Third‐Order Nonlinear Optical Properties of DA‐salen‐Type Nickel(II) and Copper(II) Complexes

Third‐order nonlinear and linear optical properties ofnickel(II) and copper(II) complexes with salen ligands, functionalised with electron donor/acceptor groups (DA‐salen), have been investigated in solution by the Z‐scan technique using an Nd:YAG laser ( λ inc = 1064 nm) and UV/Vis spectroscopy. The [M(DA‐ salen )] complexes exhibit positive nonresonant nonlinear refractive indexes ( n 2 I ) in the range27.0–8.50 · 10 –21 m 2  W –1 and nonlinear absorption coefficients ( α 2 I ) within the range 1.80–26.0 · 10 –17 m W –1 . The latter values correspond to less than 10 % of the overall magnitude of the third‐order susceptibility, | χ 3 |, which is the result of the absence of electronic transitions near λ inc = 1064 nm. For the group of Ni II complexes, the highest n 2 I values are exhibited by the complexes with aromatic diimine bridges and large π‐electron delocalisation, which leads to intense electronic charge‐transfer bands ( ϵ > 16000 mol –1  dm 3  cm –1 ) in the region λ = 275–500 nm. The complexes that exhibit charge‐transfer bands near λ inc /2 = 532 nm, also show the highest α 2 I values, a consequence of multiphoton absorption processes. For the group of Cu II complexes, the highest n 2 I value is also observed by the complex that exhibits the most intense charge‐transfer bands in the region λ = 275–475 nm. Between Ni II and Cu II complexes with the same DA‐salen ligands, those of copper show always higher n 2 I values, indicating that NLO responses can also be fine‐tuned by the metalcentre. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)