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

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(pi)(Ru-II)-->pi*(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 beta of 1-4 and 8-15, obtained from hyper-Rayleigh scattering measurements at 1064 nm, are in the range (579-1068) x 10(-30) esu. Static hyperpolarizabilities beta(0) derived by using the two-level model are also very large, with 13 having the largest at 336 x 10(-30) esu. In general, beta(0) increases as the absorption energy decreases, in keeping with the two-level model.