NIR-Emitting Cyclometalated Cp*-Ir(III) Complexes: Impact of Ligand π-Extension on Aggregation Behavior and Photophysical Properties

Half-sandwich iridium complexes exhibit poor photophysical properties. We reported the first case of half-sandwich cyclometalated iridium complexes with anticancer photodynamic activity, using two π-expansive ligands differing by one extra ring in [Cp*Ir(C^N)<i>L</i>]BF₄ complexes. Considering the ability of the ligands to aggregate through π-π interactions, which may reduce the emission energy, and the interest in NIR emitters, we envisaged to study the concentration-dependent solution and solid state photophysical properties. Besides emission, aggregation was verified by ¹H NMR, PFGSE-DOSY NMR experiments, X-ray diffraction, and DLS. It was found by X-ray diffraction the formation, with pair of enantiomers, of head-to-tail dimers that were further aggregated in some cases. The complexes with <i>L</i> = <i>N</i>-benzylimidazole became NIR emitters in solid state with a red-shift for the more π-expansive complex. The less π-expansive complex exhibited aggregation-enhanced emission. A significant effect on the emission of the additional ring was observed, that is explained by the stronger distortion of the excited state of the more π-expansive complex that favors disaggregation in solution. These are the first reported half-sandwich derivatives of any metal to exhibit NIR emission. DFT and TD-DFT studies support the experimentally observed features through the study of dinuclear model systems displaying π-π stacking interactions.