Distinct Ir(III) complexes containing unsymmetric ligands with fluorene-oxadiazole groups and their performance of organic light-emitting diodes

Abstract

1,3,4-Oxadiazole was modified with phenyl and fluorenyl groups at the 2- and 5- positions, respectively, which can be used as a new cyclometalating ligand to synthesize a series of novel Ir(III) complexes. In the preparation of picolinate iridium(III) complexes [Ir(N^C)2(pic)] (N^C = cyclometalating ligand, pic = picolinate), two isomeric products were isolated unexpectedly, among which, the iridium(III) center in one product was cyclometalated at the phenyl site of two cyclometalating ligands. In the other, the cyclometalations occurred at the fluorenyl site and phenyl site from the two ligands respectively. Another possible isomer completely coordinated by two fluorenyl groups was not obtained owing to the very low product yield. The obtained two isomers were characterized by 1H NMR and MALDI-TOF. Single-crystal X-ray structural analysis of their methyl-substituted analogues confirmed the different coordination modes. Electrochemical and photophysical investigation revealed that the excited-state properties of the two isomers were closely related to their coordination modes, and thus a facile strategy was provided that is to regulate luminescent and electrochemical properties by easily changing coordination sites. Both isomers were used to fabricate high performance electroluminescence devices using spin-coating techniques. The unsymmetric coordination complex FP-DP exhibited the highest performance among all the devices, achieving a maximum current efficiency of 12.16 cd/A and luminance of 7037.69 cd/m2.