should cause stabilization (destabilization) of thep* orbital of
the bipyridine ligand, with only a small effect on the central
metal d orbital energy.- Introducing a phosphorous ligand with a strong (weak)
LF ligand should cause both stabilization (destabilization) of
the rhenium dporbitals and destabilization (stabilization) of
the rhenium ds orbitals, it does only a small effect on the
bipyridinep orbital.
The energy of the^3 MLCT excited state (E 00 (^3 MLCT)) can be
evaluated from the emission spectrum. Emission peak wave-
length (le), emission quantum yields (Fe), emission lifetimes
(te), and reaction quantum yields of the photochemical ligand
substitution reactions (Fr) are summarized in Table III. The
modification of the bipyridine ligand caused changes in
E 00 (^3 MLCT) as large as 2400 cm^1.
The relaxation processes from the^3 MLCT state can be
evaluated quantitatively by measuring the temperature depen-
dence of the emission yield and lifetime because the^3 MLCT state
is both the lowest and emissive excited state. Higher tempera-
ture caused largerFr. However,Feandtebecame smaller and
shorter at higher temperature, respectively. These experimental
results could be well simulated by assuming that the photochem-
ical reactions occur via the thermally accessible higher excited
(^1) MLCT
(^3) MLCT
(^3) LF
Ground state
ke kd kd’
kr
Reaction
ΔE
hn
(^1) LC( (^1) pp)
(^3) LC( (^3) pp)
SCHEME2. Schematic energy diagram offac-[Re(bpy)(CO) 3 {P(OEt) 3 }]þ
(3a).
154 HIROYUKI TAKEDAet al.