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)OH2](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 gamma-cyclodextrin (gamma-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 alpha-CD and beta-CD. Additional experiments indicated that the two alkyl chains were incorporated into the cavity of gamma-CD. The photoisomerized complex, distal-1, exhibits thermal isomerization back to proximal-1 in the dark with a k(obs) = 7.26 +/- 0.01 x 10(-6) s(-1). In the presence of gamma-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 gamma-CD resulted in a reproducible and reversible host-guest complex formation.

Automated summary

The study investigated the responsive properties of a host-guest system based on proximal and distal ruthenium aqua complexes. It was found that the two alkyl chains in the complexes had different binding affinities to cyclodextrins, with proximal-1 showing higher affinity. The reversible formation of host-guest complexes could be achieved through repeated external stimuli.