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

## TL;DR

This paper explores how extending ligand π-bonds in iridium complexes affects their aggregation and light-emitting properties, especially in the near-infrared range.

## Contribution

The first half-sandwich iridium complexes showing near-infrared (NIR) emission and anticancer photodynamic activity are reported.

## Key findings

- The more π-expansive complex emits in the NIR range in the solid state with a red-shift.
- The less π-expansive complex shows aggregation-enhanced emission.
- X-ray diffraction revealed head-to-tail dimer formation with enantiomers.

## Abstract

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)L]­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 L = N-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.