFP) experiments revealed that the rate constant for quenching of lumichrome by

295

BPAs showed no trend with the electron-donating or electron-withdrawing ability of the

296

substituent at the 4’-position (Fig. S6). Given that electron-donating substituents accelerated

297

the rate of benzocoumarin formation in steady-state experiments, these LFP experiments

298

support previous observations that initial electron transfer from the carboxylic acid to triplet

299

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

303

natural water samples and organic matter isolate solutions revealed differences in the

304

production of hydroxyl radical-like oxidants compared to triplet sensitizers (Fig. 3b). All

305

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.



Page 16 of 24

322

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


Page 17 of 24


346

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



371

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


Page 18 of 24

Page 19 of 24


396

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



412

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


Page 20 of 24

Page 21 of 24


437

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



461

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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Fluorescent Molecular Probes for Detection of One-Electron Oxidants

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