Luminescence Color Tuning of the Iridium Complexes by Interligand Energy Transfer (ILET) with Ancillary Ligands for Organic Light Emitting Diode

The series of new iridium complexes, Ir(C^N)₂(LX), (main ligand, C^N = the anion of 2-phenylpyridine (ppy), 4-difluoro-2-phenylpyridine (F₂-ppy) and 4-methyl-2,3-diphenylquinoline (4-Me-2,3-dpq); ancillary ligand, LX = 2-(2-hydroxyphenyl)benzoxazolate (BOX) and 2-(2-hydroxyphenyl)benzothiazolate (BTZ)) were prepared and their luminescence properties were investigated. We expected that the relative energy levels of the main ligands and ancillary ligands in the complexes could determine the possibility of interligand energy transfer (ILET) in the complexes and thereby luminescence properties. As the main ligands, F₂-ppy, ppy and 4-Me-2,3-dpq, which have drastically different energy gap between the HOMO and LUMO energy levels, were chosen and their complexes were synthesized. BOX and BTZ were chosen as the ancillary ligands which can form a stable 6-membered metallacycle with the iridium center. The iridium complexes showed various emission ranges from 510 to 643 nm, depending upon the relative energy levels of their main and ancillary ligands. The studies of photoabsorption, electrochemistry, photoluminescence and electroluminescence revealed that ILET might contribute to absorption and luminescence process of the iridium complexes containing the ppy-based ligands and BOX.