Fluorescence Study of Pyrene and Naphthalene in Cyclodextrin-Amphiphile Complex Systems

Article abstract of DOI:10.1021/ja00302a010

Amphiphilic molecules are shown to interact with pyrene-β-cyclodextrin (Py-β-CD) complexes leading to an extremely hydrophobic environment for pyrene (Py) in aqueous solution.The three-component systems give rise to a 1:1:1 complex of Py, β-CD, and the surfactant.The binding constant of Py and β-CD increases significantly in the presence of the surfactants, which suggests an improvement in the solubility of Py in aqueous β-CD systems.Larger binding constants of Py and β-CD were obtained in the presence of shorter chain amphiphiles between C4 and C16 surfactants.Fluorescence quenching of Py in Py-β-CD-pyridinium surfactants (CnPd(1+)x(1-)) systems obeyed first-order kinetics, which were independent of the concentration of CnPd(1+)X(1-) above a certain concentration, while the quenching rate constant was markedly affected by the chain length of the pyridinium surfactants.Smaller rate constants are obtained for longer chain surfactants.The observed kinetics are explained in terms of a 1:1:1 complex formation of Py, β-CD, and CnPd(1+)X(1-), and the chain-length-dependent rate constants are interpreted by assuming a "diffusion-controlled reaction within limited space".On the other hand, Stern-Volmer kinetics were observed for Py fluorescence quenching in the Py-β-CD-C16C2V(2+) (1-ethyl-1'-hexadecyl-4,4'-bipyridinium ion) system.This is ascribed to the long-range nature of the reaction in Py(S^*₁)-viologen group systems compared with that in Py(S^*₁)-pyridinium group systems.In the naphthalene-β-CD (N-β-CD) system, reduced association constants were observed in the presence of surfactants, which is markedly different from that observed in the Py-βCD system.A determination of the dynamic parameters of the equilibrium showed that the entry rate constant of naphthalene into β-CD was reduced in the presence of surfactants, while the exit rate constant was unchanged.The exit rate was appreciably reduced in the Py-β-CD system on addition of surfactants.Quenchers such as oxygen, nitromethane, copper(II) ion, thallium(I) ion, etc., which reside in the aqueous phase also quench excited Py in β-CD.The influence of CD with and without surfactant on the rate depends on the nature of the quenching reaction, and on the degree of screening by the host system on the guest molecule.It is demonstrated in the present study that the introduction of amphiphilic molecules into the Py-β-CD complex system improves the organization of the system and simplifies the reaction mechanism.The unique types of reaction kinetics observed are due to the selective organization of reactants in the CD system.