A visible-light and temperature responsive host-guest system: the photoisomerization and inclusion complex formation...

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(C₁₀tpy)(C₁₀pyqu)OH₂]²⁺ complexes (proximal and distal-1; C₁₀tpy = 4'-decyloxy-2,2':6',2''-terpyridine and C₁₀pyqu = 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.