Tuning of charge-transfer absorption and molecular quadratic non-linear optical properties in ruthenium(II) ammine complexes †

Abstract

The ligands N-methyl-2,7-diazapyrenium (Medap+), N-(2-pyrimidyl)-4,4′-bipyridinium (PymQ+), N-methyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium (Mebpe+) and N-phenyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium (Phbpe+) have been used to prepare a series of complex salts trans-[RuII(NH3)4(LD)(LA)][PF6]3 [LD = NH3 and LA = Medap+ 1, PymQ+ 2, Mebpe+ 3 or Phbpe+ 4; LD = pyridine (py) and LA = Medap+ 8, PymQ+ 9, Mebpe+ 10 or Phbpe+ 11; LD = 1-methylimidazole (mim) and LA = Medap+ 12, PymQ+ 13, Mebpe+ 14 or Phbpe+ 15]. The salt trans-[RuII(NH3)4(py)(4,4′-bpy)][PF6]2 (4,4′-bpy = 4,4′-bipyridine) 16 has also been prepared. The dipolar complexes in 1–4 and 8–15 exhibit intense dπ(RuII)→π*(LA) metal-to-ligand charge-transfer (MLCT) absorptions in the region 560–700 nm. For a given LA, the MLCT energy decreases as the donor strength of LD increases, in the order py < NH3 < mim. Within the pairs of Medap+/PymQ+ complexes, the energy of the Ru-based HOMO is constant and the MLCT energy decreases by ca. 0.3 eV as the acceptor strength of LA increases on going from Medap+ to PymQ+. The complexes of Mebpe+ or Phbpe+ also have similar HOMO energies which are lower than those of their Medap+/PymQ+ counterparts due to the increased basicity of LA. Replacement of Mebpe+ by Phbpe+ decreases the MLCT energy by ca. 0.1 eV due to the greater electron-withdrawing ability of Phbpe+. Single-crystal structures of 8·4MeCN and 16·2MeCN have been determined. Molecular first hyperpolarizabilities β of 1–4 and 8–15, obtained from hyper-Rayleigh scattering measurements at 1064 nm, are in the range (579–1068) × 10–30 esu. Static hyperpolarizabilities β0 derived by using the two-level model are also very large, with 13 having the largest at 336 × 10–30 esu. In general, β0 increases as the absorption energy decreases, in keeping with the two-level model.