Cyclodextrin complexation of a stilbene and the self-assembly of a simple molecular device

Abstract

(E)-4-tert-Butyl-4′-oxystilbene, 1⁻, is thermally stable as the (E)-1⁻ isomer but may be photoisomerized to the (Z)-1⁻ isomer as shown by UV-vis and ¹H NMR studies in aqueous solution. When (E)-1⁻ is complexed by αCD two inclusion isomers (includomers) form in which αCD assumes either of the two possible orientations about the axis of (E)-1⁻ in αCD·(E)-1⁻ for which ¹H NMR studies yield the parameters: k₁(298 K) = 12.3 ± 0.6 s⁻¹, ΔH₁^‡ = 94.3 ± 4.7 kJ mol⁻¹, ΔS₁^‡ = 92.0 ± 5.0 J K⁻¹ mol⁻¹, and k₂(298 K) = 10.7 ± 0.5 s⁻¹, ΔH₂^‡ = 93.1 ± 4.7 kJ mol⁻¹, ΔS₂^‡ = 87.3 ± 5.0 J K⁻¹ mol⁻¹ for the minor and major includomers, respectively. The βCD·(E)-1⁻ complex either forms a single includomer or its includomers interchange at the fast exchange limit of the ¹H NMR timescale. Complexation of 1⁻ by N-(6^A-deoxy-α-cyclodextrin-6^A-yl)-N′-(6^A-deoxy-β-cyclodextrin-6^A-yl)urea, 2, results in the binary complexes 2·(E)-1⁻ in which both CD component annuli are occupied by (E)-1⁻ and which exists exclusively in darkness and 2·(Z)-1⁻ in which only one CD component is occupied by (Z)-1⁻ and exists exclusively in daylight at λ ≥ 300 nm. Irradiation of solutions of the binary complexes at 300 and 355 nm results in photostationary states dominated by 2·(E)-1⁻ and 2·(Z)-1⁻, respectively. In the presence of 4-methylbenzoate, 4⁻, 2·(Z)-1⁻ forms the ternary complex 2·(Z)-1⁻·4⁻ where 4⁻ occupies the second CD annulus. Interconversion occurs between 2·(Z)-1⁻·4⁻ and 2·(E)-1⁻ + 4⁻ under the same conditions as for the binary complexes alone. Similar interactions occur in the presence of 4-methylphenolate and 4-methylphenylsulfonate. The two isomers of each of these systems represent different states of a molecular device, as do the analogous binary complexes of N,N-bis(6^A-deoxy-β-cyclodextrin-6^A-yl)urea, 3, 3·(E)-1⁻ and 3·(Z)-1⁻, where the latter also forms a ternary complex with 4⁻.