Photochemistry and Photophysics of Coordination Compounds

144 S. Campagna et al.
Ir(III) chromophores to the Ru(II) units is inefficient at room temperature in
fluid solution. This suggests that the Ir-to-Ru photoinduced energy transfer
rate constant in this tetranuclear species is lower than the intrinsic rate constant
for Ir decay (about 3.7 × 10 5 s –1 ), in spite of the nonnegligible driving
force (about 0.3 eV, from emission data). At 77 K, energy transfer from Irbased
to Ru-based chromophores is quantitative because of the much longer
lifetime (205 µs) of the excited state of the Ir-based units. Indirectly, the
room- and low-temperature results tend to suggest that Ir-to-Ru energy transfer
in the tetranuclear mixed-metal species would occur with a rate constant
of the order of 10 4 s –1 . The apparent discrepancy with the relatively fast energy
transfer rate constant for the Ru–Os species with three phenylene unit
bridges of the meta series discussed above (having a similar bridge to the
Ir/Ru tetranuclear system here discussed) shows that the energy transfer