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 (λinc = 1064 nm) and UV/Vis spectroscopy. The [M(DA‐salen)] complexes exhibit positive nonresonant nonlinear refractive indexes (n2I) in the range27.0–8.50·10–21 m2 W–1 and nonlinear absorption coefficients (α2I) 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, |χ3|, which is the result of the absence of electronic transitions near λinc = 1064 nm. For the group of NiII complexes, the highest n2I 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 dm3 cm–1) in the region λ = 275–500 nm. The complexes that exhibit charge‐transfer bands near λinc/2 = 532 nm, also show the highest α2I values, a consequence of multiphoton absorption processes. For the group of CuII complexes, the highest n2I value is also observed by the complex that exhibits the most intense charge‐transfer bands in the region λ = 275–475 nm. Between NiII and CuII complexes with the same DA‐salen ligands, those of copper show always higher n2I values, indicating that NLO responses can also be fine‐tuned by the metalcentre. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)