Preparation of Acifluorfen-Based Ionic Liquids with Fluorescent Properties for Enhancing Biological Activities and Reducing the Risk to the Aquatic Environment

Article abstract of DOI:10.1021/acs.jafc.0c00842

In this work, 12 novel herbicidal ionic liquids (HILs) based on acifluorfen were prepared by pairing with the fluorescent hydrazides or different alkyl chains for increasing activities and reducing negative impacts on the aquatic environment. The results showed that the fluorescence of coumarin hydrazide in the HILs was applied as the internal and supplementary light source to meet the requirement of light wavelength range of acifluorfen, which improved the phytotoxicity of acifluorfen to weeds by enhancing singlet oxygen generation with increased sunlight utilization. The herbicidal activities of HILs were related positively with the length of chain of cation under high light intensity and depended mainly on the fluorescence characteristic of the cation under low light intensity, and the double salt IL forms of acifluorfen containing coumarin hydrazide and n-hexadecyltrimethylammonium had enhanced efficacies against broadleaf weeds in the field. Compared with acifluorfen sodium, HILs had lower water solubility, better surface activity, weaker mobility in soils, and higher decomposition temperature. These results demonstrated that HILs containing different cations provided a wider scope for fine-tuning of the physicochemical and biological properties of herbicides and established a promising way for the development of environmentally friendly herbicidal formulations.

Full text of DOI:10.1021/acs.jafc.0c00842

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Agricultural and Environmental Chemistry
Preparation of acifluorfen-based ionic liquids with
fluorescent properties for enhancing biological activities
and reducing the risk to the aquatic environment
Gang Tang, Junfan Niu, Wenbing Zhang, Jiale Yang, Jingyue
Tang, Rong Tang, Zhiyuan Zhou, Jianqiang Li, and Yongsong Cao
J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.0c00842 • Publication Date (Web): 11 May 2020
Downloaded from pubs.acs.org on May 18, 2020
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Preparation of acifluorfen-based ionic liquids with fluorescent properties for
enhancing biological activities and reducing the risk to the aquatic environment
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Gang Tang, Junfan Niu, Wenbing Zhang, Jiale Yang, Jingyue Tang, Rong Tang, Zhiyuan Zhou,
Jianqiang Li, Yongsong Cao*
College of Plant Protection, China Agricultural University, Beijing, China
*Corresponding author: NO.2 Yuanmingyuan West Road, China Agricultural University, Beijing,
China, 100193
Telephone: 86-10-62734302. Fax: 86-10-62734302.
Email: caoysong@126.com, caoys@cau.edu.cn
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ABSTRACT: In this work, twelve novel herbicidal ionic liquids (HILs) based on acifluorfen were
prepared by pairing with the fluorescent hydrazides or different alkyl chains for increasing activities
and reducing negative impacts on the aquatic environment. The results showed that the fluorescence
of coumarin hydrazide in the HILs was applied as the internal and supplementary light source to
meet the requirement of light wavelength range of acifluorfen, which improved the phytotoxicity of
acifluorfen to weeds by enhancing singlet oxygen generation with increased sunlight utilization.
The herbicidal activities of HILs were related positively with the length of chain of cation under
high light intensity and depended mainly on fluorescence characteristic of cation under low light
intensity. And the double salt IL forms of acifluorfen containing coumarin hydrazide and n-
hexadecyltrimethylammonium had enhanced efficacies against broadleaf weeds in the field.
Compared with acifluorfen sodium, HILs had lower water solubility, better surface activity, weaker
mobility in soils, and higher decomposition temperature. These results demonstrated that HILs
containing different cations provided a wider scope for fine-tuning of the physicochemical and
biological properties of herbicides and established a promising way for the development of
environmentally friendly herbicidal formulations.
KEYWORDS: acifluorfen; ionic liquids; physicochemical properties; fluorescence; singlet oxygen;
efficacy
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INTRODUCTION
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Ionic liquids (ILs), a kind of organic salts composed cations and anions, have more advantageous
physicochemical properties such as low volatility, high ionic conductivity, unique solvation
capabilities and good stability so on.1-3 More importantly, ILs can be designed easily to meet the
requirements of different applications due to the combination diversities of cations and anions.4-6
Over the past two decades, ILs have been studied extensively in various fields and gone through
development periods of first generation (ILs with desired physical properties), second generation
(ILs with adjustable chemical properties) and third generation (ILs with excellent biological
activities).7,8 A flexible design of ionic liquid derivatives for herbicides can pose fascinating
physicochemical properties (increased hydrophobicity and high surface activity), splendid
environmental advantages (low volatility and great adsorption capacity) and excellent biological
activity by choosing appropriate counterions.9,10 Herbicidal ionic liquids (HILs) containing
herbicidally active anions (e.g., 2,4-D, bentazone, bromoxynil, clopyralid, dicamba, dichlorprop,
fomesafen, glyphosate, mesotrione, nicosulfuron, picloram) were prepared to optimize
physicochemical properties for improving herbicide efficacies and reducing environmental risks
caused by evaporation, leaching and the use of large dosage.10-21 With the development of HILs, a
more effective herbicide formulation, “double salt herbicidal ionic liquids” (DSHILs), shows more
designable than single salt HILs.22 DSHILs contain two different herbicidal anions and can’t be
classified as simple IL “mixtures” due to the unique synergistic effect of different ions.23 The
DSHILs can improve herbicidal efficacy, further broaden the spectrum of weed control, and reduce
the risk of weed resistance.22,24 This double salt approach is not only suitable for two herbicidal
anions, it can also be used to prepare HILs by pairing one herbicidal anion with two different cations.
Therefore, the development of double salt form of single herbicide is worth exploring, which may
address the various shortcomings of herbicide and further expand the platform for ionic liquid
applications.
According to HRAC herbicide classification system, the action mechanisms of many herbicides
including triazines, bipyridyliums, diphenylethers and so on are involved in the process of
photosynthesis of plants.25 The activities of these herbicides usually required the participation of
light and were directly or indirectly influenced by light intensity.26 As a diphenyl ether herbicide,
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acifluorfen (ACI) (5-[2-chloro-4-(trifluoromethyl)-phenoxy]-2-nitrobenzoic acid) has been used
extensively in postemergence for selectively controlling broadleaf weeds in soybeans and
peanuts.27,28 It inhibits protoporphyrinogen oxidase (PPO) and consequently induces the
accumulation of protoporphyrin and other tetrapyrrole derivatives in weed cells in the presence of
light.29-33 Photo-excitation of these porphyrin derivatives in the presence of oxygen produces
reactive oxygen, lipid peroxides and hydrogen peroxide in plant tissues, all of which are very
harmful for cell membranes.30,34 Therefore, the activity of ACI was highly affected by sunlight, and
irradiating by weak light will lead to poor activity. However, the wavelengths of light required for
ACI toxicity to weeds are between 410-615 nm, which means that ultraviolet radiation in sunlight
has no impact on its herbicidal activity.35-37 ACI is a weak acid (pKa = 3.50) and usually marketed
in the form of sodium salt.38,39 It has strong leachability (leaching potential index = 3.15) because
the water solubilities of ACI acid and sodium salt are 120 mg L-1 and 250, 000 mg L-1
respectively.40,41 This herbicide is also stable to hydrolysis and difficultly photodegraded, and thus
increases the possibility of ground and surface water contamination.42 In addition, ACI exhibits a
certain extent toxicity to aquatic organisms with LC50 values from 17 mg L-1 to 61 mg L-1.43
Therefore, it is an urgent task to construct a novel form of ACI to improve efficacies by increasing
sunlight utilization and reduce negative impacts on the aquatic environment.
ILs could exhibit robust fluorescence properties by introduction of fluorescent groups and afford
a broad application prospect in solar cells and organic light-emitting diodes.44-47 In this study, novel
HILs based on ACI were prepared by paired with cations derived from natural occurring substances
with good fluorescence properties, such as salicylic acid and coumarin, to increase efficacies by
improving the sunlight utilization and reduce the negative impacts on the aquatic environment by
optimizing physicochemical properties. The thermal stability, fluorescence spectrum, singlet
oxygen, water solubility, surface activity, leaching and herbicidal activity (greenhouse and field) of
prepared HILs were determined and investigated.
MATERIALS AND METHODS
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Materials. Acifluorfen (99% purity) was purchased from Qingdao Hansen Biologic Science Co.,
Ltd. Methanol, ethanol, dichloromethane, N, N-dimethylformamide (DMF), sulfuric acid,
phosphoric acid, sodium carbonate, potassium hydroxide, hydrazine hydrate (80%),
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tetramethylammonium
(Tma)
chloride,
tetrabutylammonium
(Tba)
bromide,
dodecyltrimethylammonium (Tda) bromide and n-hexadecyltrimethylammonium (Cet) chloride
were analytical grade reagent and purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing,
China). Salicylic acid, 3-hydroxy-2-naphthoic acid, 1,3-diphenylisobenzofuran (DPBF),
protoporphyrin IX, and coumarin-3-carboxylic acid were analytical grade and purchased from
Tianjin Heowns Biochemical Technology Co., Ltd. Acetonitrile was high performance liquid
chromatography (HPLC) grade and supplied from J.T. Baker (Phillipsburg, NJ, USA). The ultrapure
water was obtained by using a Millipore water purification system (Millipore, Billerica, MA, USA).
Apparatus.1H and 13C NMR data were tested on a Bruker Avance DPX 300 MHz NMR
spectrometer (Bruker, Germany) at 25 ℃. The NETZSCH STA 449F5 (Germany) was used to
conduct the thermogravimetric analysis (TGA). Melting point values were recorded by a SGWX-
4B microscopic melting point apparatus (SPSIC, China). Surface tension was determined by a JK
99B analyzer (Powereach, China, resolution < 0.05 mN m−1). Fluorescence spectrometer
(FluoroMax®-4) was applied for the determination of scanned excitation and emission spectra of
the sample. LI-6400 portable photosynthesis system (Li-Cor Inc., USA) was used to detect light
intensity. Chlorophyll meter SPAD-502 Plus (Konica Minolta Inc., Japan) was used to measure
chlorophyll content. A HPLC system, consisting of two LC-20ATvp pumps and an SPD-20Avp
ultraviolet detector (Shimadzu, Japan), was used for analysis. A reversed phase kromasil ODS C18
column (250 mm × 4.6 mm, 5 μm) was applied for separation at room temperature. The mobile
phases were acetonitrile (eluent A) and water with phosphoric acid (pH = 2.8, eluent B) (70/30, v/v).
Injection volume was 20 μL, and the detection wavelength was 290 nm. The flow rate was constant
at 1.0 mL min-1, and the column was kept at normal temperature.
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Preparation of HILs. Synthesis of carboxylic esters (Est1-3). Sulfuric acid (1 mmol) was
dissolved in organic solvent (methanol or ethanol) (30 mL) at ambient temperature, and then the
corresponding carboxylic acid (compounds 1-3) (10 mmol) added. The mixture was heated and
stirred under reflux for 10 h. Reaction was monitored by thin layer chromatography (TLC). After
reaction being finished, the solvent was evaporated under vacuum and the residue was washed three
times with saturated solutions of sodium carbonate. Then, the aqueous solution was extracted with
dichloromethane. Finally, the desired compound was obtained after the removal of dichloromethane
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and used in the next reaction.
Synthesis of carboxylic acid hydrazides (Frc1-3) from Est1-3. Frc1-2: ester (Est1 or Est2) (3
mmol) and hydrazine hydrate (9 mmol) were solubilized in ethanol (30 mL). The mixture was heated
and stirred under reflux for 12 h. Reaction was monitored by TLC. After reaction being finished,
the solvent was evaporated and crude product was recrystallized in ethanol to obtain the desired
hydrazide.
Frc3: ethyl-3-coumarincarboxylate (Est3) (2 mmol) was added into a round-bottom flask
containing hydrazine hydrate (2 mmol) and 30 mL of ethanol as the solvent. The mixture was stirred
for 24 h at 25 ℃. The reaction was monitored by TLC. After reaction being finished, the crude
product was recrystallized in the solvent to obtain Frc3.
Synthesis of HIL1-12. The preparation method was performed following the previously reported
procedure.48,24 HIL1-3: ACI (10 mmol), hydrazides (10 mmol), and methanol were mixed in a
round-bottom flask. The mixture was heated and stirred under reflux for 3 h. After reaction finished,
HILs were obtained via the removal of methanol. HIL4-7: quaternary ammonium chloride or
bromide (3 mmol) was dissolved in anhydrous methanol (20 mL) in a round-bottom flask. Next,
potassium hydroxide (3 mmol) dissolved in anhydrous methanol (10 mL) was added. The mixture
was stirred for 2 h at 25 ℃. After the filtration of the potassium chloride or bromide, ACI (3 mmol)
was introduced into the methanolic solution of quaternary ammonium hydroxide. The reaction was
stirred for another 1 h at 25 ºC. HILs (4-7) were obtained via the removal of the solvents.
[ACI][Frc3]1-m[Cet]m: HILs(8-12) were studied for m =0.1, 0.3, 0.5, 0.7, and 0.9 respectively. ACI
(5 mmol) was dissolved in methanol in a round-bottom flask. Next, the same stoichiometric amount
(5 mmol) of a mixture of Frc3 and Cet hydroxide was added and the reaction was heated and stirred
under reflux for 3 h. Then, after the removal of the solvents, the desired HIL8-12 were obtained.
Thermal analysis of HILs. The studies of thermal stability of HILs were performed using TGA.
The analysis consisted of sampling a mass of 3-15 mg, which was heated from room temperature to
500 ℃ at a constant rate of 10 ℃ min-1 with nitrogen as an insert gas. The mass of every sample was
as follows: 8.79 mg (HIL1); 4.72 mg (HIL2); 6.10 mg (HIL3); 5.00 mg (HIL4); 7.11 mg (HIL5);
11.47 (HIL6); 6.08 mg (HIL7); 5.63 mg (HIL12).
Procedure of fluorescence measurement of HILs. HIL1-3 were diluted in methanol to obtain
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a final concentration of 20 mg L-1. Fluorescence excitation and emission spectra were recorded
using FluoroMax®-4 with a 10 × 10 mm quartz cuvette at room temperature. The excitation and
emission wavelengths were set respectively in the range of 200-400 nm and 400-600 nm. The
excitation and emission slit widths were both set to 5 nm. Moreover, the fluorescence emission
intensities of AS, HIL3, HIL7-12 were also evaluated by observing the fluorescence color at the
concentration of 300 mg L-1 of ACI in a mixture of methanol and water (> 99% of total volume)
under 366 nm UV light.
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Detection of singlet oxygen. The singlet oxygen generation of HIL3 was explored by testing the
degradation of DPBF (a singlet oxygen quencher with maximum absorption wavelength at 411
nm).49 In a typical experiment, a 1 mL of DMF solution containing HIL3 (1 μM), protoporphyrin
IX (1 μM), and DPBF (100 μM) was put into a 1 cm quartz cell in the dark. Then, the ultraviolet
lamp (Emax=366 nm) was applied as the irradiation source. After 10 minutes of irradiation, the
absorption spectra of the solution were recorded by a UV-Vis spectrophotometer.
Water solubility. The water solubilities of HILs at different pH (pH = 5, 7, and 9) at constant
temperature were evaluated in phosphate buffer solutions (PBS) with the classic saturation shake-
flask method.50 In brief, HILs were weighed into the tubes (each tube with 10 mL PBS) until the
emergence of surplus undissolved HILs. Then, these tubes were shaking on a shaker for 48 h. At
the end of shaking, the suspensions in the tubes were centrifuged at 8000 rpm for 10 min. And the
concentrations of filtered supernatants were determined by HPLC. Each treatment was carried out
four times in the test.
Surface activity. The surface tensions of the synthetized HILs were determined with the
Wilhelmy plate method at room temperature.51 Before the test, a series of aqueous solutions of HILs
with different concentrations were prepared using ultrapure water and the instrument needed to be
calibrated with ultrapure water. The sensing platinum plate rinsed and dried was placed
perpendicular and then immersed slowly into the prepared liquid sample until the formation of
equilibrium between surface tension of the sample and other related forces. The value of surface
tension of the synthetized compound was recorded by a tensiometer balance sensor. The value of
critical micelle concentration (CMC) was determined from the intersection of two straight lines of
surface tension γ versus logarithm bulk concentration (log C) of compound. Each treatment was
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carried out four times in the test.
Leaching test. The mobilities of HILs in the natural soils were evaluated through soil thin layer
chromatograph (soil TLC).52 The preparation of soil TLC plates, treatment conditions of test
samples, and definition of Rf values were similar to our previous researches.10,15 Firstly, the fine air-
dried soil particles were obtained through a 250-μm meshed sieve. Soil TLC plates with 0.5 mm
thickness were prepared by spreading the air-dried soil/water (4/3, m/m) slurry onto grass plates (20
× 20 cm) with the use of a TLC spreader. Two horizontal lines at distance of 1.5 cm and 11.5 cm
were scribed on each plate above the base when the plates had been air-dried at ambient temperature
for 24 h. Then the test compound (3 μg) dissolved in methanol was spotted on the bottom edge of
the plate with a microsyringe at 1.5 cm. After 3 hours, the plates were put into a closed
chromatographic tank with ultrapure water at a height of 0.5 cm with angle of inclination of 30º.
When the water reached the 11.5 cm line, the plates were taken out and dried at ambient temperature.
Rf values were calculated according to the furthest distance traveled by the test HILs divided by the
distance traveled by the eluent front.
Greenhouse experiment. The herbicidal activity tests were done in the experimental greenhouse
of China Agricultural University (the Haidian district, Beijing) in June 2018. Common amaranth
(Amaranthus retroflexus) was chosen to evaluate the herbicidal efficacy of HILs. Amaranthus
retroflexus were grown in square plastic pots with the size of 7 (length) × 7 (width) × 8 (height) cm
containing nutrition soil and vermiculite (v/v 3:1). The nutrition soil texture is loam with pH=7.14
and 5% of organic carbon content. The greenhouse temperature in daytime was 26 ± 2 ℃, and the
night temperature was 15 ± 2 ℃. Average day/night hours were 14/10 in the plant growth stages.
Water was used for the irrigation of the plants. Ten days after emergence, the plants were thinned
and about four seedlings that grow at the same rate were left in per pot. ACI-sodium (AS) and HILs
dissolved in a mixture of methanol and water (> 99% of total volume) were applied using a
microaerosol sprayer when the plants were at four–six leaf stage (about 15 days). Each pot was
treated with 2.5 mL of drug solution and water was used as the blank controls. The test plants were
divided into three groups (group A, group B and group C). Group A was sprayed with drug solutions
of AS and HIL1-7 at doses of 50, 150, and 300 mg L-1. Group B and group C were sprayed with
drug solutions of HIL3, HIL7-12 at dose of 300 mg L-1, respectively. Then, group A and group B
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were placed in the sunlight (PARmax = 820 μmol m-2 s-1) as high light intensity treatment and group
C was placed in the weak light (PARmax = 140 μmol m-2 s-1) as low light intensity by the use of two
layers of shade nets. Each treatment was carried out four times. At different days after treatment
(DAT), the fresh weights and the leaf SPAD values in aboveground parts of plant under different
treatments were determined and recorded. In this experiment, herbicidal activities were evaluated
by fresh weight reduction (FWR) and chlorophyll content reduction (CCR) of test plants. The FWR
and CCR were calculated as follows: (W1 – W2)/W1 × 100%, where W1 and W2 represent control
groups and treatment groups, respectively.
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Field experiments. The research was conducted in no-till farmland located in Chongqing China
(29°12′42″N 105°46′03″E) in April 2020. The environmental conditions in the region were average
temperatures of 23/15 ℃ day/night, a humidity of about 65%, and a photoperiod of 13/11 hours
day/night. Each experimental area size was designed to be identically 10 m2 (2 m wide × 5 m long)
covered with broadleaf weeds (Conyza canadensis (L.) Cronq. and Gnaphalium affine D. Don) and
gramineous weeds (Alopecurus aequalis Sobol.). The weeds with a height of about 10 to 20 cm
were in a fast growth phase. Based on the actual weed growth and the recommended doses of ACI
in practical application, AS and HILs were applied at dose of 180, 270, and 360 g AI ha-1. The
spraying solution was prepared by dissolving the test compounds in a mixture of water and methanol
(300:1 v/v). About 600 mL of solution was delivered in each area (10 m2) using a microaerosol
sprayer and a solution of water and methanol (300:1 v/v) was used as the control group. Each
treatment was carried out in three replications. In this experiment, the herbicidal activity was
evaluated by a visual method at 14 days after treatment (DAT). According to the standard developed
by the Weed Science Society of America, the weed control efficacy was presented by using a scale
of 0% (no visible damage) to 100% (complete death).
RESULTS AND DISCUSSION
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Preparation of HILs. Twelve novel HILs (Table 1) were synthesized via a multi-step process
for the first time. The major synthetic routes and the structures of cations are introduced in Scheme
1
1
1. The described compounds were also characterized by H and C NMR spectroscopy and the
spectra are provided in the supplementary information. Three carboxylic acid compounds with
fluorescence characteristic were esterified and then amidated with hydrazine hydrate to produce the
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cations containing primary amino groups. These cations were used to react with ACI to create HIL1-
3. The synthesized HIL1-3 had high yields (exceeding 90%). The synthetic method of HIL1-3 may
provide some new thoughts for the preparation of HILs with special function. HIL4-7 were prepared
by reaction with quaternary ammonium hydroxide obtained from quaternary ammonium chloride
(or bromide) using potassium hydroxide ion-exchange reaction. The counter cations were Tma, Tba,
Tda and Cet, which represented quaternary ammonium with hydrocarbons of different lengths. The
obtained HIL4-7 also had high yields (exceeding 95%) and their grease forms exhibited differences
from the solid form of ACI.
HIL8-12 based on ACI were synthesized by merging two cations with different characteristics.
ACI was reacted with a mixture of cations with the total molar amount being equal to that of ACI
resulting in formation of double salt ILs. HIL8-12 with different Frc3/Cet molar rations (9:1, 7:3,
5:5, 3:7, 1:9, respectively) were obtained in high yields (94-98%) and presented the grease forms.
HIL1 and HIL4-12 had melting points below 100 ℃, being classified as ACI-HILs. HIL2 and HIL3
were solids with melting points above 100 ℃, which were classified as new ACI salts. In 1H NMR
spectroscopy, the –COOH peak of ACI disappeared and the hydrogen protons on the benzene ring
of ACI nearing the cations shifted to higher field strengths. These changes on ACI supported the
ionized nature of HILs. There was no significant variation in the 13C NMR spectra of these HILs.
The thermal properties of HILs. Thermal stabilities of the HILs were evaluated by TGA and
the data are showed in Table 2. The melting points of all prepared HILs were less than 150 ℃
although acifluorfen has a high-melting solid (melting point being 276 ℃).41 The decomposition
temperatures of onset to 5% (Tonset5%) and 50% (Tonset) sample were determined to provide accurate
assessment of thermal stabilities for the synthesized compounds. On the basis of the obtained data,
it can be concluded that HIL1-3 with carboxylic acid hydrazides had better thermal stabilities than
HIL4-7 contained quaternary ammonium with different lengths of hydrocarbons. The data of Tonset5%
and Tonset among HIL4-7 did not show a significant effect of the alkyl chain length on their thermal
stabilities. The thermal properties of HIL 3 and HIL 7 showed that Frc3 had more effects on Tonset5%
and Tonset than Cet, and Frc3 could improve the thermal stability of ACI. HIL 8-12 have the similar
structural composition [ACI][Frc3]1-m[Cet]m and HIL 12 has the biggest amount of Cet among HIL
8-12. The Tonset5% and Tonset of HIL12 were 134 ℃ and 285 ℃ respectively, which indicated that
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double salt forms of ACI had high thermal stability.
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The fluorescence characteristics of HILs. The activities of ACI required the participation of
light and were directly influenced by light intensity. However, ultraviolet radiation in sunlight isn’t
in the scope of light required for ACI phytoxicity to weeds. Thus, the sunlight utilization for ACI
may be increased with the help of the fluorescence characteristics of HILs. Fluorescence excitation
and emission spectra of Frc1-3 and HIL1-3 recorded at different emission and excitation
wavelengths accordingly are showed in Figure 1A and Figure 1B. Although Frc1-3 were
synthesized by multistep reactions from carboxylic acid compounds, they still kept the fluorescence
properties. The maximum excitation wavelengths (λm-ex) of Frc1-3 were 319 nm, 349 nm and 379
nm, respectively. The maximum emission wavelengths (λm-em) of Frc1-3 were 425 nm, 412 nm
and 536 nm, respectively. The fluorescence intensity of Frc2 was the strongest, followed by Frc1
and Frc3 in the same experimental condition. HIL1-3 obtained by the acid-base reactions between
ACI with Frc1-3 still retained the fluorescence properties. HIL1-3 exhibited the similar λm-ex and
λm-em as Frc1-3 except slight reductions in the fluorescence intensity. Under 366 nm UV light, the
fluorescence color of AS, HIL3, HIL7-12 are presented in Figure 1C. HIL3 showed strong yellow
fluorescence, while no fluorescence was observed in AS and HIL7 at the same concentration of
ACI. HIL8-12 also exhibited yellow fluorescence and their fluorescence intensities were decreased
gradually with the decreasing of Frc3/Cet molar rations.
Detection of singlet oxygen. Usually, DPBF would be readily degraded to endoperoxides in the
presence of singlet oxygen and the formed endoperoxides could be decomposed into the irreversible
products.53 The reaction between DPBF and singlet oxygen can easily be monitored by recording
the change of the DPBF absorption at 411 nm.54 Figure 2 illustrates the UV-Vis spectra and
absorption of DPBF at 411 nm in DMF solution under the different treatments with light in 10
minutes. The results showed that DPBF treated with HIL3 and protoporphyrin IX had lower
absorptions at 411 nm than DPBF treated with protoporphyrin IX under the same light, which
indicated that DPBF treated with HIL3 and protoporphyrin IX produced more singlet oxygens.
Therefore, it could be concluded that HIL3 enhanced the singlet oxygen generation due to the paired
cation containing fluorescence characteristic.
Water solubility. The water solubilities of HIL1-12 at different pH are displayed in Table 3. The
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solubilities of the prepared HILs in PBS were remarkably lower than that of AS with pH ranging
from 5.0 to 9.0. As an important factor to measure the environmental fate of HILs, the water
solubilities of HILs could be designed by appropriate selection of counterions. Based on the
solubilities of HIL4-7, the water solubilities of HILs were distinctly decreased as the increasing of
alkyl chain length of cations. The obtained data on the water solubilities of HILs were also in accord
with the results of previous studies.10,13,55 The solubilities of HIL8-12 presented a declining trend
with the decreasing of Frc3/Cet molar rations and were not significantly affected by pH. With the
decrease of water solubilities, ionic liquids form of ACI possessing hydrophobic property could
reduce the potential risks to the natural aquatic environment.
Surface activity. The values of surface tension of the synthesized HIL1-12 at room temperature
are also showed in Figure 3. The surface tensions of HIL1-12 were lower than that of AS (63.42
mN m-1) except HIL2. Compared with AS, the surface tensions of HIL1-5 ranging in values from
50.89 to 64.15 mN m-1 had no change significantly, which indicated that the cations with
fluorescence groups and short alkyl chains had little influence on the surface activities. HIL6-12
showed the low surface tensions (< 32 mN m-1) due to the long alkyl chains in the cations. The type
of cation in the HILs played an important role in surface tension and these findings were consistent
with the previous studies.56,57 Therefore, HILs based on ACI with good surface activities were
conducive for enhancing absorption of the active ingredient and thus brought a better herbicide
activity.
Leaching test. The Rf values of the synthesized HIL1-12 are also shown in Figure 3. The Rf
values of HILs (ranging from 0.59-0.71) indicated lower leaching possibility than that of AS (0.73).
The Rf values of HIL1-7 showed that a long alkyl chain in the cation had obvious relation with the
mobility capacity of HILs in soil. HIL7 possessed the lowest mobility with Rf value being 0.63
among HIL1-7. This result was also strongly associated with the water solubility. HIL8-12 pairing
with two cations also exhibited low mobility with Rf values ranging from 0.59-0.65 and had more
potential possibilities for interacting with organic matter in soil. The low Rf values of HILs could
contribute to reducing the negative impacts of ACI on the aquatic environment.
Greenhouse experiment. The efficacies of HIL1-7 against Amaranthus retroflexus at the doses
of 50, 150, 300 mg L-1 in sunlight showed that the fluorescence characteristics in ionic liquids had
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a positive effect on activities (Figure 4). The excellent efficacies of HILs may be derived from the
improvement of the light utilization. Referring to the fluorescence characteristic of HIL3, the λm-
em at 536 nm belonged to action spectrum of the activity of ACI proposed in the previous study,
which was conducive to improvement of the herbicidal activity of ACI.35 HIL7 presented a better
efficacy against broadleaf weeds than AS at the concentration of 300 mg L-1, and its FWR and CCR
reached 86.23% and 66.68%, respectively. These results could be explained by the good
lipophilicity and low surface activity of HIL7. HILs paired with Cet had the best herbicidal activity
among HIL4-7, which was consistent with previous study.22
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Based on the excellent herbicidal efficacies of HIL3 and HIL7 derived from Frc3 and Cet
respectively, Frc3 and Cet were chosen as cations in the preparation of HIL8-12. The herbicidal
activities of HIL3, HIL7 (without fluorescence group in cation) and HIL8-12 (with different molar
rations of Frc3 and Cet) against Amaranthus retroflexus at the concentration of 300 mg L-1 under
high and low light intensities respectively are shown in Figure 5. The FWR values of AS, HIL3 and
HIL7 were 68.22%, 89.56% and 82.12% respectively in the sunlight, which indicated that HIL3 and
HIL7 had higher herbicidal activities than AS under normal lighting conditions. In the weak light,
the FWR value of HIL3 was 70.82%, while that of HIL7 was only 46.32%. These results showed
the HILs with fluorescence properties had an enhanced efficacy, while low efficacy was found for
HILs without fluorescence properties under low light intensity.
Compared to the single salt IL forms, double salt ILs with cations of different properties could
present more excellent herbicidal activities under high and low light intensity. The FWR value of
HIL11 was 94.68% in sunlight and that of HIL8 was 84.73% in the weak light. Although the cation
Cet had no effect significantly on the efficacy of ACI in weak light, the fluorescence characteristics
of Frc3 in the HIL8-12 could be used as the internal and supplementary light source to increase the
efficacy against Amaranthus retroflexus. There was a certain degree of variation in the efficacies of
HIL8-12. In the sunlight, HILs with low Frc3/Cet molar rations showed high efficacies with the
benefit of the excellent physicochemical properties (e.g., improved lipophilicity and good surface
activity). In the weak light, the utilization efficiencies of light were increased by HILs with high
Frc3/Cet molar rations, and thus high efficacies were obtained. Therefore, the efficacies of HIL8-
12 based on ACI could be improved by controlling Frc3/Cet molar ratio.
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Field experiment. According to the results from the greenhouse experiment, HIL3, HIL7, and
HIL11 were chosen as test HILs to evaluate their herbicidal activities in the field experiments. The
biological activities of AS, HIL3, HIL7, and HIL11 against different weeds in no-till farmland are
illustrated in Table 4. The results demonstrated that the single salt IL forms (HIL3 or HIL7) of ACI
had higher herbicidal activities against broadleaf weeds than AS, especially for Conyza canadensis
(L.) Cronq. The control efficacies of HIL3 and HIL7 on Conyza canadensis (L.) Cronq. were 81.3%
and 83.2% respectively at the dose of 360 g AI ha-1 while that of AS was 75.8%. However, this
improvement is not obvious in the case of nonsensitive weeds, Alopecurus aequalis Sobol. Hence,
the impacts of different counter cations on the herbicidal activity of ACI depended on the weed
species. Compared to the single salt IL forms, the double salt IL forms (HIL11) of ACI paired with
Frc3 and Cet had a better herbicidal activity. In the case of Conyza canadensis (L.) Cronq. and
Gnaphalium affine D. Don, HIL11 had higher efficacies about 5.4-11.5% than HIL3 and HIL7 at
the dose of 360 g AI ha-1. The results indicated that the double IL forms of ACI containing Frc3 and
Cet had enhanced herbicidal activities in the field.
In this study, twelve novel HILs based on ACI were synthesized using fluorescence characteristic
groups and alkylamines as cations. Five double salt ILs were prepared by using coumarin hydrazide
and n-hexadecyltrimethylammonium as the pairing cations due to their strong fluorescence
characteristic and lipophilicity, respectively. The results showed that the prepared HILs had
excellent physicochemical properties, such as good thermal stability and surface activity, low water
solubility, strong fluorescence characteristic, and weak mobility in soils, which could contribute to
the enhancement of herbicidal activity as well as minimization of the adverse impact on aquatic
environment. The phytotoxic activities against weeds in the greenhouse showed that HIL11 was the
best compound under high light conditions and HIL8 would be the best one under low light
conditions. Due to the presence of two different cations, the efficacies of HILs could be optimized
by variation of coumarin hydrazide/n-hexadecyltrimethylammonium molar ratio. The results of
field studies confirmed HILs with coumarin hydrazide and n-hexadecyltrimethylammonium as
cations had the excellent herbicidal activity against broadleaf weeds. Hence, HILs containing
different cations provided a wider scope for fine-tuning of the physicochemical and biological
properties of herbicides.
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ASSOCIATED CONTENT
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Supporting Information
The 1H and 13C NMR of synthesized compounds; the CMC value and surface tension isotherms for
HILs.
AUTHOR INFORMATION
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Corresponding Authors
* Yongsong Cao. E-mail: caoysong@126.com, caoys@cau.edu.cn.
ORCID
Yongsong Cao: 0000-0003-2110-8072
Funding
The authors acknowledge financial support of this work by the National Natural Science Foundation
of China (31672067).
Notes
The authors declare no competing financial interest.
REFERENCES
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TABLE AND FIGURE CAPTIONS
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Scheme 1. Synthetic methods of HILs and the chemical structure of cations employed in this study.
Figure 1. The scanned excitation (A) and emission (B) spectra of HIL1-3. The fluorescence of HILs
under UV light of 366 nm (C).
Figure 2. A: UV-Vis spectra (A) and absorption at 411 nm (B) of DPBF in DMF solution with light
at 10 minutes. (a) DPBF only, (b) DPBF with protoporphyrin IX, and (c) DPBF with HIL3 and
protoporphyrin IX.
Figure 3. The interfacial surface tensions and Rf values of HILs at 25 °C.
Figure 4. The chlorophyll content reduction (A) and fresh weight reduction (B) of acifluorfen
sodium (AS) and HIL1-7 under sunlight at five days after treatment. Each data point represents the
mean value from at least three independent experiments. Different letters within each column
indicate statistical differences at P < 0.05.
Figure 5. The fresh weight reductions of HILs at the concentration of 300 mg L-1 at seven days after
treatment under high and weak light (upper). The pictures of Amaranthus retroflexus seedlings at
seven days after treating with HILs at the concentration of 300 mg L-1 under high and weak light
(under). Each data point represents the mean value from at least three independent experiments.
Table 1. Synthesized HILs based on acifluorfen. Different letters within each column indicate
statistical differences at P < 0.05.
Table 2. The thermal properties of HILs.
Table 3. The water solubilities of HILs at different pH.
Table 4. The weed control efficacies of acifluorfen sodium (AS) and HILs at 14 days after treatment
in no-till farmland.
564
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565
566
567
Scheme 1. Synthetic methods of HILs and the chemical structure of cations employed in this study.
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Journal of Agricultural and Food Chemistry
568
569
570
571
Figure 1. The scanned excitation (A) and emission (B) spectra of HIL1-3; the fluorescence of HILs
under UV light of 366 nm (C).
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572
573
Figure 2. A: UV-Vis spectra (A) and absorptions at 411 nm (B) of DPBF in DMF solution with light
at 10 minutes. (a) DPBF only, (b) DPBF with protoporphyrin IX, and (c) DPBF with HIL3 and
protoporphyrin IX.
574
575
576
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Journal of Agricultural and Food Chemistry
577
578
Figure 3. The interfacial surface tensions and Rf values of HILs at 25 °C.
579
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581
Figure 4. The chlorophyll content reduction (A) and fresh weight reduction (B) of acifluorfen
sodium (AS) and HIL1-7 under sunlight at five days after treatment. Each data point represents the
mean value from at least three independent experiments. Different letters within each column
indicate statistical differences at P < 0.05.
582
583
584
585
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Journal of Agricultural and Food Chemistry
586
587
588
589
590
591
Figure 5. The fresh weight reductions of HILs at the concentration of 300 mg L-1 at seven days after
treatment under high and weak light (upper). The pictures of Amaranthus retroflexus seedlings at
seven days after treating with HILs at the concentration of 300 mg L-1 under high and weak light
(under). Each data point represents the mean value from at least three independent experiments.
Different letters within each column indicate statistical differences at P < 0.05.
592
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593
Table 1. Synthesized HILs based on acifluorfen.
No. Abbreviation
Formula [Herb][Cat]m[Cat]1-m
[ACI][Frc1]
Color
Physical State
Solid
Solid
Solid
Wax
Yield (%)
97
1
2
HIL1
HIL2
HIL3
HIL4
HIL5
HIL6
HIL7
HIL8
HIL9
HIL10
HIL11
HIL12
brown
[ACI][Frc2]
Yellow
Yellow
Yellow
Yellow
Yellow
Yellow
Yellow
Yellow
Yellow
Yellow
Yellow
95
3
[ACI][Frc3]
96
4
[ACI][Tma]
98
5
[ACI][Tba]
Wax
95
6
[ACI][Tda]
Wax
96
7
[ACI][Cet]
Wax
98
8
[ACI][Frc3]0.9[Cet]0.1
[ACI][Frc3]0.7[Cet]0.3
[ACI][Frc3]0.5[Cet]0.5
[ACI][Frc3]0.3[Cet]0.7
[ACI][Frc3]0.1[Cet]0.9
Wax
96
9
Wax
98
10
11
12
Wax
94
Wax
96
Wax
97
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595
Table 2. The thermal properties of HILs.
No.
Abbreviation
Tma (℃)
Tonset5%b [℃]
Tonsetc [℃]
1
2
HIL1
HIL2
HIL3
HIL4
HIL5
HIL6
HIL7
HIL8
HIL9
HIL10
HIL11
HIL12
72
120
140
−
205
204
272
231
164
169
136
−
326
498
323
279
295
290
277
−
3
4
5
−
6
−
7
−
8
−
9
−
−
−
10
11
12
−
−
−
−
−
−
−
134
285
c
b
596
aTm, melting point. Tonset5%, decomposition temperature of 5% sample. Tonset, decomposition
597
598
temperature of 50% sample.
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