Robust Phosphorescent Platinum(II) Complexes Containing Tetradentate O^N^C^N Ligands: Excimeric Excited State and Application in Organic White-Light-Emitting Diodes

摘要

Intra- and intermolecular metal-metal and ligand-ligand interactions are important characteristic features of platinum(II) complexes that have chelating C and/or N atom donor ligands;~([1-3])such complexes could be harnessed for self-assembly of nanostructured materials~([4]) and used for developing luminescent sensors of biological targets.~([5]) These intramolecular/intermolecular interactions give rise to lowenergy metal-metal-to-ligand charge transfer (MMLCT) and/or π(ligand)···π(ligand) excimeric excited states leading to red-shifts in both absorption and emission energies of oligomers from the corresponding monomeric counterparts.~([1-3]) In this area, cyclometalated platinum(II) complexes have been extensively studied as they exhibit dual phosphorescence in the high-energy (bluish green) and lowenergy (reddish orange) spectral regions. These two types of emission bands are often attributed to monomeric and excimeric emissions, respectively. The electronic structure of the excimeric excited states of Pt~(II) complexes (Figure 1), however, remains elusive. More than 20 years ago, Miskowski et al. suggested that the low-energy excimer emission is ascribed to 1) metal-metal-to-ligand charge transfer (~3MMLCT) for which upon excitation, there is an enhanced Pt···Pt interaction with concomitant shortening of the Pt···Pt distance or 2) transitions involving π···π interactions between aromatic ligands of interplanar separation <3.5 ?.~([6]) Brédas and Kim lately reported DFT/TDDFT calculations on FPt1 (FPt1=[2-(4',6'-difluorophenyl)pyridinato-N,C~(2'))](2,4-pentanedionato)platinum(II)) and their conclusion is in concordance with the proposal of Miskowski and co-workers, namely, an excimer is formed through the cooperative effect of both Pt···Pt and interligand π···π interactions.~([7]) On the other hand, Forrest and D'Andrade, based on the photoluminescent and electroluminescent studies on neat films of FPt1, proposed that the triplet excimer is formed through interactions between a monomer triplet exciton and a monomer ground state; the stabilization of the excimer is accounted for by a configuration interaction between exciton resonance and charge-transfer resonance states.~([8]) Recently, Kalinowski et al. made a similar conclusion on the formation of excimers from [Pt(N^C^N)Cl] (N^C^N=1,3-di(2-pyridyl)benzene and it's derivatives).~([9])