Photoswitching of ortho-Aminated Arylazopyrazoles with Red Light

Abstract

Bidirectional photoswitching of arylazopyrazoles with visible light is enabled by substitution with pyrrolidine and piperidine in the ortho-position of the phenyl ring. The absorption maxima were red-shifted and the molar absorption coefficients in the visible range increased significantly, allowing the use of blue light (λ = 465 nm) for the E → Z isomerization and red light (λ = 600 nm) for the Z → E isomerization. N-Methylation of the pyrazole leads to an excellent thermal stability of the Z isomer.

Cross-conjugation controls the stabilities and photophysical properties of heteroazoarene photoswitches

“…24,30,31 Since the early 2000s, there has been an effort to design an ideal azoarene photoswitch substituted with aryl groups other than phenyl rings. Reports have shown that the λ max can be redshifted into the visible range by replacing phenyl groups with heteroaryl rings (benzodioxanes, 32 diazinines, 31 pyrroles, 33 pyrazoles 34,35 and imidazoles [36][37][38][39] ) and by functionalizing the aryl groups. 32,[40][41][42][43][44][45][46][47][48][49][50][51][52][53][54] These heteroazoarenes have relatively high E → Z photochemical reaction yields, ranging between 46-98%, and Z-isomer half-lives ranging from 1 second up to 46 years.…”

Near-Complete Bidirectional Photoisomerization of para-Dialkylamino-Substituted Arylazopyrazoles under Violet and Green or Red Lights

“…Very recently, Ravoo and his colleagues reported ortho-pyrrolidine-and ortho-piperidine-substituted AAPZs in which the forward isomerization occurred under blue light (465 nm) and the reverse isomerization occurred under red light (600 nm). 8 However, their best switch, the orthopyrrolidine-substituted AAPZ 3, showed moderate isomerization yields in both directions. Photostationary states (PSSs) of trans−cis and cis−trans conversions consisted of 70% cis isomers and 88% trans isomers.…”

“…Nonetheless, there is a pressing need to identify red-shifted photoswitches, which ideally can be addressed with near-IR light. Very recently, Ravoo and his colleagues reported ortho -pyrrolidine- and ortho -piperidine-substituted AAPZs in which the forward isomerization occurred under blue light (465 nm) and the reverse isomerization occurred under red light (600 nm) . However, their best switch, the ortho -pyrrolidine-substituted AAPZ 3 , showed moderate isomerization yields in both directions.…”

“…The PSSs of 4 – 7 at 415 nm carry >92%, > 92%, 90%, and 89% cis isomers, respectively, which are slightly less than those of switches 1 and 2 but are fairly comparable to the state-of-the-art azobenzene switches . In comparison, 8 exhibits poor trans–cis conversion (73% cis), which is nearly identical to that of the ortho -pyrrolidine-substituted AAPZ 3 …”

“…5 In comparison, 8 exhibits poor trans−cis conversion (73% cis), which is nearly identical to that of the ortho-pyrrolidinesubstituted AAPZ 3. 8 The absorption spectra of cis isomers of 4−7 consist of two well-separated bands, one in the UV region (λ max between 361 and 368 nm) and the other in visible wavelengths (λ max between 450 and 460 nm) (Figure 2). Theoretical calculations predict that the cis HOMO and HOMO − 1 are n-type orbitals, while the HOMO − 2 and LUMO are πand π*-type orbitals, respectively (Figures S32−S34), and the above two bands appear due to n−π* electronic transitions (Tables S15− S26).…”