Electroabsorption Spectroscopic Studies of Dipolar Ruthenium(II) Complexes Possessing Large Quadratic Nonlinear Optical Responses

A series of 20 ruthenium(II) complex salts, known to exhibit large molecular static first hyperpolarizabilities β 0[HRS] from hyper-Rayleigh scattering measurements on acetonitrile solutions at 298 K (Coe, B. J.; et al. Inorg. Chem. 1997, 36, 3284; 1998, 37, 3391; J. Chem. Soc., Dalton Trans. 1999, 3617), have been studied by using electroabsorption spectroscopy in butyronitrile glasses at 77 K. The salts are of the form trans-[RuII(NH3)4(LD)(LA)][PF6]3 [LD = NH3 and LA = N-methyl-4,4‘-bipyridinium (MeQ+) (1), N-phenyl-4,4‘-bipyridinium (PhQ+) (2), N-(4-acetylphenyl)-4,4‘-bipyridinium (4-AcPhQ+) (3), N-(2,4-dinitrophenyl)-4,4‘-bipyridinium (2,4-DNPhQ+) (4), N-methyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium (Mebpe+) (5), N-phenyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium (Phbpe+) (6), or N-methyl-2,7-diazapyrenium (Medap+) (7); LD = 1-methylimidazole and LA = MeQ+ (8), PhQ+ (9), 4-AcPhQ+ (10), N-(2-pyrimidyl)-4,4‘-bipyridinium (2-PymQ+) (11), Mebpe+ (12), or Phbpe+ (13); LD = 4-(dimethylamino)pyridine and LA = MeQ+ (14), PhQ+ (15) or 4-AcPhQ+ (16); LD = pyridine and LA = 2-PymQ+ (17), Mebpe+ (18) or Phbpe+ (19); LD = 4-(dimethylamino)benzonitrile and LA = MeQ+ (20)]. The electroabsorption spectra afford dipole moment changes Δμ 12 for the visible metal-to-ligand charge-transfer (MLCT) excitations of the dipolar complexes. The transition dipole moments μ 12 cover a range of ca. 4−7 D and generally increase with the electron accepting strength of LA, most notably on replacing an N-methyl with a N-phenyl substituent. The Δμ 12 values are large (ca. 14−21 D) and generally increase with the size of LA. Comparison of experimental and calculated diabatic dipole moment changes Δμ ab suggests that the orbital(s) that receive the MLCT electron are delocalized only over the first two aryl rings. ZINDO calculations on the pentaammine complexes do not accurately reproduce the MLCT energies or β 0[HRS] values, but they can predict the dipole properties with reasonable accuracy and also indicate that the low lying π* orbitals span only the first two rings of LA. β 0[Stark] values of 1−20 calculated according to the two-state model by using β 0 = 3Δμ(μ 12)2/(E max)2 (E max = MLCT energy maximum) are mostly in good agreement with β 0[HRS]. The β 0[Stark] values are the first meaningful first hyperpolarizabilities for 18 and 20. The electroabsorption results confirm the unusually large magnitudes of β 0 in 1−20 and also that N-arylation enhances β 0, the latter effect being m