Enhancement of Molecular Quadratic Hyperpolarizabilities in Ruthenium(II) 4,4‘-Bipyridinium Complexes by N-Phenylation

The new compounds N -phenyl-4,4‘-bipyridinium (PhQ + ), N -(4-acetylphenyl)-4,4‘-bipyridinium (4-AcPhQ + ), and N -(2,4-dinitrophenyl)-4,4‘-bipyridinium (2,4-DNPhQ + ), together with the known ligand N -methyl-4,4‘-bipyridinium (MeQ + ), have been used to prepare a series of Ru(II) complex salts trans -[Ru(NH 3 ) 4 (L D )(L A )](PF 6 ) 3 [L D = NH 3 and L A = MeQ + ( 1 ), PhQ + ( 2 ), 4-AcPhQ + ( 3 ), or 2,4-DNPhQ + ( 4 ); L D = 4-(dimethylamino)pyridine (dmap) and L A = PhQ + ( 7 ) or 4-AcPhQ + ( 11 ); L D = 1-methylimidazole (mim) and L A = PhQ + ( 8 ) or 4-AcPhQ + ( 12 ); L D = 4-(dimethylamino)benzonitrile (dmabn) and L A = PhQ + ( 9 ) or 4-AcPhQ + ( 13 ); L D = phenothiazine (PTZ) and L A = PhQ + ( 10 ) or 4-AcPhQ + ( 14 )]. These complexes display intense, visible metal-to-ligand charge-transfer (MLCT) absorptions, due to dπ(Ru II ) → π*(L A ) excitations. The MLCT energy decreases as the acceptor strength of L A increases, in the order MeQ + < PhQ + < 4-AcPhQ + < 2,4-DNPhQ +, and/or as the donor strength of L D increases, in the order PTZ < dmabn < NH 3 < mim < dmap. X-ray crystal structure determinations have been carried out for [PhQ + ]Cl·2H 2 O and trans -[Ru(NH 3 ) 4 (PhQ + )(PTZ)](PF 6 ) 3 ·Et 2 O ( 10 ·Et 2 O). [PhQ + ]Cl·2H 2 O, chemical formula C 16 H 17 ClN 2 O 2, crystallizes in the triclinic system, space group P 1̄, with a = 7.675(2) Å, b = 9.895(2) Å, c = 10.175(2) Å, α = 96.003(1)°, β = 104.74(2)°, γ = 90.398(1)°, and Z = 2. 10 ·Et 2 O, chemical formula C 32 H 44 F 18 N 7 OP 3 RuS, crystallizes in the triclinic system, space group P 1̄, with a = 10.310(3) Å, b = 10.698(2) Å, c = 20.986(4) Å, α = 95.09(2)°, β = 91.49(2)°, γ = 105.53(2)°, and Z = 2. The dihedral angles between the two pyridyl rings of the 4,4‘-bipyridinium unit are 19.8° in [PhQ + ]Cl·2H 2 O and 2.6° in 10 ·Et 2 O. Molecular first hyperpolarizabilities β of the complex salts, obtained from hyper-Rayleigh scattering measurements at 1064 nm, are in the range (698−1214) × 10 - 30 esu. Static hyperpolarizabilities β 0 derived by using the two-level model are very large, with trans -[Ru(NH 3 ) 4 (4-AcPhQ + )(dmap)](PF 6 ) 3 ( 11 ) having the largest at 410 × 10 - 30 esu. When the MLCT absorption is sufficiently far from the second harmonic at 532 nm, both β and β 0 increase as the absorption energy decreases.