Azobispyrazole, 4pzMe-5pzH, derivatives with small terminal substituents (Me, Et, i-Pr, and n-Pr) are reported to undergo facile reversible photoswitching in condensed phases at room temperature, exhibiting unprecedentedly large effective light penetration depths (1400 μm of UV at 365 nm and 1400 μm of visible light at 530 nm). These small photoswitches exhibit crystal-toliquid phase transitions upon UV irradiation, which increases the overall energy storage density of this material beyond 300 J/g that is similar to the specific energy of commercial Na-ion batteries. The impact of heteroarene design, the presence of ortho methyl substituents, and the terminal functional groups is explored for both condensedphase switching and energy storage. The design principles elucidated in this work will help to develop a wide variety of molecular solar thermal energy storage materials that operate in condensed phases.

No abstract

The process of vision begins with the absorption of light by retinal, which triggers isomerization around a double bond and, consequently, a large conformational change in the surrounding protein opsin. However, certain organisms evolved different visual systems; for example, deep-sea fishes employ chlorophyll-like antennas capable of capturing red light and sensitizing the nearby retinal molecule via an energy-transfer process. Similar to retinal, most synthetic photochromic molecules, such as azobenzenes and spiropyrans, switch by double-bond isomerization. However, this reaction typically requires shortwavelength (ultraviolet) light, which severely limits the applicability of these molecules. Here, we introduce DisEquilibration by Sensitization under Confinement (DESC) – a supramolecular approach to switch various azoarenes from the E isomer to the metastable Z isomer using visible light of desired color, including red. DESC relies on a combination of a coordination cage and a photosensitizer (PS), which act together to bind and selectively sensitize E-azoarenes. After switching to the Z isomer, the azoarene loses its affinity to—and is expelled from—the cage, which can convert additional copies of E into Z. In this way, the cage⋅PS complex acts as a light-driven supramolecular machine, converting photon energy into chemical energy in the form of out-of-equilibrium photostationary states, including ones that cannot be accessed via direct photoexcitation.