A rechargeable molecular solar thermal system below 0 °C

Visible-Light-Activated Heteroaryl Azoswitches: Toward a More Colorful Future

Azobenzenes (Ph-N═N-Ph) are known as the most widely studied molecular photoswitches, and the recent rise of azoheteroarenes (Het-N═N-Ph or Het-N═N-Het) offers great opportunities to advance this already mature field. A common limitation is that azo-switches generally require harmful UV light for activation, which hinders their application across various fields. Despite great efforts in developing visible-light azobenzenes over the past few decades, the potential of visible-light heteroaryl azoswitches remains largely unexplored. This Perspective summarizes the state-of-the-art advancements in visible-light heteroaryl azoswitches, covering molecular design strategies, the structure-property relationship, and potential applications. We highlight the distinctive advantages of azoheteroarenes over azobenzenes in the research and development of visible-light switches. Furthermore, we discuss the opportunities and challenges in this emerging field and propose potential solutions to address crucial issues such as spectral red-shift and thermal half-life. Through this Perspective paper, we aim to provide inspiration for further exploration in this field, in anticipation of the growing prosperity and bright future of visible-light azoheteroarene photoswitches.

Photoresponsive heparin ionic complexes toward controllable therapeutic efficacy of anticoagulation

Controllable heparin-release is of great importance and necessity for the precise anticoagulant regulation. Efforts have been made on designing heparin-releasing systems, while, it remains a great challenge for gaining the external-stimuli responsive heparin-release in either intravenous or catheter delivery. In this study, an azobenzene-containing ammonium surfactant is designed and synthesized for the fabrication of photoresponsive heparin ionic complexes through the electrostatic complexation with heparin. Under the assistance of photoinduced trans-cis isomerization of azobenzene, the obtained heparin materials perform reversible athermal phase transition between ordered crystalline and isotropic liquid state at room temperature. Compared to the ordered state, the formation of isotropic state can effectively improve the dissolving of heparin from ionic materials in aqueous condition, which realizes the photo-modulation on the concentration of free heparin molecules. With good biocompatibility, such a heparin-releasing system addresses photoresponsive anticoagulation in both in vitro and in vivo biological studies, confirming its great potential clinical values. This work provides a new designing strategy for gaining anticoagulant regulation by light, also opening new opportunities for the development of photoresponsive drugs and biomedical materials based on biomolecules.

Visible-Light-Sensitive Photoliquefiable Arylazoisoxazoles for the Solar Energy Conversion, Storage and Controlled-Release of Heat at Room Temperature or Lower Temperatures

The photoswitchable MOlecular Solar Thermal (MOST) energy storage systems that are capable of exhibiting high energy storage densities are found to suffer from the poor cyclability, the use of less abundant UV light of the solar spectrum, or reduced charging/discharging rates and poor photoconversions in solid states. Herein, we have designed and readily synthesized a novel set of para-thioalkyl substituted arylazoisoxazoles, that undergo high trans-cis and cis-trans photoconversions under visible light, and show fast charging/discharging and impressive cyclability. Remarkably, the presence of C6-or C10-thioalkyl chainin photochromes permitted reversible solid-liquid phase transition with the formation of cis-enriched charged states by 400 nm light irradiation and trans-enriched discharged states by 530 nm light at various temperatures (10-35 °C). The solid-to-liquid phase transition enabled storage of the latent heat in addition to the isomerization energy, resulting in a high net energy storage density of 189-196 J/g, which are substantially higher than that of many recently reported azobenzene-based MOST compounds (100-161 J/g). Using a high-resolution infrared camera, we further demonstrated that a brief irradiation of green light can be employed to readily release the trapped photon energy as heat. Our results suggest that the arylazoisoxazole with C6-thioalkyl chain at para-position can serve as an effective and eco-friendly photoliquefiable MOST material.

Solar Efficiency of Azo-Photoswitches for Energy Conversion: A Comprehensive Assessment

Photoswitches can absorb solar photons and store them as chemical energy by photoisomerization, which is regarded as a promising strategy for photochemical solar energy storage. Although many efforts have been devoted to photoswitch discovery, the solar efficiency, a critical fundamental parameter assessing the solar energy conversion ability, has attracted little attention and remains to be studied comprehensively. Here we provide a systematic evaluation of the solar efficiency of typical azo-switches including azobenzenes and azopyrazoles, and gain a comprehensive understanding on its decisive factors. All the efficiencies are found below 1.0 %, far from the proposed limits for molecular solar thermal energy storage systems. Azopyrazoles exhibit remarkably higher solar efficiencies (0.59-0.94 %) than azobenzenes (0.11-0.43 %), benefiting from largely improved quantum yield and photoisomerization yield. Light filters can be used to improve the isomerization yield but inevitably narrow the usable range of solar spectrum, and these two contradictory effects ultimately reduce solar efficiencies. We envision this conflict could be resolved through developing azo-switches that afford high isomerization yields by absorbing wide-spectrum solar energy. We hope this work could promote more efforts to improve the solar efficiency of photoswitches, which is highly relevant to the prospect for future applications.

Efficient and Robust Molecular Solar Thermal Fabric for Personal Thermal Management

Molecular solar thermal (MOST) materials, which can efficiently capture solar energy and release it as heat on demand, are promising candidates for future personal thermal management (PTM) applications, preferably in the form of fabrics. However, developing MOST fabrics with high energy-storage capacity and stable working performance remains a significant challenge because of the low energy density of the molecular materials and their leakage from the fabric. Here, an efficient and robust MOST fabric for PTM using azopyrazole-containing microcapsules with a deep-UV-filter shell is reported. The MOST fabric, which can co-harvest solar and thermal energy, achieves efficient photocharging and photo-discharging (>90% photoconversion), a high energy density of 2.5 kJ m^-2 , and long-term storage sustainability at month scale. Moreover, it can undergo multiple cycles of washing, rubbing, and recharging without significant loss of energy-storage capacity. This MOST microcapsule strategy is easily used for the scalable production of a MOST fabric for solar thermal moxibustion. This achievement offers a promising route for the application of wearable MOST materials with high energy-storage performance and robustness in PTM.

Photoliquefiable Azobenzene Surfactants toward Solar Thermal Fuels that Upgrade Photon Energy Storage via Molecular Design

Photoresponsive phase change materials (PPCMs) are capable of storing photon and heat energy simultaneously and releasing the stored energy as heat in a controllable way. While, the azobenzene-based PPCMs exhibit a contradiction between gravimetric energy storage density and photoinduced phase change. Here, a type of azobenzene surfactants with balance between molecular free volume and intermolecular interaction is designed in molecular level, which can address the coharvest of photon energy and low-grade heat energy at room temperature. Such PPCMs gain the total gravimetric energy density up to 131.18 J g^-1 by charging solid sample and 160.50 J g^-1 by charging solution. Notably, the molar isomerization enthalpy upgrades by a factor of up to 2.4 compared to azobenzene. The working mechanism is explained by the computational studies. All the stored energy can release out as heat under Vis light, causing a fast surface temperature rise. This study demonstrates a new molecular designing strategy for developing azobenzene-based PPCMs with high gravimetric energy density by improving the photon energy storage.