Luminescence quenching in supramolecular assemblies of quantum dots and bipyridinium dications

We have investigated the ability of two bipyridinium dications, with either octyl or decyl groups on their nitrogen atoms, to quench the luminescence of CdSe-ZnS core-shell quantum dots coated by either tri-n-octylphosphine oxide or a tris(phenylureido) calix[6] arene. Our studies demonstrate that both bipyridinium dications adsorb on the surface of the quantum dots with association constants ranging from 10(4) to 10(7) M-1 and quench the luminescence of the inorganic nanoparticles with rate constants ranging from 10(8) to 10(9) s(-1). The association constants of these supramolecular assemblies vary significantly with the counterions of the bipyridinium dications and the ligands on the nanoparticle surface. Their quenching rate constants vary with the length of the alkyl chains appended to the bipyridinium core and, once again, the ligands on the nanoparticle surface. Furthermore, our studies show that the addition of a calix[6] arene able to compete with the quantum dots for the bipyridinium quenchers restores the original luminescence intensity of the nanoparticles. Indeed, the supramolecular association of the calix[6] arene with the bipyridinium dication removes the quencher from the nanoparticle surface and, hence, is transduced into a luminescent enhancement.