Carbene–bridge–carbene ensembles: tunable diradicaloids for singlet fission?

Platinum(II) complexes with rigid and electron donating ligands are efficient phosphorescent emitters at room temperature. However, the excited‐state lifetimes of these complexes are, in general, relatively long. This problem, which can result in the degradation of the emitter, can be overcome by exploiting the natural tendency of platinum to form metallophilic interactions and aggregates. Herein, we report platinum(II) bimetallic complexes with 100 % quantum yield and at the same time exceptionally short radiative decay times, which in some cases are up to a hundredfold faster than the analogous monometallic complexes.

The structural motif of platinum(II) complexes bearing cyclometalating N‐heterocyclic carbene ligands can be used to design deep‐blue phosphors for application in organic light‐emitting diodes. However, the photophysical properties of the resulting molecules are also highly dependent on the auxiliary ligand. These often allow molecular deformations in the excited state which contribute to non‐radiative decay processes that diminish the attainable quantum yield. The use of bis(pyrazolyl)borate‐based auxiliary ligands enforces a high molecular rigidity due to their unique geometry. The steric crowding in the coordination sphere inhibits deformation processes and results in highly efficient deep‐blue platinum(II) emitters with CIE coordinates below (0.15; 0.15).

We report a new class of C∧C* platinum(II) complexes in which excimer formation enhances the quantum yield while shortening the phosphorescence lifetime. Selective excitation of the monomer or dimer could be achieved at different wavelengths. These complexes exhibit strong phosphorescent emissions in the blue part of the visible spectrum around 450 nm with quantum yields of up to 93%. The emission behavior is controlled through the steric demand of the substituents at the 2,4-pentanedione ligand. We see dual emission with high photoluminescence quantum yields (PLQY) from monomeric and aggregated excited states.