Photochemical reduction of xanthone in an alcoholic solution flowing in a column packed with MCM-41, a mesoporous silica, was dependent on the magnetic field. The free radical intermediates were detected by the spin trapping method and the yield of spin adducts was also dependent on the magnetic field. In addition, a radicalquenching experiment was made to analyze the bulk concentrations of the free radical intermediates as functions of the magnetic field. Although these magnetic field dependencies were different from those observed in the micelle, we concluded that these observations are explained with the '' relaxation mechanism '' of the radical pair model: i.e. the intermediate radical pair in a cage has a life span comparable to or longer than the spin lattice relaxation times of these radical intermediates. The mechanism and characteristics of this cage effect in the nanotube, which is opened at the both ends, are discussed.

Abstraet.A spin-probe electron spin resonance (ESR) study was made on the alcoholic solution flowing through a quartz column packed with MCM-4I to clarify the dynamics of the liquid molecules in the nanochannel. The ESR spectra of a few hydrophobic spin probes showed that they undergo rotational diffusion preferentially along the longest molecular axis, indicating that the nanochannel is effectively narrowed further for these radicals by the influence of the solvent. Since almost identical ESR spectra were observed for the static samples, which were prepared in vacuo by introducing the solutions into the quartz tube with the MCM-41 powder and sealing off, the solution in the above-mentioned experiment should really flow through the nanochannel of MCM-41. Although a laminar flow is expected from the classical theory, the calculated flow rate is almost zero. In addition, the duration for the spin-probe mo[ecules to flow through the column was basically not dependent on their affinity to the silica surface. To explain all these phenomena, we propose a model that the liquid molecules flow collective[y by slipping on the surface of the nanochannel.

The ESR spectrum of a spin-probe in 2-propanol, which was condensed into the nano-channel of MCM-41 from its vapor, showed three well-separated nitrogen hyperfine lines. This observation indicates that the system exists in the liquid state in the nano-channel, though the rotational diffusion of the spin probe molecule is considerably suppressed and anisotropic. In a mixed solvent composed of 2-propanol and cyclohexane, on the other hand, the ESR spectrum for the system prepared by the above procedure comprised two components, indicating that the solution is phase-separated in the nano-channel. When all of the space in the MCM-41 layer, including that among the granules, is filled with the solution, the molecules obtain a higher mobility and phase separation is suppressed even in the nano-channel. The distribution and dynamics of the solution molecules are discussed.

The ability to adsorb water of ordered mesoporous silica was successfully controlled by organic modification with an organoalkoxysilane containing polyethylene oxide groups.

No abstract