Controlling the Stereospecificity of a Volume-Conserving Adiabatic Photoisomerization within a Nanotubular Self-Assembled Cage: A Reversible Light–Heat Torque Converter

Abstract

We present herein a host-guest supramolecular system by which we were able to obtain precise control of the stereospecificity of a new and unusual adiabatic photoisomerization reaction capable of restoring reversibly the original configuration. The host-guest system is composed of (a) a naphthalene ring linked centrosymmetrically-via sp(2) hybridized oxygen atoms-with methoxytriethyleneglycol chains (1) and (b) a nanotubular cage formed by four self-assembled face-to-face β-cyclodextrins threaded onto the long "axle" of 1. The compound 1 can exist in distinct cis,cis, cis,trans, and trans,trans conformations that are spectrally distinguishable (see Scheme 1 ). Spectroscopic and kinetic manifestations of the torsional isomerization of 1 in the lowest excited singlet state both in solution and within the tubular cage were investigated. The results provide clear evidence that the compact cavity completely blocks the photoisomerization pathway manifested in common solution (cis,cis* → cis,trans*), allowing observation of stereospecific, volume-conserving turning of the naphthalene ring about the two "quasidouble" bonds C(Naph)-O by φ ≈ 180° (cis,cis* → trans,trans*). The photoisomerization is purely adiabatic, and the encaged molecule restores its original configuration by generating torque thermally, when relaxing to the ground state.

Staudinger ligation towards cyclodextrin dimers in aqueous/organic media. Synthesis, conformations and guest-encapsulation ability

“…The inclusion of ada evidently helped to reduce these forces but aggregation was not totally prevented. It is known that β-CD forms aggregates in aqueous solution as shown by DLS [39] and DOSY measurements at different concentrations [38,40–41]; it is therefore reasonable to assume that a part of the aggregation of the dimer is due to the β-CD moieties. Finally, given that in any host–guest solution in the fast exchange regime in the NMR time scale,…”

Altered relaxation dynamics of excited state reactions by confinement in reverse micelles probed by ultrafast fluorescence up-conversion

“…For example a family of methoxynaphthalenes incorporated in cyclodextrins undergoes a volume conserving isomerization reaction which is not observed in free solution. 163,164 This effect thus offers a degree of control over the structure of the photoproduct. Such volume conserving pathways (such as ''crankshaft motion'' and ''hula twist'') which contrast with the solvent displacing ''one-bond flip'' rotation about a single bond, had earlier been shown to be important for isomerization reactions in glassy media, but are generally thought to proceed over higher activation barriers barriers.…”