Photoinduced Autonomous Nonequilibrium Operation of a Molecular Shuttle by Combined Isomerization and Proton Transfer Through a Catalytic Pathway

Abstract

We describe a [2]rotaxane whose recognition sites for the ring are a dibenzylammonium moiety, endowed with acidic and H-bonding donor properties, and an imidazolium center bearing a photoactive phenylazo substituent. Light irradiation of this compound triggers a network of E/Z isomerization and proton transfer reactions that enable autonomous and reversible ring shuttling away from equilibrium.

Figure 1

Structures of rotaxane E-1H²⁺, thread E-2H²⁺, and bases B1-B5 with their respective pK_a values in acetonitrile.

Figure 2

Partial ¹H NMR spectra (500 MHz, acetonitrile-d₃, 298 K) of E-1H²⁺ (center), E-1⁺ (top), and Z-1H²⁺ (bottom).

Scheme 1. (a) Closed Network of Acid–Base (Horizontal) and Photochemical (Vertical) Reactions Connecting the Four States That Can Be Obtained from Rotaxane E-1H²⁺ and (b) Qualitative Energy Diagrams of the Corresponding Rotaxane States Indicating the Changes in Energy Induced by the Deprotonation/Protonation and Isomerization/Back-Isomerization Processes^a

The inner dashed arrows indicate the bimolecular proton transfer between Z-1H²⁺ and E-1⁺. The outer colored arrows indicate the preferential anticlockwise direction during the out-of-equilibrium cycling. The red sphere indicates the preferential position of the macrocycle along the axle

Figure 3

Time-dependent absorption changes at 375 nm for (a) E-1H²⁺ solutions irradiated at 365 nm until the PSS (yellow background) and upon termination of the irradiation and concomitant addition of different amounts of E-1⁺ (gray background); (b) solutions containing E-1H²⁺ and E-1⁺ in different proportions, upon irradiation at 365 nm light (yellow background), and subsequent rest in the dark (gray background). Conditions: 18 μM E-1H²⁺, acetonitrile, 298 K.