Photosensitivity

Modern Applications of Azo Dyes on Polymers: Photosensitivity

Azo Dyes

Azo dyes are compounds that contain, in terms of molecular structure, at least one azo (-N=N-) functional group bonded to two separate symmetric or asymmetric alkyl or aryl groups. These compounds can exist in cis or trans form (Figure 1.). Used in many fields such as textiles, printing inks, and paper production, these compounds have the largest production volume in dye chemistry. [caption id="attachment_159024" align="aligncenter"] Figure 1: General chemical structure of asymmetric or symmetric azo dyes
(left trans, right cis)[/caption]

Synthesis Methods of Azo Dyes

Almost all azo dyes are obtained by converting a primary amine into a diazonium salt through diazotization, followed by coupling reactions with aromatic rings containing a mesomerically electron-donating substituent such as amino or hydroxyl groups. Other synthesis methods for azo dyes include reduction of nitro-substituted aromatic rings in basic medium, reduction of nitroso derivative compounds with LiAlH4, and condensation of hydrazine and quinone compounds. Below is shown the synthesis scheme of Sudan I azo dye (Figure 2.). [caption id="attachment_159025" align="aligncenter"] Figure 2: Synthesis of Sudan I (IUPAC: 1-(Phenyldiazenyl)naphthalen-2-
ol) compound via diazonium salt[/caption]

Photosensitivity of Azobenzenes

Azo dyes are compounds containing groups called chromophores and auxochromes. In organic chemistry, chromophore groups are molecules that absorb light of a specific wavelength and consequently emit color. Auxochrome groups are substituents on chromophore groups that change the wavelength of light absorbed by the chromophore. Since the change in the wavelength of absorbed light results in a change in the wavelength of emitted light, the colors of azo dyes can be changed to desired colors [1]. Azobenzenes, which are a type of azo dye similar to Sudan I, are azo dyes in which both sides of the azo group are aromatic. They can exist in two separate forms, cis and trans, and can change form when exposed to UV light (Figure 3.). The trans isomer, which is in equilibrium in the dark, is thermodynamically more stable. These properties depend on the chemical structure of the azobenzene molecule. The photochemical properties of azobenzenes are extremely important in terms of their potential applications in materials science and biology. These properties are related to the photosensitive system's chemical structure, particularly π N=N, π*N=N transitions. They depend largely on the energy level arrangement of orbitals of unshared electrons on the nitrogens forming the azo group and on the electron density at these levels. These properties change with the presence and variety of electron-withdrawing and electron-donating R group substituents (auxochromes) on the benzene rings shown in Figure 3. Trans-cis isomerization occurs under UV irradiation, and the reverse is also true. These properties of azobenzenes have enabled light-controlled materials, drug molecules, and molecular machine research [2]. [caption id="attachment_159026" align="aligncenter"] Figure 3: Isomerization of azobenzenes with light[/caption]

Application of Azo Dyes to Polymers

In a study [3], it was demonstrated that the glass transition temperatures of polymers could be changed in a light-sensitive manner. Using the photosensitivity property of azobenzenes, acrylic azobenzene polymers were synthesized, and through the cis-trans isomerization of azobenzenes with UV and visible light, the Tg values of the obtained polymers were made adjustable with light. Five different azobenzene acrylate polymers were obtained from two separate synthesized azobenzenes. Below is shown the synthesis scheme of the obtained azobenzene polymers. [caption id="attachment_159027" align="aligncenter"] Figure 4: Synthesis of azobenzene polymers[/caption] Azopolymers designated as P1 and P2 and their photosensitivity demonstration with changes in glass transition temperatures (Tg) were examined. For P1, Tg in trans form was 48°C, while this value decreased to -10°C in cis form. Similarly, for P2, Tg in trans form was reported as 68°C, but no definitive value was given for cis form. Exposure of the synthesized polymers to UV light causes a change in their molecular structure. During this process, azobenzene groups in the P1 polymer molecules transition from cis to trans form under UV light (Figure 3). This transition causes changes in the physical properties of the polymer. Particularly, the polymer's glass transition temperature (Tg) changes under UV light. However, when UV light is removed or under visible light, these azobenzene groups generally revert slowly or rapidly back to cis form on their own, thus the polymer's properties change again. This cyclic process reveals the polymer's light-sensitive properties. In the study, the modification of the Tg value using the photosensitivity property of azobenzenes offers a different solution highlighting the light-repairable properties of polymers. A series of experiments was conducted to demonstrate this property. The figure below shows the process followed for repairing the damaged region of the polymer. [caption id="attachment_159028" align="aligncenter"] Figure 5: Light-induced healing of damaged regions of polyazobenzene acrylates[/caption]

Conclusion

The application of azobenzenes to polymers creates a broad field of application by making polymer properties light-sensitive. According to similar studies conducted at İzel Kimya's R&D Centre and information found in the literature, through the cis-trans isomerization of azobenzene groups under UV light, changes occur in the physical and chemical properties of polymers, and thus superior properties are imparted to polymers. These changes have significant effects on the glass transition temperatures, mechanical properties, and optical properties of polymers. This innovative approach provides important contributions in fields such as smart materials, biomedical applications, and optical technologies.   References 1. Said Benkhaya et al. Classifications, properties, recent synthesis and applications of azo dyes. Heliyon, 6(1), e03271, (2020). 2. Dudek et al. Molecular design and structural characterization of photoresponsive azobenzene-based polyamide units. Dyes and Pigments, Volume 180, 108501, (2020). 3. Zhou, H., Xue, C., Weis, P. et al. Photoswitching of glass transition temperatures of azobenzene-containing polymers induces reversible solid-to-liquid transitions. Nature Chem 9, 145–151 (2017).   Dr. Cemil Dizman Research and Development Director İzel Kimya Volkan M. Akbulut Research and Development Researcher İzel Kimya