Solar Azo‐Switches for Effective E→Z Photoisomerization by Sunlight

Abstract

Natural photoactive systems have evolved to harness broad‐spectrum light from solar radiation for critical functions such as light perception and photosynthetic energy conversion. Molecular photoswitches, which undergo structural changes upon light absorption, are artificial photoactive tools widely used for developing photoresponsive systems and converting light energy. However, photoswitches generally need to be activated by light of specific narrow wavelength ranges for effective photoconversion, which limits their ability to directly work under sunlight and to efficiently harvest solar energy. Here, focusing on azo‐switches—the most extensively studied photoswitches, we demonstrate effective solar E→Zphotoisomerization with photoconversions exceeding 80% under unfiltered sunlight. These sunlight‐driven azo‐switches are developed by rendering the absorption of E isomers overwhelmingly stronger than that of Z isomers across a broad ultraviolet to visible spectrum. This unusual type of spectral profile is realized by a simple yet highly adjustable molecular design strategy, enabling the fine‐tuning of spectral window that extends light absorption beyond 600 nm. Notably, back‐photoconversion can be achieved without impairing the forward solar isomerization, resulting in unique light‐reversible solar switches. Such exceptional solar chemistry of photoswitches provides unprecedented opportunities for developing sustainable light‐driven systems and efficient solar energy technologies.

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

“…Improving the Z-isomer thermal stability of red-shifted azo molecules remains a fundamental challenge. The existing studies reveal several possible strategies: (i) selecting heteroaryl azo molecules with ultralong half-lives as the parent compound, such as phenylazopyrazole; 32 (ii) utilizing additional intramolecular attractions generated by interactions between the two sides of azoheteroarenes to "lock" the Z isomer (e.g., hydrogen bonds, dispersion forces); 20 (iii) introducing ICT transition bands 52,53,55 or selectively exciting the well-separated n−π* bands in the visible region instead of red-shifting the π−π* bands. 95 (5) Photoswitchable functions associated with heterocycles.…”

“…Recently, we achieved sunlight-induced E → Z photoisomerization by rendering the absorption of E isomers overwhelmingly stronger than that of Z isomers across a broad UV−vis range. 55 This was demonstrated using rationally designed ortho-aminated phenylazopyrazoles where (1) the visible absorption of E isomers was boosted through orthoamino substitution and (2) simultaneously, the n−π* absorption of Z isomers was suppressed by the T-shaped geometry of phenylazopyrazole. 17 A typical example is phenylazopyrazole 16c, where E-16c possessed a pronounced, broad CT band centered around 423 nm with ε max of 5.2 × 10 3 M −1 cm −1 while Z-16c showed very weak n,π* bands with ε max = 0.8 × 10 3 M −1 cm −1 (Figure 5c).…”

“…Ortho -amino-substitutions of azobenzenes have been reported to induce an intense visible absorption band in E- isomer spectra that allows visible-light photoswitching. − Such kind of visible bands was attributed to an intramolecular charge-transfer (ICT) excitation from a π-conjugated lone-pair of the amino groups to a typical π* orbital. , This strategy is also applied to develop visible-light heteroaryl azoswitches. ,− Currently, a range of ortho -aminated azoheteroarenes have been developed with several kinds of the heterocycles including pyrrole, , pyrazole, ,, isoxazole and thiazole and various amino groups, such as primary amino, dimethylamino, pyrrolidino, piperidino, morpholino and N -methylpiperazino (Figure a). The type of the amine has a major effect on the position of the ICT band.…”

“…(a) Chemical structures of ortho -aminated heteroaryl azoswitches. ,− (b) Photoswitch properties of ortho -aminated phenylazopyrazoles 16a , 16b , 16c and 16d . , λ 2 is absorption maxima of the charge-transfer band for the E isomer. The π–π* λ max were determined in MeCN and t 1/2 at 25 °C were determined in [a] MeCN; [b] DMSO.…”

“…Photographs of E - 16c and Z - 16c solutions (0.4 mM in MeCN) were shown in inset. Pure Z - 16c was isolated by column chromatography . (d) Z-isomer fractions of measured PSSs between 350 and 532 nm and the ratio of absorbance ε­( E )/ε­( Z ) .…”