A periodic mesoporous organosilica-based donor-acceptor system for photocatalytic hydrogen evolution.

A new solid-sate donor-acceptor system based on periodic mesoporous organosilica (PMO) has been constructed. Viologen (Vio) was covalently attached to the framework of a biphenyl (Bp)-bridged PMO. The diffuse reflectance spectrum showed the formation of charge-transfer (CT) complexes of Bp in the framework with Vio in the mesochannels. The transient absorption spectra upon excitation of the CT complexes displayed two absorption bands due to radical cations of Bp and Vio species, which indicated electron transfer from Bp to Vio. The absorption bands slowly decayed with a half-decay period of approximately 10 mus but maintained the spectral shape, thereby suggesting persistent charge separation followed by recombination. To utilize the charge separation for photocatalysis, Vio-Bp-PMO was loaded with platinum and its photocatalytic performance was tested. The catalyst successfully evolved hydrogen with excitation of the CT complexes in the presence of a sacrificial agent. In contrast, reference catalysts without either Bp-PMO or Vio gave no or little hydrogen generation, respectively. In addition, a homogeneous solution system of Bp molecules, methylviologen, and colloidal platinum also evolved no hydrogen, possibly due to a weaker electron-donating feature of molecular Bp than that of densely packed Bp in Bp-PMO. These results indicated that densely packed Bp and Vio are essential for hydrogen evolution in this system and demonstrated the potential of PMO as the basis for donor-acceptor systems suitable for photocatalysis.