Investigating the role of heavy metal complexes as efficient triplet emitters for OLED applications

Abstract

We present here two iridium metal complexes, namely complex A and B, based on two different cyclometalated ligands 2-(4-fluorophenyl) H-imidazo [1,2-a] pyridine (pip), and 2-phenylpyrdine (ppy), and an ancillary ligand acetylacetone (acac) ligands for their use in organic light-emitting diodes. The structural and purity analysis of the complexes have been estimated via NMR spectroscopy and elemental analysis. UV–visible and photoluminescence spectroscopy were used to investigate the optical characteristics of the molecules. Both complexes exhibit nearly identical absorption spectra comprising of sharp bands in the ultraviolet region i.e., 245 nm–300 nm for the complex A and 232 nm and 304 nm for the complex B which are assigned to spin allowed π – π* transitions of cyclometalated (C^N) ligand and relatively weaker bands in the lower energy range i.e., 300 nm to 450 nm for the complex A and 403 nm and 457 nm for the complex B, which are ascribed to metal-ligand charge transfer (MLCT) transitions. Upon excitation, both of the complexes reveal green photoluminescence with their emission peaks at 535 nm and 520 nm for the complexes A and B, respectively. The thermal analysis of both complexes performed under a nitrogen atmosphere exhibit excellent thermal stability up to 300 °C. The electrochemical behavior of the complexes has been analyzed through cyclic voltammetry measurements and the corresponding values of HOMO/LUMO levels were found to be 5.09/2.52 eV for the complex A and 5.21/2.92 eV for the complex B. Furthermore, the diffusion of ions at the surface of electrodes and the corresponding values of diffusion coefficients have been estimated using the Randles-Sevcik equation.