Development of Novel Chemical Probes for Examining Triplet Natural

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Development of novel chemical probes for examining triplet natural organic matter under solar illumination Huaxi Zhou, Shuwen Yan, Jianzhong Ma, Lushi Lian, and Weihua Song Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02828 • Publication Date (Web): 31 Aug 2017 Downloaded from http://pubs.acs.org on September 1, 2017

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Environmental Science & Technology

1 2 3 4 5 6

Development of novel chemical probes for examining triplet natural

7

organic matter under solar illumination

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Huaxi Zhou, Shuwen Yan, Jianzhong Ma, Lushi Lian, and Weihua Song* Department of Environmental Science & Engineering, Fudan University, Shanghai 200433, P. R. China

*Corresponding author: email: [email protected]; Tel: (+86)-21-6564-2040 Resubmitted to Environ. Sci. & Technol.

1

ACS Paragon Plus Environment


41

Abstract

42

Excited triplet states of chromophoric dissolved organic matter (3CDOM*) are critical transient

43

species in environmental photochemistry. In the present study, sorbic amine (2,4-hexadien-1-amine)

44

and sorbic alcohol were employed as new probe molecules for triplet measurements and compared to

45

the results measured from sorbic acid under identical conditions. Unlike sorbic acid, sorbic amine

46

and sorbic alcohol were not directly photolyzed under solar irradiation. Photosensitized isomerization

47

of the probes with the conjugated diene structure could yield four geometrical isomers. The

48

separation and quantitative determination of the geometrical isomers were accomplished using HPLC

49

and high-resolution NMR analyses. When photo-irradiated Suwannee River natural organic matter

50

(SRNOM) was employed as a source of

51

isomerization rates were observed for the diverse charged probes. The bimolecular reaction rate

52

constants between 3SRNOM* and the probes were calculated as (0.42±0.1)×109 M-1 s-1 for sorbic acid,

53

(1.1±0.1)×109 M-1 s-1 for sorbic alcohol, and (5.2±0.4)×109 M-1 s-1 for sorbic amine, respectively. The

54

average apparent Φtriplet was (0.96±0.03)% based on an irradiation range of 290 to 400 nm. We

55

developed highly selective and efficient probes for triplet determination and elucidated the different

56

reaction behaviors of these conjugated dienes containing different charged substituents within the

57

photochemical energy transfer process.

3

CDOM*, significantly different photosensitized

58

2


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Introduction

60

Chromophoric dissolved organic matter (CDOM) is ubiquitous in surface waters and plays an

61

important role in photochemical processes.1-5 CDOM could enhance organic pollutant

62

phototransformation by producing reactive intermediates (RIs), including the excited triplet states of

63

CDOM (3CDOM*)6-8 and a group of reactive oxygen species (ROS), for instance hydroxyl radical

64

(HO•),9 singlet oxygen (1O2),10 superoxide (O2•−),11 and H2O2. Investigations on the formation,

65

scavenging and steady-state concentrations of these RIs have been conducted by employing chemical

66

probes and chemiluminescence methods.12-17 However, studies on 3CDOM* have lagged behind ROS

67

due to the complexity and diversity of the CDOM chemical composition.18-22

68

In general, the formation of 3CDOM* in surface water is initiated by the absorption of

69

sunlight by CDOM. Upon illumination, the ground-states of CDOM are excited to the singlet states

70

of CDOM (1CDOM*), and a small portion of excited singlet states can transit into metastable triplet

71

states through intersystem crossing (ISC). The lifetimes of 3CDOM* are expected to be variable due

72

to the complexity of DOM and the presence of various quenchers within its macromolecules.23-25 In

73

aerated conditions, the triplet lifetimes have been considered in the range of microseconds, and the

74

steady-state concentrations of triplets have been estimated as 10-15 to 10-13 M in natural waters.26-28

75

Chemical probes have been employed to react characteristically with triplets and found to be a

76

promising tool for triplet measurements.16 Canonica et al. first provided a detailed mechanism of the

77

triplet-induced phenol transformation and found that aromatic ketone moieties within CDOM could

78

be important in phenol oxidation, which was further verified using model aromatic ketone

79

compounds as photosensitizers.29 In the phenol oxidation processes, hydrogen abstraction and

80

electron transfer pathways are involved, and the triplet states generally behave as electron acceptors.

81

Furthermore, phenols have been discussed in detail as probe molecules for transient photooxidants in

82

natural waters.30 The substance 2,4,6-trimethylphenol (TMP) was then employed as a chemical probe

83

to investigate the electron transfer pathway in the triplet-induced reactions and was then used 3



84

extensively in later studies.7, 31-34

85

It is also well known that in addition to hydrogen abstraction and electron transfer, energy

86

transfer is an important pathway involved in the triplet-induced photoreactions.2, 7, 35-37 In oxygenated

87

surface waters, 3CDOM* is quenched by ground-state molecular oxygen, resulting in the generation

88

of 1O2 simultaneously. Zepp gave the reasonable value of 2.0 × 109 M-1 s-1 for this quenching rate,

89

which was subsequently adopted by Golanoski and coworkers in their studies on the photooxidation

90

of TMP by humic substances.6, 28 As the triplet state is a precursor to 1O2 formation, 1O2 yield was

91

considered an alternative index for triplet state measurements in the triplet energy transfer pathway.2,

92

32

93

and it is generally acknowledged that triplet states with energies up to 250 kJ mol-1 are critical for

94

DOM photochemistry.16,

95

sufficiently high energies (250 kJ mol-1) to transfer energy to chemicals such as polycyclic aromatic

96

hydrocarbons, nitroaromatic compounds and conjugated dienes.28 Studies in which 1,3-dienes were

97

either used as quenching agents for acetone triplets or as probes for the detection of triplet carbonyls

98

in biological systems have also been reported.38,

99

structure was also employed as probe molecules for triplet measurements by Mitch and coworkers.

100

These researchers performed comprehensive studies on the formation rates, scavenging rate

101

constants and steady-state concentrations of 3CDOM* under irradiations above 315 nm. This sorbic

102

probe method using quantification of the isomerization products has an advantage over the TMP

103

method since other RIs such as HO• and 1O2 that potentially react with sorbic acid will not produce

104

isomerization products.40 However, the main drawback of this probe is the existence of overlaps

105

between the absorption spectrum of sorbic acid and the solar spectrum. The probe would be directly

106

photolyzed, causing it to be unsuitable for solar irradiation.

However, the energy required to promote molecular oxygen to its singlet state is only 94 kJ mol-1,

28

Zepp et al. reported that approximately half of triplet states have

39

Sorbic acid containing the conjugated diene

107

In the present study, sorbic amine (2,4-hexadien-1-amine) and sorbic alcohol were employed

108

as novel probes for examining 3CDOM*. Sorbic amine was synthesized from sorbic acid according to 4


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the method proposed by Matsumoto and coworkers.41 Unlike sorbic acid, sorbic amine and sorbic

110

alcohol were not directly photolyzed under solar irradiation. Photosensitized isomerization of the

111

probes with a conjugated diene structure could yield four geometrical isomers. The separation and

112

quantitative determination of the geometrical isomers were accomplished by high-performance liquid

113

chromatography (HPLC) and high-resolution nuclear magnetic resonance (HR-NMR) analyses.

114

Through a comparison for probe testing of 3CDOM*, we developed highly selective and efficient

115

probes for the determination of triplet excited states under simulated solar irradiation. Moreover, we

116

investigated the effects of different electronic charges on the energy transfer process for CDOM.

117

Experimental Section

118

Chemicals. Suwannee River natural organic matter (SRNOM, cat. # 1R101N) was purchased

119

from the International Humic Substances Society (IHSS). Benzophenone (BP, 99%), sorbic acid

120

(2,4-hexadienoic acid, 99%), sorbic alcohol (2,4-hexadien-1-ol, 97%), DMSO-d6 (99.96 atom% D,

121

contains 0.03% (v/v) TMS), methanol-d4 (99.8 atom% D, contains 0.03% (v/v) TMS), chloroform-d

122

(99.96 atom% D, contains 0.03% (v/v) TMS), lithium aluminum hydride (LiAlH4, powder, reagent

123

grade, 95%), trifluoroacetic acid (TFA, 99%) and phosphates (NaH2PO4 and Na2HPO4 (both 99%))

124

were

125

4,4'-dicarboxybenzophenone (4,4'-DCBP, 99%), p-nitroacetophenone (PNAP, 98%), pyridine (pyr,

126

99%), furfural (FAD, 98%), p-chlorobenzoic acid (pCBA, 99%) and Rose Bengal (RB) were

127

obtained from Tokyo Chemical Industry Co., Ltd. Thionyl chloride (99%, AR), anhydrous ether

128

(99.7%, AR), ethyl acetate (99.5%, AR), petroleum ether (AR), dichloromethane (99.5%, AR),

129

methanol (99.5%, AR), anhydrous sodium sulfate (99%, AR), NaSO4•10H2O (AR), FeSO4•7H2O

130

(99.5%, AR), H2O2 (30%, AR) and ammonium hydroxide (25%-28%, AR) were supplied by

131

Sinopharm Chemical Reagent Co. Ltd. All the above chemicals were used as received. Sorbic amine

132

was synthesized from sorbic acid according to the procedures proposed by Matsumoto and

133

coworkers. Our experimental details can be found in the Text S1 of Supporting Information (SI).

purchased

from

Sigma-Aldrich.

4-Benzoylbenzoic

5


acid

(4-BBA,

99%),


134

Ultraviolet-visible (UV-vis) absorbance spectra were collected in 1 cm quartz cuvettes on a

135

spectrophotometer (Cary 60, Agilent) using phosphate buffer (5.0 mM) as a blank.

136

Photochemical experiments. Photochemical experiments were performed using a solar

137

simulator (Suntest XLS+ Atlas) equipped with a 1700 W xenon lamp. A solar filter was employed to

138

block the irradiance below 290 nm. For the irradiation condition of λ > 315 nm, a glass filter was

139

used. The temperature was maintained at 25.0 ± 1.0 °C by a temperature control unit (Suncool). The

140

irradiation intensity on the surface of the solutions was set to 40 W m-2 (1.36 × 10-8 Einstein s-1 cm-2)

141

at 290-400 nm. The absolute irradiance spectra of the simulated solar light and natural sunlight were

142

recorded using a spectra-radiometer (USB-4000, Ocean Optics Inc.). p-Nitroacetophenone/Pyridine

143

(PNAP-pyr) actinometry has been employed to measure the irradiance under our reaction conditions,

144

and the details have been presented in the Text S2 of SI. The quantum yields have been calculated

145

based on the latest reference.42 All solutions were prepared using Milli-Q water. A stock solution of

146

SRNOM (200 mg L-1) was prepared in phosphate buffer (5.0 mM, pH 7.0) using magnetic stirring,

147

then the solution was filter with a 0.22 µm filter and stored at 4.0 °C. The experimental solutions

148

were prepared by diluting stock solutions with phosphate buffer to 5.0 mgC L-1. Dissolved organic

149

carbon (DOC) was measured using a TOC analyzer (Shimadzu® L-CPH). The concentration of

150

dissolved oxygen (DO) was measured using a DO meter (WTW, Germany), and kept constant during

151

the irradiation. Chemical probes were spiked at varying concentrations ranging from 1.0 to 520.0 µM.

152

Samples (20 mL) were placed in specially made cylindrical quartz containers (diameter = 6.0 cm,

153

height = 2.0 cm, thickness = 0.2 cm) as presented in Figure S4 of SI and were irradiated for a given

154

period under ambient conditions. Upon irradiation, aliquots were removed at various time intervals

155

and analyzed using HPLC-UV. The error bars in the corresponding figures represent the standard

156

deviation.

157

Isomers preparation. To quantify the isomers of sorbic alcohol, 200 mL of a standard

158

solution of trans, trans-sorbic alcohol (0.5 mg mL-1 in methanol) was irradiated with a 25 W 6


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low-pressure mercury lamp (Trojan Technologies Lamp, 253.7 nm) for 12 hrs. The fluency rate of

160

the lamp, which was measured based on iodide-iodate actinometry, was 7.97 × 10−4 Einstein L−1 s−1

161

(0.37 mW cm−2).43 After the photo-isomerization of the trans, trans-sorbic alcohol, the solutions

162

were further purified with silica gel chromatography eluted by ethyl acetate/petroleum ether (3:1).

163

The mixture of isomers was carefully dried under a reduced pressure and then dissolved in the

164

deuterated reagent for both 1H-NMR and HPLC-UV analyses. The relative ratios of the isomers were

165

quantified using 1H-NMR and further analyzed using HPLC-UV to obtain the molecular absorption

166

coefficients of the isomers. To quantify the isomers of sorbic amine, similar procedures were

167

followed except that dichloromethane/methanol (10:1) was employed for the silica gel

168

chromatography purification.

169

NMR analysis. The high-resolution 1H-NMR spectra of the mixtures of isomers were

170

recorded by an Agilent NMR spectrometer (800 MHz) at the Shanghai Institute of Organic

171

Chemistry at the Chinese Academy of Sciences. The mixture of sorbic alcohol isomers was dissolved

172

in methanol-d4 and the mixture of sorbic amine isomers was dissolved in DMSO-d6 for the NMR

173

analysis. Tetramethylsilane (TMS) was used as the internal standard for all NMR tests.

174

HPLC methods. The analysis was conducted on an HPLC (1260, Agilent) equipped with a

175

photodiode array detector (DAD) and a C18 column (4.6 × 250 mm, 5 µm, Luna, Phenomenex®).

176

Sorbic alcohol and sorbic amine were eluted with the isocratic mobile phase consisting of acetonitrile

177

(ACN) and water acidified with trifluoroacetic acid (TFA, 0.05%) at a flow rate of 1.0 mL min-1. The

178

percentage volume of ACN/acidified water was 20:80 for sorbic alcohol and 10:90 for sorbic amine,

179

respectively. The detection wavelength was set to 230 nm for both. The column temperature is

180

critical, and the separation can be accomplished by cooling the column to 10 °C. For sorbic acid, the

181

samples were eluted with an isocratic mobile phase consisting of 15% acetonitrile and 85% 30 mM

182

acetate buffer at pH 4.75 and a flow rate of 1.0 mL min-1. The detection wavelength was 254 nm, and

183

the column temperature was set to 30 °C. 7



184

Results and Discussion

185

Characterization of the geometrical isomers of sorbic alcohol and sorbic amine.

186

Compounds that contained the conjugated diene structure are considered excellent molecular probes

187

for measurements of triplet energy transfer.13, 28 When the energy transferred from the excited triplets

188

to dienes, isomerization occurred as described in Scheme 1, and four geometrical isomers (trans, cis-,

189

cis, trans-, cis, cis- and trans, trans-) were obtained. The qualitative and quantitative determination

190

of the geometrical isomers can be a challenging analytical task because of their similar

191

physicochemical properties. (Insert Scheme 1)

192 193

Similar to the previous method for the characterization of sorbate geometrical isomers, initial

194

studies were conducted to determine the molar absorption coefficients of the geometrical isomers.44

195

The irradiation of the solutions of the trans, trans-sorbic alcohol and trans, trans-sorbic amine under

196

UV254

197

accompanied by their partial degradation into undesirable products. After being purified by silica gel

198

chromatography, the isomer mixtures were dissolved in a deuterated solvent for high-resolution

199

1

200

in Figure S5 of the SI, there were four characteristic ethylene proton signals observed between δ 5.5

201

to 6.3 ppm, a methylene proton signal at δ 4.05 ppm and a methyl proton signal at δ 1.73 ppm when

202

calibrated with an internal standard of TMS. After UV irradiation, a sorbic alcohol mixture

203

containing four geometrical isomers was obtained. As shown in Figure 1a, three isomers in different

204

yields were produced based on the methylene proton signals of the 1H-NMR spectra of the sorbic

205

alcohol mixture. In general, the cis-isomers have a greater steric effect than the trans-isomers, which

206

leads to the lower stability of the cis-isomers relative to the trans-isomers.45 The methylene proton

207

signals at δ 4.13 ppm were therefore assigned to be the least stable cis, cis-sorbic alcohol. The

208

assignment of the cis, trans-sorbic alcohol was conducted based on data compiled by Paquette and

nm

resulted in the direct photo-isomerization of all four geometrical isomers and was

H-NMR analysis. In the 1H-NMR spectra of the parent isomer trans, trans-sorbic alcohol as shown

8


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209

coworkers with a coupling constant (J) for the methylene proton of 7 Hz.46 Thus, the three

210

geometrical isomers were successfully assigned together with the parent trans, trans-sorbic alcohol.

211

The assignments of the ethylene proton signals in Figure 1b were supported by the 1H-1H correlation

212

spectroscopy (COSY) spectra for the sorbic alcohol mixture as presented in Figure S6a of the SI.

213

Certain overlaps were identified in the signals of the ethylene and methyl protons, but the methylene

214

proton signals were well resolved. The well-resolved methylene proton signals were then employed

215

to quantify the geometrical isomers. After the methylene proton signals of individual isomers were

216

normalized with respect to the peak of trans, trans- sorbic alcohol, the relative ratios of isomers can

217

be obtained. The quantification of the geometrical isomers for the sorbic amine mixture was

218

conducted using procedures similar to those employed for the sorbic alcohol mixture except that the

219

well-resolved ethylene proton signals were used in the quantification instead of the methylene proton

220

signals. The related data are presented in Figure S6b and Figure S7 of the SI.

221

(Insert Figure 1)

222

The photoisomer mixtures were also examined using HPLC-UV. As presented in Figure 1c,

223

the four geometrical isomers were well separated. In the sorbic alcohol mixture, only the

224

commercially available trans, trans-sorbic alcohol could be determined. To distinguish other sorbic

225

alcohol isomers, three geometrical sorbic acid isomers (cis, cis-, trans, cis-, cis, trans-) were first

226

isolated from mixtures of sorbic acid isomers through semi-prep LC system, and further determined

227

through comparison with previous investigations.13, 39, 47 Each sorbic acid isomer has been further

228

converted into corresponding sorbic alcohol through selective LiAlH4 reduction. Therefore the sorbic

229

alcohol isomers determination can be achieved through comparison individual sorbic alcohol isomer

230

with the sorbic alcohol mixtures under identical HPLC conditions. The experimental details can be

231

found in the Text S3 of SI. As abovementioned, the concentration ratios of isomers have been

232

determined using 1H-NMR signals. Comparing the relative areas of individual isomers obtained by

233

the HPLC to the ratios of 1H-NMR signal, the molar absorption coefficients of the geometrical 9



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234

isomers were successfully determined as presented in Table 1. The molar absorption coefficients for

235

the isomers of sorbic alcohol were in the order trans, trans- > cis, trans- > trans, cis- > cis, cis-,

236

which agreed well with a previous work by Mitch and coworkers, who reported the same trend for

237

the isomers of sorbic acid.13 To provide further evidence, experiments regarding the

238

photostabilization point were also conducted. Isomerization is reversible in the energy transfer

239

process. A stabilization point will be reached in the irradiation process, at which the formation and

240

loss rates of each isomer become equal. As shown in Figure 1d, the trans, trans-sorbic alcohol had

241

the highest concentration and the cis, cis-sorbic alcohol had the lowest concentration at the

242

photostabilization point. The concentration of the trans, cis-sorbic alcohol was slightly higher than

243

that of the cis, trans-sorbic alcohol. The confirmation of the geometrical isomers for the sorbic amine

244

mixture in the HPLC data was conducted similarly as for the results of the sorbic alcohol mixture,

245

and the data are presented in Figure S7 and Text S3 of the SI. (Insert Table 1)

246 247

Comparison of the different chemical probes in 3CDOM* determination. Sorbic acid

248

has been previously used as a quantitative probe for triplet energy transfer measurements.13 The

249

formation rate, scavenging rate constant and steady-state concentrations of

250

investigated carefully and rigorously. A comparison of sorbic acid, sorbic alcohol and sorbic amine in

251

3

252

present as sorbate anion form in the experimental pH range. We prefer to use the name “sorbic acid”

253

for easy following. Sorbic acid presents only good trapping in the light wavelengths above 315 nm,

254

which is inappropriate for solar irradiation. There is no overlap between the absorption spectra of

255

sorbic alcohol or sorbic amine and the lamp emission spectrum (and the natural solar spectrum), as

256

presented in Figure 2a. Therefore, the direct photolysis of sorbic alcohol or sorbic amine under solar

257

irradiation could be minor. Figure 2b demonstrates that a negligible amount of sorbic alcohol (1.3%)

258

and sorbic amine (1.5%) underwent photodegradation after 4 hrs of irradiation and no

3

CDOM* were

CDOM* determination have been performed in this study. It should be noted that sorbic acid would

10


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259

photoisomerization product was detected. Meanwhile, 10.1% of the sorbic acid underwent direct

260

photodegradation, and the sum of all the isomerization products accounted for up to 80% of this loss. (Insert Figure 2)

261 262

A mathematical model has also been established by Mitch and coworkers in their studies on

263

sorbic acid as a probe for triplet determination and is cited in this study as discussed briefly below.13

264

In general, the formation of 3CDOM* (FT) in surface water is initiated by the absorption of sunlight

265

by CDOM. Upon illumination, the ground-states of CDOM are excited to the singlet states of

266

CDOM (1CDOM*) and further transit into 3CDOM* through ISC. In the presence of a probe, the

267

triplet-quenching rate will be the sum of quenching rates with scavengers, RS (Eq. 1), and with the

268

probe, RP (Eq. 2). At a steady state, FT and the quenching rates are equal (Eq. 3).

269

R = Scavengers CDOM ∗ = CDOM ∗

(1)

270

R = Probe CDOM ∗

(2)

271

F = R + R

(3)

272

where kS is the second-order rate constant for the reaction between the triplet and solution scavengers,

273

is the pseudo-first order rate constant, and kP is the second-order rate constant for the reaction

274

between the triplet and probe. Under air-saturated surface water conditions, the O2-dependent triplet

275

decay pathway is an order of magnitude more important than the O2-independent pathway.2 That is,

276

O2 served as the predominant scavenger for the triplet in our reaction conditions. The concentrations

277

of dioxygen were constant during the irradiation. During the photosensitized isomerization of probe,

278

only a small fraction (less than 5%) of probe has been isomerized. Therefore we can combine Eqs.

279

1-3 and rearrange them yielding Eq. 4:

280

CDOM ∗ = #$ & '$ %

!"

(4)

( )*+,-

281

Substitution of Eq. 4 into Eq. 2 yields Eq. 5, a nonlinear form:

282

R = $ &('$

$ )*+,!" ( )*+, %

(5)

11



Page 12 of 28

283

Rearrangement of Eq. 5 obtains Eq. 6, a linear form:

284

285

The variable RP was calculated by the sum of the isomers formation rates (Eq. 7), and the

286

calculations of the trans, trans-isomer reformation rate are discussed in the Text S4 of SI.

287

R = F0123, 567839 + F056783, 1239 + F0123, 1239 + F0t6783, 567839

)*+, ./

=

)*+, !"

+

$%&

(6)

!" $(

(7)

288

To study the different photochemical behaviors of the probe molecules in the triplet

289

determination, varying concentrations of the probe (sorbic acid, sorbic alcohol or sorbic amine) were

290

then employed to investigate the triplets of SRNOM (5.0 mgC L-1) in air-saturated solutions under

291

simulated solar irradiation with a glass filter to cut off the wavelength below 315 nm. First, the

292

reaction kinetics for the photoisomerization of the probes were investigated. As illustrated in Figure

293

S11 of the SI, both the reduction of the trans, trans-isomer and formation of the other three

294

geometrical isomers (trans, cis-, cis, trans-, and cis, cis-) were correlated linearly as a function of the

295

irradiation time, which assured pseudo-steady-state conditions in our reaction processes. A decrease

296

of 2% was observed for sorbic acid with a 60% transformation to the isomerization products in 2 hrs

297

of irradiation. Meanwhile, approximately 3% and 9% of sorbic alcohol and sorbic amine were

298

reduced in 1 hr of irradiation, with isomerization products accounting for 51% and 90% of this loss,

299

respectively. These findings indicate that the sorbic amine reacted predominantly with the triplet

300

through energy transfer, and up to 40% of the sorbic acid and sorbic alcohol reacted with secondary

301

photooxidants such as hydroxyl radical or singlet oxygen, etc., which could not produce

302

isomerization products. Thus, the sorbic amine exhibited higher selectivity in the triplet

303

determination than sorbic acid or sorbic alcohol.

304 305

(Insert Figure 3) As shown in Figure 3a, the Rp versus [probe] followed a nonlinear trend. To simplify the )*+,

306

calculation, the linear expression (Eq. 6) for

307

straight lines that were approximately in parallel and had different y-intercepts were obtained for the

./

against [Probe] has been conducted. Three

12


Page 13 of 28


308

sorbic acid, sorbic alcohol, and sorbic amine, as shown in Figure 3b. The values of FT can be

309

calculated from the slopes as shown Eq. 8. The triplet quantum yield (Φtriplet) can be further

310

developed from Eq. 9.

311

F =

312

Φ@)A>=,@ = ." 0R C is the rate of light absorption9

;

(8)

, !

B

(9)

313

The FT values, the inverse of the slopes, were nearly the same for the different probes with an

314

average value of (6.5 ± 0.5) nM s-1. The results further verified that these sorbate probes present

315

similar triplet energy (approximately 250 kJ mol-1). Moreover, the average value of Φtriplet was (0.73

316

± 0.05)%.

317 318

Meanwhile, [3CDOM*]SS can be obtained from Eq. 10. !

CDOM ∗ = $"&

(10)

%

319

Under air-saturated conditions, the relaxation of 3CDOM* 0 9 consists of the O2-independent and

320

O2-dependent deactivation pathways, as shown in Eq. 11.

321

=IJ + KL OM

322

The rate constant for the O2-independent deactivation pathway (defined as IJ ) was estimated to be

323

approximately 5 × 104 s-1, which maybe independent for CDOM origins.27 KL is the second-order

324

rate constant for the triplet reaction with dioxygen. Zepp provided a reasonable estimation of 2.0 ×

325

109 M-1 s-1 for this value.28 The O2 concentration was 250 µM under our reaction conditions. The rate

326

constant for the O2-dependent deactivation pathway given by KL OM was 5 × 105 s-1. Thus, the

327

value for could be 5.5 × 105 s-1. Then, the average [3CDOM*]SS value of (1.2 ± 0.1) × 10-14 M

328

was obtained under our experimental conditions.

(11)

329

The second-order reaction rate constant (kP) between 3CDOM* and the probes can also be

330

calculated as (0.42 ± 0.1) × 109 M-1 s-1 for sorbic acid, (1.1 ± 0.1) × 109 M-1 s-1 for sorbic alcohol,

331

and (5.2 ± 0.4) × 109 M-1 s-1 for sorbic amine from Eq. 12: 13



Page 14 of 28

AN@,)O,>@

332

=

333

It should be noted that the calculations of [3CDOM*]SS and kP are based on the estimation of KL as

334

2.0 × 109 M-1 s-1. Preliminary results from McNeill suggested that the KL value is more likely to be

335

1.0 × 109 M-1 s-1.48 While the new value of KL (1.0 × 109 M-1 s-1) is engaged, the [3CDOM*]SS will be

336

proportionately higher and the kP will be proportionately lower.

(12)

,

337

As it is known that the CDOM surface is negatively charged, sorbic acid (pKa = 4.6) is

338

present in the negative form R-COO–, sorbic alcohol in the neutral molecule and sorbic amine (pKa

339

= 9.39) in the positive form R-CH2NH3+ in the neutral solutions. Due to electrostatic repulsion,

340

inhibition of the reaction between the triplets and sorbic acid is inevitable, which results in the lower

341

second-order rate constant of the triplets with sorbic acid. Conversely, the rapid reaction rate between

342

the triplets and the sorbic amine is probably due to their electrostatic attraction. These results may be

343

useful

344

triplet-dominated reactions. In natural waters, positively charged contaminants are likely to have a

345

faster degradation rate than negatively charged contaminants with similar chemical structures.

in

predicting

CDOM

photoreactivity

toward

charged

organic

contaminants

in

346

Triplet determination under simulated solar irradiation. The energy of light is

347

inversely proportional to its wavelength. That is, shorter wavelengths possess higher energy within

348

the solar spectra and may play a more critical role in triplet photochemistry than longer wavelengths.

349

The experiment above focused on the photoreactions under irradiation with wavelengths greater than

350

315 nm. Thus, the following experiments were conducted to investigate the triplet photoreactions

351

under simulated solar irradiation (λ > 290 nm). Because of the direct photodegradation of sorbic acid,

352

only sorbic alcohol and sorbic amine were employed in the simulated solar irradiation. As shown in

353

Figure 3c, two lines that were approximately parallel and had different y-intercepts were obtained.

354

These lines agreed well with the results under irradiations of λ > 315 nm. The average FT and

355

[3CDOM*]SS values were (12.8 ± 0.4) nM s-1 and (2.3 ± 0.1) × 10-14 M, respectively, which was

356

approximately two-fold greater than the measured values under irradiations of λ > 315 nm. The 14


Page 15 of 28


357

shorter wavelengths in the solar spectra play a more important role in triplet formation than the

358

longer wavelengths. The average Φtriplet value was (0.96 ± 0.03)%, which was slightly higher than

359

that measured under irradiations of λ > 315 nm. Our apparent quantum yield data agreed well with

360

the previous estimates of triplet yields in natural waters based upon singlet oxygen yields (Φtriplet =

361

(0.4 – 1.6)%).28 The kP values obtained for the sorbic alcohol and sorbic amine with 3CDOM* were

362

nearly identical to those measured under irradiation of λ > 315 nm, which further validates this probe

363

method. Table 2 summarizes all the values of FT, Φtriplet, [3CDOM*]SS, and kP under various

364

irradiation conditions. (Insert Table 2)

365 366

Photo-isomerization of the probes using model triplet sensitizers. Benzophenone and

367

its derivatives acting as photosensitizers have been employed widely in environmental

368

photochemistry.29, 49-51 To verify the speculation on the electrostatic interactions between CDOM and

369

the probes proposed above, model triplet sensitizers including neutral benzophenone and acidic

370

derivatives 4-BBA, 4,4'-DCBP were employed to investigate the influence of charge conditions on

371

the reaction behaviors between the triplet and probes. By using the linear model of Eq. 6, fitted

372

regression lines of the

373

illustrated in Figure 4. Under identical irradiation conditions, the slopes were observed to be almost

374

consistent in the different chemical probes for the same photosensitizer. It is apparent that the FT

375

value was dependent on the light source and photosensitizer and was not influenced by the different

376

probes spiked in the solution. As presented in the insets of Figure 4, the average FT value was (29.7 ±

377

4.0) nM s-1 for BP, (25.4 ± 2.6) nM s-1 for 4-BBA, and (42.1 ± 3.4) nM s-1 for 4,4'-DCBP. Under

378

air-saturated natural waters, the O2-dependent pathway is considered the dominant triplet

379

deactivation pathway.2 Thus, by estimating a value of 5 × 105 s-1, the kP values can be calculated

380

using Eq. 12. Our results reveal that almost equivalent kP values are obtained for the 3BP* reaction

381

with the three chemical probes because the neutral form triplet 3BP* has limited electrostatic

)*+, ./

value versus [Probe] were obtained for the three photosensitizers as

15



382

interaction with the different charged chemical probes. When employing the monocarboxylic acid

383

substituted benzophenone (4-BBA) as a photosensitizer, apparent differences were observed for the

384

kP values as shown in Figure 4. The interaction between the negatively charged 34-BBA* and the

385

positively charged sorbic amine was promoted by their electrostatic attraction, while the reaction

386

between the negatively charged 34-BBA* and the negatively charged sorbic acid was inhibited by

387

electrostatic repulsion. This trend has been apparently enlarged using the dicarboxylic acid

388

4,4'-DCBP as the photosensitizer. The kP ratios of the sorbic amine, sorbic alcohol and sorbic acid

389

changed from 2.2:1.6:1 for 34-BBA* to 4.9:3.1:1 for 34,4'-DCBP*. (Insert Figure 4)

390 391

In summary, with the use of sorbic alcohol and sorbic amine as novel chemical probes, the

392

measurements of energy transfer from the excited triplet have been accomplished under the solar

393

irradiation spectrum. This sorbic probe method using quantification of the isomerization products is

394

specific since other RIs such as HO• and 1O2, which potentially react with sorbic probes, will not

395

produce isomerization products.40 The photochemical experiments for SRNOM and model

396

photosensitizers reveal that the electrostatic interactions between the negatively charged 3CDOM*

397

and charged organic contaminants are critical. The bimolecular reaction rate constants (kP)

398

dramatically decrease from the positively charged sorbic amine to the negatively charged sorbic acid.

399

The obtained kP values for probes could be further employed to evaluate (relaxation of 3CDOM*)

400

and explore how it varies as a function of environmental conditions. Although our results are based

401

on only sorbic compounds, it would be feasible to extend the results to the triplet photochemistry of

402

other organic contaminants in NOM-enriched solutions. We suggest that charge interactions be need

403

to be taken into consideration when studying the phototransformation of organic contaminants,

404

particularly when the triplet energy transfer plays a key role.

405

Supporting Information

406

The Supporting Information consist of 1 Scheme, 2 Tables, 13 Figures and five Texts including: 16


Page 16 of 28

Page 17 of 28


407

synthesis of sorbic amine (Text S1); calculation of quantum yields (Text S2); determination of the

408

geometrical configurations of sorbic alcohol and sorbic amine isomers on the HPLC (Text S3);

409

calculation of the relative rate constant for t,t-SA re-formation (Text S4); and determination of the

410

bimolecular reaction rate constants between 1O2 and HO• with the probes (Text S5). This material is

411

available free of charge at http://pubs.acs.org.

412

Acknowledgments

413

We are thankful for the partial funding support from the National Natural Science Foundation of

414

China (21607026, 21677039, and 21422702). W. S. also acknowledges support from the program for

415

Professor of Special Appointment (Eastern Scholar) at the Shanghai Institutions of Higher Learning.

416

S. Y. appreciates the financial support from the China Postdoctoral Science Foundation

417

(2016M590321).

418

References

419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440

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of aminoglycoside antibiotics in simulated natural waters. Environ. Sci. Technol. 2016, 50 (6), 2921-2930. (10) Boreen, A. L.; Edhlund, B. L.; Cotner, J. B.; McNeill, K. Indirect photodegradation of dissolved free amino acids: The contribution of singlet oxygen and the differential reactivity of DOM from various sources. Environ. Sci. Technol. 2008, 42 (15), 5492-5498. (11) Zhang, Y.; Blough, N. V. Photoproduction of one-electron reducing intermediates by chromophoric dissolved organic matter (CDOM): Relation to O2·– and H2O2 photoproduction and CDOM photooxidation. Environ. Sci. Technol. 2016, 50 (20), 11008-11015. (12) Latch, D. E.; McNeill, K. Microheterogeneity of singlet oxygen distributions in irradiated humic acid solutions. Science 2006, 311 (5768), 1743-1747. (13) Grebel, J. E.; Pignatello, J. J.; Mitch, W. A. Sorbic acid as a quantitative probe for the formation, scavenging and steady-state concentrations of the triplet-excited state of organic compounds. Water Res. 2011, 45 (19), 6535-6544. (14) Zhang, D.; Yan, S.; Song, W. Photochemically induced formation of reactive oxygen species (ROS) from effluent organic matter. Environ. Sci. Technol. 2014, 48 (21), 12645-12653. (15) Canonica, S.; Freiburghaus, M. Electron-rich phenols for probing the photochemical reactivity of freshwaters. Environ. Sci. Technol. 2001, 35 (4), 690-695. (16) Rosario-Ortiz, F. L.; Canonica, S. Probe compounds to assess the photochemical activity of dissolved organic matter. Environ. Sci. Technol. 2016, 50 (23), 12532-12547. (17) Rose, A. L.; Webb, E. A.; Waite, T. D.; Moffett, J. W. Measurement and implications of nonphotochemically generated superoxide in the equatorial pacific ocean. Environ. Sci. Technol. 2008, 42 (7), 2387-2393. (18) Coble, P. G. Characterization of marine and terrestrial DOM in seawater using excitation-emission matrix spectroscopy. Mar. Chem. 1996, 51 (4), 325-346. (19) Leenheer, J. A.; Croué, J.-P.; Benjamin, M.; Korshin, G. V.; Hwang, C. J.; Bruchet, A.; Aiken, G. R., Comprehensive isolation of natural organic matter from water for spectral characterizations and reactivity testing. In Natural Organic Matter and Disinfection By-Products, American Chemical Society: 2000; Vol. 761, pp 68-83. (20) Shon, H. K.; Vigneswaran, S.; Snyder, S. A. Effluent organic matter (EfOM) in wastewater: Constituents, effects, and treatment. Crit. Rev. Environ. Sci. Technol. 2006, 36 (4), 327-374. (21) Sharpless, C. M.; Aeschbacher, M.; Page, S. E.; Wenk, J.; Sander, M.; McNeill, K. Photooxidation-induced changes in optical, electrochemical, and photochemical properties of humic substances. Environ. Sci. Technol. 2014, 48 (5), 2688-2696. (22) Her, N.; Amy, G.; McKnight, D.; Sohn, J.; Yoon, Y. Characterization of DOM as a function of MW by fluorescence EEM and HPLC-SEC using UVA, DOC, and fluorescence detection. Water Res. 2003, 37 (17), 4295-4303. (23) Wenk, J.; Eustis, S. N.; McNeill, K.; Canonica, S. Quenching of excited triplet states by dissolved natural organic matter. Environ. Sci. Technol. 2013, 47 (22), 12802-12810. (24) Wenk, J.; Aeschbacher, M.; Sander, M.; Gunten, U. V.; Canonica, S. Photosensitizing and inhibitory effects of ozonated dissolved organic matter on triplet-induced contaminant transformation. Environ. Sci. Technol. 2015, 49 (14), 8541-8549. (25) Janssen, E. M. L.; Erickson, P. R.; McNeill, K. Dual roles of dissolved organic matter as sensitizer and quencher in the photooxidation of tryptophan. Environ. Sci. Technol. 2014, 48 (9), 4916-4924. 18


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(42) Laszakovits, J. R.; Berg, S. M.; Anderson, B. G.; O’Brien, J. E.; Wammer, K. H.; Sharpless, C. M. p-Nitroanisole/pyridine and p-nitroacetophenone/pyridine actinometers revisited: Quantum yield in comparison to ferrioxalate. Environ. Sci. Technol. Lett. 2017, 4 (1), 11-14. (43) Rahn, R. O. Potassium iodide as a chemical actinometer for 254 nm radiation: Use of iodate as an electron scavenger. Photochem. Photobiol. 1997, 66 (4), 450-455. (44) Kralj Cigić, I.; Plavec, J.; Možina, S. S.; Zupančič-Kralj, L. Characterisation of sorbate geometrical isomers. J. Chromatogr. A 2001, 905 (1–2), 359-366. (45) Zhang, L.; Borysenko, C. W.; Albright, T. A.; Bittner, E. R.; Lee, T. R. The cis-trans isomerization of 1,2,5,6-tetrasilacycloocta-3,7-dienes: Analysis by mechanistic probes and density functional theory. J. Org. Chem. 2001, 66 (16), 5275-5283. (46) Paquette, L. A.; Crouse, G. D.; Sharma, A. K. Relationship of the anionic behavior of unsaturated medium-ring alcohols to structure. Generation and antarafacial cyclization of coiled 8π-electron carbanions. J. Am. Chem. Soc. 1982, 104 (16), 4411-4423. (47) Cigic, I. K.; Plavec, J.; Zupancic-Kralj, L. Determination of the geometrical isomers of ethyl 2,4-decadienoate. J. Chromatogr. A 1999, 847 (1-2), 359-364. (48) McNeill, K. Using direct observation of singlet oxygen to determine triplet organic matter rate constants. 253th ACS National Meeting, San Francisco, CA. April 2-6, 2017. (49) Cuquerella, M. C.; Lhiaubet-Vallet, V.; Cadet, J.; Miranda, M. A. Benzophenone photosensitized DNA damage. Acc. Chem. Res. 2012, 45 (9), 1558-1570. (50) Cantau, C.; Pigot, T.; Manoj, N.; Oliveros, E.; Lacombe, S. Singlet oxygen in microporous silica xerogel: Quantum yield and oxidation at the gas-solid interface. Chemphyschem 2007, 8 (16), 2344-2353. (51) Latour, V.; Pigot, T.; Simon, M.; Cardy, H.; Lacombe, S. Photo-oxidation of di-n-butylsulfide by various electron transfer sensitizers in oxygenated acetonitrile. Photochem. Photobiol. Sci. 2005, 4 (2), 221-229.

20


Page 20 of 28

Page 21 of 28


1

R

O2, HO non-isomerization product

3

CDOM*

R

*

R = COO , CH2OH, CH2NH3+

isomerization product

R

R

R 556 557

trans, cis

cis, trans

R cis, cis

trans, trans

Scheme 1. Photosensitized isomerization of the trans, trans-dienes by the triplet under illumination.

558 559 560 561 562 563 564 565

21



566 567 568 569 570 571

Figure 1. (a) Methylene proton signals and (b) ethylene proton signals for the 1H-NMR (methanol-d4, 800 MHz) spectra of the sorbic alcohol mixture; (c) HPLC separation of the sorbic alcohol mixture and (d) the concentration of the geometrical isomers as a function of reaction time. Note: “c, c” represents the cis, cis-isomer; “c, t” represents the cis, trans-isomer; “t, c” represents the trans, cis-isomer; and “t, t” represents the trans, trans-isomer.

572 573 574 575 576 577 578

22


Page 22 of 28

150

-1

(a)

120

4

10

90

-1

-1

ε (M cm )

Sorbic acid Sorbic alcohol Sorbic amine

-2

5

10

60 3

10

Natural sunlight Solar simulator

)


30 0

200 250 300 350 400 450 500 550 600

Absolute Irradiance (µW cm nm

Page 23 of 28

Wavelength (nm)

579

ln (Probe/Probe0)

0.00 -0.03 -0.06 -0.09 -0.12 -0.15 580 581 582 583 584 585 586

Sorbic acid Sorbic alcohol Sorbic amine 0

50

100 150 Time (min)

(b) 200

250

Figure 2. (a) Overlaps between the UV-vis absorption spectra of the dienes (trans, trans-sorbic acid, trans, trans-sorbic alcohol and trans, trans-sorbic amine) and the spectra of the light sources (natural sunlight and solar simulator); (b) direct photodegradation of the dienes (trans, trans-sorbic acid, trans, trans-sorbic alcohol and trans, trans-sorbic amine) under illumination of natural sunlight (at noon time of Nov. 3, 2016). Reaction conditions: probe concentration 10 µM, air-saturated, 5.0 mM phosphate buffer, pH 7.0.

587 23



λ > 315 nm 6.0

588 589 590 591 592 593 594


λ > 290 nm Sorbic alcohol Sorbic amine

(a)

(c)

16.0 12.0

4.0

8.0

2.0

4.0

0.0 4.8

0.0

5

[Probe]/Rp (×10 s)

-1

RP (nM s )

8.0

Page 24 of 28

3.6


Sorbic alcohol Sorbic amine

(b)

(d)

1.2 0.9

2.4

0.6

1.2

0.3

0.0 0.00

0.15 0.30 0.45 [Probe] (mM)

0.00

0.15 0.30 0.45 [Probe] (mM)

0.0 0.60

Figure 3. (a, c) Total formation rates of the isomerization products as a function of the probe concentration; (b, d) the [Probe]/RP value versus the probe concentration under various reaction conditions. Reaction conditions: 5.0 mgC L-1 of SRNOM, air-saturated, 5.0 mM phosphate buffer, pH 7.0, probes were spiked at 8 different concentrations ranging from 1.7 µM to 516.0 µM, irradiation wavelengths (a, b) λ > 315 nm and (c, d) λ > 290 nm.

24


Page 25 of 28


2.8

(a)

4

[Probe]/Rp (× 10 s)

3.5

2.1 1.4

FT

kP -1

0.7 0.0


0.0

0.1

0.2

( nM s ) 28.2 ± 3.8 30.7 ± 4.7 30.3 ± 3.4

0.3

0.4

-1

9

-1

(× 10 M s ) 2.0 ± 0.3 2.3 ± 0.4 1.9 ± 0.2

0.5

0.6

[Probe] (mM) 595

2.8

(b)

4


3.5

2.1 1.4

FT

kP

-1

0.7 0.0

sorbic acid sorbic alcohol sorbic amine

0.0

0.1

0.2

(nM s ) 24.5 ± 3.1 26.2 ± 2.8 25.6 ± 1.9

0.3

0.4

-1

9

-1

(× 10 M s ) 1.6 ± 0.2 2.5 ± 0.3 3.5 ± 0.3

0.5

0.6

[Probe] (mM) 596

4


2.0

(c) 1.5 1.0

0.0

0.0

598 599 600 601 602 603

kP -1

-0.5

597

FT

0.5 sorbic acid sorbic alcohol sorbic amine

0.1

0.2

(nM s ) 38.5 ± 4.0 40.2 ± 2.7 47.5 ± 3.4

0.3

0.4

9

-1

-1

(× 10 M s ) 1.8 ± 0.2 5.6 ± 0.4 8.9 ± 0.6

0.5

0.6

[Probe] (mM) Figure 4. The [Probe]/RP value versus the probe concentration under different reaction conditions. The values for the triplet formation rate (FT, M s-1) and second-order rate constant of the triplet and probe (kP, M-1 s-1) are shown in the insets of each figure. Reaction conditions: irradiation wavelength λ > 315 nm, 10 µM of photosensitizers, air-saturated, 5.0 mM phosphate buffer, pH 7.0; probes were spiked at 5 different concentrations ranging from 7.0 µM to 511.0 µM. Photosensitizer: (a) benzophenone, (b) 4-benzoylbenzoic acid, and (c) 4,4'-dicarboxybenzophenone. 25



604 605 606 607

Page 26 of 28

Table 1. Quantitative analyses of the sorbic alcohol and sorbic amine isomers using NMR and HPLC. In this table, amount of individual isomers are normalized with respect to the peak area of trans, trans-isomer. Geometrical

Methods

Molar absorption coefficients at

Compound configuration

NMR

HPLC

230 nm (× 104 M-1 cm-1)a

trans, trans-

1

1

1.72 ± 0.09

cis, trans-

0.310

0.282

1.57 ± 0.08

trans, cis-

0.242

0.210

1.49 ± 0.06

cis, cis-

0.178

0.141

1.36 ± 0.05

trans, trans-

1

1

1.95 ± 0.10

cis, trans-

0.116

0.090

1.52 ± 0.08

trans, cis-

0.091

0.062

1.34 ± 0.07

cis, cis-

0.056

0.035

1.21 ± 0.06

Sorbic alcohol

Sorbic amine

608 609

a

The error bars showing the uncertainty of HPLC and NMR analysis (less than 5%).

26


Page 27 of 28

610 611 612 613


Table 2. The triplet formation rate (FT), triplet quantum yield (Φtriplet), steady-state triplet concentration ([3CDOM*]SS), and second-order rate constant of the triplet and probe (kP) under different reaction conditions. Φ

FT

Triplet

[3CDOM*]

SS

kP

Light No

(× 109 M-1

Probe (nM s-1)

source

(%)

(× 10-14 M) s-1)

1 2

λ > 315 nm

3

Sorbic acid

6.1 ± 0.5

0.68 ± 0.06

1.1 ± 0.1

0.42 ± 0.1

Sorbic alcohol

6.6 ± 0.5

0.75 ± 0.06

1.2 ± 0.1

1.1 ± 0.1

Sorbic amine

6.7 ± 0.3

0.75 ± 0.03

1.2 ± 0.1

5.2 ± 0.4

6.5 ± 0.5

0.73 ± 0.05

1.2 ± 0.1

Sorbic alcohol

12.8 ± 0.5

0.96 ± 0.04

2.3 ± 0.1

1.0 ± 0.1

Sorbic amine

12.9 ± 0.3

0.97 ± 0.03

2.3 ± 0.1

4.5 ± 0.2

12.8 ± 0.4

0.96 ± 0.03

2.3 ± 0.1

Mean 4

λ > 290 nm 5 Mean 614 615 616

Reaction conditions: 5.0 mgC L-1 of SRNOM, air-saturated, 5.0 mM phosphate buffer, pH 7.0; probes were spiked at 8 different concentrations ranging from 1.7 µM to 516.0 µM.

27



617

Graphical Abstract:

618

619

28


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Development of Novel Chemical Probes for Examining Triplet Natural

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