Exploring Arylazo-3,5-Bis(trifluoromethyl)pyrazole Switches

Abstract

Arylazopyrazoles stand out among the azoheteroarene photoswitches due to their excellent properties in terms of stability of the least stable isomer and conversion between isomers, leading to their use in several interesting applications. We report herein the synthesis of arylazo-trifluoromethyl-substituted pyrazoles and their switching behavior under light irradiation. UV-vis and NMR experiments showed that arylazo-1H-3,5-bis(trifluoromethyl)pyrazoles displayed very long half-lives in DMSO (days), along with reasonable values of other parameters that characterize a photoswitch. Inclusion of naphthyl moieties as aryl counterparts of the arylazopyrazoles is beneficial only in combination with trifluoromethyl groups, while extending the conjugation by grafting the pyrazole moiety with electron-donating or -withdrawing substituents positively affects the photoswitching behavior, in terms of isomerization yield and half-lives of the least stable isomer. The experimental values were correlated with theoretical calculations indicating the valuable influence of the trifluoromethyl groups onto the photoswitching behavior.

Figure 1

Examples of representative arylazopyrazoles (I, II, III) and the general structure of the compounds 1 prepared and investigated in this work.

Scheme 1. Synthesis of Compounds 1

Isolated after two steps after three steps

Figure 2

Molecular structures of compounds 1i (a), 1l (b), 1n (c), and 1o (d) revealed by single-crystal X-ray diffraction.

Figure 3

View along the crystallographic a axis of the packing diagram of crystal 1i (a) and details of the π–π interactions established between neighboring molecules within a supramolecular column (b).

Figure 4

View of the packing diagram in 1l along the crystallographic a axis. The insets show details of the π–π interactions established between the molecules along the crystallographic a axis.

Figure 5

View of the packing diagram in crystal 1o along the crystallographic a axis (a) and details of the CH−π interactions in the crystallographic ac plane (b).

Figure 6

View of the packing diagram in crystal 1n along the crystallographic a axis and details of the CH−π interactions established within a supramolecular column.

Figure 7

(Top left) UV–vis spectrum of 1a. (Top right) UV–vis spectrum of 1e. (Bottom left) NMR spectra (fragments) for compound 1a before irradiation, after irradiation at 365 nm, and after irradiation at 450 nm. (Bottom right) NMR spectra (fragments) for compound 1e before irradiation, after irradiation at 365 nm, and after irradiation at 450 nm.

Figure 8

(Top left) UV–vis spectra of 1d before and after irradiation at 365 nm. (Top right) UV–vis spectra of 1h before and after irradiation at 365 nm. (Bottom left) NMR spectra for compound 1d before irradiation, after irradiation at 365 nm, and after irradiation at 450 nm. (Bottom right) NMR spectra for compound 1h before irradiation, after irradiation at 365 nm, and after irradiation at 450 nm.

Figure 9

(a) Thermal back-isomerization of 1d, monitored using ¹H NMR in DMSO-d₆. (b) Half-life determination of 1d in DMSO-d₆. (c) Fatigue study of 1d in DMSO through irradiation cycles at 365 and 450 nm.

Figure 10

Calculated NCI surfaces for isomer (Z) and transition states (TSs) of azopyrazoles 1e (a, b) and 1i (c, d). Attractive dispersive intramolecular effects are marked by green surface, whereas repulsive ones are marked by a red surface.

Figure 11

Energy diagram for surface scan of compound 1e, using scanned variables: the dihedral angle ϕ_CNNC corresponding to a rotation mechanism, and angles τ₁ and τ₂ corresponding to two possible inversion mechanisms.

Figure 12

Molecular electrostatic potential (MEP) surfaces generated for compounds 1e (a) and 1i (b). High electron density regions are marked with red, whereas low electron density ones are in blue. Natural bonding orbital (NBO) analysis for TS type I (d) and TS type II (c) of compound 1e (for compound 1i see Supporting Information).