Solid-state NMR study on dynamics of p-nitroaniline molecules in FSM-type mesoporous silicas at high loading levels

We have investigated dynamics of deuterated p-nitroaniline (pNA-d) molecules in FSM-16 mesoporous silica (FSM) and FSM modified with trimethylethoxysilane (C1FSM) with different loading amounts ranging from 10 to 50 mass% by means of solid-state NMR. We have measured temperature dependence of 2H NMR spectra and spin-lattice relaxation times (T1) as well as 13C magic-angle-spinning (MAS) NMR spectra. The 2H and 13C MAS NMR spectra indicate that there are two modes of motions; an isotropic motion and a fast wobbling one. The rates of the wobbling motion are estimated from the 2H T1 results. pNA-d molecules in the FSM/pNA-d sample of 10 mass% undergo the wobbling motion at the low temperatures, whereas the isotropic motion is relatively suppressed at the high temperatures. In the samples of 20 and 30 mass% the wobbling motion is suppressed at the low temperatures, whereas the isotropic motion is enhanced at the high temperatures. The above changes further proceed in the sample of 50 mass%. The temperature dependences of 2H spectra in C1FSM/pNA-d of 13 and 20 mass% are similar to those of FSM/pNA-d of 20 and 30 mass%. C1FSM/pNA-d of 30 mass% shows spectra intermediate between FSM/pNA-d of 30 mass% and FSM/pNA-d of 50 mass%. We have proposed “three-layer model” to explain the above NMR results.