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 N+ 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 PTZ 2+ . 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 N+ and PTZ 2+ are only intermediates and that the thermodynamically stable end product is the occluded PTZ molecule.

Spontaneous Ionization of N‐Alkylphenothiazine Molecules Adsorbed in Channel‐Type Zeolites: Effects of Alkyl Chain Length and Confinement on Electron Transfer

“…Moreover, p-PTZ 2 + is never clearly observed. The comparison of the spectral concentrations found for the alkyl-PTZ molecules with that of the parent unsubstituted compound PTZ [24] also shows that the ionization process occurs more quickly for PTZ than for m-PTZ, e-PTZ, and p-PTZ.…”

“…Indeed, we have shown recently that the mere contact of PTZ with the zeolite inner or outer surface can induce an ionization process. [24] The contribution of solid m-PTZ could not be extracted but the strongly overlapping m-PTZ, m-PTZC + , and m-PTZ 2 + band maxima in the 200-350 nm spectral range make the band resolution very difficult to process in this region.…”

“…[24] Therefore, the methyl group is assumed to hinder partially the diffusion within the pore and consequently limit the ionization. However, unfortunately it should be noted that the absence of an absorbance coefficient for the dication in UV-visible spec- www.chemphyschem.org troscopy does not allow calculation of the absolute concentrations and that the diamagnetic behavior of the dication makes the EPR technique not useful to quantify both the radical cation and dication species.…”

“…[35] In that context, the sorption and the spontaneous ionization of phenothiazine (PTZ) in medium-pore zeolites were recently investigated to highlight the essential role of the confinement effect on electron transfer. [24] In particular, the close match between the molecule and the pore walls was shown to lead to exceptionally long-lived radical cations, which evolved readily to dications for high confinement.…”

“…[22,23] The effect of the confinement was also shown to be of high importance for electron transfer. [24,25] N-Methylphenothiazine (m-PTZ) and…”