Enhancement of intramolecular excimer formation, photodimerization and energy transfer of naphthalene end-labelled poly(ethylene glycol) oligomers via complexation of alkali-metal and lanthanide cations

Abstract

The emission spectra and photochemical dimerization of naphthalene end-labelled poly(ethylene glycol) oligomers (N-P_n-N) in methanol in the presence of alkali-metal and lanthanide cations have been investigated. Alkali-metal cations enhance or inhibit the excimer formation remarkably, depending on the cation size and the length of the polyether chain. Photo-irradiation of N-P_n-N in solution resulted in intramolecular dimerization to give a cubane-like dimer and alkali-metal cations affect the quantum yield of the photodimerization. This effect also shows ion size and chain-length dependence. In the presence of the europium(III) cation, the excitation energy of the end group of N-P_n-N is efficiently transferred to this ion, thus leading to the strong emission spectrum of Eu³⁺. All these observations reveal that the polyether chain of N-P_n-N in solution complexes with cations. Within the complex, the orientation of the terminal chromophores is dependent on the size of the cation and the length of the polyether. The binding constants for alkali-metal cations with N-P_n-N were determined. The complexation of the photochemically synthesized crown ethers from N-P_n-N with cations in non-polar solvents was also examined.