Investigations of the host properties of three fluorescent cage compounds

Supramolecular chemistry has been an emerging field over the last twenty years with host-guest inclusion being the simplest example. Usually in fluorescence-based host-guest inclusion studies the host is a non-fluorescent cage compound and the effects of inclusion on fluorescent guests are investigated. However, in this project three fluorescent cage compounds were identified for study. This enabled the investigation of their host properties using non-fluorescent guests, to determine the effect of their inclusion on the host fluorescence. The ability of such fluorescent hosts to form complexes has potential application in fluorescent nanomachines and molecular switches and sensors.

The first host investigated was the methoxy nanoball, which displayed both a ligand centered and a ligand-to-metal charge transfer band and which was found to form a weak 1:1 complex with benzene. This band also displayed significant pH dependence, indicating a potential application as a molecular controlled pH sensor. Benzene was the only guest investigated which formed a complex with the nanoball, and because it was a weak complex it was concluded that methoxy nanoball was not a good host for inclusion. However, guest encapsulation during the nanoball synthesis was attempted and proved to be promising, especially with pyrene.

The compound LB[6] also possessed fluorescence properties and formed a higher order complex with the fluorescent guest curcumin. LB[6] also gave interesting but scattered thermodynamic results with the binding constant (K) value both increasing and decreasing with temperature. Time-resolved fluorescence results confirmed previous steady-state iv results with a small increase in intensity with increasing benzene concentration.

The bistren cage compound displayed fluorescence which was sensitive to the polarity of the guests with which it formed higher order complexes. This host also gave good thermodynamic data with anisole, where the K value decreased with increasing temperature. Also, the time-resolved fluorescence showed a trend of decreasing lifetime (ns) with increasing concentration of aniline.