Azoheteroarenes are a prominent class of molecular photoswitches derived from azobenzene by substituting one or both phenyl rings with heteroaromatic units. In recent years, azoheteroarenes have attracted considerable attention due to the easy tunability of their photophysical properties enabling the development of systems exhibiting superior photoswitch characteristics such as quantitative bidirectional photoswitching and thermal isomerisation kinetics spanning from picoseconds to decades. However, despite their great potential, azoheteroarenes still remain an understudied photoswitch class. In particular, bisheteroaromatic systems where heterocycles are present on both sides of the azo bridge have not been the subject of many publications and remain largely unexplored. This work focuses on the development, investigation, and applications of the understudied azobisheteroarene photoswitches.
The novel family of azobisheteroarenes, the azobispyrazoles have been investigated. These photoswitches demonstrated excellent bidirectional addressability and thermal isomerisation kinetics ranging from hours to years depending on the exact design. The photoswitch performance of the azobispyrazoles was significantly influenced by the connectivity and the substitution pattern of the pyrazole units. Experimental observations have been supported by DFT calculations.
The superior photoswitch properties exhibited by the azobispyrazoles render them promising candidates for applications where effective spatiotemporal photocontrol is required. Thus, the azobispyrazoles have been explored as molecular solar thermal (MOST) materials. These compounds underwent outstanding photoisomerisation in the condensed phase accompanied with solid-to-liquid phase transition occurring at room temperature achieving exceptional energy storage densities. Remarkably, the azobispyrazoles demonstrated unprecedently large effective light penetration depth exceeding 1400 µm, making them suitable for large energy storage applications.
In addition, the photochromism of other azobisheteroarene switches, the azobisthiazoles and the azobisimidazoles, has been studied. Following a high-throughput virtual screen these compounds have been identified to exhibit significantly red-shifted absorbances. Experimental analysis revealed that the E and Z absorption spectra of these azobisheteroarenes were well shifted in the visible range enabling photoswitching to take place with visible light in both directions. Furthermore, in some instances the azobisimidazole scaffold demonstrated quantitative Z-E back conversion facilitated with red light.
The conformational preference of Z-azobisheteroarenes has also been explored. Through computational analysis it has been demonstrated that for more compact Z isomers a planar disposition is possible when a suitable intramolecular stabilising interaction is available. This interaction could be a stabilising chalcogen bond, evident for a series of Z-azobisthiazoles, or a stabilising hydrogen bond observed in the structures of various Z-azobisheteroarenes with suitable donor-acceptor pairs.