On the photophysical properties of Ir, Pt, and Pd (phenylpyrazole) (phenyldipyrrin) complexes

• Published in: Physical chemistry chemical physics : PCCP 2020-02, Vol.22 (6), p.3217-3233
• Main Authors: Föller, Jelena, Friese, Daniel H, Riese, Stefan, Kaminski, Jeremy M, Metz, Simon, Schmidt, David, Würthner, Frank, Lambert, Christoph, Marian, Christel M
• Format: Article
• Language: English
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c9cp05603c

The absorption and emission characteristics of (ppz) 2 (dipy)Ir III , (ppz)(dipy)Pt II and (ppz)(dipy)Pd II , where ppz stands for phenylpyrazole and dipy for a phenyl meso -substituted dipyrrin ligand, have been investigated by means of combined density functional theory and multireference configuration interaction including scalar relativistic and spin-orbit coupling effects. These results were compared with experimental spectra. The complexes exhibit a high density of low-lying electronically excited states originating from ligand-centered (LC) and metal-to-ligand charge transfer (MLCT) states involving the dipyrrin ligand. In addition, metal-centered (MC) states are found to be low-lying in the Pd complex. In all three cases, the first strong absorption band and the phosphorescence emission band stem from LC excitations on the dipyrrin ligand with small MLCT contributions. The MLCT states show more pronounced relaxation effects than the LC states, with the consequence that the first excited state with predominant singlet multiplicity is of S MLCT/LC type in the heavier Ir and Pt complexes. Substantial spin-orbit coupling between S MLCT/LC and T LC enables fast and efficient intersystem crossing (ISC) and a high triplet quantum yield. Phosphorescence rate constants are rather small in accord with the dominant LC character of the transitions. Out-of-plane distortion promotes nonradiative decay of the excited state population via the MC states thus explaining the lower phosphorescence quantum yield of the Pt complex. The spectral properties of the Pd complex are different in many aspects. Optimization of the S 1 state yields a dipyrrin intraligand charge transfer (ILCT) state with highly distorted nuclear arrangement in the butterfly conformers leading to nonradiative deactivation. In contrast, the primarily excited S LC state and the S MLCT/LC state of the twist conformer have nearly equal adiabatic excitation energies. The lack of a driving force toward the S MLCT/LC minimum, the high fluorescence rate constant of the bright S LC state and its moderately efficient ISC to the triplet manifold explain the experimentally observed dual emission of the Pd complex at room temperature. The absorption and emission characteristics of (ppz) 2 (dipy)Ir III , (ppz)(dipy)Pt II and (ppz)(dipy)Pd II , where ppz stands for a phenylpyrazole and dipy for a phenyl meso -substituted dipyrrin ligand, have been investigated by theory and experiment.