PICOSECOND STUDIES OF ENERGY-TRANSFER IN PHOTOEXCITED MIXED ORGANOMETAL CHELATES

Energy transfer in a single crystal of [Rh(phpy)(2)bpy]PF6, substituti onally doped with 0.9% [Ir(thpy)(2)bpy](+) (phpy(-) = 2-phenylpyridine , bpy = 2,2'-bipyridine, and thpy(-) = 2,2'-thienylpyridine), is studi ed from picosecond transient phosphorescence measurements in the tempe rature range from 130 up to 290 K. The cation complexes of host and gu est compounds both exhibit a lowest excited (3) pi pi state, localize d on one phpy(-) or thpy(-) ligand, respectively. The emission spectru m of the undoped crystal is characterized by luminescence from an A tr ap, 98 cm(-1) below the origin of the host absorption. In the mixed cr ystal, the emission spectrum is dominated by luminescence from the dee p Ir3+ trap (3200 cm(-1) below the host band). After picosecond pulsed laser excitation, a decay of the phosphorescence intensity of the A t rap within about 100 ns is observed; the decay of the A-trap emission is accompanied by a rise in the intensity of the Ir3+-trap phosphoresc ence on the same time scale. The A-trap luminescence decay is attribut ed to intermolecular energy transfer, and its time behavior can be sim ulated using a 3D-exchange interaction model. In addition, it is found that, upon increasing the temperature, the characteristic time for ho pping between nearest donor species is increased. It is argued that, a t higher temperatures, the excitation hopping is among spatially exten ded donor states, characteristic of eigenstates of donor clusters. Due to the longer lifetime of these cluster eigenstates intercluster ener gy transfer is effectively slowed down.