Synthetic Strategy for Preserving Sky-Blue Electrophosphorescence in Square-Planar Pt(II) Complexes

Phosphorescent Pt(II) complexes having tetradentate ligands have emerged as promising materials for use in organic light emitting devices (OLEDs). One drawback that retards the full exploitation of their electroluminescence is a strong propensity for bathochromically shifted emissions. The chromic shift results from intermolecular association. Molecular strategies that avoid the intermolecular interactions are needed, particularly for producing blue electrophosphorescence. We have designed and synthesized a series of phosphorescent cycloplatinate complexes having bis(1-pyrazolylphenyl)methane tetradentate ligands (PtSN1-3). The ligands have been systematically modified to incorporate methyl substituents at different positions of the pyrazole moieties (PtSN1, no methyl substituent; PtSN2, 4-methyl substituents; PtSN3, 3-methyl substituents) with the aim of preserving intrinsic sky-blue phosphorescence while suppressing intermolecular interactions. The synthetic modifications control the extent of out-of-plane distortions in the cycloplatinate scaffold. Excimer emission is obtained from PtSN1-2 of planar platinacycles in diluted solutions (10 mu M) or in thin mCBP:TSPO1 (8 wt %) films (1:1, wt/wt; mCBP = 3,3-di(9H-carbazol-9-yl)biphenyl; TSPO1 = diphenyl(4-(triphenylsilyl)phenyl)phosphine oxide) due to strong spontaneous excimer formation with free energy changes of -4.4 to -3.1 kJ mol(-1). By contrast, helically distorted PtSN3 is capable of preserving its inherent sky-blue phosphorescence in concentrated states as it effectively suppresses excimer formation. An additional benefit of our synthetic control is improved stability against degradation for PtSN3. Finally, electroluminescence performances were evaluated by constructing and analyzing multilayer OLEDs employing the PtSN complexes as dopants. As expected, the PtSN3 devices produced sky-blue electrophosphorescence with a Commission Internationale de l'Eclairage coordinate of (0.16, 0.24) and a peak external quantum efficiency of 8.5%. The electrophosphorescence spectra of the PtSN1-2 devices were contaminated with excimer luminescence. These results collectively demonstrated the effectiveness of the helical distortion approach for obtaining blue electrophosphorescence.