However, photocatalytic activity was lowered in the cases ofRu
(mfibpy)ReandRe(mfibpy)Ruwith conjugated bridges. This is
thought to be due to more favorable electron transfer between the
Ru and Re units (TNCO¼14, 28, respectively)( 96 ). A decrease in
the reducing power of the rhenium complex due to the use of the
conjugate bridging ligand is the cause of the attenuated photo-
catalysis. As stated in Section IV.A, rhenium complexes cannot
photocatalyze CO 2 reduction unless the redox potentialsE1/2red
are more positive than1.4 V vs. Ag/AgNO 3. AsE1/2red¼1.10V
forRu(mfibpy)ReandE1/2red¼1.10V forRe(mfibpy)Ru, this
condition is not satisfied. This is likely to be the reason why these
supramolecules demonstrated only low photocatalytic abilities.
Photocatalytic CO 2 reduction of a supramolecule with a Zn por-
phyrin unit, which is a redox photosensitizer that can absorb
even wider ranges of visible light, connected to a rhenium com-
plex (ZnTMP-ReCl) was considered(101,102). In this system,
ultrafast (1.3 1012 s^1 ) electron transfer from the S 2 excited
state of the ZnTMP unit to the rhenium unit was observed.
Reduction of CO 2 proceeded with generation of the OER species
of the rhenium unit by the reduction of this intramolecular
charge-transfer state by TEA.
E. LIGHT-HARVESTINGSYSTEM WITHPERIODICMESOPOROUS
ORGANOSILICA
The low light density of sunlight may be a problem for solar
energy conversion. Photosynthesis alleviates this problem by
capturing sunlight with light-antennae unit consisting of chloro-
phyll stacks and concentrating the energy to reaction centers by
excitation energy transfer. Artificial systems that mimic the
light-antennae's light-harvesting abilities using chlorophyll
analogs ( 103 ) and porphyrin ( 104 ) have been an active field of
research for the past two decades. However, nearly no
applications of these artificial systems to photocatalytic reactions
have been made.
Recently, photocatalytic CO 2 reduction using a periodic meso-
porous organosilica (PMO, Scheme 9) as the light-harvesting
antenna has been reported. PMO has well-ordered pores with a
narrow pore size distribution and consists of organosilica
frameworks with aromatic rings aligned in a high-density
arrangement ( 105 – 108 ). The organosilica framework can absorb
light efficiently and efficiently transfer energy to guest molecules
situated within the pores, thereby efficiently generating the
excited state of the guest molecule.
180 HIROYUKI TAKEDAet al.