Effects of pore surfaces on the electronic states of metal complexes formed on bipyridine periodic mesoporous organosilica

Abstract

Bipyridine periodic mesoporous organosilica (BPy-PMO) is a porous material in which the pore walls consist of silica frameworks bridged by 2,2′-bipyridine (bpy) functional groups, and is expected to be a novel platform for catalysts and photocatalysts. Metal complexes formed on the pore surfaces of BPy-PMO are under unusual environmental conditions that are entirely different from those in the bulk solids or in solution. To understand the effects of the unique structural features of the pore surfaces on the metal complexes, this study focused on the combination of BPy-PMO with Re(bpy)(CO)₃Cl (Re-BPy-PMO) and analyzed the metal-to-ligand charge transfer (MLCT) absorption band of this material. UV/vis and infrared analyses demonstrated that the Re-complex in Re-BPy-PMO interacts with water molecules adsorbed on the pore surfaces. Quantum chemical calculations suggest that the Re-complex also interacts with SiOH groups on the silica framework. Combined interactions are responsible for the unexpectedly short MLCT absorption wavelength of Re-BPy-PMO. This hypothetical mechanism is consistent with a red shift of the MLCT absorption following trimethylsilylation, which transforms SiOH groups to SiOSiMe₃ (TMS) groups and removes adsorbed water. Calculations also suggest that the TMS group is more electron-donating than the SiOH group. Therefore, the trimethylsilylation should result in a blue shift of the MLCT absorption with regard to the substituent effect, which is inconsistent with the experimental observations. Evidently, the effect of the removal of water and SiOH groups is sufficiently powerful to counteract the blue shift. Thus, the pore-surface features have a significant influence on the metal complex.