cleavage (Y 1 ) and CO elimination (Y 2 ) was evaluated as Y 1 /
Y 2 ¼0.13 ( 35 ).
Irradiation of LnMRe(CO) 3 (L–L) (LnM¼(CO) 5 Mn, (CO) 5 Re,
Ph 3 Sn, Me 3 Sn; a diimine ligand (L–L) coordinated on one of the
metal centers) complexes also causes Re M bond homolysis, but
the reaction pathway is different. The longest wavelength absorp-
tion band of this type of complex was attributed to a^3 MLCT(Re
dp!p(L–L)) transition. Stufkenset al. reported that the Re M
bond homolysis proceeds via thesb!p(L–L) excited state pro-
duced by internal conversion from the^3 MLCT excited state( 36 ).
It has been also reported that Re R bond homolysis can be pro-
moted by the photoexcitation of mononuclear rhenium(I)
complexes with a Re alkyl bond, fac-[Re(CO) 3 (a-diimine)R]
(R¼alkyl ligand). This type of the reaction also proceeds via a
(^3) sp state, which is produced by the relaxation from the (^3) MLCT
excited state. In the cases of R¼ethyl (Et) and benzyl (Bz), the
homolysis reaction proceeds efficiently. However, the reaction effi-
ciency is much lower in the case R¼methyl (Me) because the tran-
sition from^3 MLCT to^3 sp requires extra energy (Scheme 1).
Using transient absorption spectroscopy, the^3 sp state was
directly detected as a reaction intermediate with an absorption
band at 500 nm. It was concluded that the potential curve of the
(^3) sp excited state should have a distinct minimum and that
the homolysis reaction will proceed competitively with the
nonradiative decay process to the ground state (Scheme 1) ( 37 ).
C. PHOTOCHEMISTRY OFRe(I) DIIMINETETRACARBONYLCOMPLEXES
Rillemaet al. have presented a comprehensive report on the
rhenium diimine tetracarbonyl complexes, [Re(L–L)(CO) 4 ]
Ea
kd
kd
kr
kr
Re• + R•
knr
G.S.
hn
isc
(^3) MLCT
(^1) MLCT
sp(Me)
sp(Et, Bz)
SCHEME1. Energy diagram of [Re(a-diimine) (CO) 3 R].
148 HIROYUKI TAKEDAet al.