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) (1 and 2) and phenylpyridine (C∧N) (3 and 4) 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 1π,π* transitions with admixtures of 1ILCT (π­(benzothiazolylfluorene) → π*­(bpy)) and 1MLCT (metal-to-ligand charge transfer) characters below 475 nm, and very weak 1,3MLCT and 1,3LLCT (ligand-to-ligand charge transfer) transitions above 475 nm. The emission of these complexes at room temperature in CH2Cl2 solutions is ascribed to be predominantly from the 3MLCT/3LLCT states for 1 and from the 3π,π* state for 2, while the emitting state of 3 and 4 are assigned to be an admixture of 3MLCT, 3LLCT, and 3π,π* characters. The variations of the photophysical properties of 14 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. 14 all possess broadband transient absorption (TA) upon nanosecond laser excitation, which extends from the visible to the NIR region. Therefore, 14 all exhibit strong reverse saturable absorption (RSA) at 532 nm for ns laser pulses. However, the TA of complexes 1, 2, and 3 are much stronger than that of 4. 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: 123 > 4. Therefore, complexes 13 are strong reverse saturable absorbers at 532 nm and could potentially be used as broadband nonlinear absorbing materials.