Theoretical Study on Cationic Iridium(III) Complexes with a Diphosphane Ligand - Geometry, Electronic Properties, and Application for Light-Emitting Electrochemical Cells

A DFT/time-dependent DFT (TD-DFT) investigation was conducted on a series of cationic iridium(III) complexes with 2-phenylpyridine (ppyn) derivatives and a diphosphane (PPn) ancillary ligand to shed light on the effects of stereoisomerism and ligand substituents on the photophysical properties. The geometries, electronic structures, lowest-lying singlet-singlet absorptions, vertical singlet-triplet excitations, and triplet-singlet emissions of N,N-cis-[Ir(ppy0)2(PP)]+ (1), N,N-trans-[Ir(ppy0)2(PP)]+ (2) and their derivatives were investigated with DFT-based approaches [ppy0 = 2-phenylpyridine, PP = 1,2-bis(diphenylphosphanyl)ethene]. The complex N,N-trans-[Ir(ppy2)2(PP2)]+ (3b) shows high quantum phosphorescence efficiency (phi PL) of 91%, whereas an extremely low phi PL (<1%) was observed for N,N-trans-[Ir(ppy4)2(PP1)]+ (2d). To clarify this behavior, the S1-Tn splitting energy (ES1-Tn), the transition dipole moment (S1) upon the S0S1 transition, and the energy gap between the triplet metal-to-ligand charge transfer (3MLCT) -* and triplet metal-centered (3MC) d-d states (EMC-MLCT) were calculated. A drastically small ES1-T3 and large S1 for 3b (<0.05 eV and 1.38 D, respectively), compared to those for 2d (>0.2 eV and 1.26 D, respectively), were found to be closely linked to the substituents on the ppyn ligands. The remarkably small ES1-T3 and similar S1 for N,N-cis 1c (<0.05 eV and 1.41 D, respectively), compared to those for N,N-trans 2c (>0.1 eV and 1.42 D, respectively), could be attributed to the effects of the trans-cis structural isomerism. On the basis of these parameters, the higher phi PL of 3b with respect to that of 2d was explained, and 1c, 1d, 2b, and 2e were considered to have better physical properties than the experimentally synthesized complexes 2, 2d, and 3b. The newly designed 1c, 1d, 2b, and 2e are expected to be highly emissive in the blue-green region for light-emitting electrochemical cell (LEC) applications.