Double-Threaded Dimer and Supramolecular Oligomer Formed by Stilbene Modified Cyclodextrin: Effect of Acyl Migration and Photostimuli

We observed changing supramolecular structures of stilbene-alpha-cyclodextrin (StiO-alpha-CD) by photoirradiation and migration. Stilbene derivatives show photoinduced isomerization under irradiation with lambda = 340 nm to give 2-cis-StiO-alpha-CD and with lambda = 254 nm to give 2-trans-StiO-alpha-CD. Photoisomerization of StiO-alpha-CD shows the photostationary state during 30 min. 2D NMR and diffusion coefficient studies revealed that 2-trans-StiO-alpha-CD forms a double-threaded dimer but 2-cis-StiO-alpha-CD changes to a supramolecular oligomer by photoirradiation. We found that the mutual migration of a stilbene group (StiO) on alpha-CD occurs under neutral conditions. The StiO group of alpha-CD (StiO-alpha-CD) moves between the C2 and C3 positions on the secondary hydroxyl group of StiO-alpha-CD (the wider rim of alpha-CD) to give 3-trans-StiO-alpha-CD. 3-trans-StiO-alpha-CD forms a supramolecular oligomer, whereas 3-cis-StiO-alpha-CD changes to a double-threaded dimer, indicating that 3-StiO-alpha-CDs gives the opposite results in the supramolecular structures of 2-StiO-alpha-CDs. The thermal isomerization (migration) is very slow. It takes about 300 h to reach the equilibrium state. Moreover, the migration rate constant (k(trans3 -> 2)) of the trans-StiO group from the C3 position to the C2 position of alpha-CD is faster than k(trans2 -> 3) from the C2 position to the C3 position of alpha-CD. On the other hand, k(cis2 -> 3) of the cis-StiO group from the C2 position to the C3 position of alpha-CD is faster than k(cis3 -> 2) from the C3 position to the C2 position, meaning k(cis2 -> 3) > k(cis3 -> 2), which is the opposite result for k(trans3 -> 2) > k(trans2 -> 3). The formation of a stable double-threaded dimer would suppress the migration of the StiO group of StiO-alpha-CDs in aqueous solutions.