Two-Photon Absorption Properties in “Push-Pull” Ruthenium Nitrosyl Complexes with various Fluorenylterpyridine-Based Ligands

Using the compound [Ru^II(FT)(bipy)(NO)](PF₆)₃ (FT is the electron-rich 4’-(2-fluorenyl)-2,2’:6’,2’’-terpyridine ligand and bipy is 2–2’bipyridine) as a reference, two new compounds are presented in which carbon-carbon double and triple bonds are inserted between the fluorenyl substituent and the terpyridine to provide an extended conjugation path. The electronic properties of the three complexes are compared experimentally by UV-visible spectroscopy and computationally by means of the density functional theory. All of them exhibit a capability for NO release under irradiation on their low-energy transition located in the 400–500 nm range, with a quantum yield around 0.01. Their two-photon absorption (TPA) cross sections are investigated by the Z-scan technique at λ=800 nm. While the reference compound exhibits a cross-section equal to 108 GM, the introduction of double and triple bonds leads to increased cross-sections equal to 131 GM and 150 GM, respectively. These values are discussed in reference to the two-level model in use for “push-pull” dipolar TPA chromophores.