Direct photolysis mechanism of pesticides in water

Abstract

Photodegradation is one of the most important abiotic transformations for pesticides in the aquatic environment, and the high energy of sunlight causes characteristic reactions such as bond scission, cyclization, and rearrangement, which are scarcely observed in hydrolysis and microbial degradation. This review deals with direct photolysis via excitation of a pesticide by absorbing natural or artificial sunlight in order to know its basic photochemistry, and indirect photolysis meaning either sensitization by dissolved organic matters or oxidation by reactive oxygen species is basically excluded. Several experimental approaches including spectroscopic techniques together with theoretical calculations are first discussed from the viewpoint of the reaction mechanisms in direct photolysis. Then, the typical photoreactions of pesticides are summarized by chemical classes and/or functional groups and discussed as far as possible in relation to their mechanisms.

Introduction

A pesticide applied to a field is generally distributed to edge-of-field water bodies via spray drift and runoff, and it dissipates by hydrolysis, photolysis, and microbial degradation, depending on its chemical structure, water characteristics, and bioavailability. The toxicological impacts of a pesticide and its major degradates on aquatic organisms are generally assessed by comparing their potential toxicity and predicted environmental concentrations in the US and EU registration.^1,2) When the chemical structure of a pesticide is markedly altered by photoreactions such as cyclization and rearrangement, the toxicity of a photoproduct is sometimes difficult to infer from the toxicophore of a pesticide, as exemplified by polychlorinated cyclodiene insecticides.³⁾ Photoproducts are generally less toxic to aquatic organisms than pesticides, but their enhanced toxicity, by about an order of magnitude, has been reported for bifenazate via hydrazine oxidation, fenhexamid via cyclization, and fipronil via sulfur oxidation/reduction or desulfinylation.⁴⁾ Direct photolysis of a pesticide proceeds via excited state(s) in the environment when it has a UV-visible absorption at >290 nm, its threshold wavelength is that of sunlight at the earth’s surface, and it is possible for a photoproduct with a structure very different from the original pesticide to be formed.^5–7) In contrast, the reaction of a pesticide with a photogenerated reactive oxygen species (ROS) in indirect photolysis takes place especially in natural water containing dissolved organic matter (DOM) and/or NO₃⁻, irrespective of its absorption profile.^5–7) The most reactive hydroxyl radical (∙OH) oxidizes an alkyl group and aromatic ring in a relatively nonselective manner.^6,7) The singlet oxygen (¹O₂) reacts with an electron-rich unsaturated moiety to give hydroperoxide, 1,2-dioxetane or endoperoxide, and it also participates in the sulfur oxidation and oxygenation of heterocycles.^8,9)

The excited states of pesticides via direct absorption of light may be quenched by DOM abundant in natural waters, and at the same time, DOM may generate ROS, causing the oxidation of pesticides. Therefore, the limited contribution of direct photolysis has been reported in sunlit natural water, with the exception of some pesticide classes¹⁰⁾; however, the knowledge of direct photolysis is indispensable for evaluating the basic photochemistry of pesticides in the aquatic environment. In this review, we first discuss several approaches to investigating the reaction mechanisms in direct photolysis. The photodegradation behavior of pesticides mainly in water has been examined through a survey of the literature and EFSA regulatory reports,⁴⁾ while keeping the solar radiation at the earth’s surface in mind. The mechanisms proposed for the typical reactions are discussed by referring to spectroscopic, theoretical, and product analyses, and, then, the general scope of aqueous photolysis is summarized for each pesticide class and/or functional group. Finally, an overview summary is provided, including issues that should be kept in mind for elaborating on the degradation pathways of pesticide in direct photolysis.

1. Experimental Approach

1.1. Absorption and emission spectra

The profiles of absorption and emission spectra should be examined first to best understand the excited state(s) of a pesticide relevant to its photoreactions. Upon the absorption of light, a pesticide molecule in the ground state (S₀) is excited with a certain probability to its excited singlet states (S*), and usually that in the lowest one (S₁) undergoes further processes, as shown in Fig. 1.^7,11) The contribution of direct photolysis can be easily evaluated by the extent to which the absorption and emission spectra of a pesticide overlap. S₁ is deactivated to S₀ through nonradiative internal conversion (IC) or emission of fluorescence (Fl), and partially undergoes further reaction(s) to produce photoproducts. Increasing solvent polarity generally causes hypsochromic (blue) shift of an absorption peak due to n,π* transition but bathochromic (red) shift for π,π* transition, which helps to characterize the electronic transition involved in photolysis. Since many pesticides have heavy atoms such as halogen, a spin-forbidden intersystem crossing (ISC) from S₁ to the excited triplet state (T*; T₁, lowest) may favorably proceed, T₁ then undergoes photoreaction(s) or is deactivated to S₀ through ISC or by emitting phosphorescence (Ph). The energy levels of S₁ (E_S) and T₁ (E_T) can be conveniently estimated from the Fl and Ph spectra, and they are useful for evaluating either sensitization or quenching via energy transfer toward a pesticide.⁷⁾ Furthermore, the pK_a values not only in S₀ but also in S₁ help us to understand the photochemistry of a pesticide with a dissociative functional group. The pK_a value in S₁ can be estimated by the electronic transitions in molecular and dissociated forms, based on the Förster cycle¹²⁾; pK_a (S₁)=pK_a (S₀)+0.625Δν/T. Δν is the difference in a frequency (cm⁻¹) of “0–0 transition” between two forms, estimated from the crossing point of absorption and Fl spectra, and T is an absolute temperature (K). The pK_a (S₁) value was estimated to be higher by 0.4–2.0 than pK_a (S₀) for the several sulfonylurea herbicides.^13,14)

Fig. 1. Simplified state energy diagram. S₀, ground singlet state; S₁, excited singlet state; T₁, excited triplet state; IC, internal conversion; ISC, intersystem crossing; Fl, fluorescence; Ph, phosphorescence.

1.2. Steady-state photolysis

Many photolysis studies of pesticides have been conducted in organic solvents because of their low water solubility, and a mercury lamp having strong line-shaped emission at <290 nm is conveniently utilized to enhance their photochemical reactivity.^5–7) The above experimental conditions, different from the natural aquatic environment, may result in different photochemical profiles, that is photolysis rates and photoproducts. For example, mecoprop-P mainly underwent either photo-induced substitution of 4-Cl by OH at a pH of 5.5 or photo-Claisen rearrangement at a pH of 2.2 by UV irradiation at 254 nm, while the cleavage of ether to form 4-chloro-2-methylphenol was the sole photoreaction in pure water under sunlight.¹⁵⁾ The OECD 316 guideline prescribes that a pesticide solution in sterile and buffered pure water should be exposed under air to natural sunlight or artificial light simulating sunlight emission from a filtered Xe arc lamp.¹⁶⁾ The usage of a radio-labelled pesticide, generally with ¹⁴C, is desirable to quantify each photoproduct, and setting a dark control is necessary for evaluating the contribution of hydrolysis. Unless a satisfactory material balance is achieved by setting a general volatile trap, such as an alkaline solution, a specific trap to recover a target volatile had better be equipped, as reported for thiamethoxam, to collect carbonyl sulfide.¹⁷⁾

A sufficient amount of a photoproduct for its chemical identification is not usually obtained in aqueous photolysis of a pesticide due to its low water solubility. Recent technical advances in HPLC quadrupole time-of-flight mass spectrometry (QTOF-LC-MS), including its high mass accuracy and high sensitivity in a full-scan mode, have enabled us to identify an unknown product in a trace amount.^18,19) Scant information on mass fragmentation rules for photoproducts sometimes becomes a barrier to focusing on the most probable structure from many candidates based on the elemental composition identified by QTOF-LC-MS, and so the additional information from LC-MS/MS and GC-MS after derivatization is very useful. However, the chromatographic and spectroscopic comparisons of a photoproduct with synthetic standards are the best approach in chemical identification.

Since molecular oxygen in the ground state (³O₂) not only quenches T* and radicals originating from a pesticide but also is activated to ¹O₂ via energy transfer from excited molecules in T*,^7,20) the effects of deaeration by N₂ or argon bubbling on a photolysis rate and product distribution give valuable information for considering a photolysis mechanism. Furthermore, the various sensitizers and quenchers with known E_S and E_T values are conveniently utilized to estimate the energy level of an excited state of a pesticide by examining their effects on a photolysis rate and emission spectra of the pesticide.¹¹⁾ Deuterated solvent, such as D₂O, as a reaction medium helps us in examining a reaction mechanism through MS analysis, as reported in the photo-induced aryl methyl oxidation of fenitrothion.²¹⁾

1.3. Laser flash photolysis (LFP)

The transient absorption of T*, ¹O₂, solvated electron (e⁻_aq), radical, or carbene can be measured in a nano- to microsecond scale by the flash photolysis technique using a nanosecond laser pulse with a pulsed Xe lamp as a probe light.²⁰⁾ This technique is very useful for characterizing these reactive species formed in photolysis, generally from the maximum wavelength (λ_max) and decay profiles of transient absorption. A pulsed beam at 266 nm (fourth harmonic) from a Nd:YAG laser is used in many studies, but the difference between its excitation wavelength and one of polychromatic sunlight at >290 nm should be kept in mind when evaluating the importance of an observed transient species in the direct photolysis of a pesticide. The LFP of triazine herbicides at a pH of 7 under argon and oxygen showed the photoejection of an electron followed by the formation of a superoxide anion radical (O₂⁻) only by excitation at 193 nm but no reaction at 248–308 nm, indicating that the observed reaction is of minor importance under sunlight.²²⁾ After the end of a pulse, multiple transient species may be formed and decayed in a different time window. The following quenchers are useful for characterizing each transient species in LFP: ³O₂ and N₂O for e⁻_aq; 1,3-cyclohexadiene and ³O₂ for T*; NaN₃, DABCO (1,4-diazabicyclo[2.2.2]octane), and β-carotene for ¹O₂; and Cl⁻ and nucleophiles for carbene.²⁰⁾ The reaction mechanism of a photo-induced bond cleavage can be examined by LFP. The different dechlorination mechanisms in the aqueous photolysis of monochlorinated phenols and anilines were confirmed by LFP at 266 nm; ring contraction to form ketene (λ_max, ca. 260 nm), photo-induced substitution of Cl by OH, and the formation of carbene for 2-, 3-, and 4-Cl derivatives, respectively.²³⁾ C- and S-centered radical species formed from phenylthioacetic acid via homolytic C–S and C–C bond cleavage were detected in acetonitrile by excitation at 266 nm under argon and oxygen.²⁴⁾ The LFP of cyanomethyl 1-naphthyl ether in acetonitrile at 308 nm from an excimer laser showed that a 1-naphthoxyl radical was generated via C–O bond cleavage from T*.²⁵⁾ The photo-induced cyclization of N-(2-halophenyl)pyridinecarboxamide to benzoxazole was examined by LFP at 266 nm, and the S_N(ET)Ar* mechanism to form the anion radical was strongly supported.²⁶⁾

LFP studies of pesticides using a Nd:YAG laser at 266 nm are summarized in Table 1. Sulfacetamide with a structure analogous to that of asulam showed three transient peaks at 330, 450, and 720 nm at pH 7 under N₂.⁵¹⁾ The peak at the longest wavelength was assigned to e⁻_aq by its disappearance in the presence of N₂O, and the quenching study showed the former two originating from oxygen-sensitive T₁ absorption. The T₁ absorption of cyanophos at 400 nm after the end of a pulse decayed to form a long-lived species appearing after 10 μsec, which was supposed to be the protonated amide intermediate from the product analysis.⁴²⁾ The benzyl radicals formed via the homolytic cleavage of the C–S bond were detected in 20% aqueous acetonitrile for ethiofencarb and thiobencarb after pulse end.³⁵⁾ Long-lived species insensitive to oxygen successively appeared at 320 nm and might be assigned from the product analysis to the corresponding carbanion (ethiofencarb) and carbocation (thiobencarb) via heterolytic cleavage of the C–S bond. The secondary photolysis of a main photoproduct has also been studied by LFP. Benzazimide from azinphos-methyl was photodegraded via T* with λ_max of 340 and 390 nm to anthranilic acid.⁵²⁾ The difference spectrum estimated by comparing the spectra under N₂ and oxygen in LFP showed the involvement of T* in the photolysis of 2-aminobenzimidazole from carbendazim.⁵³⁾ As described above, the LFP technique can shed light on the mechanisms of photolysis when the transient species participating in photoreactions are well characterized by their spectroscopic profiles and quenching studies.

Table 1. Transient species in laser flash photolysis (LFP) of pesticide ^a)

Pesticide	Medium b)	Transient c)	λmax (nm)	Lifetime d)	Quencher e)	Ref.
Chlorothalonil	n-heptane (N2)	T*	320	4.5	O2, An	27
	pH 7, aq. ACN (Ar)	T*	320	4	O2, IPA	28
2,4,5-T	H2O (Ar)	R (cation)/e−aq	500/720	—	—	29
Pyrethroids f)	ACN (O2)	R (Bz)	360–380	>10	—	30
Fenvalerate	ACN (–O2)	R (Bz)×2	340/440	1–3/37	—	31
Carbaryl	H2O (–O2)	R (Np)/T*/e−aq	340/410/725	—/2.9/—	—/O2/O2	32
Carbofuran	H2O (Ar)	R (Ph)	380, 420	1.5	O2	33
Propoxur	H2O (N2)	R (Ph)	380	5.0	—	34
Thiobencarb	aq. ACN (–O2)	R (Bz)	270, 318	—	O2	35
Ethiofencarb	aq. ACN (–O2)	R (Bz)	257, 320	10	O2	35
Asulam	H2O (–O2)	T*	320, 440	0.47	O2	36
Metobromuron	H2O (air)	C/R(x)	300, 405/300, 460	—/100	IPA, O2	37, 38
Chlorsulfuron	pH 3, aq. ACN (–O2)	T*	300–310	2.7	O2	39
Thifensulfurong)	pH 7 (–O2)	T*	320	24	O2	40
Cycloxydim	ACN (Ar)	R (iminyl)	280, 350, 500	ca. 110	—	41
Cyanofos	H2O (Ar)	T*/e−aq	280, 370, 500/720	5.9/—	O2, Aa, N2O	42
Fenthion	H2O (Ar)	T*/e−aq	320, 540/ca.700	17/—	O2, N2O, An	43
Coumaphos	aq. ACN (N2)	T*	390, 430	20	O2	44
Triadimefon	cyclohexane (Ar)	R (Ph)	400	—	—	45
/Triadimenol	aq. CH3OH (Ar)	R (Ph)	310	—	—	46
Fenarimol	CH3OH (air)	T*/NI	315/350–450	—	—	47
Imazaquin	pH 4	T*	380	11	O2	48
Metamitron	H2O (Ar)	T*×2	<400/560	<3/10	O2, MV2+	49
Thiabendazole	aq. ACN (N2)	T*	570	1	O2	50

^a) Excitation wavelength at 266 nm except for coumaphos (355 nm) from a Nd:YAG laser. ^b) Solvent (atmosphere). ACN, acetonitrile. ^c) NI, not characterized; T*, excited triplet state; R, radical (Bz, benzyl; Np, naphthoxyl; Ph, phenoxy); e⁻_aq, solvated electron; C, carbene; x, iminoquinone-O-oxide. ^d) in μsec. ^e) Aa, acrylamide; An, anthracene; IPA, isopropanol; MV²⁺, methyl viologen. ^f) Type-II having an α-cyanobenzyl moiety. ^g)Methyl ester.

1.4. Electron spin resonance (ESR)

Either homolytic bond cleavage or electron transfer in photolysis should generate radical species that can be theoretically measured by ESR. However, an extremely low concentration of a radical and its high reactivity with solvent and ³O₂ have limited the application of this technique to the aqueous photolysis of pesticides. Although the LFP of cycloxydim in acetonitrile indicated the formation of the corresponding iminyl radical via homolytic N–O bond cleavage, no ESR signal could be detected.⁴¹⁾ By lowering the measurement temperature, iminyl and anilino radicals, photoproduced, respectively, from oxime carbamates and ethyl N-phenylcarbamates, could be detected in oxygen-free inert organic solvents.^54,55) In contrast, a stable phenoxy radical formed in photolysis can be detected by ESR at room temperature.^56,57)

An unstable NO₂ radical formed in the photolysis of N-nitroguanidine can be derivatized with DMSO-d₆ to a more stable one for an ESR measurement,⁵⁸⁾ and similarly for the radicals from chlorobenzene in cyclohexane or acetonitrile.⁵⁹⁾ This is just the concept of “spin trapping” to prepare a stable spin adduct, frequently by using nitroso and nitrone derivatives such as 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and N-tert-butyl-α-phenylnitrone (PBN).⁶⁰⁾ The photo-induced homolytic cleavage of the N–S bond of sulfanilamide in D₂O⁶¹⁾ and the P–S bond of O-ethyl S-n-propyl phenylphosphonothioate in benzene⁶²⁾ generated amino and PhP∙(=O)OC₂H₅ radicals, respectively, which were successfully trapped by DMPO and PBN, respectively. The decarboxylation of fenvalerate under irradiation was examined by using nitrosodurene and PBN as spin traps, and the spin adducts of two benzyl radicals separated by HPLC were individually detected by ESR.⁶³⁾ The further derivatization of a spin adduct with fluorescamine makes its identification and quantitation possible by HPLC-Fl combined with MS, and this technique was successfully applied to the photolysis of type II pyrethroids having the α-cyano-3-phenoxybenzyl moiety.^30,64) Incidentally, ¹O₂ may be formed even in direct photolysis by energy transfer from a pesticide in T* to ³O₂,¹¹⁾ and it can participate in various oxidation reactions. The selective reaction of 2,2,6,6-tetramethyl-4-piperidone with ¹O₂ to form a nitroxide radical has been conveniently used for its detection by ESR.⁶⁵⁾ These examples of the application of ESR show that the usage of a spin trapping reagent suitable for C-, N-, or O-centered radicals is indispensable for identifying the radical formed in photolysis.

2. Theoretical Approach

The three-dimensional structure and electron configuration of an excited molecule together with relevant electronic transitions are very important for investigating reaction mechanisms in direct photolysis. Semi-empirical molecular orbital (MO) calculations with configuration interaction were first applied to pesticide photolysis due to computational limitations.^21,66,67) Recent progress in time-dependent density functional theory (TD-DFT) has made it possible to directly examine excited states by using an exchange-correlation functional such as B3LYP and refined ab initio basis sets, such as 6-311++G(d,p), within reasonable precision.⁶⁸⁾ Not only electronic transitions with E_S and E_T values relevant to direct photolysis^50,69–71) but also three-dimensional structures in excited states and of reactive intermediates^41,70–72) can be examined using TD-DFT calculations. Since the calculations generally suppose an isolated molecule in a vacuum, a solvation effect has been taken into account in some cases, indirectly by using a polarized continuum^28,33,50) or Onsager’s cavity model,^70,73) or directly by introducing water molecules into the calculations.⁴⁹⁾

A bond liable to be photocleaved^53,73,74) together with the preference of a homolytic or heterolytic mechanism^33,75) can be conveniently evaluated by its dissociation energy. Electronic excitation generally changes the electronic overlap population between two atoms participating in bond cleavage or formation, and the changes can be used as a photochemical index. These methods using MO calculations were applied to explain either the C–O bond cleavage of pyrethroid⁷⁶⁾ or the C–C bond formation in electrocyclic reactions.^77,78) The TD-DFT calculations qualitatively predicted the favorable N–S bond cleavage in the sulfonylurea bridge of chlorsulfuron⁷⁹⁾ and photocyclization of styryl oxazoles.⁸⁰⁾

The energy diagrams along supposed reaction pathways are very useful for discovering the most probable reaction mechanism. The homolytic cleavage of the N–S bond in acibenzolar-S-methyl was found energetically more favorable than that of the S–C(=O) bond,⁷²⁾ and the deamination of metamitron was likely to proceed by the reaction of its enol form in T₁ with ³O₂.⁴⁹⁾ Incidentally, reaction paths are not always the same between ground and excited states, and the decay of S* in the Franck-Condon (FC) region is considered to occur not at an avoided crossing minimum but at a conical intersection (CI).⁸¹⁾ For example, butadiene in S₁ was found to undergo either cis–trans isomerization or electrocyclization via twisted structure about the C₂–C₃ bond at the CI. The TD-DFT calculations are insufficient for evaluating these processes, and more detailed methods, such as CASSCF (complete active space self-consistent field), are applied to optimize the molecular geometry at the CI. The stepwise photoreactions of the cis-trans isomerization of stilbene and the intramolecular cyclization of the cis-isomer to 4a,4b-dihydrophenanthrene were examined by spin-flop DFT and CASSCF calculations, which identified the twisted-pyramidalized CI point for isomerization and the other CI lying on the cyclization path.⁸²⁾ The S₀→S₂ excitation weakening the amide linkage is dominant for propanil, but the facilitated decay of S₂ to S₁ via the CI in the FC region accounts for its photostability.⁶⁹⁾ The isomer-dependent photolysis of tetrazoline oxime ether fungicide in water was successfully explained by the TD-DFT and CASSCF calculations of its lower-energy conformers.⁷¹⁾ Z-isomers were mainly isomerized from either S₁ via the twisted structure around the C=N bond at the CI or T₁, while the N–O bond cleavage from S₁ was predominant for E-isomers. The mechanism of a photo-induced structural rearrangement can also be examined by CASSCF calculations. The photo-induced valence isomerization of pyridine to Dewar pyridine, 2-azabicyclo[2.2.0]hexa-2,5-diene, was most likely to proceed from S₁.⁸³⁾ CASSCF calculations suggested the direct conversion of isoxazole to oxazole from S₁ via the CI and the ring contraction–expansion pathway via azirin was energetically less favorable.⁸⁴⁾

As shown by many previous applications, the computational methods are found to be a powerful tool for scrutinizing photolysis mechanisms of pesticides. First, they are useful for primarily obtaining the character of its electronic transition and information on the susceptibility of a pesticide to direct photolysis under sunlight. Secondly, the examination of reaction pathways is feasible by the theoretical approach, but it should be noted that the very sophisticated method including CASSCF is necessary to identify a transition state and CI point. Furthermore, it is noted that the obtained results depend on the computational methodology and basis sets chosen, and the effect of a reaction medium should be carefully taken into account.

3. Photochemical Reaction Mechanisms

Since direct photolysis in the aquatic environment is discussed, the reactions of a pesticide in water or aqueous organic solvents by exposure to natural or artificial sunlight at >290 nm are basically considered. Photo-induced cleavage of a specific bond, intramolecular bond formation, rearrangement and oxidation/reduction are summarized for representative pesticides on either a chemical class or a functional moiety basis.

3.1. Photo-induced bond cleavage

3.1.1. Carboxylic acid derivatives and carbamates

The excited states of an aryl–C(=O)X (X=O, S, N) moiety generally have π,π* character, and either α- or β-cleavage to the carbonyl group proceeds, depending on their chemical structure and the irradiation wavelength.⁸⁵⁾ The ester cleavage of pyrethroid by irradiation generally proceeds at either the α- or β-bond to C=O, judging from their identified photoproducts.⁸⁶⁾ Two benzyl radicals were formed from the type II pyrethroids through β-cleavage of the ester linkage followed by the release of carbon dioxide, and these radicals were confirmed by ESR^63,64) and LFP.³⁰⁾ The recombination of these radicals between the benzyl carbons resulted in the formation of a decarboxylated derivative, while the electron delocalization to nitrogen of the α-cyanobenzyl radical additionally gave the amide derivative as a recombination product of esfenvalerate.⁴⁾ Decarboxylation via α-cleavage is the main photoreaction of acifluorfen. The homolytic mechanism via S₁ was proposed in acetonitrile,⁸⁷⁾ while the heterolytic mechanism via short-lived T₁ was considered in water.⁸⁸⁾ Acaricide cyflumetofen was rapidly photodegraded to the decarboxylated derivative with carboxylation at the 2-position of the 4-tert-butylphenyl ring,⁴⁾ which may be accounted for by the successive homolytic α-cleavage and rearrangement. The photolysis of acibenzolar-S-methyl in acetonitrile produced the corresponding benzaldehyde and methanesulfonic acid,⁷²⁾ suggesting the α-cleavage of the thioester moiety.

Aryl amides undergo the photo-Fries type α-cleavage of an N–C(=O) bond by irradiation at <290 nm;^85,89) however, this cleavage is generally a minor pathway under sunlight. In contrast, the amide bond of niclosamide was efficiently cleaved under sunlight by effective light absorption up to 450 nm due to the electron-withdrawing substituents introduced to the phenyl rings.⁹⁰⁾ The faster degradation at lower pH implies that intramolecular hydrogen bonding may participate in this bond scission. The β-cleavage of the C–NC(=O) bond was reported in the aqueous photolysis of fluopicolide, isofetamid, propyzamide, and zoxamide,⁴⁾ and Norrish type II β- or γ-hydrogen abstractions by the excited carbonyl oxygen might finally result in the formation of amide in an enol form (Fig. 2a). The different β-cleavage of the O–CC(=O) bond was reported in sunlight photolysis of naproanilide,⁹¹⁾ and the other study in air showed propenanilide and 2-hydroxypropananilide as the main photoproducts.⁹²⁾ The photo-induced cleavage of the C–O-aryl bond, followed by fragmentation or reaction with ³O₂, may account for their formation.

Fig. 2. Photoreaction mechanisms. a) Norrish type II reaction. R₁=alkyl and aryl, R₂ and R₃=H, alkyl, and aryl. b) Cyclization. Ox, oxidant. c) S_N(ET)Ar* mechanism. R=aryl. d) Photo-Claisen rearrangement: X=O; R=aryl, allyl, alkyl, and benzyl. Photo-Fries rearrangement: X=O, NH; R=acyl. [ ] means a solvent cage.

Incidentally, the photolysis of sulfonamides results in a homolytic α-cleavage at either bond to SO₂, but with the N–S bond scission being major.^93,94) The aqueous photolysis of asulam produced sulfanilic acid as the main product by the N–S bond cleavage, together with p-formylaminoaniline and a carbamate derivative of 1,4-benzenediamine.⁴⁾ The C–S bond cleavage via T* might lead to the formation of the latter two products, as supported by LFP³⁶⁾ and semi-empirical INDO calculations.⁶⁷⁾ The triazolopyrimidine herbicides such as florasulam and pyroxsulam mainly underwent N–S bond cleavage, while the C–S bond was more favorably cleaved for penoxsulam.⁴⁾

The O–C(=O) bond of O-aryl N-alkylcarbamate is homolytically cleaved in aqueous photolysis. The LFP study of carbaryl at 266 nm confirmed the formation of a 1-naphthoxyl radical via S*, and its steady-state photolysis at >280 nm produced 1-naphthol.³²⁾ DFT calculations on the photolysis of carbofuran indicated that homolytic cleavage is energetically more favorable than heterolytic cleavage.³³⁾ A similar photoreaction was also reported for pirimicarb.⁴⁾ The N–C(=O) bond of O-alkyl N-arylcarbamate is cleaved through S₁⁹⁵⁾ and the corresponding anilino radical has been detected by LFP,⁹⁶⁾ but mostly by irradiation at <290 nm. When the phenyl ring is substituted with an electron-donating group, the N-phenyl bond is favorably cleaved,⁹⁷⁾ as reported for diethofencarb.⁴⁾

3.1.2. Urea derivatives

Urea linkage is basically resistant to photolysis at >290 nm, as reported for various phenylurea herbicides.⁹⁸⁾ However, the sunlight photolysis of thiourea insecticide diafenthiuron efficiently produced the corresponding carbodiimide.⁹⁹⁾ The accelerated photolysis in water under oxygen with its rate reduced by isopropanol suggested the involvement of ∙OH. Since the dye-sensitized oxidation of the phenylthiourea derivative produced urea,¹⁰⁰⁾ this insecticide’s reaction with ¹O₂ was unlikely. Desulfurization was also reported for prothioconazole,⁴⁾ probably via an oxygen-independent process, judging from the photochemistry of 1,2,4-triazoline-3-thione.¹⁰¹⁾

The photolysis of diflubenzuron and dichlorbenzuron in methanol at 300 nm resulted in C(=O)N–C(=O)N bond cleavage, and a free radical mechanism was denied by MS detection of the corresponding isocyanate and benzamide derivatives.¹⁰²⁾ A six-membered transition state of the benzoylurea linkage was proposed for the Norrish type II abstraction of aniline hydrogen by the excited benzoyl oxygen followed by disproportionation. In contrast, the radical mechanism via the homolytic C–N bond cleavage was proposed for chlorbenzuron from the solvent effect on the photolysis rate.¹⁰³⁾ Irrespective of the mechanism, the C–N bond cleavage is the main pathway in the aqueous photolysis of flufenoxuron⁴⁾ and hexaflumuron.¹⁰⁴⁾

Sulfonylurea herbicides are susceptible to acid hydrolysis, but many of them are resistant to direct photolysis unless they have an absorption at >290 nm due to the substituted arylsulfonamide moiety. Irradiation of chlorsulfuron at 254 nm with the triazine moiety being mainly excited resulted in a bond scission in the sulfonylurea bridge, while the excitation of the benzene moiety at 280 nm caused dechlorination.³⁹⁾ The slower aqueous photolysis in the presence of triplet quenchers and ³O₂ showed the involvement of T* for imazosulfuron,¹⁰⁵⁾ thifensulfuron-methyl,¹⁰⁶⁾ and triasulfuron.¹⁰⁶⁾ The additional involvement of S* was suggested for thifensulfuron-methyl, since the oxygen quenching rate estimated from the quantum efficiency of its steady-state photolysis was much lower than that from triplet decay in its LFP.⁴⁰⁾ The photo-induced hydrolysis of the sulfonylurea bridge is most likely accounted for by higher pK_a(S₁) than pK_a(S₀), as estimated for cinosulfuron,¹⁴⁾ iodosulfuron,¹³⁾ metsulfuron-methyl,¹⁰⁷⁾ thifensulfuron-methyl,⁴⁰⁾ and triasulfuron.¹⁴⁾ The main formation of the urea derivatives showed the N–S bond cleavage in the aqueous photolysis of chlorimuron-ethyl,¹⁰⁸⁾ iodosulfuron¹³⁾ and thifensulfuron-methyl,^4,106,109) also as supported by the detection of a sulfonic acid derivative of thifensulfuron-methyl.⁴⁰⁾ The cleavage of either the C–S or N–S bond was reported for cinosulfuron,^14,110) foramsulfuron,⁴⁾ imazosulfuron,¹¹¹⁾ sulfosulfuron,⁴⁾ and triasulfuron,^4,110) with the former cleavage more favorable, in most cases. The C–S cleavage was supported by the detection of corresponding sulfamic acid derivatives. Less formation of methyl benzoate than its 2-hydroxy derivative in the aqueous photolysis of metsulfuron-methyl indicated that both heterolytic and homolytic C–S bond cleavage had occurred, but with the former preferable.¹⁰⁷⁾

3.1.3. Ethers

Phenoxyacetic herbicides generally undergo ether cleavage to form the corresponding phenol, as reported for 2,4-D,¹¹²⁾ 2,4,5-T,¹¹³⁾ MCPA⁴⁾ and mecoprop-P¹⁵⁾; diaryl ether linkages in clodinafop-propargyl, haloxyfop-P, and fenoxaprop-P-ethyl were also cleaved.⁴⁾ The ether cleavage to produce 4-chlorophenol was one of the main photoreactions of triadimefon.¹¹⁴⁾ Its maximum Fl at 420 nm originating from n–π* transition with a much weaker one at 330 nm from π, π* indicated the fast IC from S₂ (π, π*; phenyl) to S₁ (n, π*; C=O).⁴⁵⁾ LFP studies of triadimefon at 266 nm showed the generation of a phenoxy radical at λ_max of 400 nm in cyclohexane,⁴⁵⁾ while the new transient at λ_max of 300 nm appeared by increasing the water content in aqueous methanol.⁴⁶⁾ Therefore, heterolytic C–O cleavage to form phenoxide is the most likely reaction mechanism in water. The other pesticides, etofenprox, pyridalyl, and pyriproxyfen, also showed ether cleavage in aqueous photolysis.⁴⁾ The formation of 4-tert-butylstyrene as the major photoproduct of fenazaquin⁴⁾ may indicate the Norrish type II abstraction of a benzyl hydrogen by nitrogen at the 3-position of the excited quinazolyl ring, followed by disproportionation.

A radical process via S₁ has been proposed for the photolysis of diphenyl ethers in alcohols, and the electronic character of the substituents in each ring is likely to control which C–O bond is favorably cleaved.¹¹⁵⁾ The diphenyl ether C–O linkages of acifluorfen,⁸⁸⁾ bifenox,⁴⁾ nitrofen,¹¹⁶⁾ and cyhalofop-butyl¹¹⁷⁾ were photocleaved at either 254 nm or >290 nm in water. The photonucleophilic mechanism was proposed for nitrofen, since the photolysis rate increased at an alkaline pH and the addition of a cyanide or bromide ion produced the corresponding 2,4-dichlorobenzene derivative.¹¹⁸⁾ Although several energy quenching and radical scavenging experiments of acifluorfen suggested homolytic C–O cleavage from S₁ in acetonitrile,⁸⁷⁾ photo-induced base-catalyzed ether cleavage was proposed instead in water.⁸⁸⁾ Therefore, the reaction mechanism seems to be dependent on the solvent and the pH. The ether linkages of strobilurin fungicides azoxystrobin, dimoxystrobin, and mandestrobin were also photocleaved.⁴⁾ In the case of triclosan rearranging at 300 nm to the diphenyl derivative, a different biradical mechanism was proposed via one-electron reduction, followed by the release of Cl⁻.⁷⁵⁾

Cyclohexene oxime herbicides such as alloxydim¹¹⁹⁾ generally undergo photo-induced N–O cleavage to form the corresponding imines. The succeeding hydrolysis produced the ketone derivative in the case of tralkoxydim.⁴⁾ The homolytic N–O cleavage to form an iminyl radical was suggested by the LFP study of cycloxydim at 266 nm⁴¹⁾ and the increased reaction rate of sethoxydim in acetonitrile over that in water.¹²⁰⁾ In the case of aldoxime ethers such as fenpyroximate^4,121) and oxamyl,¹²²⁾ the corresponding cyanides were formed via N–O cleavage. Incidentally, pyraclostrobin underwent N-demethoxylation,⁴⁾ probably via a homolytic N–O bond scission from S₁.¹²³⁾

Either homolytic or heterolytic C–S bond cleavage is proposed in the aqueous photolysis of thioether pesticides. The main photoproducts of flutianil were the corresponding benzenesulfonic acid and (1,3-thiazolidin-2-ylidene)acetonitrile,⁴⁾ possibly formed via homolytic C–S bond cleavage followed by oxidation and hydrogen abstraction, respectively. In the case of pyridaben,⁴⁾ the heterolytic C–S bond cleavage was most likely caused by the formation of (4-tert-butylphenyl)methanol via a benzyl carbonium ion, as reported for thiobencarb.³⁵⁾ Both mechanisms may be operative in the aqueous photolysis of ethiofencarb by product analysis and LFP studies.³⁵⁾ The phenylpyrazole pesticide fipronil with the trifluoromethylsulfinyl group rapidly underwent desulfinylation in the aqueous photolysis⁴⁾; however, its reaction mechanism is still unclear.¹²⁴⁾

3.1.4. Organophosphorus (OP) esters

The photo-induced hydrolysis of a P–O-aryl bond to form the corresponding phenol is known as the typical reaction of OP pesticides.¹²⁵⁾ The involvement of S* has been proposed for cyanophos,⁴²⁾ and the weakening of the P–O-aryl bond by excitation may account for this reaction, as theoretically estimated for fenitrothion.²¹⁾ In the case of coumaphos upon exposure to UV light at 334 nm, either dimerization at the C₃–C₄ position of the coumarin moiety or oxidative ring cleavage was the main reaction instead of ester hydrolysis.¹²⁶⁾ Azinphos-methyl mainly underwent photo-induced C–N bond cleavage to produce benzazimide,¹²⁷⁾ which was further degraded via T₁ to anthranilic acid.⁵²⁾

3.1.5. Dehalogenation

3.1.5.1. Dechlorination

The photodegradation of aryl chlorides in various organic solvents has been extensively studied at 254 nm by applying ESR and LFP techniques.⁵⁹⁾ Photosubstitution and reductive dechlorination are proposed to proceed via the T* state in the heterolytic and homolytic C–Cl bond cleavage mechanisms, respectively; the former reaction is favorable in a protic solvent. In general, the heterolytic mechanism for forming hydroxylated derivatives is likely for 2- or 3-halogenated phenol, aniline and urea, while the formation of carbene has been proposed for 4-halogenoaromatics.¹²⁸⁾ In the latter case, either reductive dehalogenation or OH substitution is possible, depending on the solvent. The aqueous photolysis of chlorpropham¹²⁹⁾ and metoxuron¹³⁰⁾ produced the corresponding OH derivatives via heterolytic C–Cl bond cleavage. The effect of oxygen on the photolysis rate and product distribution together with the LFP study indicated the involvement of both S* and T* in the aqueous photolysis of chlorsulfuron at 280 nm, and either homolytic or heterolytic C–Cl bond cleavage was proposed.³⁹⁾ Reductive dechlorination was predominant for propanil,¹³¹⁾ fenhexamid,¹³²⁾ and norflurazon,¹³³⁾ and photo-induced substitution was additionally observed for metconazole.⁴⁾ Incidentally, chloroacetanilide herbicides, such as alachlor,¹³⁴⁾ metolachlor,¹³⁵⁾ and propisochlor,¹³⁶⁾ generally produced hydroxymethylcarbonyl derivatives by photonucleophilic substitution.

The photoreaction of polyhalogenated aromatics is more complicated. The 4-OH derivatives of diuron¹³⁷⁾ and linuron¹³⁸⁾ were mainly produced by irradiation at 365 nm, while the predominant formation of 3-OH derivatives was observed at 254 nm. Dicamba¹⁴¹⁾ and pentachlorophenol¹⁴⁰⁾ mainly underwent the photosubstitution of Cl by OH, while the radical mechanism was most likely for methoxychlor¹⁴¹⁾ and DDE.¹⁴²⁾ The LFP study with quenching by ³O₂ showed T* as a key intermediate in the photolysis of chlorothalonil, and reductive dechlorination was the main reaction in water.^27,28) Photosubstitution seems more favorable in the aqueous photolysis of pesticides that have a chlorinated aryloxy moiety. Phenoxyacetic herbicides, such as 2,4-D,¹⁴³⁾ 2,4,5-T,¹¹³⁾ and triclopyr,¹⁴⁴⁾ generally produced the corresponding 2-OH derivatives; however, reductive dechlorination at the 4-position was more favorable for dichlorprop.¹⁴³⁾

3.1.5.2. Other dehalogenation

The homolytic cleavage of a C–F bond is unlikely under sunlight due to its high bond dissociation energy around 110 kcal mol⁻¹, corresponding to the excitation energy at ca. 250 nm.¹¹⁾ Defluorination in an aromatic moiety has rarely been reported, with the exception of the photo-induced cyclization of teflubenzuron.⁴⁾ The photonucleophilic substitution of benzyl fluoride by water was reported for flubendiamide.⁴⁾ The trifluoromethyl group of fluometuron¹⁴⁵⁾ was photolytically hydrolyzed to COOH, most likely via the stepwise release of F⁻, as reported for flufenamic acid.¹⁴⁶⁾

Easier cleavage is expected for C–Br and C–I bonds with a lower dissociation energy of 50–70 kcal mol⁻¹.¹¹⁾ The photo-induced homolytic cleavage of a C–Br bond in the radical mechanism was proposed for bromoxynil in buffers^4,147) and deltamethrin in aqueous acetonitrile,¹⁴⁸⁾ and the corresponding debrominated products were identified. In contrast, the dihydroxy derivative of bromoxynil was mainly formed in the unbuffered system without any oxygen effect.¹⁴⁹⁾ In the aqueous photolysis of metobromuron in a buffer with a pH of 7, reductive debromination was more favorable than the formation of a 4-OH derivative,⁴⁾ while the latter product predominated in slow sunlight photodegradation in pure water.³⁷⁾ The LFP study at 266 nm indicated the formation of the corresponding carbene via C–Br cleavage^23,37,128); therefore, the proton concentration likely affects the product distribution of these herbicides. Both 4-H and 4-OH derivatives were formed from profenophos,¹⁵⁰⁾ and a similar mechanism might be operative. Either a homolytic or a heterolytic mechanism is possible for deiodination. Reductive deiodination was reported for flubendiamide and proquinazid.⁴⁾ In contrast, the heterolytic cleavage to form the OH-substituted derivative with the release of I⁻ dominated the aqueous photolysis of ioxynil,¹⁵¹⁾ and it was most likely for iodosulfuron.^4,13)

3.1.6. Denitration and deamination

Neo-nicotinoid insecticides have an N-nitroguanidine moiety susceptible to photolysis, and the corresponding imine by denitration is one of the main photoproducts of imidacloprid, clothianidin and thiamethoxam.⁴⁾ Either homolytic or heterolytic cleavage of the N–N bond has been proposed in the photolysis of N-nitroguanidine, and the solvent effect may control the mechanism. The former mechanism was supported by ESR observation of the NO₂ radical adduct,⁵⁸⁾ while the detection of NO₂⁻ by ion chromatography during sunlight aqueous photolysis suggested the latter mechanism.¹⁵²⁾ Furthermore, the stepwise reduction mechanism was proposed from the MS detection of an N-nitroso derivative in the aqueous photolysis of imidacloprid.¹⁵³⁾ Some pesticides that have a nitrophenyl moiety undergo the photo-induced nucleophilic substitution of NO₂ by OH. The aqueous photolysis of oxyfluorfen produced the corresponding phenol derivative as one of the main products.^4,154) Although DFT calculations suggested that the triplet process via σ-complex formed by attacking OH⁻ at the nitro-bearing carbon was favorable in the photo-induced hydrolysis of 4-nitroanisole,¹⁵⁵⁾ the quenching study in the photolysis of oxyfluorfen showed the involvement of neither excited triplet states nor radicals; therefore, the mechanism is still unclear.¹⁵⁶⁾ A similar photosubstitution is also known for parathion-methyl¹⁵⁷⁾ and the benzoic acid photoproduct of mesotrione.¹⁵⁸⁾

Deamination is the main photoreaction of metamitron in water with the oxidative opening of its hetero ring as a minor pathway.^4,159) Another steady-state photolysis study showed the concomitant formation of NO₂⁻ with deamination, but without any contribution of ¹O₂.⁴⁰⁾ The LFP study at 266 nm showed the involvement of two types of T₁, one of which would react with ³O₂, as supported by the TD-DFT calculations.⁴⁹⁾ Energy transfer from T₁ may result in the generation of ¹O₂, which is involved in the oxidative ring opening through endoperoxide.¹⁵⁹⁾ Deamination is also observed for metribuzin,⁴⁾ and the contribution of ³O₂ with the formation of NO₂⁻ suggests a similar reaction mechanism to that of metamitron.¹⁶⁰⁾ Photo-induced N–N bond cleavage was also reported for famoxadone and pymetrozine,⁴⁾ but the mechanism is unclear.

3.1.7. Hetero rings

Many kinds of 2-substituted 4,6-dialkylamino-1,3,5-triazines have been developed as herbicides. The direct photolysis of 2-chloro derivatives, and of 2-methylthio derivatives at slightly acidic pH, is feasible due to their weak absorption at >290 nm.¹⁶¹⁾ Atrazine, the representative former derivative, underwent photo-induced hydrolysis at the 2-position via T* by the addition-elimination mechanism, as supported by the TD-DFT calculations.¹⁶²⁾ In contrast, 2-methylthiotriazines were likely to either be hydrolyzed as above or degraded via homolytic C–S cleavage in S*, followed by hydrogen abstraction.¹⁶²⁾

Rapid direct photolysis of imazamox by using a Xe lamp mainly proceeded via the hydrolysis of the C=N bond of the imidazolinone ring,⁴⁾ while two N-alkenyl pyridinecarboximidamides having the C=C bond at different positions, probably formed via ring opening with decarbonylation, were estimated by MS analysis with exposure to UV light at >290 nm.^163,164) Irrespective of the primary photoproducts differently proposed, the herbicide was finally degraded to 2-carbamoyl or 2-carboxypyridine. Similar uncertainty in product identification was reported for other imidazolinone herbicides.^164–167) Further studies using authentic standards of photoproducts are necessary to determine their reaction pathways. In any case, both S* and T* are likely to be involved in the photolysis of these herbicides, based on LFP and quenching studies on imazethapyr.⁴⁸⁾

3.2. Intramolecular bond formation-cyclization

3.2.1. C–C bond formation

The electrocyclic ring closure of cis-stilbene proceeds from S₁ to the transient dihydrophenanthrene followed by oxidation to phenanthrene,^83,168) as shown in Fig. 2b. Similar cyclization can proceed for o-halostilbene¹⁶⁹⁾ and styryl hetero rings.^80,170) Diniconazole⁷⁸⁾ and uniconazole¹⁷¹⁾ were rapidly isomerized by irradiation to their Z-isomers which underwent this type of cyclization very efficiently between the phenyl C₂ and triazolyl C₅. The C–C(Cl) bond formation between two aromatic rings connected by various linkages has been reported in the aqueous photolysis of pesticides. Either the oxidative cyclization as observed for stilbene or a radical mechanism via energy or electron transfer¹⁷²⁾ is possible, but the mechanism has not been confirmed in most cases. The favorable orientation of two rings may be necessary for cyclization, as represented by the formation of the five-membered ring from diclofop-methyl,¹⁷³⁾ haloxyfop-P,⁴⁾ and fenarimol,¹⁷⁴⁾ or the six-membered ring from propiconazole,¹⁷⁵⁾ pyridaben⁴⁾ and quinoxyfen.⁴⁾ An eight-membered ring was formed in the main photoproduct of fluopyram.^4,176) Incidentally, the radical process via homolytic cleavage of the C–Cl bond was proposed for the cyclization of acetochlor¹⁷⁷⁾ and alachlor.¹³⁴⁾

3.2.2. C–O bond formation

The photo-induced attack of carbonyl oxygen at a Cl-bearing aryl carbon has been reported for several pesticides. A benzoxazole derivative was one of the main photoproducts of fenhexamid, with more rapid formation under an alkaline pH.^4,132) As compared to the photochemistry of N-(2-halophenyl)pyridinecarboxamide,²⁶⁾ cyclization is likely to start from an imidolate anion in S₁, and the oxygen radical formed via intramolecular one-electron transfer attacks the anionic 2-chlorophenyl moiety, followed by the release of Cl⁻ [S_N(ET)Ar* mechanism], as shown in Fig. 2c. Similar cyclization between the central carbonyl oxygen and 3-Cl-pyridin-2-yl ring to form a 1,4-oxazine derivative was shown for diamide insecticides chlorantraniliprole and cyantraniliprole in their aqueous photolysis.^4,178) A couple of benzoxazolone derivatives were efficiently photoproduced from oxadiazon at a hydrolytically stable pH of 5.⁴⁾ The photo-induced hydrolytic opening of the oxadiazolone ring produces an N-carboxyl derivative,¹⁷⁹⁾ whose carboxyl oxygen would attack the Cl-bearing phenyl carbon in a similar manner to that described above. The aqueous photolysis of sulcotrione produced a chromone derivative by cyclization between oxygen in a 1,3-cyclohexanedione moiety and a 2-Cl-benzoyl ring.¹⁸⁰⁾ The efficient reaction at acidic pH suggests that the intramolecular hydrogen bonding of the protonated β-diketone in an enol form with benzoyl oxygen makes the reaction sites closer for cyclization. A similar pH dependence of cyclization was reported for tembotrione.¹⁸¹⁾ Mesotrione having 2-NO₂ group did not undergo photocyclization because NO₂⁻ is a poorer leaving group than Cl⁻, and photo-induced hydrolysis was the main reaction instead to produce the benzoic acid derivative.¹⁵⁸⁾ The intramolecular nucleophilic attack of methoxy oxygen at the chloromethyl carbon was facilitated for metolachlor by sunlight to form a morpholin-3-one derivative, likely via carbonyl excitation.¹³⁵⁾ Although an example is rare, intramolecular S–C(Cl) bond formation was induced for prothioconazole by irradiation.⁴⁾ In the photolysis of N-(2-chlorophenyl)thioacetamide, the existence of Cl∙ confirmed by LFP but without the formation of the dechlorinated derivative suggested the S_RN1 mechanism including the intramolecular one-electron transfer from sulfur to the phenyl ring, followed by cyclization with the release of Cl∙.¹⁸²⁾ A similar mechanism is probable for this fungicide.

3.2.3. C–N bond formation

Dinitroaniline herbicides such as benfluralin, flumetralin, oryzalin and trifluralin were generally photodegraded to benzimidazole and/or its N-oxide.⁴⁾ From the photochemistry of N,N-disubstituted 2-nitroanilines,¹⁸³⁾ two mechanisms can be postulated: an excited nitro oxygen abstracts hydrogen from an N-alkyl moiety, and the resulting biradical is cyclized; oxygen transfers from the excited nitro group to the N-alkyl nitrogen, followed by cyclization. Chloridazon was first dimerized in aqueous photolysis via the intermolecular nucleophilic attack of the amino nitrogen at the Cl-bearing carbon, followed by the intramolecular cyclization between the remaining same sites,¹⁸⁴⁾ probably by the S_N(ET)Ar* mechanism as described in the previous section. Although the reaction mechanism is unclear, photo-induced cyclization was reported for fluazinam⁴⁾ to the carbazole derivative, and for teflubenzuron⁴⁾ between the aniline nitrogen and the 2-position of the benzoyl moiety.

3.3. Rearrangement

3.3.1. Isomerization

3.3.1.1. Geometrical isomerization

Geometrical isomerization around a C=C or C=N bond is one of the most popular photochemical reactions. Macrolide insecticides abamectin and emamectin were rapidly converted to their Z-isomers at the diene moiety under sunlight.⁴⁾ Rapid equilibrium to more stable Z-isomers was observed for diniconazole⁷⁸⁾ and uniconazole.¹⁷¹⁾ E-isomer is energetically preferable for azoxystrobin (E/Z=2/1), and the twisted structure around the C=C bond has been theoretically proposed for its CI point connecting S₁ and S₀.¹⁸⁵⁾ A similar E/Z isomerization was reported for other strobilurin fungicides dimoxystrobin, fluoxastrobin, kresoxim-methyl, picoxystrobin, and trifloxystrobin.⁴⁾ The photoisomerization from anti (E) to syn (Z) of the oxime ether clethodim¹⁸⁶⁾ and fenpyroximate^4,121) proceeded rapidly, while it was of minor importance for alloxydim¹¹⁹⁾ and sethoxydim¹²⁰⁾ due to a more favorable N–O bond cleavage. Incidentally, pyrethroids that have a cyclopropyl moiety are known to undergo cis–trans isomerization via the homolytic cleavage of the C₁–C₃ bond, followed by the recombination of the resulting biradical.^86,187)

3.3.1.2. Thiono–thiolo rearrangement

In their aqueous photolysis, several OP pesticides showed thiono-thiolo rearrangement, 1,3-migration in an O–P=S or S–P=O moiety.¹²⁵⁾ The migration of an alkyl group was observed for fenitrothion,¹²⁵⁾ profenofos,¹⁵⁰⁾ and quinalophos,¹⁸⁸⁾ while an aryl moiety migrated in coumaphos⁴⁴⁾ and fenthion.⁴³⁾ Although the reaction mechanism is not clear, the LFP of coumaphos at 355 nm proposed T₁ as an intermediate.⁴⁴⁾ The aqueous photolysis of fenthion at 300 nm mainly produced the S-phenyl isomer and corresponding phenol; however, it produced 4-chloro-2-methyl-1-(methylthio)benzene instead in the presence of Cl⁻, indicating σ aryl cation as a reactive intermediate.⁴³⁾ The heterolytic cleavage of an O–aryl bond from T₁ to form a σ aryl cation is likely, as reported for OP esters that have an electron-rich aromatic group.¹⁸⁹⁾

3.3.1.3. Ring systems

Photo-induced valence isomerization of a pesticide has been rarely reported. Acetamiprid, flupyradifurone and fluazifop-P-butyl having substituted pyridinyl rings were photochemically rearranged to the corresponding Dewar pyridine derivatives.⁴⁾ The photo-induced nucleophilic substitution of Cl by OH and ether cleavage should primarily proceed for the former two pesticides and the latter one, respectively. The subsequent valence isomerization would proceed via 2-hydroxy derivative similarly to 2-halopyridines,¹⁹⁰⁾ and its mechanism has been thoroughly examined by CASSCF calculations.⁸³⁾ In the case of the main hydrolysate from pyridate, the valence isomerization from pyridazine to pyrimidine was observed.⁴⁾ The rearrangement of a five-membered heterocycle is also known. Sunlight photolysis of isoxaben in water produced the corresponding oxazole via an azirin derivative as an intermediate.¹⁹¹⁾ CASSCF calculations of 3,5-dimethylisoxazole have shown its one-step barrierless isomerization to oxazole through CI⁸⁴⁾; this difference may be due to the neglect of a solvent effect. Recently, the aqueous photolysis of thifensulfuron-methyl⁴⁰⁾ has been revisited by the chromatographic comparison of photoproducts with authentic standards.¹⁰⁹⁾ The estimated chemical structure of a photoproduct formed via contraction of the sulfonylurea bridge was revised from N-(3-thienyl)-1,3,5-triazine-2-amine to the N-(2-thienyl) derivative, and photo-induced ring rearrangement was proposed. The slow photolysis of dichlobenil produced 4-Cl-2(3H)-benzoxazolone.⁴⁾ 1,2-Benzisoxazolinone, formed by the photo-hydrolysis of dichlobenil at both the C–Cl and C≡N bonds with their successive condensation, is likely to isomerize via stepwise free radical reactions.¹⁹²⁾

Incidentally, ring contraction is reported for some pesticides and degradates. Flumioxazin was rapidly hydrolyzed in water, and azetidin-2-one derivatives were efficiently produced at the same time in its aqueous photolysis.¹⁹³⁾ The mechanism reported in the photolysis of a serotonin receptor agonist Y-25130 is likely to explain this reaction.¹⁹⁴⁾ The spiro intermediate 2,9-dioxo-1-azaspiro[3.5]nona-5,7-diene is first formed via stepwise photo-induced O–C(=O) bond cleavage and intramolecular cyclization, and it is further degraded to ketene, followed by hydrolysis. Although a concrete mechanism is unclear, sunlight photolysis of bupirimate afforded 4-n-butylpyradolidine-3,5-dione,⁴⁾ possibly via contraction of the pyrimidyl ring. 2-Chlorinated phenol and aniline, possible photoproducts of some pesticides, produced cyclopentadiene derivatives in their aqueous photolysis, and ketene was proposed as the key intermediate for 2-chlorophenol, based on LFP.²³⁾

The aqueous photolysis of fenarimol produced a benzophenone derivative with the pyrimidyl ring migrating to one phenyl ring.⁴⁾ The photo-induced homolytic cleavage of the bond between the carbinol carbon and the pyrimidyl ring was proposed as an initial step^47,174); however, a concrete mechanism is unclear. Similar photo-induced migration was reported for flurprimidol⁴⁾ and the anti-histamine drug fexofenadine.¹⁹⁵⁾ The anilinopyrimidine fungicide cyprodinil was photolyzed in water via C–N bond cleavage to produce aniline derivatives by the Hofmann-Martius rearrangement.¹⁹⁶⁾

3.3.2. Photo-Claisen and photo-Fries rearrangements

In the photo-Claisen rearrangement, the C–O bond of aryl benzyl (or alkyl) ether is first cleaved homolytically, as shown in Fig. 2d. The resulting radicals are recombined to 2- and 4-substituted cyclohexadienones in a solvent cage followed by tautomerization to the corresponding phenols, while the escaped radicals abstract hydrogen. The radical process from S* has been suggested by quenching, ESR, and LFP studies.⁵⁷⁾ This rearrangement efficiently proceeded by irradiation at 254 nm for aryl alkyl ethers such as MCPA,¹⁹⁷⁾ cyhalofop-butyl,¹¹⁷⁾ and fenoxaprop-P-ethyl,¹⁹⁸⁾ but it was of minor importance under sunlight. In contrast, a higher yield of rearrangement, around 50% in total, was reported in the aqueous photolysis of napropamide at >290 nm,⁴⁾ which may be accounted for by its higher absorbance at longer wavelengths and the stability of 1-(N,N-diethylaminocarbonyl)ethyl radical. The rearrangement is the main photolysis pathway for aryl benzyl ethers via homolytic cleavage of an O-benzyl bond. Mandestrobin rearranged to the 2-benzylphenol derivative, followed by intramolecular cyclization between the resulting phenol oxygen and α-methoxybenzyl carbon.⁴⁾ In the case of pyraclostrobin, the attack of the benzyl radical formed via C–O bond cleavage at either the N₂ or C₄ atom of the pyrazolyl ring led to the formation of the rearranged products.⁴⁾ An acyl C–O or C–N bond can be first cleaved homolytically in the photo-Fries rearrangement (Fig. 2d). The aqueous photolysis of O-aryl N-methylcarbamates by UV irradiation at >220 nm produced the rearranged products and phenol via C–O-aryl bond cleavage forming phenoxy radicals;^33,34,199) however, the reaction was of minor importance under sunlight. In the sunlight photolysis of phenisopham having absorption at >290 nm, two 2- and one 4-substituted phenols were efficiently formed via the cleavage of the C–O-aryl bond.¹²⁹⁾ The photo-Fries rearrangement via the cleavage of an acyl C–N bond was reported for aromatic amides,⁸⁹⁾ alkyl N-arylcarbamates,⁹⁵⁾ and phenylurea herbicides,¹²⁸⁾ but only under exposure to UV light at <290 nm in organic solvents. Incidentally, oxazole derivatives were formed in the aqueous photolysis of cycloxydim^4,41) and sethoxydim;¹²⁰⁾ the Beckmann rearrangement, followed by the intramolecular reaction of carbocation with the enol oxygen, is one possible mechanism. Alternatively, in comparison to the photolysis of salicylaldoxime,²⁰⁰⁾ the photo-induced attack of the enol oxygen at the oxime nitrogen followed by rearrangement via azirine may be also a candidate mechanism.

3.4. Oxidation

One of the oxidation mechanisms in the direct photolysis of pesticide is the reaction of a radical with ³O₂ to finally produce oxidized products. The Norrish type II abstraction of an alkyl hydrogen by an excited carbonyl oxygen plays a role in forming the radical, as proposed for diuron¹³⁷⁾ and metobromuron.³⁷⁾ This mechanism would operate for the photooxidation of acequinocyl,⁴⁾ mepronil,²⁰¹⁾ and thiobencarb²⁰²⁾ in water. Hydrogen abstraction from an anilino nitrogen results in the delocalization of an electron to the 4-position of the phenyl ring, where the reaction with ³O₂ gives the corresponding quinone imide, in the aqueous photolysis of fenamidone⁴⁾ and sustar.²⁰³⁾ Photo-induced hydrogen abstraction by the adjacent nitro oxygen would initiate the aryl methyl oxidation of fenitrothion, as evidenced by ¹H-NMR and MS product analyses through its photolysis in D₂O-CD₃OD.²¹⁾ The aqueous photolysis of trifluralin was greatly retarded in the absence of oxygen.²⁰⁴⁾ The radical formed similarly as above is likely to react with ³O₂, leading to stepwise N-dealkylation, as observed for various dinitroaniline herbicides.⁴⁾ Oxidation at the 3-phenoxybenzyl carbon was the main photoreaction of etofenprox.⁴⁾ Either radical mechanism via hydrogen abstraction followed by reaction with ³O₂ or reaction with O₂⁻ after one-electron transfer from the pesticide was proposed.²⁰⁵⁾ The bicyclopropyl moiety of sedaxane was slowly oxidized to the corresponding 1,3-diol⁴⁾; however, its mechanism is unclear.

Oxidative desulfurization to oxon has been reported in the photolysis of many OP pesticides, but it is of minor importance in water.¹²⁵⁾ LFP and/or quenching studies have shown T* being involved in the P=S oxidation of cyanophos⁴²⁾ and parathion.¹²⁵⁾ Dye-sensitized photolysis in aqueous alcohol under oxygen did not give concrete evidence of ¹O₂ as a key reactant for OP pesticides.¹²⁵⁾ A radical species would be involved in oxidative desulfurization because hydrogen peroxide was produced through the aqueous photolysis of fenitrothion,²¹⁾ and either Fenton’s reagent²¹⁾ or a photo-generated peroxide radical¹²⁵⁾ produced the oxon of OP. The nitroso-oxon derivative was primarily formed in the photolysis of butamifos.²⁰⁶⁾ The UV and IR spectroscopies of butamifos showed the presence of a steric hindrance around NO₂ at the 2-position of the phenoxy group, strongly suggesting an intramolecular photoredox reaction between the P=S and NO₂ groups.

It should be noted that the excitation of a pesticide under air may generate ¹O₂ or O₂⁻ via energy or electron transfer and thus, various oxidative reactions can proceed at susceptible moieties in a pesticide molecule. Spectrophotometric analysis using N,N-dimethyl-4-nitrosoaniline/L-histidine and nitro blue tetrazolium in sunlight photolysis of quinalphos showed the generation of ¹O₂ and O₂⁻, which were quenched by sodium azide and superoxide dismutase, respectively.²⁰⁷⁾ The pH-dependent formation of these ROS was reported in the aqueous photolysis of oxolinic acid at >310 nm.²⁰⁸⁾ The existence of these ROS was also confirmed either by reaction with tetramethylethylene or by ESR in the photolysis of experimental herbicide LS82556²⁰⁹⁾ and diphenyl ethers.⁵⁶⁾

The photooxidation of a thioether moiety is one of the typical reactions of ¹O₂,⁹⁾ and the successive transformation to sulfoxide and sulfone was reported for some pesticides including OP’s.¹²⁵⁾ Several quenching studies²¹⁰⁾ with the application of a spin trapping technique⁶⁵⁾ in the aqueous photolysis of fenthion clearly showed the involvement of ¹O₂ in its oxidation to sulfoxide, and energy transfer from its T* to ³O₂ was proposed by LFP.⁴³⁾ A similar mechanism is most likely for S-oxidation of fenamiphos⁴⁾ and propaphos.¹²⁵⁾ Photo-induced S-oxidation to the corresponding sulfoxide was reported for cycloxydim⁴⁾ and sethoxydim.¹²⁰⁾ The accelerated photolysis by acetone but retardation in the presence of sodium azide strongly suggested that ¹O₂ generated by energy transfer from sethoxydim in T* oxidized the thioether sulfur.¹²⁰⁾ The rapid photolysis of carboxin produced sulfoxide as one of its major photoproducts, most likely via reaction with ¹O₂.^4,211) A sulfonic acid derivative as the major photoproduct of fipronil may imply that S-oxidation is a major pathway and the sulfone derivative is rapidly hydrolyzed;²¹²⁾ however, the involvement of ¹O₂ is not clear.

Oxidative ring cleavage via 1,2-dioxetane by reaction with ¹O₂ was reported for carboxin²¹¹⁾ and coumaphos,¹²⁶⁾ and endoperoxide was a likely intermediate in the photolysis of fuberidazole.^4,213) In the case of thiabendazole, 2-carbamoyl, 2-carboxy, and 2-hydroxybenzimidazole were detected as the main products.⁴⁾ Since the E_T value of thiabendazole as estimated by LFP and TD-DFT studies⁵⁰⁾ is high enough to generate ¹O₂ by energy transfer, the former two products are likely to be formed by the reaction with ¹O₂ via endoperoxide.⁹⁾ The detection of the latter product may indicate the homolytic cleavage of the C–C bond connecting two hetero rings.²¹⁴⁾ Although a reaction mechanism is unclear, a similar oxidation mechanism⁹⁾ is a possible candidate for the photo-induced ring cleavage of bupirimate, fludioxonil, and prochloraz.⁴⁾

3.5. Reduction

Photo-reduced products were detected for pesticides that have a carbonyl or nitro group. Sunlight photolysis of triadimefon in water gave the alcohol analog triadimenol,¹¹⁴⁾ and the reduction would be initiated by n, π* excitation of C=O being able to abstract hydrogen.¹¹⁾ A similar mechanism is likely to operate in the photolysis of spinosad whose C=C bond conjugated with C=O is reduced.⁴⁾ The nitro groups of dinitroaniline herbicides flumetralin⁴⁾ and pendimethalin,²¹⁵⁾ and fluazinam⁴⁾ were reduced to NH₂ in their photolysis. Diphenyl ether herbicides, such as nitrofen¹¹⁶⁾ and oxyfluorfen,⁴⁾ produced the corresponding aniline derivatives as major photoproducts. Since nitroso¹¹⁶⁾ and hydroxyamino²¹⁵⁾ derivatives were detected, hydrogen abstraction by an excited NO₂ group would be their primary mechanism in water.

Conclusion

Many photolysis studies of pesticides have been conducted at <290 nm in organic, sometimes water-miscible, solvents at higher concentrations than its water solubility, and conditions different from the natural aquatic environment raise concerns if the estimated photolysis mechanism is operative in sunlit water. In contrast, the aqueous photolysis of pesticides for regulatory purposes is conducted in line with the guideline where a natural aquatic environment is simulated. Although the dissipation profiles with the formation of major photoproducts are extensively examined by using radiolabeled pesticides, many of these studies lack sufficient information to explain a photochemical mechanism. Through a literature survey on the proposed mechanisms in the direct photolysis of pesticides, typical photoreactions are conveniently summarized for each chemical class in Table 2. Since the chemical class characteristic of each reaction cannot be specified, oxidation, reduction and ring contraction of pesticides are not listed here. Pesticides generally have a few functional moieties possibly participating in photolysis, and the experimental conditions of photolysis, such as medium and spectral irradiance of a light source, usually control the contribution of photoreaction at each moiety. Although the spectroscopic and theoretical analyses taken in many photolysis studies gave valuable information on reaction mechanisms, these different conditions sometimes make it difficult to definitively determine a photolysis mechanism.

Table 2. Reaction mechanisms in direct photolysis of pesticide in water

Photoreactions	Bond / mechanism		Pesticide class a)
Photo-induced hydrolysis			sulfonylurea (C–S, N–S), organophosphorus (P–O)
			chlorinated triazine herbicide (C–Cl)
Homolytic cleavage of	C–O(S)	Norrish type-I	pyrethroid a), thioester, O-aryl N-alkylcarbamate
C(=O)X (X=O, N, S)		β-cleavage	pyrethroid (type-II)
	C–N	Norrish type-I	aryl amide a), O-alkyl N-arylcarbamate a), benzoylurea b)
		Norrish type-II	aryl amide
Homolytic bond cleavage	C–O (ether) c)		diphenyl ether, phenoxyacetic acid
	C–S		sulfonamide, thioether c)
	N–O		oxime ether
	N–S		sulfonamide
Dehalogenation	homolytic		organochlorine (DDT, methoxychlor)
	heterolytic		2- or 3-halogenated phenol d), aniline & their derivatives
			chloroacetanilide d), 4-halogenated aromatics e)
Denitration, deamination	C–N		neo-nicotinoid, triazinone herbicide
Isomerization	geometrical		pyrethroid, alkene, oxime ether
	valence		pyridine, pyridazine & isoxazole rings
Cyclization	C–C f)		pesticides having aromatic rings adequately oriented
	C–O g)		diamide insecticide, benzoylcyclohexanedione herbicide
	C–N		dinitroaniline herbicide
Rearrangement	photo-Fries a)		O-alkyl N-arylcarbamate, O-aryl N-alkylcarbamate, phenylurea
	photo-Claisen		phenoxyacetic acid a), aryl benzyl ether
	Beckmann		oxime ether
	thiono–thiolo a)		organophosphorus

^a) Minor pathway in direct aqueous photolysis under sunlight. ^b) Norrish type II process followed by disproportionation is alternatively proposed. ^c) Heterolytic bond cleavage is an alternative mechanism. ^d) Homolytic bond cleavage is an alternative mechanism. ^e) Formation of carbene is proposed for some compounds. ^f) Electrocyclic or oxidative process. ^g) S_N(ET)Ar* mechanism is most probable.

By considering these circumstances, the effect of each experimental condition on photolysis rates and product profiles should be examined in a preliminary steady-state photolysis of a pesticide. Sensitization and/or quenching methods also afford valuable information on key intermediates such as excited states and radicals. LFP and ESR approaches greatly help to focus on these key species; however, they are not so familiar and are too expensive to be applied to routine photolysis. By gathering as much of the above information together as possible with the accumulated knowledge of photolysis, the aqueous photolysis of pesticides should be conducted in accordance with the experimental conditions prescribed in the guidelines. Recent advances in QTOF-LC-MS make it easier to obtain structural information on a photoproduct in a trace amount. One disadvantage of this approach is that there is no concrete information on the formation ratio of each photoproduct if neither the corresponding standard is available nor a radiolabeled pesticide is used. Since the degree of ionization is not always parallel to the formation ratio, the importance of some photoproducts for further risk assessment may be missed. Furthermore, the most important concern not to be overlooked is the uncertainty in estimating a chemical structure only by MS. The MS fragment profiles of pesticide and relevant chemicals can reduce candidate structures to a minimum; however, the estimated structure is not conclusive. Therefore, synthetic standards had better be used for structural identification, especially for a main photoproduct amounting to >10% of a pesticide. If the standard of a photoproduct that has a proposed structure is not available due to difficulty of synthesis, the photochemical mechanism to form a proposed product should at least be suggested, based on the experimental and accumulated evidence to reduce the uncertainty of its identification.

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