RESUMO
We apply photodetachment-photoelectron spectroscopy to measure the electron affinities and the energetics of the lowest excited electronic states of the neutral molecules para-terphenyl (p3P), para-quaterphenyl (p4P) and para-quinquephenyl (p5P), including especially the triplet states below S1. The interpretation of the experimental data is based on the comparison to calculated 0-0 energies and Dyson norms, using density functional theory and multireference configuration interaction methods, as well as Franck-Condon patterns. The comparison between calculated and experimental vibrational fine-structures reveals a twisted benzoid-like molecular structure of the S0 ground state and nearly planar quinoid-like nuclear arrangements in the S1 and T1 excited states as well as in the D0 anion ground state. For all para-oligophenylenes (ppPs) in this series, at least two triplet states have been identified in the energy regime below the S1 state. The large optical S0-S1 cross sections of the ppPs are rationalised by the nodal structure of the molecular orbitals involved in the transition. The measured electron affinities range from 380 meV (p3P) over 620 meV (p4P) to 805 meV (p5P). A saturation of the electron binding energy with the increasing number of phenyl units is thus not yet in sight.
RESUMO
The absorption and emission characteristics of (ppz)2(dipy)IrIII, (ppz)(dipy)PtII and (ppz)(dipy)PdII, 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 SMLCT/LC type in the heavier Ir and Pt complexes. Substantial spin-orbit coupling between SMLCT/LC and TLC 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 S1 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 SLC state and the SMLCT/LC state of the twist conformer have nearly equal adiabatic excitation energies. The lack of a driving force toward the SMLCT/LC minimum, the high fluorescence rate constant of the bright SLC state and its moderately efficient ISC to the triplet manifold explain the experimentally observed dual emission of the Pd complex at room temperature.