Photoionization of <i>N</i>-Alkylphenothiazines in Mesoporous SiMCM-41, AlMCM-41, and TiMCM-41 Molecular Sieves

Abstract

Electron spin resonance and diffuse reflectance spectroscopy have been used to monitor the photoionization yields of N-alkylphenothiazine cation radicals (PCn+) generated by UV irradiation within mesoporous SiMCM-41, AlMCM-41, and TiMCM-41 molecular sieves. Mesoporous SiMCM-41 and AlMCM-41 molecular sieves are efficient for accomplishing stable photoproduced electron transfer of N-alkylphenothiazine molecules (PCn) at room temperature and at 77 K. A series of mesoporous TiMCM-41 molecular sieves with variable amounts of framework titanium have been hydrothermally synthesized and used for PCn photoionization in comparison with SiMCM-41 and AlMCM-41 molecular sieves. The addition of Ti into the framework of SiMCM-41 increases the net PCn+ cation radical photoyield at room temperature and 77 K. Thus, Ti in the TiMCM-41 framework is suggested to act as a better electron acceptor in competition with SiMCM-41 and AlMCM-41 molecular sieves. The photoionization efficiency to form PCn+ cation radicals increases in the order SiMCM-41 < AlMCM-41 < TiMCM-41. The photooxidation yields decrease as the alkyl chain lengths of the PCn molecules increase from methylphenothiazine (PC1) to hexadecylphenothiazine (PC16). Diffuse reflectance (DR) UV−visible spectra support the formation of some radical cation dimer as the alkyl chain length increases from PC1 to PC16 in all MCM-41 materials and thus the photoionization yields decrease from PC1 to PC16. The photoyield is higher by about 2.5 times at 77 K compared to room temperature. The PCn+ cation radicals are stable in SiMCM-41, AlMCM-41, and TiMCM-41 at room temperature for several hours and do not decay at 77 K. The observed photoyields of PCn molecules in mesoporous MCM-41 molecular sieves suggest that these materials can be utilized effectively for solar energy conversion and storage.