Electron paramagnetic resonance and diffuse reflectance spectroscopic studies of the photoionization of N-alkylphenothiazines in synthetic microporous M-clinoptilolite (M=Na++K+, H+, Li+, Na+, K+, Ni2+, Co2+, Cu2+) molecular sieves at room temperature

Abstract

Methylphenothiazine and other N-alkylphenothiazines were incorporated into M-clinoptilolite (M=Na⁺+K⁺, H⁺, Li⁺, Na⁺, K⁺, Ni²⁺, Co²⁺, Cu²⁺) channels by impregnation. Photoionization of N-alkylphenothiazines (PC_n) doped into dehydrated M-clinoptilolite samples at room temperature by ultraviolet irradiation results in the formation of N-alkylphenothiazine radical cations (PC_n^•+). The radicals were identified by the electron paramagnetic resonance (EPR) g-factor and by diffuse reflectance spectroscopy. Dehydrated M-clinoptilolites are efficient hosts for the formation and stabilization of PC_n⁺ at room temperature. As-synthesized M-clinoptilolite that was not dehydrated and then doped with PC_n was EPR silent. The PC_n⁺ yield was determined by double integration of the EPR spectrum. The PC_n⁺ yield is highest in H-clinoptilolite among the other ion-exchanged M-clinoptilolites. The photoionization efficiency to form PC_n⁺ decreases in the order H-clinoptilolite>Ni-clinoptilolite>K-clinoptilolite>Na-clinoptilolite>Co-clinoptilolite>Na,K-clinoptilolite>Li-clinoptilolite>Cu-clinoptilolite. The photooxidation yields decrease as the PC_n alkyl chain length increases from methylphenothiazine (PC₁) to decylphenothiazine (PC₁₀). The photoionization efficiency depends on the nature of the metal ion and the channel size of the M-clinoptilolite material.