Discovering physicochemical principles for simultaneous harvesting of multiform energy from the environment will advance current sustainable energy technologies. Here we explore photochemical phase transitionsa photochemistry−thermophysics coupled regimefor coharvesting of solar and thermal energy. In particular, we show that photon energy and ambient heat can be stored together and released on demand as high-temperature heat, enabled by room-temperature photochemical crystal↔liquid transitions of engineered molecular photoswitches. Integrating the two forms of energy in single-component molecular materials is capable of providing energy capacity beyond that of traditional solar or thermal energy storage systems based solely on molecular photoisomerization or phase change, respectively. Significantly, the ambient heat that is harvested during photochemical melting into liquid of the low-melting-point, metastable isomer can be released as high-temperature heat by recrystallization of the high-melting-point, parent isomer. This reveals that photon energy drives the upgrading of thermal energy in such a hybrid energy system. Rationally designed small-molecule azo switches achieve high gravimetric energy densities of 0.3–0.4 MJ/kg with long-term storage stability. Rechargeable solar thermal battery devices are fabricated, which upon light triggering provide gravimetric power density of about 2.7 kW/kg and temperature increases of >20 °C in ambient environment. We further show their use as deicing coatings. Our work demonstrates a new concept of energy utilizationcombining solar energy and low-grade heat into higher-grade heatwhich unlocks the possibility of developing sustainable energy systems powered by a combination of natural sunlight and ambient heat.

Recent advances in photoswitchable molecular devices based on single molecules or self-assembled monolayers of photochromic molecules are summarized and discussed.

Azobenzenes are classical molecular photoswitches that have received widespread application. In recent endeavors of molecular design, replacing one or both phenyl rings by heteroaromatic ones is emerging as a strategy to expand the molecular diversity and to access improved photoswitch properties. However, the currently available heteroaryl azo switches generally show limitations on E ⇆ Z photoisomerization yields and/or Z-isomer stability. Here we report a family of azobispyrazoles as new photoswitches, which combine (near-)quantitative bidirectional photoconversions and widely tunable Z-isomer thermal half-lives (t 1/2 ) from hours to years. A visible-light-activated photoswitch is also obtained. Systematic experimental and theoretical investigations reveal the different geometric and electronic structures of azobispyrazoles from those of phenylazopyrazoles, overcoming the conflict existing in the latter between effective photoconversion and Z-isomer stability. Our work shows the great potential of azobispyrazoles in developing photoresponsive systems and can inspire the rational design of new photoswitches making use of bis-heteroaryl azo architecture. File list (2)download file view on ChemRxiv Azobispyrazole family as photoswitches combining (near-)... (1.39 MiB) download file view on ChemRxiv Azobispyrazole family as photoswitches -SI.pdf (7.94 MiB) Synthesis 1.1 General methodsAll reagents and solvents were obtained commercially (Bide Pharmatech Ltd, Shanghai Titan Technology Ltd, and J&K Scientific Ltd). All reactions were monitored by thin-layer chromatography (TLC) performed on silica gel F254 coated glass plates (HSGF254, Huanghai) and visualized by irradiation under UV light (254 nm). Column chromatography was performed using silica gel (300-400 mesh, Huanghai). 1 H NMR and 13 C NMR spectra were recorded on Bruker AVANCE III HD 400 spectrometers at 400 MHz and 101 MHz, respectively. Chemical shifts (δ) were internally referenced to residual solvent signals: 1 H δ = 7.26 (CDCl3), 4.79 (D2O), 2.50 (DMSO-d6) ppm; 13 C δ = 77.06 (CDCl3), 39.53 (DMSO-d6) ppm. 1 HRMS data were obtained on Bruker Impact II quadrupole time of flight mass spectrometry instrument. UV-Vis absorption spectra were recorded on Shimadzu UV-2700 spectrophotometer with slit width of 2.0 nm. Melting points (m.p.) were determined on SGW X-4B digital melting point apparatus (Shanghai INESA Physical Optics Instrument Ltd). Synthetic proceduresMalonaldehyde sodium salt (MDA-Na)The synthesis of malonaldehyde sodium salt (MDA-Na) followed the method from literature. 2

Molecular photoswitches (e.g., azobenzenes) can reversibly interconvert between their thermodynamically stable and metastable isomers upon light irradiations. However, it remains challenging to integrate both high bidirectional photoconversion and long metastable‐state lifetime into a photoswitchable functionality. Here, we introduce pyrazolylazophenyl ethers (pzAzo ethers) as a class of azo photoswitches that provides quantitative (>98 %) trans–cis photoisomerization (365 nm light), near‐quantitative (95–96 %) reverse isomerization (532 nm light), and a long cis‐isomer half‐life of three months. They can be easily synthesized in high yields and readily functionalized at one or both sides with a broad scope of substituent groups. Molecular systems incorporating pzAzo ethers can be endowed with high responsiveness, robust reversibility, and long persistent metastable states. Such superior yet pragmatic azo switches hold high promise for upgraded photoregulation in many light‐responsive applications.