Long-lived electron-hole pair formation through photoionization of diphenylacetylene occluded in medium pores of aluminum rich M_(6.6)ZSM-5 zeolite (M = Li~+, Na~+, K~+, Rb~+, Cs~+) Influence of the counterbalancing cations on the recombination rate

摘要

Diffuse reflectance UV-vis and Raman spectroscopies show complete sorption of neutral diphenylacetylene (DPA) in the void space of M_(6.6)ZSM-5 zeolites (M = Li~+, Na~+, K~+, Rb~+ Cs~+) after several months of exposure of solid DPA to empty zeolite at room temperature. After a long organization period, the laser photolysis of DPA occluded in M_(6.6)ZSM-5 generates long-lived DPA~(·+) radical cation as primary phenomenon. Charge recombination occurs mainly through electron transfer and DPA@M_(6.6)ZSM-5~(·-·+) electron-hole pair formation. This subsequent electron transfer takes place between the electron deficient radical cation DPA~(·+)and the electron donor oxygen atom of zeolite framework. The multivariate curve resolution analysis of the DRUVv spectra recorded during the reaction sequence including charge separation, electron transfer and charge recombination provide the specific absorption spectra and respective spectral concentrations of all species as function of time. The DRUVv spectrum assigned to the long-lived DPA@M_(6.6)ZSM-5~(·-·+) electron-hole pair exhibits broad bands between 450 and 550 nm. The electron-hole pair recombination depends on M~+ and appears to be in relation with the electron donor properties of the framework. The charge recombination rate decreases in the order Cs~+ > Rb~+ ~ K~+ > Na~+ > Li~+. The electron-hole pair lifetime exceeds several hours at room temperature. The stabilization of DPA~(·+)-electron pair and DPA@M_(6.6)ZSM-5~(·-·+) electron-hole pair depends on the combined effects of confinement which dramatically reduces the DPA mobility in the zeolite void space and on the intrazeolithe electrostatic field of DPA@M_nZSM-5 (M = Li~+, Na~+, K~+, Rb~+, Cs~+).