Source: https://pubs.rsc.org/en/content/articlehtml/2016/em/c6em00408c
Timestamp: 2019-04-24 04:13:48+00:00

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Excited triplet states of chromophoric dissolved organic matter (3CDOM*) play a major role among the reactive intermediates produced upon absorption of sunlight by surface waters. After more than two decades of research on the aquatic photochemistry of 3CDOM*, the need for improving the knowledge about the photophysical and photochemical properties of these elusive reactive species remains considerable. This critical review examines the efforts to date to characterize 3CDOM*. Information on 3CDOM* relies mainly on the use of probe compounds because of the difficulties associated with directly observing 3CDOM* using transient spectroscopic methods. Singlet molecular oxygen (1O2), which is a product of the reaction between 3CDOM* and dissolved oxygen, is probably the simplest indicator that can be used to estimate steady-state concentrations of 3CDOM*. There are two major modes of reaction of 3CDOM* with substrates, namely triplet energy transfer or oxidation (via electron transfer, proton-coupled electron transfer or related mechanisms). Organic molecules, including several environmental contaminants, that are susceptible to degradation by these two different reaction modes are reviewed. It is proposed that through the use of appropriate sets of probe compounds and model photosensitizers an improved estimation of the distribution of triplet energies and one-electron reduction potentials of 3CDOM* can be achieved.
Photochemical processes are critically important in driving biogeochemical element cycling and in the breakdown of contaminants in surface waters. One of the most important and least understood sets of photochemical pathways involves triplet chromophoric dissolved organic matter (3CDOM*), a form of electronically excited DOM. There has been a recent surge in the study of the properties and reactivity of 3CDOM* and this review article is an attempt to organize and synthesize what has been discovered about 3CDOM* over the past few decades.
Sunlight-driven processes are central to both the buildup of complex molecules through photosynthesis and their breakdown through photodegradation reactions. These photodegradation processes may be initiated not only directly by the absorption of light, but also indirectly through reactions involving a menagerie of exotic chemical species such as free radicals and electronically excited molecules, referred to here collectively as photochemically produced reactive intermediates (PPRI).1–4 Triplet excited states of chromophoric dissolved organic matter (3CDOM*) are an important subset of the larger pool of PPRI formed in sunlit waters that also include singlet oxygen (1O2, 1Δg), superoxide (O2−˙), hydrogen peroxide, hydroxyl radical (OH˙), and others.1–43CDOM* has been implicated in the degradation of contaminants, such as pesticides5 and pharmaceuticals,6,7 and holds a special position among the PPRI for at least two reasons. First, 3CDOM* is known or suspected to be a precursor of other PPRI.1–4 For example, 3CDOM* is the primary source for 1O2 in sunlit natural waters.8,9 Second, unlike other PPRI, 3CDOM* is not a well-defined species; rather, it is an infamously ill-defined mixture of triplet states, which vary in their excited state energies and excited state redox potentials.
The goal of this review article is to outline the reactivity modes of 3CDOM* and to summarize what is known or can be reasonably inferred about both the triplet energy and redox potential of 3CDOM*. In addition, some back-of-the-envelope calculations are presented that give rough answers to questions that often arise when discussing 3CDOM*: why are triplet states more important than singlet states in CDOM-sensitized processes? And, what is the steady-state concentration of 3CDOM*?
Attempting to quantify the steady-state concentration of 3CDOM* in an aquatic system would seem to be more challenging than other PPRI, due to the abovementioned problem that 3CDOM* is a mixture of triplet states of diverse molecules. Therefore, it may seem surprising that we can actually estimate the steady-state concentration of 3CDOM* within about a factor of two with a high degree of confidence. This is thanks to the inextricable link between 1O2 and 3CDOM*.
To understand this, it is helpful to consider the simplified kinetic scheme that connects 3CDOM* and 1O2 (Fig. 1). CDOM is excited by the absorption of a photon (symbolized by hν) to form the excited singlet state of CDOM, 1CDOM*. Under optically thin conditions, the rate of light absorbance (Rabs), in units of M s−1, is given by the product of the irradiance (mmol photons cm−2 s−1), the Naperian absorption coefficient of CDOM (natural log-based absorption coefficient in units of cm−1), and a conversion factor (mol L−1 (mmol cm−3)−1 = 1). The efficiency of the conversion of 1CDOM* to 3CDOM* (i.e., the intersystem crossing efficiency) is given by ΦISC. The rate constants for the O2-independent and O2-dependent deactivation pathways of 3CDOM* are given by kTd and kO2[O2], respectively. Under normal air-saturated surface–water conditions, O2-dependent relaxation almost certainly dominates over O2-independent relaxation. Sharpless has estimated the O2-independent lifetime of triplets through O2-dependent formation kinetics of 1O2 using Suwannee River and Pony Lake isolates, and determined a lifetime around 20 μs (kTd ≈ 5 × 104 s−1).10 Zepp has made a reasonable estimate of kO2 = 2 × 109 M−1 s−1, based on O2 quenching rate constants of well-defined sensitizers.9 While there is certainly some variation in the individual kO2 values among the numerous sensitizers that comprise 3CDOM*, they are all expected to be quite high and near the diffusion-controlled limit. For air-saturated freshwater at 25 °C (258 μM O2), kO2[O2] is thus approximately 5 × 105 s−1 (τ = 2 μs), which suggests that the O2-dependent relaxation pathway is an order of magnitude more important than the O2-independent pathway.
Fig. 1 Kinetic scheme illustrating the connection between 3CDOM* and 1O2. Definition of variables and symbols: CDOM, chromophoric dissolved organic matter; 1CDOM*, singlet state dissolved organic matter; 3CDOM*, triplet state dissolved organic matter; 1O2, singlet oxygen (1Δg); hν, photon; Rabs, rate of light absorbance; ΦISC, intersystem crossing quantum yield; kO2, bimolecular rate constant for the quenching of 3CDOM* by O2; fΔ, fraction of O2-dependent quenching that produces 1O2; kTd, rate constant for O2-independent relaxation of 3CDOM*; kΔd, rate constant for relaxation of 1O2 to O2.
Expressions for the steady-state concentrations of 3CDOM* and 1O2 based on the scheme depicted in Fig. 1 are given by eqn (1) and (2).
One can rearrange eqn (2) to arrive at an expression for the ratio of the steady-state concentrations of 1O2 and 3CDOM* (eqn (3)).
Substituting values for kΔd (2.5 × 105 s−1 for H2O),15kO2 (2 × 109 M−1 s−1),9 and [O2] (258 μM at 298 K), one arrives at eqn (4).
This result indicates that for 25 °C, air-saturated water, the ratio of 1O2 to 3CDOM* is linearly dependent on the yield of 1O2 from the O2-dependent quenching of 3CDOM* (fΔ) with a maximum [1O2]ss value of two times [3CDOM*]ss. While we do not know the value of fΔ for 3CDOM*, eqn (4) nevertheless suggests a useful rule-of-thumb of [3CDOM*]ss ≈ [1O2]ss. To reiterate, this will hold when the value for kO2 is close to the estimate of 2 × 109 M−1 s−1 and the average fΔ value is near 0.5. Under noon-time clear summer sky conditions, [1O2]ss, has been found to be between 10−14 and 10−12 M in natural waters, depending on the concentration of DOM (1–100 mgC L−1; see for example Peterson et al.16). Based on the above argumentation, we can therefore adopt this same concentration range for [3CDOM*]ss.
Why triplet states and not singlet states?
Could singlet excited state CDOM moieties (1CDOM*) act as reactive intermediates in a similar manner to 3CDOM*? After all, the lowest lying singlet excited state of a given sensitizer is higher in energy than its lowest lying triplet state and would therefore be expected to be more reactive than the triplet. While this is true, it is counteracted by the fact that the steady-state concentration of 1CDOM* is much lower than that of 3CDOM*.
To determine exactly how much lower [1CDOM*]ss is than [3CDOM*]ss, we need estimates of the relative formation and decay rate constants for both species. Formation quantum yields for 3CDOM* have been estimated to be in the range of 1–2%,9,17 but could be as high as 6% or higher for some DOM samples, based on 1O2 quantum yield measurements.16,18 This indicates that 1CDOM* formation rates are 15–100 times faster than those for 3CDOM*. On the decay side, the 1CDOM* lifetime is much shorter than that of 3CDOM*, which has a lifetime of about 2 μs (i.e., the inverse of kO2[O2]; see previous section). Fluorescence lifetime studies give a direct measurement of the decay of 1CDOM*, and, as expected, the mixture of fluorophores do not display a single lifetime. Rather, the data suggest a dominant pool of short lifetime 1CDOM* species (τ < 150 ps), with contributions from two other pools of 1CDOM* (τ ≈ 1 and 3 ns).19 For simplicity, we consider 100 ps to be the typical lifetime of 1CDOM*.
Fig. 2 Compounds that have been shown to act as energy acceptors with 3CDOM*. Triplet energies (ET) are given for each compound, except in the case of O2, where the lowest singlet energy (ES) is given.
The energy required to promote ground state O2 to 1O2 is 94 kJ mol−1 (980 meV).11 Since most triplet excited states of organic chromophores are much higher (typically 180–320 kJ mol−1), O2 has been proposed to be a universal energy acceptor, capable of accepting energy from all 3CDOM* moieties.9 This is an oversimplification as discussed above in the section on the concentration of 3CDOM* in natural waters, but to a first approximation, it is a reasonable statement.
Fig. 3 Distributions of triplet energies found for different classes of organic molecules containing functional groups that are thought to be present in CDOM and a hypothetical normal distribution of 3CDOM* triplet energies that fit the observation of 37% having ET ≥ 250 kJ mol−1.9 The data used for this figure is compiled in Table 1.
a Balance is H2O when a percentage co-solvent is specified. b Abbreviations: CBBP = 4-carboxybenzophenone; 3MAP = 3-methoxyacetophenone; EtOH = ethanol; MeOH = methanol; iPrOH = isopropanol; DMF = N,N-dimethylformamide; MCIP = 5 : 1 methylcyclohexane : isopentane; EPA = 5 : 5 : 2 diethyl ether : isopentane : ethanol; PMMA = polymethylmethacrylate; EtOAc = ethyl acetate; EA = 3 : 1 diethyl ether : ethanol; MTHF = 2-methyltetrahydrofuran; IPMC = 5 : 1 isopentane : methylcyclohexane; CFC–EA = mixture of CFC-113 (1,1,2-trichlorotrifluoroethane) and EtOAc; EM = 9 : 1 ethanol : methanol; MeCf = 3 : 1 methanol : CHCl3. c Matrix not specified. d No reported value found.
Another piece of information that could be obtained by Zepp and coworkers in the CDOM-sensitized isomerization of 1,3-pentadiene was the apparent ET of CDOM from the final cis–trans ratio, or the photostationary state, of 1,3-pentadiene.9 This photostationary state was shown to reflect the sensitizer's ET, and the values obtained for CDOM solutions were consistent with an apparent ET of 250 kJ mol−1.9 Similar experiments conducted with functionalized carbon nanotubes41 and petroleum54 found the apparent ET values to be lower and higher than CDOM, respectively. The petroleum value was estimated to be 288–303 kJ mol−1, suggesting that the triplet photochemistry relevant to oil spills may differ substantially from CDOM-based photochemistry.54 The high average ET value found in petroleum stands in contrast to the low average triplet energies of PAH molecules in Table 1 and Fig. 3. This may mean that the small selection of PAHs (triphenylene, phenanthrene, naphthalene, pyrene, and anthracene) is not representative of the PAH mixture in petroleum or that other higher ET species present in petroleum (e.g., ketones formed from oxidation)55,56 are dominating the sensitization of the diene probes.
At least two spectroscopic estimates of the ET value of 3CDOM* have been made. Bruccoleri et al. applied magnetic circular dichroism (MCD) spectroscopy to an organic matter isolate and assigned an absorbance transition as S0 → T1, and the wavelength for this transition (714 nm; 14 000 cm−1) corresponded to an energy of 170 kJ mol−1.57,58 Mazhul et al. used room temperature phosphorescence spectroscopy to identify the opposite transition (T1 → S0), with an onset near 405 nm, corresponding to the highest energy (phosphorescing) triplets having an ET value of 300 kJ mol−1.59 In both of these studies, the estimates of ET must be viewed with caution, as both techniques are almost certainly confounded by the complex mixture of DOM. Indeed, Mazhul et al. explicitly point out that they believe they are only observing phosphorescence from a minority of the 3CDOM* components in their mixture.59 Additionally, these spectroscopic values do not seem reasonable as average or representative values, since one is at the extreme low end and one is at the far high end of the range of triplet energies normally found for organic sensitizers.
Redox reactions are the dominant reaction type between organic substrates and 3CDOM*, with 3CDOM* primarily acting as the oxidant. The oxidation reactions have been reviewed elsewhere60 and the discussion here will be mostly confined to the substrate scope and the reduction potential of 3CDOM*.
Fig. 4 Selected compounds that have been shown to react with 3CDOM*. Aniline (and other aminoarene), phenol (and aryl ether), sulfide and related substructures are highlighted.
Examining the structures in Fig. 4, one can see that anilines and phenols are well represented. Anilines and compounds containing aniline substructures are especially susceptible to oxidation by 3CDOM*. This includes simple aniline structures, such as N,N-dimethylaniline,66p-aminobenzoic acid,67 and p-cyano-N,N-dimethylaniline,68 but also more complex structures, such as found in phenylurea herbicides,28,69–72 sulfa drugs,45,46,73–75 chloroacetamide herbicides,28,72 diarylamines (e.g., mefenamic acid44), and arguably within the structure of tryptophan47,76 and indole77 (Fig. 4). There is also some evidence that this reactivity extends to aniline analogues that are amino-substituted aromatic heterocycles. For example, triazine herbicides atrazine and cyanazine have been shown to react with 3CDOM*, and both of these compounds contain a diaminotriazine functional group.28,72 The structurally similar diaminopyrimidine group in trimethoprim45 and ormetoprim78 may be responsible for the reactivity of these compounds toward 3CDOM*, but these compounds also contain electron-rich methoxy-substituted benzene rings that could instead be the locus of reactivity. 3CDOM* is capable of oxidizing both electron-rich and electron-poor anilines, although the rates of aniline oxidation are clearly modulated by their electron-richness.60,69 By contrast, only electron-rich phenols appear to be susceptible to oxidation by 3CDOM*.
Relatively simple alkyl- and methoxy-substituted phenols, such as p-cresol, 3,4-dimethoxyphenol, tyrosine, and 2,4,6-trimethylphenol, have been shown to be oxidized by 3CDOM* (Fig. 4).47,79–81 This reactivity has been found in more complicated phenol-containing compounds, including the polycarbonate constituent bisphenol A,82,83 the oral contraceptive 17α-ethinylestradiol,42 agricultural hormones zeranol, β-zeranol, and zeranolone,84 and phytoestrogens daidzein, genistein, and equol32,85 (Fig. 4). Presumably, the phenol functionality is the site of reactivity toward 3CDOM* in these compounds.
Excited triplet states are both better oxidants and better reductants than their ground states. The reason for this can be seen visually in Fig. 5a, which shows the ground state and lowest triplet state electronic configurations of the frontier orbitals of a generic molecule. One can see that for the molecule to act as an oxidant (receive an electron), it requires less energy in the excited state than the ground state. Instead of the incoming electron having to occupy the high-energy lowest unoccupied molecular orbital (LUMO) in the ground state, it can occupy the lower energy singly occupied molecular orbital (SOMO) (formerly the highest unoccupied molecular orbital [HOMO]) in the excited state. Similarly, to act as a reductant (release an electron), it requires less energy for this process in the excited state than the ground state as one electron has already been promoted to the higher SOMO (former LUMO).
Fig. 5 (a) Frontier orbital occupancy diagram showing the difference between the ground state and its lowest triplet excited state, illustrating that the triplet state is both a stronger reductant and oxidant. HOMO = highest occupied molecular orbital; LUMO = lowest unoccupied molecular orbital; SOMO = singly occupied molecular orbital. (b) Example of rose bengal (RB2−), showing that the potential for reduction of RB2− (i.e., RB2− as oxidant) moves from −0.54 to +1.23 V upon excitation to its triplet state, 3RB2−. Similarly, the potential for oxidation of RB (i.e., RB2− as reductant) moves from +1.33 to −0.44 V upon excitation to 3RB2−. RB2− is thus a better oxidant and better reductant in its triplet state than in its ground state, and the difference in potential comes from the triplet energy, ET/F = 1.77 V.
A concrete example is shown in Fig. 5b for the case of rose bengal dianion (RB2−). RB2− in its triplet state has been shown to act as both a reductant and an oxidant.93 The potential associated with RB2− as a reductant, E°(RB˙−/RB2−), decreases by 1.77 V (from 1.33 to −0.44 V SHE) upon excitation to its triplet state, 3RB2−.93 The potential associated with RB2− as an oxidant, E°(RB2−/RB˙3−), increases by 1.77 V (from −0.54 to 1.23 V SHE).93 The value 1.77 V is the triplet energy converted to potential, ET(RB2−)/F = 1.77 V, and the reason this is combined with the ground state potential is discussed further in the next section. Note that RB2− in its ground state exhibits a window of redox stability between −0.54 and 1.33 V SHE, but has no such window in its triplet state. Triplet state RB2− is thermodynamically unstable with respect to oxidation above −0.44 V SHE and to reduction below 1.23 V SHE. Between these values, 3RB is thermodynamically unstable with respect to both processes, and can thus act as both an oxidant and a reductant.
How strong are these 3CDOM* reductants? Some information potentially comes from Krogh who examined the photoreduction of a suite of halogenated compounds sensitized by CDOM.104 CCl4 (E°(CCl4/CCl3˙,Cl−) ≈ −0.1 V)105 underwent facile photoreduction sensitized by Christina River water (18 mgC L−1) exposed to 310 nm radiation. This makes sense given that CCl4 is thermodynamically easier to reduce than O2 (−0.18 V).94 Importantly, however, tetrachloroethylene (PCE) (E°(PCE/C2Cl3˙,Cl−) ≈ −0.60 V)105 was not reduced under the same conditions. This gives an effective oxidation potential of the photoreductants produced by 310 nm radiation between the reduction potentials of O2 (−0.18) and PCE (−0.60 V SHE).
Returning to the topic of 3CDOM* as an oxidant, the excited state reduction potential of 3CDOM* (E°*(3S*/S−˙)) is a critical value that determines not only the thermodynamics, but also the kinetics of its electron transfer reactions.105 The connection between the thermodynamics and kinetics of electron transfer are discussed in the following section. As with all other parameters involving DOM, there is no single value for E°*(3S*/S−˙), but rather a distribution. It is important to realize that the excited state potential is a sum of the ground state potential and the excited state energy, divided by the Faraday constant (F = 96.485 kJ V−1) to convert from energy to potential (eqn (5)).
This is shown visually in Fig. 6 for the half-wave reduction of an example aromatic ketone, 3-methoxyacetophenone (3MAP). While the ground-state reduction reaction is unfavorable in this example (E°(S/S−˙) = −1.50 V), the excited-state reaction is favorable (E°*(3S*/S−˙) = +1.64 V), and the difference between the two is the triplet energy of the ketone (ET = 303 kJ mol−1; ET/F = 3.14 V) (see Table 1). This means that compounds that are good oxidizers in the ground state (e.g., quinones) and compounds that have high triplet energies (e.g., ketones) are often powerful oxidants in their triplet state. We will return to this point below.
Fig. 6 Schematic representation of the half-wave reduction reactions of 3-methoxyacetophenone (3MAP) in its ground state and its triplet excited state, showing the relationship between the ground-state reduction potential (E°(S/S−˙) = −1.50 V vs. the standard hydrogen electrode, SHE), the excited-state reduction potential (E°*(3S*/S−˙) = +1.64 V) and the triplet energy (ET/F = 3.14 V).
where kd is the diffusion-controlled second-order rate constant for the formation of the precursor complex, Kd is the corresponding equilibrium constant, Z is the universal collision frequency factor according to transition-state theory (often taken to be 6 × 1011 s−1 for solution reactions105), and λ is the reorganization energy. The latter may be interpreted as the Gibbs free energy, related to bond and solvent reorganization, needed by the precursor complex to reach the equilibrium configuration of the successor complex. For organic redox reactions λ can vary over a broad range (20 to several hundreds of kJ mol−1).105 The reader should be aware that analogous relationships derived from Marcus' theory of electron transfer,105 or Sandros–Boltzmann type relationships108,109 could also be used.
Both ΔrG0et and λ determine the activation energy of the electron transfer process. A basic qualitative feature of eqn (6) (see the thin lines in Fig. 7) is that for highly exergonic electron transfer reactions, ket approaches the diffusion-controlled rate constant kd. For highly endergonic reactions, the denominator of eqn (6) simplifies and log ket decreases linearly with increasing ΔrG0et, with a slope of −(2.3 × RT)−1 (corresponding to −(5.7 kJ mol−1)−1 or −(0.059 eV)−1 at 25 °C). The Rehm–Weller, Marcus or Sandros–Boltzmann equations were found to adequately fit sets of second-order rate constants obtained in aqueous solution for the quenching of the excited triplet state of individual acceptor photosensitizers using series of electron donor quenchers.106,110 Moreover, in the case of electron-rich phenols as the electron donor quenchers, such triplet quenching rate constants106 were almost equal to the second-order rate constants measured for phototransformation.79 Thus, provided that each quenching event leads to transformation of the quencher, Rehm–Weller relationships of the type of eqn (6) could be used to predict the photooxidation rate constants of any organic contaminant in the aquatic environment.
Fig. 7 Rehm–Weller plots (gray thin curves) for a set of five functions having the same parameters (eqn (6); kd = 6.2 × 109 M−1 s−1; kd/(KdZ) = 0.1; λ = 65 kJ mol−1; values from a typical fit in ref. 106 were used) and an offset of 0, 5, 10, 15 and 20 kJ mol−1 (from left to right) for ΔrG0et. The arithmetic average of these curves is shown as a thick blue line, showing that a hypothetical equimolar mixture of five sensitizers would be expected to show a smoother transition from the diffusion-controlled plateau to the log-linear kinetic regime.
Unfortunately the distributions of one-electron reduction potentials in excited triplet CDOM are not known, and one has to rely on model calculations to predict the impact of such distributions on ksensTC. Let us assume that ket for the electron transfer reaction between TC and 3CDOM*i can be expressed by eqn (6) using constant values for kd and λ. In Fig. 7, Rehm–Weller plots are shown for five hypothetical 3CDOM*i having F × E°*(3CDOM*i/CDOM−˙i) that differ by 5 kJ mol−1 (ΔE°* = 52 mV). Assuming equal [3CDOM*i]ss for all chromophoric units, one can use the average of these five curves to represent ket for this group of five chromophores. The resulting curve (in the logarithmic representation, see thick line in Fig. 7) has a similar shape but a smoother transition between the diffusion-controlled plateau and the steep linear decrease compared to the single Rehm–Weller curves. We therefore refer to this as a pseudo-Rehm–Weller curve. With these considerations in mind, one can conclude that the determination of E°*(3S*/S−˙) for CDOM will remain fuzzy.
A possible approach to empirically determine the shape of the pseudo-Rehm–Weller curve for 3CDOM* consists of using a suite of probe compounds (PCs) with different (and exactly known) oxidation potentials and unit product yield for excited triplet state quenching, as recently proposed elsewhere.111 Thereby, it is suitable to define an “effective” concentration of 3CDOM* capable of oxidizing a given PC by dividing an experimentally determined ksensPC through the best guess for the maximum second-order rate constant for the electron-transfer reaction from the PC to 3CDOM* (e.g., ≈3 × 109 M−1 s−1, but an optimized value might be obtained from a consistent set of quenching data for model photosensitizers in aqueous solution). The “effective” concentration of 3CDOM* obviously decreases with increasing PC oxidation potential. In such a way, a function of [3CDOM*] vs. oxidation potential of PC can be constructed and used for the prediction of the “effective” concentration of 3CDOM*, and consequently of a pseudo-first-order transformation rate constant, for the transformation of any TC by 3CDOM* (provided that the one-electron oxidation potential of the TC is known).
After the kinetic considerations made in this section, one might ask why a corresponding analysis is not available for triplet energy transfer rate constants. Indeed, energy transfer kinetics can be treated in the frame of analogous models, which lead to equations of the same or similar form as those derived for electron transfer processes.112,113 Thereby, the difference in triplet energy between donor and acceptor assumes the same role as ΔrG0et in electron transfer processes. To our knowledge, there has been no application of these concepts to the photochemistry of CDOM to date, but this approach appears to be promising.
Another way to look at the question of the triplet state one-electron reduction potential of CDOM is to consider known values from well-defined compounds. Table 1 gives E°*(3S*/S−˙) values for a series of compounds that have structures that could plausibly be similar to constituents of CDOM. These E°*(3S*/S−˙) values come from the compounds triplet energies (ET) and ground state reduction potentials (E°(S/S−˙)) (eqn (5)), which are also listed in Table 1. These data are visualized in Fig. 8, with a plot of ETvs. E°*(3S*/S−˙).
Fig. 8 Triplet energies (ET, kJ mol−1) vs. triplet state one-electron reduction potentials (E°*(3S*/S−˙), V SHE) for a selection of 23 DOM-like model compounds and a selection of widely used sensitizers. The horizontal lines correspond to the triplet excited state energy of a typical diene energy transfer probe and the singlet state energy of O2. The vertical lines correspond to the one-electron oxidation potentials of TMP, E°(ArOH+˙/ArOH), and hydroxide at pH 7, E°′(OH˙/OH−). Selected estimates for 3CDOM* values from Zepp,9 Canonica,79,106 and Parker and Mitch48 are also shown. The data used for this figure is compiled in Table 1 along with the abbreviation definitions.
A word of caution about excited state redox potentials is in order. There are several difficulties associated with obtaining accurate (ground state) aqueous one-electron reduction potentials for the various compounds listed, which lead directly to difficulties in calculating accurate excited state reduction potentials.105 First, most of the compounds (excluding the quinones) are poorly behaved electrochemically, displaying irreversible redox couples, which necessitates some estimation of the true reduction potential. Second, the observed couples are also often not associated with pure one-electron transfers, but rather have an associated protonation process. For predicting the kinetics of electron transfer, the potential associated with just the one-electron process is needed. Third, the compounds are often poorly soluble, leading to the use of co-solvents or non-aqueous conditions, which can drastically alter the potentials. In compiling the data for Table 1, every effort was made to find values in water or water–alcohol mixtures. In the case of the polycyclic aromatic hydrocarbons, one value was only found in DMF and the others were from experiments in 75 : 25 dioxane : water mixtures. Additionally, for reduction potentials of the ketones and other carbonyl-containing compounds, values from the highest pH conditions were taken to get as close to the pure one-electron potential as possible. The values collected here differ somewhat from other compilations, for example the excellent compilation of Loeff, et al.114 All of this is to say that, while we believe the values in Table 1 are the best available, they should be used with some caution.
Caveats aside, it can be seen that this relatively small selection of compounds covers a wide range of E°*(3S*/S−˙), from 0.15 V for anthracene to 2.42 V for benzoquinone, suggesting that triplet CDOM oxidants will be found across the entire range of possible potentials in aqueous solution.
Further support for the importance of ketone- and aldehyde-containing sensitizers in CDOM comes from experiments in which the CDOM-sensitized photooxidation rates of trimethylphenol (TMP, a probe molecule for triplet oxidants) were significantly reduced following removal of the ketone and aldehyde functional groups by treatment of the CDOM samples with sodium borohydride.128 Similarly, Sharpless showed that borohydride-treated DOM formed 1O2 at lower rates than (but with the same quantum efficiency as) untreated DOM.10 In most cases, treatment with borohydride led to incomplete loss of photosensitization ability, suggesting that non-ketone and -aldehyde photosensitizers are also involved.10,128 Quinones, which are reduced by borohydride but quickly revert under aerated conditions, are candidates for a part of this other pool of photosensitizers. Flavones, which are not easily reduced by borohydride129 and have similar triplet state properties to aromatic ketones (Table 1 and Fig. 8), are also possible candidates.
A second observation concerns a potential noted in Fig. 8 as a vertical line at 1.22 V. The line corresponds to the one-electron oxidation potential for TMP, E°(ArOH+˙/ArOH),106 which is a popular probe molecule for 3CDOM*.130 One-electron transfer reactions between TMP and any of the triplets to the right of this line are exergonic. This does not necessarily forbid reactions between TMP and the triplets with E°*(3S*/S−˙) < 1.22 V, but rather means that strict one-electron transfer oxidations of TMP by these sensitizers will be thermodynamically unfavorable. The way around this problem for weaker oxidants is to oxidize TMP via hydrogen atom transfer or some other proton-coupled electron transfer (PCET) reaction that yields a phenoxy radical directly. For example, 2AN (E°*(3S*/S−˙) = 1.10) oxidizes TMP and one strong piece of evidence favoring PCET as the oxidation mechanism comes from the isotope effect on this reaction. Photooxidation of TMP by 2AN in D2O was 3.4 times slower than in H2O, which can be interpreted as a result of the phenolic O–H/D bond being broken in the rate-determining step.79 When Suwannee River fulvic acid or Fluka humic acid was used as the sensitizer for TMP photooxidation, isotope effects of only kH/kD = 1.1 ± 0.1 and 1.2 ± 0.1, respectively, were observed.79 This suggests that the majority of the oxidants responsible for the oxidation of TMP in these two DOM isolates did not undergo PCET, and the most obvious reason is that their E°*(3S*/S−˙) values were significantly greater than 1.22 V.
A final set of observations regards the sensitizers that are commonly used in laboratory studies. Perinaphthenone (PN), rose bengal (RB), and methylene blue (MB) are widely employed for generating 1O2, but all three have also been found to be triplet oxidants, with E°*(3S*/S−˙) ranging from 1.03 to 1.50 V (Table 1). Flavin-type photosensitizers, such as riboflavin (E°*(3S*/S−˙) = 1.88 V) and lumichrome (E°*(3S*/S−˙) = 1.91 V), are even stronger triplet oxidants, with potentials near the most oxidizing triplet ketone sensitizers. Near the far end of the spectrum is AQ2S, a powerful triplet oxidant (E°*(3S*/S−˙) = 2.28 V), which has been reported to give very low yields of either 1O2 or hydroxyl radical.125 Thus AQ2S might model some of the most oxidizing triplet states found in 3CDOM*, but is a considerably stronger oxidant than the average 3CDOM* species.
Despite the importance of 3CDOM* in the transformation of organic molecules, its study has lagged behind other important PPRI, especially 1O2, ˙OH, H2O2, and O2−˙. This is clearly because 3CDOM* is a complex mixture, and its complexity confounds both direct (i.e., spectroscopic) and indirect (i.e., molecular probe methods) methods to observe and/or quantify these states. While this certainly provides a challenge, the situation is far from hopeless. Some strategies for attaining a clearer picture of 3CDOM* are outlined below along with some of the most pressing research problems.
A critical strategy for studying a complex mixture like 3CDOM* is to use methods that integrate the disparate signals arising from the mixture's components and give a single signal that is more easily detected. The best and most accessible example is the use of 1O2 as a proxy for 3CDOM*. As mentioned above, quenching of triplet states by O2 to yield 1O2 is not quantitative, but it is the best universal triplet detection method of any available. Singlet oxygen formation quantum yields provide solid lower bounds for 3CDOM* formation quantum yields. Additionally, the steady-state concentrations of 1O2 and 3CDOM* must be within a factor of two of each other (when 0.25 < fΔ < 1; see eqn (4)).
While O2 quenching of 3CDOM* gives a picture of essentially all of the component triplets, using energy transfer quenchers of different energies is a clear way to probe the distribution of triplet energies in 3CDOM*. For example, HDA (sorbic acid), being a diene, is an excellent probe for quantifying the high energy triplet states capable of transferring energy to diene-containing contaminants such as domoic acid.48 At the moment, there is a large gap between the energy of 1O2 (94 kJ mol−1) and the diene quenchers that have been employed (ET ≈ 250 kJ mol−1; see Fig. 2), giving us only a rudimentary idea of the distribution (e.g., Fig. 3). While triplet energy acceptors with intermediate energies are certainly known, such as 1,3-cyclohexadiene (ET = 221 kJ mol−1), anthracene (178 kJ mol−1), ferrocene (167 kJ mol−1), azulene (163 kJ mol−1), and tetracene (123 kJ mol−1), they pose technical challenges including long wavelength absorbance, poor aqueous solubility, and/or susceptibility to photooxidation. All of these challenges can and will eventually be overcome.
Finally, the composition of 3CDOM* is clearly different for different sources of organic matter. In particular, there has been growing evidence of significant variability in the nature of 3CDOM in DOM of terrestrial origin (e.g., surface waters with input from soil organic matter) and of microbial origin (e.g., surface waters dominated by algal DOM or wastewater effluent DOM).134 The photochemistry of sulfa drugs serves to illustrate the point. Sulfa drugs are widespread contaminants in wastewater-impacted surface waters that have been the subject of several recent studies seeking to understand their phototransformation.45,46,73,74,135,136 In three separate studies, with three different sulfa drugs, significantly better photosensitization by autochthonous (algal) than allochthonous (terrestrial) CDOM has been observed. Chin observed that sulfadimethoxine undergoes enhanced degradation when sensitized by Pony Lake (Antarctica) fulvic acid (PLFA, a standard for microbially derived organic matter) and eutrophic lake water, but not terrestrial isolates (e.g., Suwannee River fulvic acid, SRFA).73,135 Arnold found that sulfamethoxazole degraded much more rapidly in the presence of effluent organic matter than with CDOM from other sources.45 Canonica found that sulfadiazine undergoes more rapid degradation when sensitized by PLFA than SRFA.74 In each case, convincing evidence that 3CDOM was responsible for the indirect transformation was obtained.
Why do autochthonous-dominated DOM samples (e.g., PLFA) seem to show increased reactivity compared to SRFA and other terrestrially derived organic matter samples? One possibility is that both PLFA and SRFA photooxidize compounds similarly, but SRFA contains many more antioxidants which repair some of the photooxidation damage (intermolecular or intramolecular) and slow down the macroscopic transformation rate. This idea definitely has support from studies showing the antioxidant properties of DOM in photoreactions.66,137,138 Another possibility is that the PLFA-derived 3CDOM* is a stronger oxidant (higher E°*(3S*/S−˙)) than the SFRA-derived 3CDOM*. The only way to answer the question is to determine the fundamental photophysical properties of both terrestrially derived and microbially (algal) derived CDOM.
It is clear that the study of 3CDOM* is both important and difficult. Despite the challenges, a fair amount of information about its reactivity, steady-state concentrations, and physical properties can already be inferred from existing data. Future studies, taking advantage of energy transfer-based probe methods (e.g., O2 and HDA) and oxidation-based probe methods (e.g., TMP) will only further our understanding of the scope, reactivity, and variability of 3CDOM*. There is an especially important link between 1O2 and 3CDOM* that makes 1O2 probe methods (both spectroscopic and reaction-based) particularly useful in this regard. With additional study, a clearer and more detailed picture of the components contributing to 3CDOM* and their reactivity patterns will come into view, which will in turn allow a better understanding of the role of 3CDOM* in the photochemical fate of contaminants and sunlight-driven biogeochemical processes.
We thank Caroline Davis, Paul Erickson, Elisabeth Janssen, Douglas Latch, Vivian Lin, Rachele Ossola, Kimberly Parker, Fernando Rosario-Ortiz, and Markus Schmitt for helpful feedback on the manuscript. We gratefully acknowledge funding from the Swiss National Science Foundation (Grant number 200021_156198).
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