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

Abstract

Four new mononuclear alkynyl complexes each featuring a terminal 2‐fluorenyl group, namely Fe(η5‐C5Me5)(CO)2[C≡C(2‐C21H25)] (2), Ru(κ2‐dppe)2Cl[C≡C(2‐C21H25)] (3), Ru(κ2‐dppe)2[C≡C(4‐C6H4NO2)][C≡C(2‐C21H25)] (4), and [Fe(η5‐C5Me5)(κ2‐dppe){C≡C(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.