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-acetylphen yl)-4,4'-bipyridinium (4-AcPhQ(+)), and N-(2,4-dinitrophenyl)-4,4'-bip yridinium (2,4-DNPhQ(+)), together with the known ligand N-methyl-4,4' -bipyrihinium (MeQ(+)), have been used to prepare a series of Ru(II) c omplex salts trans-[Ru(NH3)(4)(L-D)(L-A)](PF6)(3) [L-D = NH3 and LA = MeQ(+) (1), PhQ+ (2), 4-AcPhQ+ (3), or 2,4-DNPhQ(+) (4); LD = 4-(dimet hylamino)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-AcP hQ(+) (13); L-D = phenothiazine (PTZ) and L-A = PhQ(+) (10) or 4-AcPhQ (+) (14)]. These complexes display intense, visible metal-to-ligand ch arge-transfer (MLCT) absorptions, due to d pi(Ru-II) --> pi(LA) excit ations. The MLCT energy decreases as the acceptor strength of LA incre ases, in the order MeQ(+) < PhQ(+) < 4-AcPhQ(+) < 2,4-DNPhQ(+), and/or as the donor strength of LD increases, in the order PTZ ( dmabn ( NH3 ( mim ( dmap. X-ray crystal structure determinations have been carrie d out for [PhQ(+)]Cl . 2H(2)O and trans-[Ru(NH3)(4)(PhQ(+))(PTZ)] (PF6 )(3). Et2O (10 . Et2O). [PhQ(+)]Cl . 2H(2)O, chemical formula C16H17Cl N2O2, crystallizes in the triclinic system, space group Pi, with a = 7 .675(2) Angstrom, b = 9.895(2) Angstrom, c = 10.175(2) Angstrom, alpha = 96.003(1)degrees, beta = 104.74(2)degrees, gamma = 90.398(1)degrees , and Z = 2. 10 . Et2O, chemical formula C32H44F18N7OP3RuS, crystalliz es in the triclinic system, space group P (1) over bar, with a = 10.31 0(3) Angstrom, b = 10.698(2) Angstrom, c = 20.986(4) Angstrom, alpha = 95.09(2)degrees, beta = 91.49(2)degrees, gamma = 105.53(2)degrees, an d Z = 2. The dihedral angles between the two pyridyl rings of the 4,4' -bipyridinium unit are 19.8 degrees in [PhQ(+)]Cl . 2H(2)O and 2.6 deg rees in 10 . Et2O. Molecular first hyperpolarizabilities beta of the c omplex salts, obtained from hyper-Rayleigh scattering measurements at 1064 nm, are in the range (698-1214) x 10(-30) esu. Static hyperpolari zabilities Bo derived by using the two-level model are very large, wit h trans-[Ru(NH3)(4)(4-AcPhQ(+))(dmap)] (PF6)(3) (11) having the larges t at 410 x 10(-30) esu. When the MLCT absorption is sufficiently far f rom the second harmonic at 532 nm, both beta and beta(0) increase as t he absorption energy decreases.