Two-photon absorption properties of iron(II) and ruthenium(II) trischelate complexes of 2,2′:4,4″:4′,4‴-quaterpyridinium ligands

A series of Ru^II or Fe^II trischelate complex salts containing N-methyl/aryl-2,2′:4,4″:4′,4‴- quaterpyridinium ligands that has previously been subjected to quadratic nonlinear optical studies (Coe, B. J. et al. J. Am. Chem. Soc. 2005, 127, 13399) has now been investigated for two-photon absorbing behavior. Z-scan measurements using a 750 nm laser afford reasonably large two-photon absorption (2PA) cross-sections σ₂ of ca. 62-180 GM for the Ru ^II complexes, but only very weak 2PA is observed for the Fe ^II compounds. The excited-state and 2PA properties of the representative chromophore [Ru^II(Me₂Qpy²⁺) ₃]⁸⁺ (Me₂Qpy²⁺ = N″,N‴-dimethy]-2,2′:4,4″:4′,4‴- quaterpyridinium) have also been investigated by using semiempirical intermediate neglect of differential overlap/multireference-determinant single and double configuration interaction computations with the optimized geometry obtained via density functional theory. The calculated σ₂ value of ca. 624 GM at 1.70 eV for this metal-to-ligand charge-transfer chromophore is about 10 times larger than that obtained from the Z-scan studies.