A multi-mode-driven molecular shuttle: Photochemically and thermally reactive azobenzene rotaxanes

The shuttling process of α-CyD in three rotaxanes (1-3) containing α-cyclodextrin (α-CyD) as a ring, azobenzene as a photoactive group, viologen as an energy barrier for slipping of the ring, and 2,4-dinitrobenzene as a stopper was investigated. The trans-cis photoisomerization of 1 by UV light irradiation occurred in both DMSO and water due to the movement of α-CyD toward the ethylene group, while the photoisomerization of 2 occurred in DMSO, but not in water. No photoisomerization was observed for 3 in both water and DMSO. The activation parameters of 1 and 1-ref in DMSO are subject to a compensation relation between ΔS‡ and ΔH‡; however, in water, the ΔS‡ terms are not compensated by the ΔH‡ terms. Alternating irradiation of the UV and visible lights resulted in a reversible change in the induced circular dichroism (ICD) bands of trans-1 and cis-1. In contrast, after the UV light irradiation, the ICD band of trans-2 decreased without the appearance of any bands of c/s-2. The NMR spectra of 2 in DMSO showed coalescence of the split signals for the methylene and for the viologen protons due to the shuttling of α-CyD. Both the NOE differential spectra for cis-1 in water after UV light irradiation and 2 in DMSO after heating to 120 °C showed the negative NOE peaks assigned to interior protons of α-CyD, suggesting that α-CyD in cis-1 exists at the one ethylene moiety, and α-CyDs in cis-2 and 2 heated in DMSO exist at the propylene moieties.