Electronic nature of the emitting triplet in SF5-substituted cationic Ir(III) complexes

Abstract

A theoretical density functional theory study has been performed on a family of cationic iridium(III) complexes of the form [Ir(C^N)2(dtBubpy)]+ (dtBubpy = 4,4′-di-tert-butyl-2,2′-bipyridine), that incorporate 2-phenylpyridine (1, 2) and 1-phenylpyrazole (3, 4) cyclometallating C^N ligands functionalized with SF5 groups. The goal is to investigate the effect that the inclusion of SF5 groups in meta (1, 3) and para position (2, 4) with respect to the Ir–C bond has on the electronic nature of the emitting triplet state and the emission wavelength. The attachment of the electron-withdrawing groups induces the stabilization of the molecular orbitals localized on the C^N ligands and, in particular, of the highest-occupied molecular orbital (HOMO). This stabilization enlarges the energy gap between the HOMO and the lowest-unoccupied molecular orbital (LUMO), and shifts to higher energies the metal-to-ligand charge transfer (MLCT) triplet described by the HOMO → LUMO excitation. As a consequence, a triplet state of ligand-centered (LC) nature becomes the lowest-energy triplet excited state for all the four complexes. For complexes 1 and 2, the state is centered on the C^N ligands (3LCC^N) and the introduction of the SF5 groups in para causes a greater effect than their insertion in meta. Substitution of the pyridine ring by a pyrazole ring in complexes 3 and 4 destabilizes the 3LCC^N states and the lowest-energy triplet involves the diimine N^N ligand (3LCN^N). Theoretical calculations therefore predict that the electronic nature of the lowest-lying triplet drastically changes in passing from the unsubstituted complex (3MLCT) to complexes 1 and 2 (3LCC^N) and to complexes 3 and 4 (3LCN^N). These changes help to rationalize the aspect of the emission spectra and the shift to the blue of the emission wavelength. The appearance of low energy 3LC triplets centered on the ancillary ligand, whose energy is hardly affected by changes in the C^N ligands structure, constitutes a limit to blue-shift the phosphorescence emission in this kind of complexes.