Effects of geometric isomerism and regioisomerism on photophysical properties of iridium(iii) complexes.

Herein, six cyclometalated Ir(iii) complexes have been systematically designed and investigated via density functional theory (DFT) and time-dependent DFT (TD-DFT) to unravel the effects of geometric and regional isomerism on the optoelectronic properties of organic electroluminescent materials. The complexes comprise three isomers each of Ir(tfmppy)2(tpip) and Ir(ptfmpy)2(tpip), where tfmppy = 2-[4-(trifluoromethyl)phenyl]pyridine, ptfmpy = 2-fluoro-5-trifluoromethylpyridine, and tpip = tetraphenylimidodiphosphate. Geometric isomerism arises from the cis/trans configurations of N and C donor atoms in the two C^N chelating ligands, whereas regional isomerism stems from different substitution positions of the trifluoromethyl group on the phenylpyridine main ligand. Comprehensive calculations of geometric structures, absorption and emission spectra, frontier molecular orbitals (FMOs), spin-orbit coupling (SOC) constants, and a series of radiative and non-radiative rate constants reveal that geometric isomerism modulates the FMO energy gap by altering the spatial arrangement of ligands around the Ir metal center, thereby tuning the emission energy and wavelength through modifications of the microscopic electronic structure. Regional isomerism fine-tunes the FMO energy levels via substituent positional variations on the ligand framework, significantly influencing the absorption spectral profiles and optical bandgap. Analysis of spin-orbit coupling and key rate constants reveals that the studied complexes are predominantly phosphorescent emitters, with complex 6 showing a certain tendency towards thermally activated delayed fluorescence (TADF). This work may provide a theoretical reference for experimental synthesis, and the relevant luminescent materials are expected to find applications in OLEDs, thereby supporting their subsequent use in organic electroluminescence.