than that of the lowest-energy empty d orbital of Re(I), making
the MLCT state the lowest excited state in many cases (Fig. 2).
The electronic transition to the MLCT andpp excited states
are optically allowed transitions, and they have relatively large
transition moments. They do not involve the population of an
orbital that is antibonding with regard to the ML bonds, in
contrast to the forbidden transitions to the LF excited states.
This is one of the reasons for photostability of Re(I) complexes.
However, as discussed in the following sections, photoinduced
chemical reactions have been reported in some cases, where
transitions to reactive higher-energy states arise from photoexci-
tation with shorter wavelength irradiation or thermal activation
from lowest excited state.
In reality, the electronic states of Re(I) complexes are mixed
with each other by configuration interactions, and the“real”elec-
tronic states are best viewed as composites of the“virtual”pure
electronic states such as MLCT andpp excited states, etc. In
this chapter, the“real”electronic states will be represented by
the “virtual” pure state which has the largest contribution to
the excited state properties.
B. PHOTOPHYSICALRELAXATIONPROCESS OFRHENIUM(I)
DIIMINECOMPLEXES
As the lowest excited state of many Re(I) diimine complexes is
the emissive^3 MLCT state, it can be tuned by chemical modifica-
tion of the diimine ligand and the identity of the monodentate
ligands. Figure 3 shows the UV/Vis absorption and emission
spectra of a typical rhenium(I) diimine complex, fac-Re(bpy)
(CO) 3 Cl (bpy¼2,2^0 -bipyridine) (1a), measured in CH 3 CN.
(^1) pp∗
(^3) pp∗
(^1) MLCT
(^3) MLCT
ic
isc ic
Absorption
Phosphorescence
G.S.
FIG. 2. The schematic energy diagram of the electronic states of the
rhenium(I) diimine complexes.
RHENIUM(I) DIIMINE COMPLEXES 141