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-[Ru^II(NH₃)₄(L^D)(L^A)][PF₆]₃ [L^D = NH₃ and L^A = Medap⁺ 1, PymQ⁺ 2, Mebpe⁺ 3 or Phbpe⁺ 4; L^D = pyridine (py) and L^A = Medap⁺ 8, PymQ⁺ 9, Mebpe⁺ 10 or Phbpe⁺ 11; L^D = 1-methylimidazole (mim) and L^A = Medap⁺ 12, PymQ⁺ 13, Mebpe⁺ 14 or Phbpe⁺ 15]. The salt trans-[Ru^II(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 d_π(Ru^II)→π*(L^A) metal-to-ligand charge-transfer (MLCT) absorptions in the region 560–700 nm. For a given L^A, the MLCT energy decreases as the donor strength of L^D 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 L^A 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 L^A. 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.

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