Synthesis, Photophysical Properties and Optical Limiting Behavior of Organometallic Complexes: Influence of Metal Centers and Conjugated Diimine Ligands

Four organometallic complexes featuring identical conjugated diimine ligands and varying transition metal centers (Rh(III), Ir(III), Ru(II), and Os(II)) were synthesized. A comprehensive investigation of the photophysical properties of these complexes and their corresponding diimine ligands was conducted by UV-vis absorption, emission, and transient absorption (TA) spectroscopy, as well as optical power limiting (OPL) measurements. The incorporation of fluorene components with alkyl chains enhanced the π-conjugation of the ligand, prevented intermolecular π-π stacking, and improved complex solubility. The diimine ligands showed a slight solvation effect in their emission spectra, indicating that the main property of their emissive state was intramolecular charge transfer (CT). According to the valence states of the four complexes, they can be divided into two classes (Rh(III), Ir(III)) and (Ru(II), Os(II)). For TA spectrum, the former group exhibited positive ΔOD values across the 400~800 nm range, while the latter showed large inverted peaks at 420~500 nm due to significant ground-state absorption in this wavelength range. Additionally, the optical limiting conduct of the complexes observed the order: 1 d>1 a>1 c>1 b. Complex 1 d exhibits powerful reverse saturable absorption (RSA) properties at 532 nm and could probably be used as an wonderful nonlinear absorbing material.