Effect of pore confinement and molecular orientation on hydrogen transfer during a free-radical reaction in mesoporous silica

Mesoporous silicas with controllable pore size are providing important platforms finding a multitude of applications such as in catalysis, separations, and as nanoreactors for molecular transformations. In this study, we probe the influence of a coattached hydrogen donor molecule, fluorene, on the free-radical pyrolysis of 1,3-diphenylpropane (DPP) in MCM-41 silica as a function of pore size (1.6-2.8 nm). The influence of surface orientation of the fluorene molecule on the pyrolysis rate is examined through use of isomeric 2-hydroxyfluorene (2-FL) and 3-hydroxyfluorene (3-FL) precursors for surface attachment. The DPP pyrolysis rates are found to be sensitive to both the surface orientation of the fluorene molecule and the pore size. Furthermore, whereas the 2-FL led to faster DPP pyrolysis rates compared with 3-FL on the exterior surface of a nonporous silica nanoparticle (Cabosil), the opposite effect was observed in the mesoporous silica with the smallest pore size. The results are interpreted based on the influence of molecular orientation of the isomeric fluorene molecules on the key bimolecular hydrogen transfer steps to intermediate free-radicals on the surface and suggest that differences in surface curvature between the Cabosil particles and MCM-41 cylindrical pores may play a key role.