The Case Against Charge Transfer Interactions in Dissolved Organic

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The Case Against Charge Transfer Interactions in Dissolved Organic Matter Photophysics Garrett McKay, Julie Ann Korak, Paul R. Erickson, Douglas E. Latch, Kristopher McNeill, and Fernando L. Rosario-Ortiz Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03589 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 14, 2017

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Environmental Science & Technology

The Case Against Charge Transfer Interactions in Dissolved Organic Matter Photophysics

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Garrett McKay 1‡, Julie A. Korak 1,2‡, Paul R. Erickson 3, Douglas E. Latch 4, Kristopher McNeill 3*, and Fernando L. Rosario-Ortiz 1*

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Abstract

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The optical properties of dissolved organic matter influence chemical and biological processes in

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all aquatic ecosystems. Organic matter optical properties have been attributed to a charge-

16

transfer model in which donor-acceptor complexes play a primary role. This model was

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evaluated by measuring the absorbance and fluorescence response of organic matter isolates to

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perturbations in solvent temperature, viscosity, and polarity, which affect the position and

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intensity of spectra for known donor-acceptor complexes of organic molecules. Absorbance and

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fluorescence spectral shape were largely unaffected by these perturbations, indicating that the

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distribution of absorbing and emitting species was unchanged. Overall, these results call into

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question the wide applicability of the charge-transfer model for explaining organic matter optical

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properties and suggest that future research should explore other models for organic matter

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photophysics.

1

Department of Civil, Environmental and Architectural Engineering, University of Colorado, Boulder, CO 80309 2 Bureau of Reclamation, Department of the Interior, PO Box 25007, Denver, CO 80225 3 Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, 8092 Zurich, Switzerland 4 Department of Chemistry, Seattle University, Seattle, WA 98122 * Corresponding authors

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‡

*

These authors contributed equally to this work. Corresponding authors: [email protected]; [email protected] 1

ACS Paragon Plus Environment


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Introduction

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Organic matter, the complex mixture of organic compounds present in terrestrial and

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aquatic ecosystems, is an active component of the global carbon cycle.1,2 Organic matter in

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aquatic systems is derived from allochthonous sources, such as the leaching of plant material and

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soil organic matter, as well as autochthonous sources, such as microbial exudates and other

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cellular components.3 Dissolved organic matter (DOM) constitutes the fraction of organic matter

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dissolved in aquatic systems and is ubiquitous in inland and marine waters.3 The variety of

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organic matter sources and environmental transformation mechanisms results in a diverse

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collection of chemical functionalities that consequently play significant roles in aquatic systems.

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The importance of DOM has led to the development of a suite of characterization methods to

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understand both its chemistry and function in the environment.4-8 DOM also has important public

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health implications due to the formation of toxic disinfection byproducts in drinking water.9

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Chromophoric DOM (CDOM) is the light-absorbing fraction of DOM and is the main

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absorber of sunlight in aquatic systems,10-12 thus influencing the dynamics of photosynthetic

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organisms in these environments. An abundance of evidence suggests that aromatic, phenolic,

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and carbonyl-containing groups are the main chromophoric units in CDOM.13 CDOM

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absorbance spectra are characterized by an exponential decrease in absorbance with increasing

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wavelength. CDOM fluorescence is characterized by significant Stokes shifts and relatively

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small fluorescence quantum yields (Φf) that exhibit a maximum Φf of approximately 1-3% at an

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excitation wavelength (λex) of ~370 nm and decrease monotonically with increasing λex.14 CDOM

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absorbance and fluorescence have been used remotely, in situ, and in whole water samples to

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track organic matter concentration, composition, and dynamics at varying spatial and temporal

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scales.15-17 In addition, CDOM photophysical properties influence many processes in aquatic

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photochemistry, including the fate of environmental contaminants18 and the oxidation and

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mineralization of organic matter.2

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Despite its importance, the photophysics of CDOM optical properties is not fully

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understood, due in part to the complexity of CDOM and its structural variability. Two models

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have been applied to CDOM. The first states that observed CDOM optical properties are a

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superposition of non-interacting chromophores ( a( λ ) =

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Alternatively, an electronic interaction model has been proposed in which electron rich donors

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(D) associate with electron poor acceptors (A) to form complexes capable of undergoing charge-

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transfer excitation (D + A ⇌ DA + hν → [D+A–]*).13,14 This model states that CDOM optical

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properties include contributions from individual chromophores in addition to DA complexes (

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a(λ ) = ∑ i=1 ai (λ ) + ∑ i

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chromophores).

N

N

∑

N

∑

N

a ( λ ) ; where ai is absorbance).

i=1 i

a (λ ) where ai,j refers to absorbance due to interacting

j≠i i , j

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Many studies have adopted the charge-transfer (CT) model as a broad framework for

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interpreting CDOM optical properties and photochemistry in aquatic systems.13,19-21 The CT

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model has also been applied to organic matter sources other than aquatic CDOM, including soil20

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and atmospheric22 systems. The humic and fulvic acid fractions of soil organic matter are

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typically more aromatic and higher in molecular weight than their aquatic counterparts, which

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according to the CT model, explains their more intense color. Recent studies of atmospheric

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systems containing so-called brown carbon, a mixture of compounds resulting from biomass

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burning that has important implications in radiative forcing, have suggested that CT interactions

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may play a role.22-24 Some atmospheric studies have drawn verbatim from the CT model

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developed for aquatic CDOM, arguing that brown carbon optical properties are largely explained

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by CT interactions.22,25,26 Taken together, it is not well understood whether the similarities in 3



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optical properties observed between these different pools of organic matter can be explained by

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the CT model or by the superposition model, especially because of the known differences in

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structure between aquatic CDOM and these other organic matter sources.

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Testing the Working CT Model

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Some of the key aspects of the CT model are shown in Figure 1. There are three possible

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arrangements of D and A groups that show CT behavior. First, D and A groups may be

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independent molecules that associate to form a ground state intermolecular DA complex with a

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characteristic CT absorbance (Figure 1a). Examples of this type include complexes between

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acceptors, such as electron-poor quinones (e.g., chloranil) or alkenes (e.g., tetracyanoethylene),

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and electron-rich aromatics (e.g., hexamethylbenzene) or amines.27 Second, D and A groups may

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be tethered, but are otherwise electronically independent, non-conjugated moieties, such as in

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naphthalene-pyridinium or porphyrin-quinone systems (Figure 1b).28-30 These groups could form

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intramolecular DA complexes that exist in equilibrium with their respective D and A groups.

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Finally, the D and A moieties may be linked as interacting, conjugated groups, such as p-cyano-

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N,N-dialkylanilines31 or pyridinium N-phenolate betaine dyes (Figure 1c).32 Fluorescence can

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occur from each of these CT excited states, which could form as a result of excited state electron

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or proton transfer from a local excited (LE) state D or A moiety.

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Each of these arrangements has an expected sensitivity to solvent conditions, including

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temperature, polarity, and viscosity, which can be used to test for the presence of these

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complexes. In fact, a common test for the presence of CT states is the role of solvent polarity on

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the observed position and intensity of spectral transitions.33

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94 95 96 97 98 99 100 101 102 103

Figure 1. Scheme depicting the possible photophysical processes of CDOM, including local excited (LE) states and charge-transfer (CT) interactions and the expected effects of temperature, solvent polarity, and viscosity on CDOM absorption (a-c) and fluorescence (d-f). Scenarios are depicted for independent D and A molecules (a, d), covalently tethered D and A (b, e) and conjugated D and A moieties (c, f). Only some of the critical dependencies are indicated. For example, the wavelength of fluorescence (hνF,CT) is dependent on solvent viscosity due to dipole reorientation effects.34 Note also that temperature also perturbs solvent viscosity and solvent polarity and is thus expected to have effects beyond DA complex equilibrium position.

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To investigate the CT model for CDOM photophysics, a diverse set of organic matter

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isolates was examined, including isolates from soil (ESHA, PPHA, PPFA), terrestrial aquatic

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(SRFA, NRFA, SRHA, SRNOM, MRNOM, NRNOM, YHPOA), and microbial/marine (PLFA,

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GMHPOA, POFA) sources. (SI provides additional details on sources and isolation procedures.)

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With respect to the soil isolates, we note that it is common practice to extract soil organic matter

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into aqueous solution to characterize its optical properties using similar approaches as aquatic

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organic matter.35 Optical properties including absorbance spectra, fluorescence spectra, and

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fluorescence quantum yields were measured as a function of temperature (10-40 °C), solvent

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viscosity (0.38–1.31 cP), and solvent polarity (dielectric constant, 8–80). This paper compares

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the results obtained following these perturbations with the hypotheses shown in Figure 1

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developed for well-defined DA complexes of organic molecules,33 thus providing a direct

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assessment of the CT model’s ability to explain CDOM optical properties.

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Materials and Methods

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Experimental Matrix. Thirteen DOM isolates were studied (see Table S2 for a list of sample

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names and sources). Absorbance spectra of all isolates were measured on a Cary Bio 100 UV-

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Vis spectrophotometer (Agilent). Fluorescence spectra were measured for all thirteen isolates

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using an Aqualog spectrofluorometer (Horiba Scientific) in H2O, acetonitrile (ACN), and

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tetrahydrofuran (THF). Temperature-dependent absorbance and fluorescence spectra were

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measured on a subset (n=7) of these samples using a Fluoromax-4 spectrofluorometer (Horiba

124

Scientific), and absorbance and fluorescence spectra at varying temperature/glycerol

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combinations were measured on a subset (n=3) of these samples on the Fluoromax-4.

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Preparation of solutions. Aqueous stock solutions (~ 100-300 mg isolate L-1) were prepared by

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dissolving isolates in lab grade water and adjusting to pH 7.0 using 0.1 M NaOH. Samples were

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maintained at this pH for two days, after which they were filtered through 0.7 µm glass fiber

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filters (muffled at 500 ºC for 4 hr). These stocks were used to prepare dilute solutions in H2O,

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ACN, and THF for absorbance and fluorescence measurements. Aqueous solutions were

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prepared by diluting aqueous stocks to a target concentration of 2.5 - 10.6 mgC L-1 depending on

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instrument sensitivity (Table S4). For organic solvent-based solutions, a standard dilution factor

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was applied across all isolates (%v/v = 95/5 organic/water). Spectroscopic grade glycerol was

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used to alter the viscosity during some experiments; the exact volume percent of glycerol was

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determined using its density. All solutions were well mixed prior to analysis. For temperature-

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dependent studies, isolates were dissolved directly in 10 mM phosphate buffer (pH 7.2) at their

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respective target concentrations. Control experiments indicated no difference in absorbance or

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fluorescence spectra between buffered and unbuffered solutions. Once prepared, samples were

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stored in muffled, amber glass bottles at 4°C.

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The 2.5 - 10.6 mgC L-1 target concentration represents an optimized value for our

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experimental system, which necessitates having enough signal to obtain good absorbance data

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while concomitantly avoiding saturation of the emission detector of the Fluoromax-4 instrument.

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In addition, DOC concentrations of samples in this study are relevant for a range of

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environmental systems.

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Dissolved organic carbon (DOC) concentrations of stock solutions (diluted from ~ 100-

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300 mg L-1 to ~ 1-4 mgC L-1) and aqueous dilutions were measured using a total organic carbon

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analyzer (M5310C, Sievers) that utilizes UV-persulfate oxidation. Potassium hydrogen phthalate

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was used as a QA/QC standard. DOC concentrations measured on the aqueous dilutions were

149

used to calculate molar extinction coefficients for these samples on a molar carbon basis,

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whereas molar extinction coefficients for organic solvent-based measurements were calculated

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using DOC concentrations determined for stock solutions and known dilution factors.

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Data collection and processing. Absorbance spectra were collected on a Cary-100 Bio UV-Vis

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spectrophotometer from 800-200 nm and baseline corrected to the appropriate solvent.

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Fluorescence spectra collected on the Fluoromax-4 spectrofluorometer were measured at

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excitation wavelengths from 240-550 nm. Emission intensity was collected from 300-800 nm

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using a 0.25 s integration time and a 5 nm bandpass for both excitation and emission

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monochromators. Fluorescence spectra collected on the Aqualog spectrofluorometer were

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measured at excitation wavelengths from 240-600 nm and a bandpass of 5 nm. Emission

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intensity for this instrument was collected using a charge-coupled device (CCD) at 4.64 nm/pixel

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leading to an emission scan range of 245 to 824 nm. Integration times varied depending on

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sample but did not exceed 10 s. Fluorescence intensity for both instruments was collected in

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signal over reference (S/R) mode. Fluorescence data were corrected according to published

163

methods incorporating instrument-specific correction factors, blank subtraction, inner filter

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correction and Raman normalization 36. Table S3 provides a comparison of the Fluoromax-4 and

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Aqualog instruments.

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Absorbance spectra were collected in triplicate and fluorescence spectra were collected in

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either duplicate or triplicate. Although the Aqualog measures absorbance and fluorescence

168

spectra simultaneously, the Aqualog absorbance data were not used for quantum yield

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calculations due to unacceptable coefficients of variance (>10%) and consequently large

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propagated errors in Φf at longer excitation wavelengths.

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E2/E3, S300-600, SR, and FI were calculated as previously described.6,37,38 Fluorescence

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quantum yields were calculated as previously described39 with some modifications that are

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explained in the Supplemental Information.

174 175

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1500

a)

0.015

ESHA PPHA SRHA SRFA MRNOM PLFA GMHPOA POFA

1000 500

b)

0.006 0.003

0 240 300 0.015

400

500

600

c)

0 250

700

0.015

f

0.006

0.003

0.003 350

400

450

350

400

450

500

550

500

550

PPFA ESHA

PPHA SRHA

0.009

0.006

300

d)

0.012

0.009

0 250

300

Excitation Wavelength (nm) MRNOM SRFA NLNOM

0.012 f

PLFA POFA GMHPOA

0.009

Wavelength (nm)

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0.012 f

Coefficient (L mol -1 cm -1 ) C

Molar Extinction


0 250

Excitation Wavelength (nm)

300

350

400

450

500

550


177 178 179 180 181

Figure 2. Optical properties of organic matter isolates in aqueous solution. a) Absorbance spectra normalized to carbon concentration and b) – d) fluorescence quantum yields for PLFA, POFA, GMHPOA (microbial/marine aquatic), SRHA, SRFA, MRNOM, NRNOM (terrestrial aquatic) and ESHA, PPHA, PPFA (soil).

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Results & Discussion

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Absorbance of CDOM. Figure 2a presents absorbance spectra of select organic matter isolates in

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aqueous solution. Spectra exhibited an exponential decrease in absorbance with increasing

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wavelength, as has been observed universally for CDOM.11 Marine and microbial-derived

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isolates (i.e., GMHPOA, POFA and PLFA) had lower molar extinction coefficients than

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terrestrial aquatic and soil isolates. Table 1 summarizes compositional metrics that have been

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used as surrogates of organic matter molecular weight, aromaticity and genesis. Specific

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ultraviolet absorbance at 254 nm (SUVA254), a widely used parameter correlated to aromatic

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carbon content, spanned a large range for aquatic samples (0.7 to 4.9 L mgC-1 m-1).40 Spectral

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slope (S300-600) and the E2/E3 ratio, two surrogates inversely proportional to organic matter

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molecular weight, were greater for marine- and microbial-derived isolates compared to soil and

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terrestrial aquatic isolates.38 YHPOA, a terrestrial isolate, was an exception and exhibited high

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E2/E3 and S300-600, and low SUVA254 values. This sample was isolated from the Yukon River

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(Alaska, USA) during winter and was reported to be enriched in carboxyl-rich aliphatic moieties,

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depleted in lignin phenols, and more indicative of microbial-derived organic matter.41 As a

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whole, this data set represents the wide range of properties in the studied organic matter isolates. Table 1. Optical properties for samples in aqueous solutiona.

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Isolate

Origin

GMHPOA POFA PLFA MRNOM NRNOM SRNOM NRFA SRFA YHPOA SRHA ESHA PPFA PPHA

Marine/ Microbial

Terrestrial Aquatic

Soil

SUVA254b (L mgC-1 m-1)

E2/E3c

S300-600d (nm-1)

FIe

1.7 0.7 2.5 3.4 4.2 3.0 4.9 4.2 2.6 3.8 8.9f 6.3f 8.3f

7.0 8.7 4.9 5.4 4.7 4.7 4.2 4.5 7.1 3.5 2.4 4.3 2.9

0.0173 0.0161 0.0155 0.0161 0.0149 0.0147 0.0151 0.0152 0.0186 0.0130 0.0075 0.0134 0.0098

1.43 1.54 1.58 1.45 1.44 1.42 1.23 1.29 1.53 1.09 0.65 1.06 0.74

Φf (%) at Max Φf (%) λex =370 nm 1.06 1.09 1.31 1.21 0.72 1.00 0.65 0.84 2.14 0.35 0.49 1.08 0.39

1.06 1.11 1.32 1.21 0.74 1.00 0.68 0.84 2.18 0.35 0.61 1.24 0.54

λex at max Φf 370 355 385 370 350 370 390 360 380 430 460 460 460

a

Measurement uncertainty not shown for clarity. Coefficient of variations for all parameters were less than 5%, and values are included in Table S-4. SUVA254 is calculated as Abs254/[DOC]×100; cE2/E3 is calculated as Abs250/Abs365; dS300-600 (nm-1) is calculated by a non-linear least squares fit to Abs(λ) = Absref×exp[-S(λ-λref)] ; eFI is the ratio of emission intensity at 470 to 520 nm at an excitation of 370 nm; fSUVA254 values > 6 L mgC-1 m-1 should be interpreted with care due to the known interference of iron. 42 b

199 200

The CT model posits that DA complex excitation begins to dominate at λex ≈ 375 nm,

201

resulting in the formation of an excited CT state, [D+A–]*, of lower energy than 1D*A.43 The

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exponential decrease in absorbance with increasing wavelength would thus suggest that potential

203

DA complexes that are excited to lower energy CT excited states have lower molar extinction

204

coefficients than species absorbing at shorter wavelengths. The exact nature of potential DA

205

complexes has not been firmly defined, but previous studies suggest that covalently tethered D

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and A moieties (Figure 1b) are most likely.14

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In this study, no changes in absorbance spectral shape or intensity were observed at

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different temperatures (10-40 °C) or solvent viscosities (0.38-1.31 cP), while minor changes

11



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(primarily at λex < 375 nm) were observed between spectra in H2O, ACN, and THF (Figures S3,

210

S7, S9-10), indicating that the dynamics of potential DA complexes are unchanged over these

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conditions.

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These results are striking in that the concentration of DA complexes giving rise to CT

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absorbance (AbsDA), formed either from independent D and A molecules or covalently tethered

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D and A moieties (Figure 1a and b, respectively), is determined by an equilibrium constant (D +

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A ⇌ DA, KDA), which is expected to be temperature-dependent as described by the van’t Hoff

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equation (). For example, an approximately 25% increase in AbsDA is expected for a complex

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with a formation enthalpy (∆HDA) of -2 kcal mol-1 for a temperature decrease from 35 to 10 °C

218

(see SI Text 3.2).33 This example ∆HDA value is similar to the value for p-benzoquinone and

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benzene measured in heptane (-1.8 kcal mol-1).44 Thus, the lack of temperature-dependence in

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absorbance spectra at long wavelengths suggests that DA complexes are not significant

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contributors to CDOM absorbance. Conjugated DA chromophores remain a possibility (Figure

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1c) but are also unlikely due to the lack of significant solvatochromism observed in CDOM

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absorbance spectra. Solvatochromic shifts occur widely in CT absorbance spectra of well-

224

defined organic DA complexes due to the large change in molecular dipole moment between

225

ground and excited states.33 If the dipole decreases upon excitation, a blue shift is expected with

226

increasing solvent polarity (hypsochromic), whereas a red shift is expected if the dipole increases

227

upon excitation (bathochromic). In effect, these absorbance data suggest that the DA complexes

228

shown in Figure 1a-c are not major contributors to long wavelength absorbance. There is another

229

explanation, in which all of the hypsochromic shifts are exactly balanced by bathochromic shifts

230

of the same intensity. We reject this latter possibility as it seems unlikely to occur across all three

231

solvents and thirteen organic matter isolates investigated.

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Fluorescence of CDOM. Maximum Φf values spanned almost an order of magnitude for the

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different isolates (Figure 2b, 2c and 2d, Φf = 0.0035-0.022 at λex = 370 nm). All aquatic isolates

234

exhibited an increase in Φf with increasing λex up to a maximum value of ~370 nm, followed by a

235

decrease with increasing λex. This behavior has been used as evidence in favor of the CT model.14

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Φf values for soil isolates (ESHA, PPHA) and one aquatic isolate (SRHA) were largely

237

independent of λex up to ~480 nm, followed by a monotonic decrease with increasing λex, notably

238

distinct from aquatic samples. The characteristic dependence of Φf on excitation wavelength

239

commonly observed in aquatic organic matter isolates, which has been attributed to the relative

240

contribution of LE state versus CT state fluorescence as λex increases to 375 nm, does not apply

241

to soil isolates and SRHA. Yet, soil isolates exhibit the greatest abundance of long wavelength

242

absorbance, which has also been used as evidence supporting the CT model. These results

243

contradict models that ascribe CT transitions as a significant contributor to the optical properties

244

of soil isolates. Thus, the same line of reasoning used to support the CT model for aquatic

245

samples cannot be applied to soil isolates.

246

CDOM exhibits large apparent Stokes shifts that decrease with increasing excitation

247

wavelength.14,43 In most cases, emission spectra exhibit a behavior in which spectra at longer

248

excitation wavelengths are lower in intensity and narrower in width, as if fitting inside emission

249

spectra at shorter excitation wavelengths (Figures S16). The CT model rationalizes this

250

observation by suggesting that CDOM fluorescence occurring at emission wavelengths (λem) >

251

470 nm results from the charge-recombination-induced luminescence of [D+A–]* and that these

252

states are highly coupled, resulting in a near continuous energy distribution. Reduction of

253

carbonyl moieties in DOM with sodium borohydride has been shown to increase Φf values,45,46

254

and it has been argued that this is due to either a decrease in excited-state electron transfer or

13



255

direct excitation into CT states. The relative importance of fluorescence and CT deactivation

256

pathways should be affected by temperature, solvent viscosity, and solvent polarity for

257

independent and covalently tethered DA complexes shown in Figure 1d-f. Emission from CT

258

states formed from conjugated DA moieties (Figure 1f) would only be affected by solvent

259

polarity. The underlying physics are that the rate of solvent reorientation about the excited state

260

dipole is slower at lower temperatures, which leads to increased emission from the LE state and

261

manifests as a blue shift in the emission spectrum.34 Solvent polarity similarly influences the

262

energy of the emitting state relative to the ground state. This solvent dependence can be due to

263

general solvent polarizability or fluorophore-specific effects such as the presence of red-emitting

264

CT states.34

265

Increasing temperature consistently decreased fluorescence intensity and Φf at all λex

266

(Figure 3), with notable exceptions being soil humic acids (Figures S4-6). Emission spectral

267

shape did not change as a function of temperature (Figure 3b inset), which indicates that the

268

energy distribution of fluorophores is unchanged over the temperature range studied.

269

Fluorescence spectra of selected samples were measured at -10 °C and 70 °C in approximately

270

70% v/v glycerol in water solution and exhibited no difference in spectral shape (Figure S26).

271

These results call into question the significance of fluorescence from CT excited states. A

272

temperature-dependence would be expected since the rates of DA exciplex formation from LE

273

states and subsequent conversion to lower-lying CT states are temperature-dependent, each with

274

presumably different activation energies. These rates, in part, determine the contribution of each

275

excited state to observed fluorescence at a given emission wavelength. Thus, for emission

276

spectral shape to be invariant with temperature, thermodynamic parameters for these processes

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would need to be approximately equal, which seems highly improbable given the chemical

278

diversity within a given organic matter isolate.

279

Fluorescence intensity measured by steady-state methods is dominated by species with

280

larger fluorescence quantum yields, leading to criticism on the use of this method compared to

281

time-resolved measurements.43 That spectral shape was unchanged on the red edge of the

282

emission, which the CT model hypothesizes to be due to CT excited state emission, supports our

283

conclusions based on steady-state emission data.

284 Absolute Data

Normalized Data

0.003

0.39

Ex=370 nm

0.26 0.13 0 300

0 300

400

500

600

700

400

450

500

0.50 0.25

0 300

550

Excitation Wavelength (nm) c)

H2O ACN THF

Normalized

f

0.009 0.006 0.003 0 300

285 286 287 288 289 290

350

400

450

500

25 °C 30 °C 40 °C

1 Ex=370 nm 0.75 0.50 0.25 0 300

400

500

600

700

800

Emission Wavelength (nm)

350

400

450

500

550

Excitation Wavelength (nm) 1.0 f

0.012

10 °C 20 °C

0.75

800

Emission Wavelength (nm)

350

b)

DOM

0.006

IDOM (RU)

f

0.009

1.0

Normalized I

10 °C 20 °C 25 °C 30 °C 40 °C

f

a)

Normalized

0.012

d)

0.75 0.50 0.25 0 300

550


H2O ACN THF

350

400

450

500

550


Figure 3. Temperature and solvent effects for SRFA by comparing fluorescence quantum yields as a function of temperature and solvent polarity. a) Φf as a function of excitation wavelength over 10-40 °C. Inset: emission spectra at an excitation wavelength of 370 nm. b) Data from a) normalized to maximum value. c) Φf as a function of excitation wavelength in H2O, ACN, and THF. d) Normalized Φf data from c).

291

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A change in temperature affects the rate of multiple photophysical processes, including

293

the rate of excited singlet state decay,47 and in the case of DA complex photophysics, the rate of

294

CT excited state formation from a LE state.34 Temperature also affects the viscosity of the

295

solvent, and thus the rate of solvent reorientation about the excited state dipole. To separate the

296

effects of changing temperature and viscosity, absorbance and fluorescence spectra were

297

measured for a subset of samples at varying water:glycerol ratios and temperatures (Table S5).

298

There were no significant changes in absorbance spectra over the temperature-solvent

299

combinations studied (Figure S7). However, we observed a significant decrease in Φf (Table S6

300

and Figure S8) with increasing temperature (10-40 °C; corresponding to a viscosity range of

301

0.83-1.31 cP) at a constant solvent composition, except for ESHA. Increased viscosity due to

302

increasing glycerol content (10-30% v/v; corresponding to a viscosity range of 0.83-1.46 cP) at

303

constant temperature did not affect Φf (Table S6). This observation suggests that decreased

304

fluorescence intensity with increasing temperature is due specifically to the effect of temperature

305

on the rate of radiationless decay mechanisms47 and not the indirect effect of temperature on

306

solvent viscosity.

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308 309 310 311 312

Figure 4. Solvatochromic shifts in peak emission wavelength in ACN and THF relative to water for a) GMHPOA, b) SRNOM, c) ESHA, and d) PPFA. ESHA in THF was not evaluated. Data within the gray region (± 15 nm) indicates no significant shift in peak emission wavelength.

313

To further test the importance of CT excited state emission, fluorescence spectra were

314

measured in ACN and THF. Fluorescence spectra containing contributions from CT emission are

315

expected to be red-shifted (increase in λem) due to the greater stabilization of the excited state

316

species by high polarity solvents in CT excited states relative to LE states. In contrast to this

317

hypothesis, few samples showed red-shifted emission spectra in H2O relative to ACN or THF

318

(Figure 4). Most samples exhibited no statistically significant change in emission wavelength (±

319

15 nm based on number of replicates and bandpass). Note that the ± 15 nm over which statistical

320

significance was assessed falls within the region of solvatochromic shifts that could be expected

321

for LE states but is less than what would be expected for fluorescence from CT excited states.

322

While low concentrations (5% v/v) of water in an organic solvent can impact fluorescence

323

spectra,34 the lack of change in emission maximum was also evident for SRFA when dissolved

17



324

directly in THF (Figure S27 and S28), corroborating the results obtained at %v/v 95/5 organic

325

solvent to water. Changes in emission maxima were only observed for PPFA, PPHA, and ESHA

326

(all soil isolates) in ACN (Figure 4, and Figure S25). Importantly, SRFA, a terrestrial organic

327

matter isolate that has been used as an exemplar for the CT model,14,43,45 exhibited no spectral

328

shift at any excitation wavelength across the tested solvents. In the CT model, it is thought that

329

excitation at λex > 375 nm leads to [D+A–]* states, however, the absence of solvatochromism in

330

fluorescence spectra calls into question the presence of these states.

331

The observation that fluorescence emission maxima are mostly unchanged by solvent

332

polarity (∆λem,max < 15 nm) may indicate that DOM fluorophores, including CT excited states,

333

are protected from the solvent in some type of hydrophobic microenvironment; although this

334

idea has some shortcomings. Such a microenvironment could not be the result of non-covalent

335

interactions since both non-polar solvents, such as THF, and increasing temperature would

336

disrupt such interactions. For example, dimethyl formamide has been shown to disrupt non-

337

covalent interactions (π-π stacking) in eumelanin that normally occur in water.48Future research

338

on this topic could further investigate the role of encapsulated fluorophores by measurement of

339

fluorescence lifetimes (expected to be increased in such a hydrophobic microenvironment) both

340

as a function of temperature and in solvents of varying polarity.

341

It is also noteworthy that soil isolates exhibited solvatochromism at λex < 375 nm,

342

whereas aquatic isolates did not (Figure 4c,d vs. a,b). Assuming that these excitations lead to LE

343

states, this result indicates that polar solvents stabilize these states more for soil isolates

344

compared to aquatic organic matter. The only sample that showed systematic solvatochromism

345

in fluorescence spectra at λex > 375 nm was ESHA. Although this observation could be

18


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346

interpreted as a potential role for DA complexes in soil isolates, we do not think that this is likely

347

given both the insensitivity of these samples’ absorbance spectra to solvent polarity and

348

fluorescence spectra to changing temperature. This could, however, indicate a role for exciplexes

349

formed from LE states in soil isolates because these complexes are formed only in the excited

350

state, unlike ground-state DA complexes. This would manifest as solvatochromism in

351

fluorescence spectra but not absorbance spectra, consistent with our observations for soil

352

isolates. In addition, one might interpret the results in Figure 4c,d as the presence of some type of

353

supramolecular assembly of soil humic substances in solution that is disrupted by decreasing

354

solvent polarity. However, this explanation is inconsistent with the both fact that DOM,

355

including soil humic substances, obey the Beer-Lambert law and the constancy of fluorescence

356

spectra with temperature. Such a supramolecular structure, and consequently its fluorescence

357

spectra, would likely be affected by the two aforementioned factors.

358

Φf for all samples decreased in THF and ACN relative to H2O (Figure 3) at all excitation

359

wavelengths. Furthermore, Φf decreased in THF relative to ACN for all isolates besides

360

MRNOM (Figure S18). Although viscosity varies widely between the solvents, this decrease in

361

Φf is attributed to solvent polarity based on the minimal change in Φf observed by changing

362

solvent viscosity using glycerol. Spectra for the soil isolates PPHA and ESHA in THF showed

363

evidence of light scattering due to particles and were not considered in the analysis. The

364

wavelength-dependence of Φf is largely the same in different solvents (Figure 3 and Figures S17-

365

S20). The decrease in fluorescence intensity and Φf with decreasing solvent polarity is opposite

366

to the behavior observed of intermolecular CT complexes of organic molecules (e.g.

367

trimethylbenzene + pyromellitic dianhydride, or 4-(N,N)-(dimethylamino)benzonitrile),49-52 but

368

consistent with observations of intramolecular CT fluorescence.53 Regardless, solvatochromism

19



369

is observed in these inter- and intramolecular CT model systems, which is generally not observed

370

for the organic matter isolates examined in this study.

371 372

Implications for CDOM photophysics

373

This study questions the validity and wide applicability of the CT model to explain the

374

photophysical properties of CDOM. The presence of CT-derived transitions is not supported in

375

the examined isolates due to the absence of expected changes in absorbance and fluorescence

376

responses as a function of temperature, solvent polarity and viscosity that are characteristic of

377

DA complexes of organic molecules. Although we cannot categorically reject the presence of CT

378

transitions within CDOM, our results suggest that these transitions are not prominent

379

photophysical mechanisms. Rather, this work suggests that organic matter contains unidentified

380

chromophores that absorb and emit at long wavelengths. Future work should focus on the

381

coupling of high-resolution DOM characterization techniques with optical and photochemical

382

behavior of DOM in order to identify the chromophores responsible for long wavelength

383

absorbance and fluorescence. Identification and characterization of the photophysics and

384

photochemistry of these long wavelength-absorbing chromophores remains a challenge for the

385

years ahead and is necessary if we are to correctly understand how organic matter is

386

photochemically degraded, how it sensitizes the degradation of pollutants, and how the optical

387

properties of natural waters will be altered by changing climate and land use.

388 389 390

Acknowledgments

391

Funding for this work came from the US National Science Foundation grant CBET #1453906

392

and CBET #1434313. We thank Johanna Rinaman for help collecting fluorescence spectra. Any

20


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393

use of trade, firm or product names is for descriptive purposes only and does not imply

394

endorsement by the United States Government. The views, analysis, recommendations, and

395

conclusions in this report are those of the authors and do not represent official or unofficial

396

policies or opinions of the United States Government and the United States takes no position

397

with regard to any findings, conclusions, or recommendations made.

398

Dedication

399

This paper is dedicated to the memory of George R. Aiken, whose curiosity of dissolved organic

400

matter fluorescence has inspired us all. George graciously provided three isolates included in this

401

study and helped develop the ideas behind measuring CDOM absorbance and fluorescence in

402

organic solvents.

403

Supporting Information Available

404

Additional text on the charge-transfer model, application of the charge-transfer model to DOM,

405

fluorescence quantum yield calculations, DOM optical properties in aqueous solution,

406

temperature-dependence of DOM absorbance and fluorescence, differentiating the effects of

407

viscosity and temperature, solvent effects on DOM absorbance and fluorescence; tables

408

including data on DOM photophysical and photochemical properties and their explanation via

409

the charge-transfer model, samples included in this study, comparison of Aqualog and

410

Fluoromax-4 instruments, photophysical data in aqueous solution, experimental matrix and data

411

for temperature/glycerol experiments, solvent physicochemical properties, optical data for model

412

chromophores; figures including the potential donor-acceptor complexes in DOM, data on

413

fluorescence quantum yields calculated using linear approximation and non-linear forms,

414

absorbance and fluorescence spectra as a function of temperature, absorbance and fluorescence

21



415

spectra at different temperature/glycerol combinations, absorbance spectra and fluorescence

416

quantum yields as a function of solvent, fluorescence peak emission and quantum yields as a

417

function of solvent, absorbance and fluorescence spectra in 100% tetrahydrofuran, absorbance

418

spectra of model chromophores water and water/acetonitrile combinations.

419

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The Case Against Charge Transfer Interactions in Dissolved Organic

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