Comparison between Spontaneous and Photoinduced Ionization Mechanisms for p-Quaterphenyl in M-ZSM-5 (M = H+, Na+) Zeolites

Abstract

The electron transfers following the initial ionization of a probe molecule (p-quaterphenyl) adsorbed in the channels of M-ZSM-5 (M = H+, Na+) zeolites are investigated using various complementary spectroscopic techniques. Under the same reaction conditions, ionization occurs spontaneously during molecule diffusion in the acid H-ZSM-5 whereas charge separation needs to be photoinduced within the pores of Na-ZSM-5. The electron transfer processes are found to be identical for both the cases in terms of transient species formed before the final charge recombination. The initial ionization leads to the formation of a radical cation which evolves gradually toward an electron/hole pair associated with a charge transfer complex. However, the reaction kinetics depend dramatically on the ionization way. As spontaneous ionization is associated with the sorption process, it is closely correlated with the molecule diffusion and, thus, is very slow due to the bulky size of the molecule. In this case, radical cations and subsequent charge transfer complexes are stabilized for months in high yield and are clearly characterized by diffuse reflectance UV–vis absorption, EPR, and Raman scattering in resonance and in off-resonance conditions. After photoionization, the evolution is followed as a function of time by using time-resolved UV–vis absorption spectroscopy on a large scale of time extending from a few microseconds to several days.