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

Abstract

The mere mixing of N-alkylphenothiazines with three channel-type acid zeolites with various structures (ferrierite, H-MFI, and mordenite) induces the spontaneous ionization of the heterocyclic molecule in high yield upon adsorption. The diffuse reflectance UV-visible absorption and Raman scattering spectra show that the accessibility of the highly polarizing acid sites is not indispensable to induce the spontaneous ionization process. Due to their particularly low ionization potential values (6.7 eV), the adsorption of the molecules on the external surface or in the inner volume is the key parameter to generate the radical cation. However, the ionization yield and charge stabilization are intimately correlated to the possibility of the zeolites accommodating molecules inside their channels. Moreover, the higher electrostatic field gradient induced by high confinement is required to favor the second ionization and dication formation. The alkyl chain length plays a decisive role by either slowing down the diffusion process or blocking the molecule at the pore entry. Therefore, the efficiency of the ionization process that depends on the number of adsorbed molecules decreases significantly from phenothiazine to the N-alkylphenothiazines. The spectral data demonstrate that deformation of the alkyl group is necessary to allow the diffusion of the molecules into the channels.

Electron Transfers Induced by t-Stilbene Sorption in Acidic Aluminum, Gallium, and Boron Beta (BEA) Zeolites

“…The spontaneous ionization of trans -stilbene, − anthracene, − phenothiazine, − and para -terphenyl , after adsorption on acid zeolites has been reported by us. These studies have included aluminum and gallium forms of zeolite ZSM-5, , zeolites with different aluminum concentrations and with different pore structures.…”

Electron transfers in a TiO2-containing MOR zeolite: synthesis of the nanoassemblies and application using a probe chromophore molecule

“…[35][36][37][38][39] Amongst the already tested solids, we recently demonstrated the ability of porous zeolites to stabilize such charge separated states for especially long periods after adsorption of aromatic molecules having dimensions similar to the pore size in the zeolite, which is commonly between 4 and 8 Å in diameter. [40][41][42] In addition to their organized microporous frameworks, an important advantage of zeolites is to offer a high surface area and a cationic exchange capacity that is expected to facilitate the ionization of the adsorbed aromatic molecules and to help creating persistent charge separated states. Moreover, even though the ionization process can be initiated either electrochemically 43 or by photoexcitation 42,[44][45][46][47][48] or radiolysis, 49,50 it can also occur spontaneously if the ionization potential of the molecule is sufficiently low and if the energy of polarization in the zeolite channel is high enough.…”

“…Moreover, even though the ionization process can be initiated either electrochemically 43 or by photoexcitation 42,[44][45][46][47][48] or radiolysis, 49,50 it can also occur spontaneously if the ionization potential of the molecule is sufficiently low and if the energy of polarization in the zeolite channel is high enough. 40 As a whole, stabilization of the separated charge states in the zeolite can thus be considered as an intrinsic property of the inner charged surface of the zeolite micropores space.…”