A kinetic study on energy transfer between guest aromatic species and rare earth charge-compensating cations within zeolites

Abstract

The real-time observation is made of energy transfer (ET) between guest aromatic species, benzophenone and naphthalene, and a rare-earth charge compensating cation, Tb³⁺ within a faujasite zeolite Y by the measurement of a time-dependent rise and decay of emission from the ⁵D₄ excited-state of Tb³⁺ on excitation of the guest molecules with a pulsed-laser light. Confinement of the guest species within the cages leads to a greatly enhanced emission intensity relative to solution systems, thus facilitating the ET by enforcing the formation of excited-state complexes. Besides, the kinetic behavior of the emission reveals that while benzophenone molecules are immobilized at the protonic sites within the cages, naphthalene molecules are still mobile leading to a remarkable observation of the loading-dependent rise and decay of the Tb³⁺ emission. Consistent with the assumption of the excited-state complex, a direct excitation of the absorption band of ⁷F₆ → ⁵D₄ band (488 nm) of Tb³⁺ results in a single-exponential emission decay with a lifetime of 2.4 ms both in the absence and in the presence of benzophenone or naphthalene. The rate constants of the ET are obtained by analyzing the rise and decay of the emission and the ET is envisaged to take place between guest species residing in the supercage to the Tb³⁺ ions located in the neighboring sodalite cages or D6R over a distance of ∼0.5 nm.