Derivatized 2-phenylbenzothiazole cyclometalating ligands for reverse saturable absorption (RSA) materials development

Transition metal chromophores (TMCs) are a widely studied class of materials due to their synthetic tunability and photophysical properties. Second- and third-row d6 TMCs, such as RuII or IrIII, are of particular importance due to their large spin-orbit coupling constants and the prevalence of metal-to-ligand charge transfer (MLCT) excited states. TMCs have found broad application in organic light-emitting diodes (OLEDs), photoredox catalysis, photodynamic therapy, and non-linear optics (NLO). Recent photophysical studies on organometallic iridium complexes of the form [IrIII(C^N)2(acac)]0, where C^N is a cyclometalating ligand and acac is acetylacetonate, have demonstrated their potential as reverse saturable absorption (RSA) materials. The photophysical properties, including photoluminescence and transient absorption spectra, are reported for [Ir(pbt)2(acac)], where pbt is 2-phenylbenzothiazole. In an attempt to engender new excited state absorption (ESA) bands, a triphenylamine (TPA) moiety was installed on the pbt ligand via microwave-assisted Suzuki coupling. The spectroscopic properties of the new TMC were compared to the parent [Ir(pbt)2(acac)] complex with particular emphasis on their potential application as RSA materials.