Molecular association of benzene with a new cyclophane receptor

Host/guest interactions in the cyclophane-2/benzene system have been investigated by absorption and fluorescence spectroscopy in dichloromethane. The cyclophane serves as a host and the benzene as a guest. Absorption and fluorescence titration experiments are carried out by holding either the concentration of the host or guest constant while varying the concentration of the other component. When the concentration of benzene is kept constant, an isostilbic point at 288 nm is observed in the fluorescence spectral data, suggesting that only two absorbing species are present in equilibrium. Keeping the concentration of cyclophane-2 constant while increasing the concentration of benzene results in a hyposchromic shift of the emission peaks in the range 275–360 nm. The shift is attributed to interaction of the cyclophane with benzene. The average association constant of cyclophane-2 with benzene, K _a = 425 ± 54 M^?1, obtains from fitting the absorption and the fluorescence spectral data to the Bourson et al. equation using non-linear regression analysis.