Bulk and confined alkanols by spin probe ESR: Effects of pore size, pore surface composition and pore topology on n-propanol in a series of mesoporous silica-based matrices

Abstract The spectral and dynamic behavior of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) in n-propanol (n-PrOH) as a representative of protic polar guests confined in a series of five silicas differing in pore size, surface and topology using electron spin resonance is presented. The hyperfine splitting constants Azz‘ and AN are higher compared with apolar hydrocarbons and the slow to fast motion regime transition at T50G(bulk) occurs in the liqudi state. Under confinement in the virgin silicas, the relative changes in both quantities with respect to the bulk n-PrOH are smaller and more sensitive to the pore size than those for hydrocarbons due to the preferential interaction of the polar pore wall of the virgin silicas with the polar n-PrOH over that with the polar TEMPO. This is supported by reductions of both quantities for the modified matrix. Finally, the T50G is essentially higher due to the very strong effect of the regular pore topology.