Evidence for enhanced dipolar interactions between Pt centers in binuclear phosphorescent complexes

Transient studies are used to examine the radiative decay dynamics in a series of phosphorescent platinum binuclear complexes. The complexes studied consist of square planar (2-(4′,6′-difluorophenyl)pyridinato-N,C^2′)Pt units bridged by either pyrazole or thiopyridine ligands. We observe an increase in radiative lifetime as temperature is reduced from 300 K to 4 K when the binuclear complexes, named 1, 2 and 3 with Pt-Pt spacings 3.19 Å, 3.05 Å, and 2.83 Å, respectively, are doped into a p-bis(triphenylsilyly)benzene wide energy gap host. The lifetimes for 1, 2 and 3 are τ = 6.3 ± 0.1 μs, τ = 2.3 ± 0.1 μs, and τ = 2.0 ± 0.1 μs at T = 295 K, respectively. At T = 4 K, those values increase to τ = 8.6 ± 0.1 μs, τ = 14.4 ± 0.1 μs, and τ = 17.0 ± 0.1 μs, suggesting that the neighboring heavy metal centers in compounds 2 and 3 have significant orbital overlap. A three-level zero-field splitting model yields the lowest triplet energy splittings of 28 ± 3 cm^-1, 142 ± 9 cm^-1, and 113 ± 10 cm^-1 for compounds 1, 2 and 3, respectively.