reduction. Conversely, many of the complexes with
E1/2red<1.4V versus Ag/AgNO 3 show high photocatalytic abil-
ity. Although3f,2a, and 8 are exceptions, the reasons for this have
been clarified. For 3f, the PPh 3 ligand dissociated relatively
quickly from the one-electron reduced (OER) species [Re(bpy)
(CO) 3 (PPh 3 )], which is produced by photochemical electron trans-
fer from the reductant TEOA (Eq. 16). Because the OER species
of the solvent complex [Re(bpy)(CO) 3 (S)] ( 15 ,S¼DMF, TEOA)
has a more negativeE1/2redthan that of3f, electron transfer from
15 to3fproceeds (Eq. 17). Due to Eqs. (16) and (17) occurring
repeatedly,3fis converted into 15 in a chain reaction. A similar
chain reaction occurs with2a. The quantum yields of this ligand
substitution via light-induced electron transfer are reported as
16.9 for3fand 50.3 for2a(29,62). Due to the short lifetime of its
excited state, 15 shows only a low photocatalytic ability.
Re bpyðÞðÞCO 3 ðÞPPh 3
þS! Re bpyðÞðÞCO 3 ðÞS
þPPh 3
ð 16 ÞRe bpyðÞðÞCO 3 ðÞPPh 3þ
þ Re bpyðÞðÞCO 3 ðÞS! Re bpyðÞðÞCO 3 ðÞPPh 3
þ Re bpyðÞðÞCO 3 ðÞSþ ð^17 ÞThe reason why 8 does not work as a photocatalyst will be
discussed later.
The photocatalytic CO 2 reduction abilities of the biscarbonyl
complexes cis,trans-[Re(4,4^0 -X 2 -2,2^0 -bpy)(CO) 2 (PR 3 )(PR^03 )]þ have
been also reported. Generally, switching of the strongly elec-
tron-attracting CO ligand by other ligands results in a decrease
of the excitation energy of the lowest^3 MLCT excited state and
leads to shortening of the excited state lifetime according to the
Energy Gap Law( 5 ). Although this is favorable considering the
use of the solar energy as such complexes are able to absorb light
of longer wavelengths, it also means that light-induced electron
transfer, which is the initiating reaction in the photocatalytic
reaction, is suppressed due to the lowering of the oxidation power
of the excited state. In fact, CO 2 reduction does not proceed
withcis,trans-[Re(LL)(CO) 2 {P(OEt) 3 } 2 ]þ(11a, LL¼bpy, 4,4^0 -Me 2 -
2,2^0 -bpy) even under the same conditions as the photocatalytic
reaction of 3a( 66 ). However, biscarbonyl complexes with two
triphenylphosphine derivatives show exceptional photocatalytic
behavior. Photocatalytic CO 2 reduction proceeded with quantum
yield of 0.20 using11e, which has electron-withdrawing F atoms
at thep-positions of the phenyl groups (33,67). This is a result of
the emergence ofp–pinteractions between the aryl groups of the
170 HIROYUKI TAKEDAet al.