Monocationic Iridium(III) Complexes with Far-Red Charge-Transfer Absorption and Near-IR Emission: Synthesis, Photophysics, and Reverse Saturable Absorption: Monocationic Iridium(III) Complexes with Far-Red Charge-Transfer Absorption and Near-IR Emission: Synthesis, Photophysics, and Reverse Saturable Absor

Liu, Bingqing; Lystrom, Levi; Cameron, Colin G.; Kilina, Svetlana; McFarland, Sherri A.; Sun, Wenfang

doi:10.1002/ejic.201900156

Two heteroleptic monocationic Ir( iii ) complexes bearing 6,6′-bis(7-benzothiazolylfluoren-2-yl)-2,2′-biquinoline as the diimine ligand with different degrees of π-conjugation were synthesized and their photophysics was investigated by spectroscopic techniques and first principles calculations. These complexes possessed two intense absorption bands at 300–380 nm and 380–520 nm in toluene that are predominantly ascribed to the diimine ligand-localized 1 π,π* transition and intraligand charge transfer ( 1 ILCT)/ 1 π,π* transitions, respectively, with the latter being mixed with minor 1 MLCT (metal-to-ligand charge transfer)/ 1 LLCT (ligand-to-ligand charge transfer) configurations. Both complexes also exhibited a spin-forbidden, very weak 3 MLCT/ 3 LLCT/ 3 π,π* absorption band at 520–650 nm. The emission of these complexes appeared in the red spectral region ( λ em : 640 nm for Ir-1 and 648 nm for Ir-2 in toluene) with a quantum yield of <10% and a lifetime of hundreds of ns, which emanated from the 3 ILCT/ 3 π,π* state. The 3 ILCT/ 3 π,π* state also gave rise to broad and moderately strong transient absorption (TA) at ca. 480–800 nm. Extending the π-conjugation of the diimine ligand via inserting CC triplet bonds between the 7-benzothiazolylfluoren-2-yl substituents and 2,2′-biquinoline slightly red-shifted the absorption bands, the emission bands, and the TA bands in Ir-2 compared to those in Ir-1 that lacks the connecting CC triplet bonds in the diimine ligand. The stronger excited-state absorption with respect to the ground-state absorption at 532 nm led to strong reverse saturable absorption (RSA) for ns laser pulses at this wavelength, with the RSA of Ir-2 being slightly stronger than that of Ir-1, which correlated well with their ratios of the excited-state to ground-state absorption cross sections ( σ ex / σ 0 ). These results suggest that extending the π-conjugation of the 2,2′-biquinoline ligand via incorporating the 7-benzothiazolylfluoren-2-yl substituents retained the broad but weak ground-state absorption at 500–650 nm, meanwhile increased the triplet excited-state lifetimes, which resulted in the much stronger triplet excited-state absorption in this spectral region and strong RSA at 532 nm. Thus, these complexes are promising candidates as broadband reverse saturable absorbers.

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