Nonlinear Absorbing Cationic Iridium(III) Complexes Bearing Benzothiazolylfluorene Motif on the Bipyridine (N∧N) Ligand: Synthesis, Photophysics and Reverse Saturable Absorption

Four new heteroleptic cationic Ir­(III) complexes bearing benzothiazolylfluorene motif on the bipyridine (N∧N) (<b>1</b> and <b>2</b>) and phenylpyridine (C∧N) (<b>3</b> and <b>4</b>) ligands are synthesized and characterized. The influence of the position of the substituent and the extent of π-conjugation on the photophysics of these complexes is systematically investigated by spectroscopic methods and simulated by time-dependent density functional theory (TDDFT). The complexes exhibit ligand-centered <sup>1</sup><i>π</i>,<i>π</i>* transitions with admixtures of <sup>1</sup>ILCT (π­(benzothiazolylfluorene) → π*­(bpy)) and <sup>1</sup>MLCT (metal-to-ligand charge transfer) characters below 475 nm, and very weak <sup>1,3</sup>MLCT and <sup>1,3</sup>LLCT (ligand-to-ligand charge transfer) transitions above 475 nm. The emission of these complexes at room temperature in CH<sub>2</sub>Cl<sub>2</sub> solutions is ascribed to be predominantly from the <sup>3</sup>MLCT/<sup>3</sup>LLCT states for <b>1</b> and from the <sup>3</sup><i>π</i>,<i>π</i>* state for <b>2</b>, while the emitting state of <b>3</b> and <b>4</b> are assigned to be an admixture of <sup>3</sup>MLCT, <sup>3</sup>LLCT, and <sup>3</sup><i>π</i>,<i>π</i>* characters. The variations of the photophysical properties of <b>1</b>–<b>4</b> are attributed to different degrees of π-conjugation in the bipyridine and phenylpyridine ligands induced by different positions of the benzothiazolylfluorenyl substituents on the bipyridine ligand and different extents of π-conjugation in the phenylpyridine ligands, which alters the energy and lifetime of the lowest singlet and triplet excited states. <b>1</b>–<b>4</b> all possess broadband transient absorption (TA) upon nanosecond laser excitation, which extends from the visible to the NIR region. Therefore, <b>1</b>–<b>4</b> all exhibit strong reverse saturable absorption (RSA) at 532 nm for ns laser pulses. However, the TA of complexes <b>1</b>, <b>2</b>, and <b>3</b> are much stronger than that of <b>4</b>. This feature, combined with the difference in ground-state absorption and triplet excited-state quantum yield, result in the difference in RSA strength, which follows this trend: <b>1</b> ≈ <b>2</b> ≈ <b>3</b> > <b>4</b>. Therefore, complexes <b>1</b>–<b>3</b> are strong reverse saturable absorbers at 532 nm and could potentially be used as broadband nonlinear absorbing materials.