Photophysical properties of iminopyrrolyl boron complexes: A DFT interpretation

The three compounds [BPh2(kappa N-2,N'-NC4H3C(H)=N-C6H5)] (A), [BPh2(kappa N-2,N'-NC4H3C(H)=N-1,4-C6H4-N=(H)C-H3C4N-N,N'kappa(2))BPh2] (B) and [BPh2(kappa N-2,N'-NC4H3C(H)=N-4,4'-C6H4-C6H4N=(H)C-H3C4N-N,N'kappa(2))BPh2] (C) are blue to green light emitters with average to excellent quantum yields. DFT and TD-DFT calculations were performed in order to understand their behaviour. The geometry of their ground and singlet excited state was optimised, and their absorption patterns and emission were calculated. The large dihedral angles between the planes of the N-aryl and 2-formiminopyrrolyl moieties of the chelating ligand drop considerably, leading to an almost planar geometry in the singlet state. In the three species, the lowest energy absorption occurs at 383, 428, and 419 nm, being calculated at 377, 502, and 529 nm in the gas-phase (slightly shifted in the right direction in THF). It results from a HOMO to LUMO excitation, and was assigned to a transition between pi and pi* orbitals (ILCT) of the iminopyrrolyl ligand, despite small differences in the exact composition of the HOMO and LUMO in A, B, and C. Introduction of the LB94 functional did not improve the calculated absorption wavelengths, but PBE0 with an all electron basis set within the SOPERT approximation led to better agreement with the experimental absorption maxima and a good reproduction of the excited state lifetimes. The estimated emission wavelengths reproduce the experimental trends.