Fast thermal cis–trans isomerization depending on pH and metal ions of water-soluble azobenzene derivatives containing a phosphate group

Azobenzene-Based Solar Thermal Fuels: A Review

The energy storage mechanism of azobenzene is based on the transformation of molecular cis and trans isomerization, while NBD/QC, DHA/VHF, and fulvalene dimetal complexes realize the energy storage function by changing the molecular structure. Acting as "molecular batteries," they can exhibit excellent charging and discharging behavior by converting between trans and cis isomers or changing molecular structure upon absorption of ultraviolet light. Key properties determining the performance of STFs are stored energy, energy density, half-life, and solar energy conversion efficiency. This review is aiming to provide a comprehensive and authoritative overview on the recent advancements of azobenzene molecular photoswitch system in STFs fields, including derivatives and carbon nano-templates, which is emphasized for its attractive performance. Although the energy storage performance of Azo-STFs has already reached the level of commercial lithium batteries, the cycling capability and controllable release of energy still need to be further explored. For this, some potential solutions to the cycle performance are proposed, and the methods of azobenzene controllable energy release are summarized. Moreover, energy stored by STFs can be released in the form of mechanical energy, which in turn can also promote the release of thermal energy from STFs, implying that there could be a relationship between mechanical and thermal energy in Azo-STFs, providing a potential direction for further research on Azo-STFs.

Light-responsive biomaterials for ocular drug delivery

Light-responsive biomaterials can be used for the delivery of therapeutic drugs and nucleic acids, where the tunable/precise delivery of payload highlights the potential of such biomaterials for treating a variety of conditions. The translucency of eyes and advances of laser technology in ophthalmology make light-responsive delivery of drugs feasible. Importantly, light can be applied in a non-invasive fashion; therefore, light-triggered drug delivery systems have great potential for clinical impact. This review will examine various types of light-responsive polymers and the chemistry that underpins their application as ophthalmic drug delivery systems.

Photosensitive drug delivery systems for cancer therapy: Mechanisms and applications

Over the past three decades, various photosensitive nanoparticles have been developed as potential therapies in human health, ranging from photodynamic therapy technologies that have already reached clinical use, to drug delivery systems that are still in the preclinical stages. Many of these systems are designed to achieve a high spatial and temporal on-demand drug release via phototriggerable mechanisms. This review examines the current clinical and experimental applications in cancer treatment of photosensitive drug release systems, including nanocarriers such as liposomes, micelles, polymeric nanoparticles, and hydrogels. We will focus on the three main physicochemical mechanisms of imparting photosensitivity to a delivery system: i) photochemical reactions (oxidation, cleavage, and polymerization), ii) photoisomerization, iii) and photothermal reactions. Photosensitive nanoparticles have a multitude of different applications including controlled drug release, resulting from physical/conformational changes in the delivery systems in response to light of specific wavelengths. Most of the recent research in these delivery systems has primarily focused on improving the efficacy and safety of cancer treatments such as photodynamic and photothermal therapy. Combinations of multiple treatment modalities using photosensitive nanoparticulate delivery systems have also garnered great interest in combating multi-drug resistant cancers due to their synergistic effects. Finally, the challenges and future potential of photosensitive drug delivery systems in biomedical applications is outlined.

Metal Ion Mediated Instant Z → E Isomerization of Azobenzene Macrocycles in the Absence of Light

The classical photoswitch azobenzenes reversibly interconvert between the E- and the Z-isomers with light. Here, we report a pair of new macrocyclic azobenzenes characterized thoroughly by spectroscopic methods and single crystal X-ray diffraction structures, and one of the compounds displays a quantitative conversion of the E- to the Z-form. These compounds, besides their normal photoswitching behavior, display an unusual instant switching of the Z-form to the E-isomer in the presence of Cu²⁺ ions in the dark under 273 K. The Cu²⁺ complex can stay in the Z-form under constant UV radiation. However, it reverts to the E-form as soon as the exposure to the UV is ceased. The same phenomenon is also observed with Ag⁺ ions albeit it is a bit slower. This unusual instant switching of the azobenzene systems with metal ions prompted the detailed studies to unravel the reason behind this behavior.

Catalysis of Thermal Isomerization of Methyl Yellow by Salts

Cu²⁺ ions are reported to catalyze the thermal isomerization (TI) of cis-azobenzene. It was found that some alkali halides (NaCl, KCl, NaBr, and KBr) also catalyze the TI of 4-dimethylamino-azobenzene in ethanol. These tendencies were attributed to the interaction between the azo group and the cation based on experimental data and density functional theory (DFT) calculations. Some Na salts (CH₃COONa, Na₂HPO₄, and Na₂SO₃) were found to inhibit the TI of 4-dimethylamino-azobenzene in ethanol. It is inferred that the weak acid ions in these salts decrease the concentration of H⁺ ions that catalyze the TI process.

Impact of temperature changes and pH on aqueous solutions of TCAB, a derived propanil contaminant

3,4,3',4'-tetrachloroazobenzene known as TCAB is an unwanted product derivate from the degradation of propanil herbicide. In this work UV-visible and infrared spectroscopies were used to experimentally explore the impact of pH and temperature changes of TCAB in aqueous and ethanol solutions. Two isomeric forms, cis and trans, are present in solution. The density functional theory (DFT) with PCM methodology was used to analyze the stability of each isomer in solution phase by the evaluation of solvation energy and frontier orbital energies of TCBA at 25 and 40 °C. This compound has been studied from room temperature to 50 °C, revealing the weakening of trans form with an increment of cis form in ethanol and high temperature. Interestingly, under acid conditions the protonated azo compound was evidenced in solution. We found that the cis form is predominant in aqueous solution at 40 °C and 30 min. Finally, FTIR studies show that the increasing of the temperature promote irreversible structural changes via a trans to cis interconversion process. The derivative results from this study may contributed to understanding of transformation of TCAB in aqueous solution by pH and temperature changes.

Polysilsesquioxane Nanowire Networks as an "Artificial Solvent" for Reversible Operation of Photochromic Molecules

Efficient isomerization of photochromic molecules often requires conformational freedom and is typically not available under solvent-free conditions. Here, we report a general methodology allowing for reversible switching of such molecules on the surfaces of solid materials. Our method is based on dispersing photochromic compounds within polysilsesquioxane nanowire networks (PNNs), which can be fabricated as transparent, highly porous, micrometer-thick layers on various substrates. We found that azobenzene switching within the PNNs proceeded unusually fast compared with the same molecules in liquid solvents. Efficient isomerization of another photochromic system, spiropyran, from a colorless to a colored form was used to create reversible images in PNN-coated glass. The coloration reaction could be induced with sunlight and is of interest for developing "smart" windows.