A spectroscopic, electrochemical and structural study of polarizable, dipolar ruthenium(II) arsine complexes as models for chromophores with large quadratic non-linear optical responses

The new series of complex salts trans-[(RuCl)-Cl-II(pdma)(2)L][PF6](2) [pdma = 1,2-phenylenebis(dimethylarsine); L = N-methyl-4,4'-bipyridinium (MeQ(+)) 2, N-phenyl-4,4'-bipyridinium (PhQ(+)) 3, N-(4-acetylphenyl)-4,4'-bipyridinium (4-AcPhQ(+)) 4, N-(2,4-dinitrophenyl)-4,4'-bipyridinium (2,4-DNPhQ(+)) 5 or N-(2-pyrimidyl)-4,4'-bipyridinium (2-PymQ(+)) 6] have been prepared. The known complex salt trans-[(RuCl)-Cl-II(pdma)(2)(4,4'-bpy)]PF6 (4,4'-bpy = 4,4'-bipyridine) 1 exhibits an intense d(pi)(Ru-II)-->pi*(4,4'-bpy) metal-to-ligand charge-transfer (MLCT) absorption with lambda(max) at 418 nm in acetonitrile, whilst 2-6 display d(pi)(Ru-II)-->pi*(L) MLCT bands with lambda(max) values in the region 486-544 nm. The MLCT energy decreases as the electron-accepting ability of L increases, in the order L = MeQ(+) < PhQ(+) < 4-AcPhQ(+) < 2,4-DNPhQ(+) < 2-PymQ(+). Cyclic voltammetric studies show that within the series 2-6, the energy of the Ru-based HOMO is almost constant, whilst that of the L-based LUMO decreases by ca. 0.4 eV moving from 2 to 6. Single-crystal structures of the complete series 1 . DMF, 2, 3 . MeCN, 4 . Me2CO, 5 . MeCN and 6 have been determined. Analysis of bond lengths and dihedral angles provides no evidence for ground state charge-transfer, despite the strongly dipolar, polarizable nature of these complexes.