The changes to the photophysical properties can be rationalized
by the following energy surfaces of the ground and^1 MLCT or
(^3) MLCT excited states of Re(I) complexes. It is reasonable to assume
that the interligand interaction decreases the lateral displacement
DQeof the MLCT excited state from the ground state by the delocal-
ization of the electron that is transferred onto the diimine ligand in
theMLCT excited state throughthe interaction (Fig. 7).Inthiscase,
less energy should be required for excitation process, and the
(^3) MLCT excited state can emit a higher-energy photon. In other
words, interligand interactions induce bathochromic and
hypsochromic shifts in absorption and emission spectra, respec-
tively. Further, the smaller lateral displacement gives a smaller
Franck–Condon factor, that is, a smaller nonradiative decay con-
stant, which results in an increase in the excitation lifetime.
III. Photochemistry of Rhenium(I) Complexes
A. PHOTOCHEMICALCO ELIMINATIONREACTION OF ARe(I)
HEXACARBONYLCOMPLEX
[Re(CO) 6 ]þbelongs to theOhpoint group and has a low-spind^6
electron configuration. This configuration is the same as the neu-
tral hexacarbonyl complexes of the Group 6 elements (Cr, Mo,
W), and they show similar photochemical reactivities.
GS 1 GS
MLCT
ΔQe(Ar)
with
p–p interaction
ΔQe(OR)
without
p–p interaction
or^3 MLCT^1 MLCTor^3 MLCT
FIG. 7. Energy surfaces of the ground state and the^1 MLCT or
(^3) MLCT excited state depending on the absence and presence of the
interligandp–pinteraction.
146 HIROYUKI TAKEDAet al.