Double-threaded dimer and supramolecular oligomer formed by stilbene modified cyclodextrin: Effect of acyl migration and photostimuli

摘要

We observed changing supramolecular structures of stilbene-α- cyclodextrin (StiO-α-CD) by photoirradiation and migration. Stilbene derivatives show photoinduced isomerization under irradiation with λ =340 nm to give 2-cis-StiO-α-CD and with λ =254 nm to give 2-trans-StiO-α-CD. Photoisomerization of StiO-α-CD shows the photostationary state during 30 min. 2D NMR and diffusion coefficient studies revealed that 2-trans-StiO-α-CD forms a double-threaded dimer but 2-cis-StiO-α-CD changes to a supramolecular oligomer by photoirradiation. We found that the mutual migration of a stilbene group (StiO) on α-CD occurs under neutral conditions. The StiO group of α-CD (StiO-α-CD) moves between the C2 and C3 positions on the secondary hydroxyl group of StiO-α-CD (the wider rim of α-CD) to give 3-trans-StiO-α-CD. 3-trans-StiO-α-CD forms a supramolecular oligomer, whereas 3-cis-StiO-α-CD changes to a double-threaded dimer, indicating that 3-StiO-α-CDs gives the opposite results in the supramolecular structures of 2-StiO-α-CDs. The thermal isomerization (migration) is very slow. It takes about 300 h to reach the equilibrium state. Moreover, the migration rate constant (ktrans3→2) of the trans-StiO group from the C3 position to the C2 position of α-CD is faster than k trans2→3 from the C2 position to the C3 position of α-CD. On the other hand, kcis2→3 of the cis-StiO group from the C2 position to the C3 position of α-CD is faster than kcis3→2 from the C3 position to the C2 position, meaning kcis2→3 > kcis3→2, which is the opposite result for k trans3→2 > ktrans2→3. The formation of a stable double-threaded dimer would suppress the migration of the StiO group of StiO-α-CDs in aqueous solutions.