(CF 3 SO 3 ) (L–L¼2,2^0 -bipyrimidine (bpm), 4,4^0 -dimethyl-2,2^0 -
bipyridine (dmb), 2,2^0 -bipyridine (bpy), and 1,10-phenanthroline
(phen)), including their electronic states, photophysics, and pho-
tochemistry ( 38 ). These complexes have a dp!p charge-trans-
fer absorption band in the region of 300–400 nm and pp
(diimine) transition bands in the shorter wavelength region of
300 nm. Even at room temperature, these complexes emit phos-
phorescence from the lowest triplet excited state (Table II).
Because the bpm complex shows a nonstructured broad emission
band in dichloromethane solution at room temperature, the low-
est excited triplet state was identified as the^3 MLCT state. How-
ever, bpy, dmb, and phen complexes showed a vibrational
structured emission spectra, and therefore, the lowest excited
state was attributed to the^3 pp* state localized on the diimine
ligands.
Irradiation of these tetracarbonyl complexes caused the
elimination of a CO ligand (Eq. 3).
Re diimineðÞðÞCO 4þ
þCF 3 SO 3 !
hn
Re diimineðÞðÞCO 3 ðÞCF 3 SO 3
þCO ð 3 ÞThe reaction quantum yields were varied from 0.03 to 0.9
(Table I).
It was suggested that the photoinduced ligand substitution
reactions proceed via the^3 LF state as a reactive state, but the
relative energy of this state compared to the lowest excited state,
that is,^3 LC, was not clearly estimated. The^3 MLCT state was
also proposed as another candidate of the reactive state because
the Re C bond strength should be lower in the^3 MLCT state
compared to the ground state owing to the removal of an electron
from a bonding dporbital.
TABLE II
EMISSIONPEAK(lE), EMISSIONQUANTUMYIELD(FE), EMISSIONLIFETIME(tE),AND
PHOTOREACTIONQUANTUMYIELD(FR)OF[RE(DIIMINE)(CO) 4 ](CF 3 SO 3 )INCH 2 Cl 2
UNDERCOATROOMTEMPERATURE.L–L le(nm) Fe te(ms) Fr
bpm 521 0.037 0.75 0.5
bpy 448, 476, 506 0.031 4.7 0.3
dmb 444, 473, 505 0.033 2.3 0.9
phen 459, 489, 525 0.008 3.8 0.05
RHENIUM(I) DIIMINE COMPLEXES 149