Azo(hetero)arene Photoswitches as Probes for Light-induced Phase Transition and Modulating Photoswitching Characteristics, and Evaluation of Multiple Azo-units Connected Systems

Abstract

Photoswitches are reversibly interconvertible systems between two isomeric states in response to light and thus exhibit bistability. Azobenzenes are one of the classic photoswitches toggling between trans and cis isomers. These are widely studied because of their robustness, ease of synthesis, photostability over multiple light irradiation steps, and tunable thermal stability of the photoswitched state. Replacing one or both aryl rings with a five-membered heterocycle ring led to azoheteroarenes, which recently gained widespread attention because of their improved performance and application potential. Currently, tuning the absorption of the azo chromophore to induce forward E-Z isomerization with longer wavelengths of light and controlling their thermal reverse isomerization (cis to trans) are important challenges. Along the line, imparting distinct physical properties to the isomers through suitable functionalization and controlling them by light are useful in diverse applications. In the first part, we designed and synthesized azopyrazole photoswitches decorated with variable N-alkyl and alkoxy chains (for hydrophobic interactions) and phenyl substituents on the pyrazoles to enable − stacking. These derivatives showed efficient bidirectional photoswitching and reversible light-induced phase transition (LIPT). Extensive spectroscopic, microscopic, and diffraction studies confirmed the manifestation of molecular-level photoisomerization into macroscopic changes leading to the LIPT phenomena. Using differential scanning calorimetric (DSC) studies, the energetics associated with those accompanying processes were estimated. The long half-lives of Z isomers, high energy contents for isomerization and phase transitions, and the stability of phases over an extended temperature range (-60 to 80 oC) make them excellent candidates for energy storage and release applications. Also, the difference in the solubility of the distinct phases in one of the derivatives allowed us to utilize it as a photoresist in photolithography applications on diverse substrates. In the next part, we have designed and synthesized a family of arylazopyrazoles and arylazoisoxazoles photoswitches by extending the -conjugation. In this regard, we systematically introduced phenyl, styryl, (triphenylvinyl)styryl, phenylethynyl groups at ortho, meta, and para positions relative to the azo unit to extend the -conjugation and varied their electronic coupling. Also, to bring diversity and enable solid-state photoswitching, we introduced methyl and phenyl groups at 3,5 positions of the heterocyclic rings. Through spectroscopic studies, insights into the structure-property relationship towards effectiveness in the solution phase and solid-state photoswitching, as well as the half-lives of the Z isomers, were established. In addition, we also demonstrated the fundamental aspects of photochromism and mechanochromism through reversible solid-state photoswitching and mechanical grinding, respectively. In the last part of the thesis, we connected azobenzene to electron-deficient triazine heptazine cores through -NH linkers to generate multiple azo-unit connected C3 tripodal systems. The electronic coupling was varied by making connections at ortho, meta, and para positions relative to the azo unit. Four such tripodal targets were primarily subjected to extensive spectroscopic studies and evaluated for varied properties such as solvatochromism and cooperative/step-wise photoswitching. Various spectroscopic studies were performed to analyze their photoswitching aspects and thermal stability. Among all the tripodal targets, the heptazine derivative exhibits supramolecular assemblies and can form a gel in DMSO. This gelation behavior and the effect of light irradiation and heat were also investigated and characterized using spectroscopy and microscopic techniques such as scanning electron microscopy (SEM) and polarized optical microscopy (POM). Besides that, the heptazine derivative exhibited base sensing ability that was explored using UV-vis 1 spectroscopy. Furthermore, we attempted to synthesize triazine and heptazine-based long-alkyl chain-connected tripodal systems through a direct connection. We also explored their liquid crystalline properties with and without forming the H-bonding complexes. Through these investigations, we explored the importance of key structural features that can control the properties such as phase transition, absorption properties, and supramolecular assemblies that can be controlled by light. All these insights are valuable for relevant applications. 2