Two Ru(II) complexes, [Ru(bpy)2L](ClO4)2 (1) and [Ru(bpy)2L′](BF4)2 (2), where bpy is 2,2′-bipyridine, L is diacetyl dihydrazone, and L′ 1:2 is the condensate of L and acetone, are synthesized. From X-ray crystal structures, both are found to contain distorted octahedral RuN6 2+ cores. NMR spectra show that the cations in 1 and 2 possess a C2 axis in solution. They display the expected metal-to-ligand charge transfer ( 1MLCT) band in the 400-500 nm region. Complex 1 is nonemissive at room temperature in solution as well as at 80 K. In contrast, complex 2 gives rise to an appreciable emission upon excitation at 440 nm. The room-temperature emission is centered at 730 nm (λem max) with a quantum yield (Φem) of 0.002 and a lifetime (τem) of 42 ns in an air-equilibrated methanol-ethanol solution. At 80 K, Φem = 0.007 and τem = 178 ns, with a λem max of 690 nm, which is close to the 0-0 transition, indicating an 3MLCT excited-state energy of 1.80 eV. The radiative rate constant (5 × 104 s-1) at room temperature and 80 K is almost temperature independent. From spectroelectrochemistry, it is found that bpy is easiest to reduce in 2 and that L is easiest in 1. The implications of this are that in 2 the lowest 3MLCT state is localized on a bpy ligand and in 1 it is localized on L. Transient absorption results also support these assignments. As a consequence, even though 2 shows a fairly strong and long-lived emission from a Ru(II) → bpy CT state, the Ru(II) → L CT state in 1 shows no detectable emission even at 80 K.
ASJC Scopus subject areas
- Inorganic Chemistry