Bromo-Substituted Phenylbenzothiazole Cyclometalating Ligands for the Development of Reverse Saturable Absorption Materials

Transition metal complexes have seen increased use in applications in the fields of photoredox catalysis, photodynamic therapy, biological sensing, and as phosphors for organic light-emitting diodes. Interest in transition metal complexes as nonlinear optical materials has increased as a result of advances in two-photon absorption and reverse saturable absorption (RSA) characterization techniques and observed characteristics. Iridium(III) organometallic chromophores are of particular interest as transition metal absorbers due to their strong spin-orbit coupling, which provides access to triplet excited states and ligand-based tuning of the molecular orbitals, which enables broad spectral absorption. Herein, new microwave-assisted synthetic methods, physical characterization, and photophysical characterization of a novel series of iridium-cyclometalated with bromo-substituted phenylbenzothiazole (pbt) ligands of the form [Ir(pbt)₂(acac)], where acac is acetyl acetonate, are reported. At an excitation wavelength of 532 nm, all complexes demonstrate RSA behavior with the ratio of the triplet absorption cross-section to the ground state absorption cross-section (σ_T/σ_g) ranging from 2.8 to 7.2, the characteristics of which correlate well with electrochemical, computational, and transient absorption data.