Simple tuning of the optoelectronic properties of Ir^III and Pt^II electrophosphors based on linkage isomer formation with a naphthylthiazolyl moiety

By tuning the substitution position of a thiazolyl group on the naphthalene system (α or β site), the distinctive electronic structures associated with these functional ligands have a substantial influence on both the photophysical behavior and electroluminescent (EL) performance of the resulting linkage isomers for two series of Ir^III and Pt^II phosphorescent emitters. The photoluminescence wavelength can be redshifted by ca. 23 nm for the linkage isomers upon replacing the β-substituted thiazolyl-based ligand with its α-substituted counterpart in the homoleptic series of Ir^III phosphors. Furthermore, the bathochromic effect can be as much as ca. 42 nm for the heteroleptic Ir^III phosphors and ca. 59 nm for the Pt^II compounds. Similarly, metallophosphors that bear β-substituted ligands exhibit a different EL performance with respect to that of their linkage isomers with α-substituted ligands. The best EL results associated with the triplet emitters chelated with β-substituted ligands show a maximum brightness (L_max) of 22563 cd m^-2, an external quantum efficiency (η_ext) of 12.88 %, a luminance efficiency (η_L) of 30.84 cd A^-1, and a power efficiency (η_p) of 26.17 lm W^-1, whereas the EL performance of their α-counterparts was characterized by a peak L _max of 8653 cd m^-2, η_ext of 6.18 %, η_L of 8.55 cd A^-1, and η_p of 6.54 lm W^-1. Owing to its unique electronic structure, the thiazolyl group is a good alternative to the pyridyl moiety to improve the EL performance of the metallophosphor. We have also demonstrated a simple and useful route to tune the functional properties of cyclometalated triplet emitters for EL applications. New thiazole-based phosphorescent emitters with iridium and platinum have been developed and applied in organic light-emitting devices. By simply changing the substitution position of the thiazolyl moiety on the naphthalene ring in each pair of linkage isomers, the optoelectronic and electroluminescent properties of these metallophosphors can be easily tuned.