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 ms 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.
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