DFT/TDDFT investigation on the electronic structures and photophysical properties of phosphorescent Ir(iii) complexes with conjugated/non-conjugated carbene ligands

Abstract

A DFT/TDDFT investigation was performed on the electronic structures, basic photophysical properties and potential OLED applications of a series of Ir(III) complexes [(fpmb)₂Ir(fptz)] (1a), [(dfpmb)₂Ir(fptz)] (2a), [(fpmb)₂Ir(bptz)] (3a), [(dfpmb)₂Ir(bptz)] (4a) [where H₂fpmb = 1-(4-fluorophenyl)-2,3-dihydro-3-methyl-1H-benzo[d]imidazole; H₂dfpmb = 1-(2,4-fluorophenyl)-2,3-dihydro-3-methyl-1H-benzo[d]imidazole; fptzH = 2-(5-(trifluoromethyl)-2H-1,2,4-triazol-3-yl)pyridine; bptzH = 4-tert-butyl-2-(5-(trifluoromethyl)-2H-1,2,4-triazol-3-yl)pyridine] and [(fbmb)₂Ir(fptz)] (1b), [(dfbmb)₂Ir(fptz)] (2b), [(fbmb)₂Ir(bptz)] (3b), [(dfbmb)₂Ir(bptz)] (4b) [where H₂fbmb = 1-(4-fluorobenzyl)-2,3-dihydro-3-methyl-1H-benzo[d]imidazole; H₂dfbmb = 1-(2,4-fluorobenzyl)-2,3-dihydro-3-methyl-1H-benzo[d]imidazole] bearing two conjugated and nonconjugated carbene ligands respectively and one blue-emitting chromophore that possesses a much larger ligand energy gap. It is found that the photophysical properties of these complexes are greatly affected by the properties of the adopted ligands. From 1b to 4b, the insertion of a saturated σ-bond methylene spacer into the carbene ligand renders an increase of the HOMO–LUMO energy gap, but relatively weak absorption intensities. The emission spectra are slightly influenced by the switch between conjugated and nonconjugated ligands. Furthermore, on the basis of the calculations, the significantly higher quantum yield of 2b and 3b with respect to 3a is also explained with the assistance of their larger metal contribution to the lowest excited states (MLCT%) and smaller S₁–T₂ splitting energies (ΔE_S1–T2). Besides, the designed complexes 1b and 4b are considered to be potential candidates as blue-emitting materials with good charge transfer properties and high quantum efficiency.