Pyrazolylazophenyl Ether‐Based Photoswitches: Facile Synthesis, (Near‐)Quantitative Photoconversion, Long Thermal Half‐Life, Easy Functionalization, and Versatile Applications in Light‐Responsive Systems

Abstract

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.

Light‐Triggered, Non‐Centrosymmetric Self‐Assembly of Aqueous Arylazopyrazoles at the Air–Water Interface and Switching of Second‐Harmonic Generation

“…The space group was determined as Pca 2 1 (Figure 2 a, Table S1, SI), which is consistent with the previous report [41] . We, however, note that the observed space group indicates that trans ‐ 1 is self‐assembled to form a noncentrosymmetric polar crystal (Figure 2 a), which is in contrast to trans ‐ 2 that showed a centrosymmetric P 2 1 / c space group [41] . As shown in Figure 2 b, the asymmetric crystal packing is seemingly directed by the intermolecular ‐O−C−H⋅⋅⋅N (2) hydrogen bonding (H⋅⋅⋅N (2) distance; 2.52 Å, C−H⋅⋅⋅N (2) angle; 163°).…”

“…The UV‐vis spectra of aqueous 1 and 2 (concentration, 50 μM) in the photostationary states (PSS) are shown in Figures 3 a and S6 (SI), respectively. Trans ‐ 1 in water showed a π‐π* absorption at 347 nm ( trans ≈90 %), which is red‐shifted from those observed for trans ‐ 1 in acetonitrile (342 nm) [41] . This is ascribed to greater stabilization of the LUMO in polar media as compared to the HOMO, which is a general trend for π‐π* transitions [43] .…”

Efficient Co-Harvesting of Solar Energy and Low-Grade Heat by Molecular Photoswitches for High Energy Density, Long-Term Stable Solar Thermal Battery

“…A class of heteroaryl azo-switches, pyrazolylazophenyl ethers (pzAzo ethers, Figure 2a), were selected as the energy harvesting molecules instead of azobenzenes. As previously hreported by us, [17] pzAzo ethers have a very long cis-isomer half-life t1/2 of 3 months (t1/2 are within several days for common azobenzenes) and also retain a high ΔHisom of 52 kJ/mol (49 kJ/mol for azobenzene). This makes pzAzo ethers precious azo-switches that can address the conflict between energy density and storage stability.…”

Tuning the Thermal Stability and Photoisomerization of Azoheteroarenes through Macrocycle Strain**

“…Our selection is based on the fact that these heteroarene families showed E / Z thermal stabilities that were at the edge of the explored values in a recent computational screening [30] . Moreover, azopyrazoles and azoimidazoles are two of the azoheteroarenes families that have received the most attention in recent years [31–38] . Herein, we present the evolution of the half‐life times associated with both the inversion and rotation thermal relaxation mechanisms with bridge length.…”

“…[30] Moreover, azopyrazoles and azoimidazoles are two of the azoheteroarenes families that have received the most attention in recent years. [31][32][33][34][35][36][37][38] Herein, we present the evolution of the half-life times associated with both the inversion and rotation thermal relaxation mechanisms with bridge length. Although bridges tend to shorten t 1=2 , we could identify some cases in which the E-isomer remains as the most stable, and t 1=2 is enhanced with respect to the acyclic compound.…”