Iron and Ruthenium Alkynyl Complexes with 2-Fluorenyl Groups: Some Linear and Nonlinear Optical Absorption Properties

Four new mononuclear alkynyl complexes each featuring a terminal 2-fluorenyl group, namely Fe((5)-C5Me5)(CO)(2)[CC(2-C21H25)] (2), Ru((2)-dppe)(2)Cl[CC(2-C21H25)] (3), Ru((2)-dppe)(2)[CC(4-C6H4NO2)][CC(2-C21H25)] (4), and [Fe((5)-C5Me5)((2)-dppe){CC(C5H4N)-CH2(2-C13H9)}][PF6] (5[PF6]), have been synthesized and characterized, and their redox, absorption, and emission properties have been studied. For the two ruthenium derivatives 3 and 4, these studies are complemented by spectroelectrochemical investigations, Z-scan measurements, and DFT calculations. Fluorimetric studies reveal that these compounds are poorly or not luminescent, and, when luminescent, that the detected weak luminescence most likely originates from a higher lying ligand-centred (LC) excited state presumably located on fluorene. Finally, the third-order nonlinear optical (NLO) properties of 3 and 4 are reported. It is shown that the bis-alkynyl complex 4 is significantly more active than 3 and that both compounds exhibit two-photon absorption (TPA) around 860-1050 nm, with TPA cross-sections above 350 GM. In addition, it is shown that both species should give rise to a marked switching of their cubic NLO properties in this spectral range upon oxidation.