Visible-Light Responsive Sucrose-Containing Macrocyclic Host for Cations

Abstract

Chiral photoresponsive host 1 was prepared by a high-yield Cs₂CO₃-templated macrocyclization. Trans-1 transforms into long-lived cis-1 (25 days) upon irradiation with green light, and the backward transformation is triggered by blue light. Both isomers prefer potassium among alkali metal cations, and cis-1 binds cations stronger than trans-1 (K_cis/K_trans ≤ 4.1). ¹H NMR titration experiments as well as density functional theory studies reveal that sucrose ring oxygen residues and azobenzene nitrogen atoms in 1 contribute to cation coordination.

Scheme 1. Synthesis of Photoresponsive Chiral Host 1

Scheme 2. (a) Proposed Mechanism of Alkali Metal Cation-Induced Stabilization of Intermediate Acyclic Polyether Leading to 1 and (b) Dependence of Reaction Yield on the Size of the Cation

Figure 1

Structures and macrocyclization yield (in parentheses) of reported systems containing an azobenzene switch.^,,,−

Scheme 3. Photoswitching of Host 1 (a–c and d) and Complexes of 1 with 10 equiv of Alkali Metal Triflates (b, c, and e)^[a] in MeCN (50 μM) at 298.0 ± 0.1 K Using Green Light (LED 530 nm) and Blue Light (LED 410 nm), Respectively

Figure 2

Comparison of the chemical shift changes (Δδ) for anomeric proton CH(1) upon addition of cation triflates to the solution of hosts (a) trans-1 and (b) cis-1 in CD₃CN. Fitted binding isotherms (gray lines). For the proton label, see Scheme 1.

Figure 3

Models of the energy-minimized host–guest complexes (a) [K⊂trans-1]⁺ and (b) [K⊂cis-1]⁺, where ⊂ denotes encapsulation.