Unveiling photophysical properties of cyclometalated iridium(iii) complexes with azadipyrromethene and dipyrromethene ancillary: a theoretical perspective

Abstract

A density functional theory/time-depended density functional theory was used to investigate a series of heteroleptic Ir(III) complexes (1–4) employing azadipyrromethene and closely related dipyrromethene derivatives as N^N ancillary ligands, in an effort to explore the underlying reasons of non-radiative behaviour of 1 and further adjust the photophysical properties by the modification of N^N ancillary ligands. The results reveal that the non-emissive phenomenon of 1 can be attributed to the weak ³ILCT character of the emissive excited state and large structure distortion, as well as the small T^opt₁–S^opt₀ energy gap. Upon tailoring the N^N ancillary ligands, the geometry distortion of 2–4 becomes obviously smaller in comparison with 1, accordingly, the spectrum properties are also markedly affected. For instance, the enlargement of frontier molecular orbital energy gaps from 1 to 4 results in the blue-shift of their absorption and emission spectra, which is considered to be dominated by the ancillary ligand, while there is a little contribution from the Ir(III) center. Importantly, further analysis on the quantum yield (Φ_PL) of these complexes also indicates a vital role of N^N ancillary ligands. It is intriguing to note that the designed complex 4 without pendant phenyl rings substituent in the ancillary ligand, possesses an efficient indirect spin-orbital coupling route, larger transition electric dipole moment (μ_S3), higher T^opt₁–S^opt₀ energy gap and smaller S₃–T₁ splitting energy (ΔE_(S3–T1)), which ensure its higher Φ_PL compared to other complexes.