Photoinduced charge separation at room temperature of N-alkylpyrenes in transition metal ion containing mesoporous SiMCM-41 and MeMCM-41 silica molecular sieves where Me = Ti(iv), V(v), Ni(ii), Co(ii) and Cu(ii)

Abstract

The photoinduced charge separation of N-alkylpyrenes in transition metal ion containing mesoporous SiMCM-41 and MeMCM-41 materials [Me = Ti(IV), V(V), Ni(II), Co(II) and Cu(II)] with ultraviolet irradiation at room temperature is investigated. N-alkylpyrene cation radicals (PyC_n⁺) are generated in SiMCM-41 and MeMCM-41 mesoporous silica materials and are characterized by electron spin resonance (ESR) spectroscopy. Mesoporous SiMCM-41 and MeMCM-41 materials are shown to be effective heterogeneous hosts for the photoinduced formation and stabilization of PyC_n⁺ cation radicals at room temperature indicating stable photoinduced charge separation for several hours. The long lifetimes for the cation radicals indicate the utility of the environment of these MCM-41 materials for controlling back electron transfer. A series of TiMCM-41 materials are synthesized that increase the PyC_n⁺ photoionization efficiency in comparison with VMCM-41, NiMCM-41, CoMCM-41 and CuMCM-41. The photoyield efficiency is decreased significantly when the concentration of pyrene is increased above about 10⁻² M. The photoionization efficiency depends on the type and concentration of the metal ions in the mesoporous MeMCM-41 materials which suggests that certain metal ions can act as electron acceptors. Also, as the N-alkylpyrene alkyl chain length increases from methyl to hexadecyl the photoionization yield decreases. The photoionization efficiency increases in the order TiMCM-41 > NiMCM-41 > VMCM-41 > SiMCM-41 > CoMCM-41 > CuMCM-41. The photoyield is higher by about 1.5 times at 77 K compared to room temperature. The observed photoyields of PyC_n (n = 1, 4, 8, 12, or 16) in mesoporous MeMCM-41 materials suggest that these materials are useful models for long-term photoinduced charge separation at room temperature.