- The^3 MLCT excited states of the complexes have strong oxida-
tion and/or reduction power.
For example, rhenium(I) complexes have been used as emitter
materials in electroluminescent devices ( 17 – 20 ) and biological
probes ( 11 ), as dye for dye-sensitized solar cells ( 21 ), as
chromophores for photochemical electron or energy transfer
studies (14,17,22), and as a redox photosensitizer ( 23 ).
In addition, the family of rhenium(I) diimine carbonyl
complexesfac-[Re(LL)(CO) 2 (X)(Y)]nþ(LL¼a-diimine) has unique
properties and application as follows:
(a) They can be used as photocatalysts for multielectron transfer
reactions, especially for CO 2 reduction.
(b) They have been used as building blocks of emissive multinu-
clear complexes because they have at least three different
ligands.
(c) Some of them show unique photoreactivities that are strongly
dependent on the ligands.
(d) They have been used as target molecules for time-resolved IR
measurements because the CO ligands have strong
stretching absorption bands that are very sensitive to elec-
tron density at the central rhenium atom.
In this chapter, we focus on these unique photochemical pro-
perties of rhenium(I) diimine carbonyl complexes (Fig. 1), espe-
cially photochemical reactions and photocatalysis.
II. Photophysics of Rhenium(I) Diimine ComplexesA. ELECTRONICSTRUCTURE OFRHENIUM(I) DIIMINECOMPLEXES
Rhenium diimine carbonyl complexes generally have octahe-
dral coordination geometries. Their electronic states are
structured by the electronic interactions between the d orbitals
of the central rhenium ion and the molecular orbitals of the lig-
ands, such as CO, which has a strongp-accepting character,
and a-diimines, which can act as an electron pool (Fig. 2)
(24,25). The excited states pertaining to the useful photophysical
and photochemical properties of these complexes are (a) the
ligand-field (LF) excited states of the central rhenium, (b)pp*
excited states localized on the diimine ligand (LC), and (c) the
MLCT state from the rhenium to the diimine ligand. The empty
d orbital energy of the central rhenium(I) is high due to the large
LF splitting resulting from the low-oxidation state of the central
RHENIUM(I) DIIMINE COMPLEXES 139