Theoretical Studies on [Ru(bpy) 2 (NN)] 2+ [NN = Hydrazone and Azine]: Ground‐ and Excited‐State Geometries, Electronic Structures, Absorptions, and Phosphorescence Mechanisms

The ground‐ and excited‐state geometries, electronic structures, absorptions, and emissions of two ruthenium(II) complexes Ru(bpy) 2 (NN) [bpy = 2,2′‐bipyridine, NN = hydrazone ( 1 ) and azine ( 2 )] were investigated theoretically. Their ground and the excited state geometries were fully optimized at the B3LYP/MP2/LANL2DZ and UB3LYP/UMP2/LANL2DZ levels, respectively, and the calculated geometries are consistent with the X‐ray results. At the TD‐DFT level with the PCM model, the absorptions and phosphorescence properties of 1 and 2 were calculated on the basis of the optimized ground‐ and excited‐state geometries, respectively. Thecalculated lowest‐lying absorptions of 1 (512 nm) and 2 (598 nm) are attributed to a {[d   x   2 – y   2(Ru) + d xy (Ru) + π(NN)] → [π*(bpy)]} transition with MLCT/LLCT transition characters and a {[d   z   2(Ru) + d xy (Ru)] → [π*(NN)]} transition with dominant MLCT transition character, respectively. The calculated phosphorescence of 1 (638 nm) and 2 (731 nm) can be described as originating from a 3 {[d   x   2 – y   2(Ru) + d xy (Ru) + π(NN)] [π*(bpy)]} excited state with 3 MLCT/ 3 LLCT character and a 3 {[d   z   2(Ru) + d   x   2 – y   2(Ru)] [π*(NN)]} excited state with dominantly 3 MLCT character, respectively. The calculated results showed that the modulation of the lowest 3 MLCT excited state of this kind of Ru complexes can be achieved by changing the NN ligand from hydrazone to azine. Moreover, the fact that 2 displays phosphorescence but 1 does not can be interpreted by the different properties of the 3 MLCT excited state: the 3 MLCT excited state of 2 is more than 60 % occupied, whereas that of 1 is less than 20 % ( k r 1  <  k r 2 ). The lowest‐lying excited state of 1 is localized on the bpy ligand, whereas that of 2 lies on the NN ligand, and the nonradiative decay pathways of 1 are easier than those of 2 ( k nr 1  >  k nr 2 ). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)