Abstract

This article explores the intriguing phenomenon of color instability in certain pesticide molecules, closely linked to their unique molecular structures, which exhibit characteristics like organic dyes, resulting in similar color dynamics. The key findings were presented at the 5th Ag Formulation & Application Technology Congress (FAT 2024), held on November 26–27 in Hangzhou, China.

A video report of the conference is available at my Scientific Talks page here
https://michberk.com/ScientificTalks.aspx.
For more information about FAT 2024, visit:
https://news.agropages.com/News/NewsDetail---51758.htm
For my lecture section in the FAT2024 report, visit:
https://news.agropages.com/News/NewsDetail---52330.htm

1. Introduction

Color is a fascinating and often overlooked aspect of chemistry that plays a significant role in pesticide molecules. Many of these molecules contain chromophore structures, similar to those found in organic dyes, which give them vibrant colors. However, like dyes, pesticides with chromophore structures frequently encounter challenges related to color instability in their formulations. This instability can impact not only the visual appearance but also raise concerns about the pesticide's performance and shelf life.

This article delves into the complex relationship between chromophore structures and color dynamics in pesticide formulations. By understanding the mechanisms behind color instability, particularly under varying conditions like protonation and deprotonation, we can develop strategies to stabilize these sensitive formulations. These strategies are critical not only for maintaining the appearance but often for ensuring the efficacy and longevity of pesticide products.

2. The mechanism of color visualization.

The mechanism of color visualization in substances, including pesticide molecules with chromophore structures, involves the interaction between light, the substance, and our perception. Light, whether from the sun or an artificial source, contains all visible wavelengths. The first diagram illustrates the electromagnetic spectrum. It shows that visible light, ranging from approximately 400 nm to 700 nm, is just a small part of the spectrum. At the lower end, violet light (~400 nm) is visible, while red light (~700 nm) occupies the higher end of the spectrum.

When light strikes a substance, its molecular structure determines which wavelengths are absorbed and which are reflected. The unabsorbed, reflected wavelengths are detected by our eyes, and our brain interprets this reflected light as a specific color. The color we perceive is complementary to the absorbed wavelengths, as shown on the color diagram.

For instance, if a substance absorbs blue light with wavelengths between 435-480 nm, it reflects the complementary yellow-orange color, which is why we perceive it as yellow-orange. Similarly, if the absorbed light falls in the red range (650-780nm), the complementary color is blue-green. This principle explains how the color we see results from the combination of all the wavelengths that the substance does not absorb.

In the second table, three key elements are presented:

Wavelength of Absorbed Light (in nanometers): This column lists the specific wavelengths of light absorbed by a substance.

Absorbed Light Color: This column corresponds to the color of the absorbed light.

Complementary Visible Color: This column shows the color perceived as a result of the absorption. The complementary visible color is what our eyes detect after certain wavelengths are absorbed.

3. Role of Chromophore Groups Determining Color of Substances

Chromophore groups are specific structural components within molecules responsible for their color. Here's how they determine a substance's color:

Definition:
Chromophores are atoms or groups of atoms in a molecule that absorb light at specific wavelengths within the visible spectrum (approximately 380-750 nm).

Structure: Common chromophores include:

• Azo group (-N=N-): Found in many dyes, absorbing visible light and producing intense colors.
• Carbonyl group (C=O): Present in ketones and aldehydes, responsible for UV absorption.
• Nitro group (-NO₂): Common in yellow dyes.
• Benzene ring and other aromatic systems: Conjugated π-electron systems in benzene and its derivatives absorb UV light.

Electron Behavior: Chromophores possess delocalized electrons that become excited when they absorb light energy.

Light Absorption: As light interacts with the molecule, the chromophore absorbs specific wavelengths.

Color Determination: The wavelengths that are not absorbed are reflected or transmitted, which defines the perceived color of the substance.

Intensity and Shade: The number and types of chromophores within a molecule influence the intensity and shade of the color.

Environmental Factors: Factors such as pH, solvent, and temperature can affect how chromophores interact with light, potentially altering the observed color

The presence and arrangement of chromophores within a molecule's structure ultimately determine which wavelengths of light are absorbed and which are reflected, giving substances their characteristic colors we see.

4. Role of Conjugated Systems in Determining Color of Substances

A conjugated system is a key factor in determining a substance's color. Here's an explanation of what it is and how it affects color:

Definition: A conjugated system is a molecular structure with alternating single and multiple bonds (usually double bonds), allowing electrons to delocalize across the system.

Structure: Conjugated systems typically feature:

• Alternating single and double bonds
• Single bonds with adjacent lone pairs
• Often include aromatic rings

Example of the molecular structure with conjugated system:

Electron Behavior: Electrons in the p-orbitals of these alternating bonds overlap, forming a delocalized π-electron system.

Light Interaction: Conjugated systems interact with light in distinct ways:

• They can absorb light in the visible spectrum.
• The energy needed for electron excitation corresponds to specific light wavelengths.

Color Determination: The degree of conjugation influences the energy gap between molecular orbitals:

• Longer conjugated systems absorb longer wavelengths, making the substance appear more red or blue.
• Shorter conjugated systems absorb shorter wavelengths, resulting in yellow or green appearances.

Relationship to Chromophores: Many chromophores are themselves conjugated systems or are part of larger conjugated structures.

Impact on Color Intensity: More extensive conjugation leads to more intense colors due to stronger light absorption.

The length and nature of a molecule's conjugated system significantly influence which light wavelengths are absorbed.

5. Interaction of Chromophore Groups and Conjugated Systems in Color Phenomena.

Chromophore groups and conjugated systems work together in molecules to create color at the molecular level. Chromophores function as electron-rich centers, while conjugated systems facilitate electron delocalization. This synergy enables electron transitions when the molecule interacts with light. The specific combination of chromophores and conjugated systems determines which wavelengths of light are absorbed and which are reflected or transmitted, resulting in the color we see.

6. Structural Similarity Between Pesticide Molecules and Organic Dyes.

Examining mode of action classification tables offers a useful way to compare pesticide molecules with organic dyes, as they reveal intriguing similarities. Let's look at the molecular structures in the tables below for herbicide, fungicide, and insecticide mode of action. Yes, you see that many of these structures contain chromophore groups and conjugated double-bond systems—the same structural elements responsible for the vibrant colors of many dyes. These chromophore groups allow pesticide molecules to interact with light similarly to dyes, absorbing and reflecting specific wavelengths that produce their characteristic colors. This shared structural feature also makes pesticides prone to color changes due to environmental factors, as their hues can shift in response to pH variations, light exposure, or oxidation, much like organic dyes.

7. Mechanisms of color Change

Deprotonation
Adding or removing protons (H⁺) can change the electron distribution in the chromophore, altering the conjugation and thus shifting the wavelength of light absorbed. This is often seen in pH indicators, where a substance changes color in response to pH due to protonation or deprotonation of its chromophore groups.

Figure 1. Example of deprotonation of a chromophore molecule of an organic dye MY5:

Figure 1 illustrates how deprotonation influence chromophore molecules by altering their electronic structure and, consequently, their color properties. The deprotonation process and the resulting electronic shifts are indicated with arrows, showing the redistribution of electron density across the chromophore molecule.

Deprotonation Process:
The neutral acyl hydrazone molecule of dye features two chromophore groups, marked by green and blue circles. Initially, the neutral molecule has a proton (H⁺) attached to the nitrogen in the hydrazone group. Upon deprotonation in an alkaline medium, this proton is removed, resulting in electron redistribution within the molecule.

Electronic Shifts:
The loss of the proton leaves a lone pair of electrons on the hydrazonoic nitrogen, introducing a negative charge at this site and increasing electron density along the conjugated system.

Electron Pair Movement:
Two possible electron shifts can occur toward the chromophore groups:
Arrow 1: Indicates electron pair movement from the hydrazonoic nitrogen to the adjacent nitrogen atom, demonstrating a shift in electron density.
Arrow 2: Represents electron pair movement from the hydrazonoic nitrogen toward the adjacent carbon atom, showing a corresponding shift in electron density.

Resonance Structures I and II:
This redistribution results in two coexisting resonance structures:
In Structure I, the negative charge delocalizes toward the nitro group (-NO₂), inducing a new resonance structure in the benzene ring. This change is reflected in the green-highlighted circle, while the blue-circled second chromophore remains unaffected.
In Structure II, the negative charge extends into the carbonyl group (-CO), establishing full conjugation across the entire molecule. This change, highlighted in blue, shows conjugated double bonds throughout the molecule, with the negative charge appearing at the carbonyl group, while the green-circled first chromophore remains unaffected.

Summary: Deprotonation alters electron distribution, generating two coexisting resonance structures. These shifts in electron density impact the molecule's light absorption properties, leading to a change in color.

Figure 2. Example of deprotonation of fungicide chromophore molecule Fluazinam:

Figure 2 illustrates the effect of deprotonation on the chromophore structure of the fungicide Fluazinam, which contains a dinitroaniline chromophore group. In an acidic medium, the molecule remains in its protonated form, with a proton attached to the nitrogen of the benzene amine group. In this state, Fluazinam absorbs light in the visible spectrum at wavelengths range 410-480 nm, corresponding to blue light. The complementary color, yellow, is characteristic of the nitro group (-NO₂). However, in an alkaline medium, the proton (H⁺) is removed from the benzene amine nitrogen, leaving a lone pair of electrons and creating a negative charge that increases electron density across the conjugated system. This negative charge delocalizes toward the nitro group, introducing a new resonance structure in the benzene ring. This shift in deprotonated form alters the molecule's light absorption properties toward longer wavelengths, causing a color change from yellow in acidic conditions to red in alkaline conditions.

Figure 3. Example of deprotonation of herbicide chromophore molecule Dicamba:

Figure 3 presents another example of chromophore molecule deprotonation, this time in the benzoic acid herbicide Dicamba, highlighting how deprotonation influences its color.

Protonated Form: In acidic conditions (pH < 7), the molecule remains in its protonated state, with the carboxylic group (-COOH) retaining its proton. In this form, the molecule does not absorb light in the visible spectrum (absorption occurs in the UV range of 230-280 nm), and it appears white.

Deprotonated Form: In alkaline conditions (pH > 7), the carboxylic group loses a proton (H⁺), forming a negatively charged carboxylate group (-COO⁻). This deprotonation extends conjugation within the molecule, shifting light absorption to longer wavelengths, resulting in a yellow appearance.

Dicamba formulations are available in their acidic form as GR, SC, and SL types, as well as in their deprotonated salt forms with pH > 7, such as dicamba-dimethylammonium, dicamba-potassium, dicamba-sodium, and dicamba-diglycolamine. These alkaline formulations are typically of the SL type. This explains why dicamba alkaline formulations appear yellow, while acidic formulations are white. However, problem may arise when the formulation is in a near-neutral solution with pH about 7. Even a slight shift in the formulation's pH can lead to a noticeable color change.

Figure 4. Example of crystal polymorphism in herbicide chromophore molecule Pendimethalin:

Now, let’s examine how changes in electron delocalization within the dinitroaniline chromophore of the herbicide Pendimethalin lead to an intriguing phenomenon of crystal polymorphism, accompanied by a crystal color change. Pendimethalin exists in two polymorphic crystal forms with distinct colors. The triclinic form, orange in color, represents the thermodynamically stable crystal structure (form I), while the monoclinic form, bright yellow, corresponds to the metastable structure (form II). Typically, the yellow form (II) is produced first upon cooling the molten compound, characterized by light absorption in the 200-480 nm range, which corresponds to blue light absorption with a complementary yellow color. The orange form (I) emerges more slowly during a polymorphic phase transition over long-term storage at temperatures below its melting point.

The striking color change from yellow to orange-red during this transition is driven by competing inter- and intra-molecular electronic effects. The transition from the yellow (II) to the orange (I) polymorph is attributed to increased electronic delocalization, which occurs as the hydrogen bond between the secondary amine hydrogen and the oxygen of the 6′-nitro group is shortened, strengthened, and partially straightened.

This modification enhances the overlap between the lone pair of electrons on the amine nitrogen and the π-electron orbitals of the aromatic ring, causing the lone pair on the nitrogen of the benzene amine group to shift toward the nitro group. This shift introduces a new resonance structure within the benzene ring, leading to a shift in light absorption toward longer wavelengths. As a result, the observed color changes from yellow to orange-red. During crystallization, the formation of the more compact conformation required for the stable orange (I) polymorph is energetically more favorable but more difficult to achieve than the yellow (II) polymorph.

Mechanisms of color changes. Summary.
These examples of chromophore molecules illustrate how deprotonation significantly alters electron distribution within the molecule. Changes in conjugation due to deprotonation shift the light absorption spectrum, causing notable color changes. This effect is clearly observed in the pH-dependent transformations of chromophore-containing pesticide molecules, where changes in protonation status lead to distinct shifts in the perceived color.

8. Examples of Chromophore Pesticide Molecules with pH-Driven Color Sensitivity

Pesticide	Molecular structure	Absorbance Wavelength nm	Appearance	Chromophore	Color Change with pH	Pesticide type / formulation types
TRIFLURALIN	C13H16F3N3O4:2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl) benzenamine	397-400 nm	Yellow -Orange crystals	yes	Orange-Red in strongly acidic conditions (pH < 3) Color may change in mixtures with other A.I.s and may depend on a kind of solvent	Dinitroaniline Herbicide /EC, GR
FLUAZINAM	C13H4Cl2F6N4O4: 3-chloro-N-[3-chloro-2,6-dinitro-4-(trifluoromethyl)phenyl]-5-(trifluoromethyl)-2-pyridinamine	410-480 nm	Yellow crystalline solid	yes	Red in alkaline conditions (pH > 7)	Dinitroaniline Fungicide / SC, WP, DP
PENDIMETHALIN	C13H19N3O4: N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine	200-480 nm	Orange-yellow crystals	yes	Yellow-to-Orange in acidic conditions (pH < 5) Deeper Orange or Reddish conditions (pH > 9) Color may change in mixtures with other A.I.s and may depend on a kind of solvent	Dinitroaniline Herbicide / GR, WG,CS, EC, SC
DICAMBA	C8H6Cl2O3: 3,6-dichloro-2-methoxybenzoic acid	230-280 nm	White granular	in deprotonated form	Yellow – deprotonated carboxylic group in alkaline solutions (pH > 7)	Benzoic acid Herbicide / SC, SL, GR In alkaline solutions – SL: dicamba-diglycolamine, dicamba-dimethyl ammonium, dicamba-potasium, dicamba-sodium
IMAZAMOX	C15H19N3O4 : 2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-(methoxymethyl)-3-pyridinecarboxylic acid	203-275 nm	Off-white solid	in deprotonated form	Yellow – deprotonated form in alkaline solutions (pH > 7)	Imidazolinone Herbicide / EC, SL In alkaline solutions- SL: imazamox-ammonium, imazamox-sodium
FOMESAFEN	C15H10ClF3N2O6S: 5-[2-chloro-4-(trifluoromethyl)phenoxy]-N-(methylsulfonyl)-2-nitrobenzamide	290 nm	White solid	in deprotonated form	Red – in deprotonated form in alkaline solutions (pH > 7)	Diphenyl Ether Herbicide / ME In alkaline solutions- SL : fomesafen-sodium

This table highlights some more examples of pesticide molecules containing chromophore groups that are sensitive to pH changes:

Trifluralin: A dinitroaniline-based molecule, exhibits a yellow-orange color in its neutral form in EC and GR formulations. When protonated or exposed to polar solvents, the absorption spectrum shifts, turning the color to orange-red.

Fluazinam and Pendimethalin: Both belong to the dinitroaniline class, featuring nitroaniline groups commonly found in organic dyes. Fluazinam in SC formulations shifts from yellow at pH < 7 to red at pH > 7. Pendimethalin appears yellow-to-orange at acidic pH levels (< 5) and turns deeper orange or red in strongly alkaline conditions (> 9).

Like Dicamba, also Imazamox and Fomesafen are formulated in both protonated forms and as deprotonated salts. Protonated Imazamox and Fomesafen appear white due to UV light absorption. As salts, Imazamox-Ammonium, Imazamox-Sodium in SL formulations turn yellow and Fomesafen-Sodium SL becomes red due to absorption shifts into the visible spectrum.

9. Color Instability in Pesticide Formulations with Chromophore Structures of A.I.s

Color changes in chromophore molecules aren't solely caused by protonation and deprotonation. Such changes result from alterations in the chromophore's electronic structure, which influence how the molecule absorbs and reflects light. Various factors can trigger these alterations:

Conjugation Changes

• Increased conjugation (more alternating double and single bonds): This extension of electron delocalization leads to absorption at longer wavelengths (red shift), resulting in a shift toward more intense colors (e.g., yellow to red). For instance, in the first example Fig.1, the formation of resonance structure 2 extended conjugated double bonds throughout the molecule. This effect can often be observed in aqueous formulations.
• Decreased conjugation: This shortens electron delocalization, causing absorption at shorter wavelengths (blue shift) and often resulting in lighter or less intense colors.

Changes in conjugation, such as adding or removing conjugated double bonds, can cause significant color shifts.

pH Changes Leading to Protonation/Deprotonation

• Chromophores with acidic or basic groups: These can change their protonation state in response to pH variations, altering electron distribution and shifting the absorption wavelength. This effect commonly can be observed in aqueous formulations.
• Common examples: This effect is seen in indicators like phenolphthalein, which changes from colorless in acidic conditions to pink in basic conditions.

Polymorphism
In chromophore-containing pesticide molecules exhibiting polymorphism, distinct crystal forms can manifest different colors. The transition between polymorphic structures arises from variations in electronic delocalization within the chromophore system. This structural shift alters the molecule's light absorption characteristics, leading to noticeable color changes, as shown in Fig. 4.

Solvent Effects
Polarity influence: The polarity of a solvent alters the chromophore's electronic environment, affecting its absorption properties. Polar solvents can stabilize different electronic states, leading to shifts in the absorption spectrum, a phenomenon known as solvatochromism. This effect can occur in EC, CS, and OD formulations and may also manifest when residual solvent impurities remain in the technical material after the final stages of active ingredient synthesis. Temperature changes can exacerbate this issue.

Metal Complexation

• Metal ion binding: Chromophores capable of binding to metal ions can form complexes that alter their electron configurations, leading to different absorption properties and resulting in color changes.
• Such effect can occur in formulations containing ion-rich active ingredients or inert components, which can interact with chromophores and induce color changes through metal complexation.
• Metal complexation of pesticide molecules often results in not only a change in color but also alterations in solubility, which can lead to formulation instability.

Light Exposure and Redox Reactions

• Light excitation: Exposure to light can excite chromophore molecules, altering their electronic states and causing changes in absorption. Similarly, redox reactions can change the oxidation state, modifying the chromophore structure.
• Impact on pesticides: This effect is observed in pesticide molecules sensitive to UV exposure, leading not only to color changes but also potential degradation of the pesticide molecule.

While conjugation, pH changes, and solvent effects can cause color changes, they generally do not impact the biological efficiency of the pesticide active ingredient or the formulation's stability. In contrast, metal complexation and sensitivity to UV and light exposure can indicate molecular degradation, leading to a reduction in active ingredient concentration and compromised storage stability of formulations.

10. Strategies for Color Stabilization in Pesticide Formulations

Now that we understand the factors impacting the color stability of chromophore-containing pesticide molecules, we can develop strategies to stabilize the color in such formulations. To achieve color stability in pesticide formulations containing active ingredients with chromophore groups and conjugated double bonds, which are sensitive to factors like pH changes, solvatochromism, metal complexation, light exposure, and redox reactions, the following key strategies can be implemented:

pH Control: Maintain a stable and controlled pH environment within the formulation. Adjust the pH to a level where the AI molecule is not prone to pH-induced conformational changes that alter its color. This can be achieved by selecting appropriate buffering agents or designing the formulation to maintain the desired pH.

Formulation Optimization: Select and optimize formulation components to minimize interactions with AI molecules. This involves choosing excipients like solvents, surfactants, and stabilizers that are compatible with the AI and prevent chemical reactions or pH changes that could lead to color alterations.

Solvent Selection: Carefully choosing solvents or solvent mixtures that reduce solvatochromic effects, ensuring that the chromophore's light absorption characteristics remain stable under different conditions.

Assessment of Metal Complexation: Conduct a preliminary evaluation to identify potential metal complexation with other active molecules or inert ingredients, as such interactions may affect stability, solubility, coloration, or efficacy. This evaluation should include analyzing the presence of metal ions, assessing chelating tendencies, and performing compatibility studies to identify and mitigate the risks of complex formation. Use metal chelators to prevent unwanted complexation reactions with metal ions, which can alter the electronic structure of the chromophore and lead to color changes.

Antioxidant Incorporation: Add antioxidants to the formulation to inhibit oxidation reactions, a common cause of color instability in AIs with conjugated double bonds. Antioxidants can help prevent the degradation of AI molecules, maintaining color stability.

UV Filters Inclusion: Incorporate UV filters, either chemical (organic compounds) or physical (inorganic particles like zinc oxide or titanium dioxide), that absorb, reflect, or scatter UV radiation. These filters prevent UV-induced decomposition reactions, helping maintain the formulation's stability and efficacy.

Encapsulation Technologies: Consider employing encapsulation or microencapsulation techniques to isolate sensitive chromophore groups from environmental factors such as light, oxygen, or moisture, further enhancing color stability.

Structural Modification: Where feasible, chemical modifications to the chromophore structure can be considered to enhance stability without affecting efficacy. This might involve selecting a acid or salt form of the active ingredient for use in the formulation. Such modifications can reduce sensitivity to environmental factors like pH fluctuations, metal ion interactions, or oxidative conditions, ultimately contributing to better color stability while maintaining the pesticide's effectiveness.

Temperature Control: Provide storage and handling guidelines to maintain the product within a stable temperature range.

Packaging and Storage: Use appropriate packaging materials that protect against light and air exposure. Properly sealed and stored formulations can minimize environmental factors that induce color changes in AIs.

Stability Testing: Conduct comprehensive stability testing to assess long-term color stability under various conditions, including high temperature, light exposure, and storage duration. This helps identify potential issues and guides formulation adjustments to improve both color and AI molecule stability.

Monitoring Residual Solvent Effects on Technical A.I. Color Stability:
Monitor the coloration of technical active ingredients. Analyze technical active ingredients for residual solvent impurities from synthetic manufacturing processes and evaluate potential color changes during storage and further formulation processes.

Quality Control Measures: Implement stringent quality control measures to ensure consistent production and minimize batch-to-batch variations. Adhering to manufacturing protocols, regularly monitoring formulation characteristics, and evaluating color stability during product development and production are crucial.

Regular Monitoring and Evaluation: Continuously monitor the formulation's color stability throughout its shelf life and make necessary adjustments. Periodic testing for color changes and prompt corrective actions can maintain product consistency.

By incorporating these strategies, pesticide formulations containing A.I.s with chromophore groups and conjugated double bonds can achieve enhanced color stability, ensuring product consistency and reliability throughout their shelf life.

11. Summary

This article examines the phenomenon of color instability in pesticide molecules, linked to their chromophore-containing molecular structures, which resemble organic dyes. By exploring the mechanisms behind color changes—such as deprotonation, polymorphism, solvent effects, metal complexation, and light exposure—it reveals how these factors influence electron distribution and light absorption, causing visible color shifts.

The structural similarities between pesticides and dyes underline their shared sensitivity to environmental conditions like pH, solvents, and light. Case studies, including Fluazinam, Dicamba, and Pendimethalin, illustrate how such changes affect both color and stability.

Based on these insights, the article proposes a comprehensive approach to managing color instability. This approach has enabled the development of effective stabilization strategies, including pH control, solvent selection, metal chelation, UV filters, and encapsulation, ensuring long-term color stability and maintaining product efficacy in pesticide formulations.