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 (σT/σg) ranging from 2.8 to 7.2, the characteristics of which correlate
well with electrochemical, computational, and transient absorption
data.