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Wenfang Sun1 Li Wang1 Hui Li1

1, North Dakota State University, Fargo, North Dakota, United States

Iridium(III) complexes are promising opto-electronic materials due to their intriguing electrochemical and photophysical properties. Because of the large spin-orbit coupling constant of the Ir(III) ion, Ir(III) complexes typically possess high triplet quantum yields and thus intense phosphorescence at room temperature. This makes Ir(III) complexes good candidates for applications in organic light-emitting diodes (OLEDs), luminescent biological reagents, molecular sensors, and light-emitting electrochemical cells (LEECs). In addition, many Ir(III) complexes exhibited broad and strong excited-state absorption in the visible to the near-IR region, which makes it possible to utilize these complexes for optical limiting application based on their reverse saturable absorption. For practical application, optical limiting materials are required to exhibit broadband spectral and temporal responses. For this purpose, we designed and synthesized two series of Ir(III) complexes with different π-conjugated cyclometalating C^N ligands. We found that the natures of the lowest singlet and triplet excited states were switched to the C^N ligand based 1,3CT (charge transfer) /1,3π,π* transitions when the π-conjugation of the core C^N ligand is larger than that of the core diimine (N^N) ligand. The increased π-conjugation of the C^N ligand red-shifted the spin-forbidden 3CT transitions in the UV-vis absorption spectra of the Ir(III) complexes, but reduced the intensity of the triplet excited-state absorption and the triplet lifetimes. Consequently, the optical limiting performance at 532 nm for ns laser pulses is reduced for the complexes with more extended π-conjugated C^N ligand. However, these complexes potentially could be used as broadband optical limiting materials in the near-IR region.

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