Synthesis of a cysteine-linked cyclophane dimer having two rhodamine moieties and its reduction-responsive degradation as studied by fluorescence spectroscopy

Cysteine-linked cyclophane dimer having two rhodamine moieties (2) was synthesized as a reduction-responsive host. Owing to self-quenching property of the two rhodamine moieties, cyclophane dimer 2 showed weak fluorescence intensity relative to that of the rhodamine B moiety itself. The cleavage of disulfide bond of 2 was performed by a treatment with reducing agents such as dithiothreitol, to give the corresponding monomeric cyclophanes having a rhodamine moiety. Such reductive degradation of 2 was detected by the increase on fluorescence intensity. As a host, cyclophane dimer 2 was found to show a stronger guest-binding affinity than the monomeric cyclophanes due to concentration effects of the macrocycles. In addition, reduction-responsive release of entrapped guest molecules by 2 was also monitored by fluorescence spectroscopy.