Photoinduced charge separation of methylphenothiazine in microporous titanosilicate M-ETS-10 (M=Na++K+, H+, Li+, Na+, K+, Ni2+, Cu2+, Co2+) and Na+, K+-ETS-4 molecular sieves at room temperature

Abstract

The photoionization of methylphenothiazine in microporous titanosilicates M-ETS-10 (M=Na⁺+K⁺, H⁺, Li⁺, Na⁺, K⁺, Ni⁺, Cu²⁺ and Co²⁺) and Na,K-ETS-4 molecular sieves with UV irradiation at room temperature is studied. Methylphenothiazine cation radicals (PC₁⁺) are produced in M-ETS-10 molecular sieves and are characterized by electron paramagnetic resonance (EPR) and diffuse reflectance (DR) UV-vis spectroscopy. Microporous titanosilicate Na,K-ETS-10 and its ion-exchanged molecular sieves with alkali and transition metal ions are shown to be efficient heterogeneous hosts to accomplish stable net photoinduced electron transfer from methylphenothiazine molecules at room temperature. The photoyield of PC₁⁺ cation radicals was enhanced by incorporation of protons into ion-exchange sites of Na,K-ETS-10. The photoionization efficiency of methylphenothiazine cation radicals decreases in the order H-ETS-10>Na,K-ETS-10>K-ETS-10>Na-ETS-10>Ni-ETS-10>Li-ETS-10>Cu-ETS-10>Co-ETS-10. For Na,K-ETS-4–PC₁, no PC₁⁺ radical is detected either by EPR or by DR spectroscopy. As the alkyl chain length of the phenothiazine increases from methyl to hexyl the photoionization yield of H-ETS-10 decreases. Thermogravimetric analysis results support this trend. It is found that the photochemistry in M-ETS-10 and Na,K-ETS-4 molecular sieves is sensitive to the metal ion, pore size and internal void space.