A visible-light and temperature responsive host-guest system: the photoisomerization and inclusion complex formation of a ruthenium complex with cyclodextrins.

In the present study, we investigated the visible-light- and thermal-stimuli-responsive properties of a host-guest system based on proximal - and distal -[Ru(C 10 tpy)(C 10 pyqu)OH 2 ] 2+ complexes ( proximal and distal -1; C 10 tpy = 4'-decyloxy-2,2':6',2''-terpyridine and C 10 pyqu = 2-[2'-(6'-decyloxy)-pyridyl]quinoline). The analogs of such ruthenium aqua complexes are well-known as metallodrugs and catalysts. The proximal isomer has a dicationic ruthenium center and hydrophobic alkyl chains on both ligands, with the two alkyl chains located close together. According to titration experiments, proximal -1 binds to γ-cyclodextrin (γ-CD) in aqueous media with a binding constant of K 1:1 = 520 ± 60 M -1 , which is much higher than the corresponding values for α-CD and β-CD. Additional experiments indicated that the two alkyl chains were incorporated into the cavity of γ-CD. The photoisomerized complex, distal -1, exhibits thermal isomerization back to proximal -1 in the dark with a k obs = 7.26 ± 0.01 × 10 -6 s -1 . In the presence of γ-CD, the corresponding rate constant is 1.3 times higher, which is attributed to the steric repulsion of cyclodextrin and the aqua ligand by the inclusion complex formation between distal -1 and the cyclodextrins. The distal isomer has a lower affinity for CDs because the two alkyl chains are more separated. The repeated application of external stimuli to a mixture of proximal -1 and γ-CD resulted in a reproducible and reversible host-guest complex formation.