Steady-state fluorescence emission studies on polyazacyclophane macrocyclic receptors and on their adducts with hexacyanocobaltate(III)

The steady-state fluorescence emission spectra of the azacyclophanes 2,5,8,11-tetraaza[12] paracyclophane (L¹), 2,6,9,13-tetraaza[14]paracyclophane (L²), 14,15,17,18-tetramethyl-2,5,8,11-tetraaza[12]paracyclophane (L³) and 16,17,19,20-tetramethyl-2,6,9,13-tetraaza[14]paracyclophane (L⁴) as a function of pH have been measured. The fully protonated species of each cyclophane gives the highest fluorescence-emission quantum yield. The shapes of the titration curves have been explained by the existence of an electron-transfer quenching effect from a non-protonated amine to the benzene chromophore. This effect is greater for macrocycles in which the first deprotonated amine group is closer to the benzene. The association constants for the interaction of the four fully protonated macrocycles with K₃[Co(CN)₆] have been measured either by potentiometry or from fluorescence-emission measurements, and increase in the order L³ ≈ L⁴ < L¹ ≈ L². The photoaquation quantum yields of K₃[Co(CN)₆] have been measured in the presence of the macrocycles L¹ and L², and indicate that three of the CN nitrogens of the complex are involved in adduct formation with the fully protonated macrocycles, as supported by molecular modelling.