Sorption and spontaneous ionization of phenothiazine within channel type zeolites: Effect of the confinement on the electron transfers.

Abstract

Diffuse reflectance UV-visible absorption and Raman scattering experimental data show evidence of the phenothiazine (PTZ) sorption and spontaneous ionization in the straight channels of three medium pore acid zeolites with various topologies (ferrierite (H-FER), H-ZSM-5 and mordenite (H-MOR)) but analogous Si/Al contents. The spectral data highlight the combined effects of confinement and local electrostatic field on the sorption and charge separation kinetics. The PTZ incorporation and ionization appeared to be quicker in the larger pore H-MOR than in H-ZSM-5 and in H-FER. However, sorption and ionization are almost complete in the three zeolites after about one year. The low ionization potential value of PTZ (I.P. = 6.73 eV) induced quasi instantaneous formation of the radical cation PTZ^•+ in high yield within the internal space of each channel structure. Nevertheless, the higher confinement effect and higher polarizing effect offered by the 10-membered rings (10-MR) channels of H-FER favoured the PTZ second ionization to form the dication PTZ²⁺. The very long lifetimes of these charge separated states are probably due to the restricted mobility of PTZ in the narrow channels and to the compartmentalization of the trapped electron away from the initial site of PTZ ionization. However, a very slow charge recombination process is observed within the three zeolite morphologies after about one year. This reaction is only partial in the narrower pores of H-FER and H-ZSM-5 whereas the faster diffusion process within the larger pore H-MOR induces quasi total cation disappearance after 2 years. Therefore, the reaction mechanism indicates clearly that PTZ^•+ and PTZ²⁺ are only intermediates and that the thermodynamically stable end product is the occluded PTZ molecule.