Multiple control of azoquinoline based molecular photoswitches

Abstract

Multi-addressable molecular switches with high sophistication are creating intensive interest, but are challenging to control. Herein, we incorporated ring-chain dynamic covalent sites into azoquinoline scaffolds for the construction of multi-responsive and multi-state switching systems. The manipulation of ring-chain equilibrium by acid/base and dynamic covalent reactions with primary/secondary amines allowed the regulation of E/Z photoisomerization. Moreover, the carboxyl and quinoline motifs provided recognition handles for the chelation of metal ions and turning off photoswitching, with otherwise inaccessible Z-isomer complexes obtained via the change of stimulation sequence. Particularly, the distinct metal binding behaviors of primary amine and secondary amine products offered a facile way for modulating E/Z switching and dynamic covalent reactivity. As a result, multiple control of azoarene photoswitches was accomplished, including light, pH, metal ions, and amine nucleophiles, with interplay between diverse stimuli further enabling addressable multi-state switching within reaction networks. The underlying structural and mechanistic insights were elucidated, paving the way for the creation of complex switching systems, molecular assemblies, and intelligent materials.

Fig. 1. (a) Proposed strategy for merging azoquinoline photoswitches with ring–chain tautomerism toward the goal of multi-addressed and multi-state switching. (b) Synthesis of target compounds E-1 and E-2 and their corresponding X-ray structures, with the distance (Å) of hydrogen bonding listed.

Fig. 2. Photoswitching behavior of E-1. (A) ¹H NMR spectra of E-1 in DMSO-d₆ (10 mM, a), and after irradiation for 1.5 h at 365 nm (b) and then for 0.5 h at 425 nm (c). (B) UV-vis spectra of E-1 (75 μM in DMSO) after alternative irradiation at 365 and 425 nm. The inset shows the multiple cycles of switching in response to 365 and 425 nm light.

Fig. 3. (A) Controlling the ring–chain tautomerization with acid/base. (B) X-ray structure of E-1-H⁺, with the distance (Å) of hydrogen bonding listed. (C) ¹H NMR spectra of E-1-H⁺ in DMSO-d₆ (10 mM, a), its mixture with DBU (1 equiv., b), and following addition of DBU (1 equiv., c). (D) UV-vis spectra of E-1-H⁺ (50 μM in DMSO) after alternative irradiation at 365 and 425 nm, with the inset showing the multiple cycles of switching in response to light.

Fig. 4. (A) Illustration of the modulation of E-1 with coordination and light. (B) ¹H NMR spectra of E-1 in DMSO-d₆ (10 mM, a), its mixture with Zn(OAc)₂·2H₂O (1.5 equiv., b), and after irradiation for 1.5 h at 365 nm (c). (C) X-ray structure of E-1-Zn²⁺. (D) ¹H NMR spectra of Z-enriched solution of 1 (created by irradiation of E-1 for 1.5 h at 365 nm) in DMSO-d₆ (10 mM, a), its mixture with Zn(OAc)₂·2H₂O (2.5 equiv., b), and after 20 min (c).

Fig. 5. (A) Illustration of the modulation of E-1 with primary amine, light, and coordination. (B) ¹H NMR spectra of E-1-Zn²⁺ in DMSO-d₆ (10 mM, a), its mixture with 1-butylamine (3 equiv., b), and after irradiation for 1.5 h at 365 nm (c). (C) ¹H NMR spectra of E-3 in DMSO-d₆ (10 mM, a), after irradiation for 1.5 h at 365 nm (b), and its mixture with Zn(OAc)₂ (2.5 equiv., c).

Fig. 6. (A) Illustration of the modulation of E-1 with secondary amine and coordination. (B) ¹H NMR spectra of E-1 in DMSO-d₆ (10 mM, a), its mixture with piperidine (1.5 equiv., b), and its mixture with Zn(OAc)₂ (1.0 equiv., c). (C) ¹H NMR spectra of E-1-Zn²⁺ in DMSO-d₆ (10 mM, a), its mixture with piperidine (1.5 equiv., b), as well as the mixture of E-4 and Zn(OTf)₂ (3 equiv., c).

Fig. 7. Illustration of the modulation of E-1 with light, pH, coordination, and amine nucleophiles, and the reaction network created by the interaction between different stimuli.