Red to blue OLEDs based on highly luminescent 1,3-bis(4-phenylpyridin-2-yl)-4,6-difluorobenzene platinum(ii) complexes: the key role of substituents on the 4-phenylpyridine unit

Four 1,3-bis(4-phenylpyridin-2-yl)-4,6-difluorobenzene platinum(ii) complexes, bearing various substituents on the phenyl group, were employed for the preparation of novel organic light-emitting diodes (OLEDs): the complex with a simple phenyl substituent, Pt(1,3-bis(4-phenylpyridin-2-yl)-4,6-difluoro-benzene) (PtL1Cl), Pt(1,3-bis(4-(4-tert-butylphenyl)pyridin-2-yl)-4,6-difluoro-benzene)Cl (PtL3Cl), Pt(1,3-bis(4-(3,5-di-tert-butylphenyl)-pyridin-2-yl)-4,6-difluoro-benzene)Cl (PtL4Cl), and Pt(1,3-bis(4-(4-methoxy-2,6-dimethylphenyl)pyridin-2-yl)-4,6-difluoro-benzene)Cl (PtL6Cl). OLEDs fabricated with PtL4Cl as the emitter (6% wt in the emitting layer) produce the highest performing devices, putting in evidence the remarkable effect caused by the steric hindrance of the two tert-butyl groups on the 4-phenylpyridine moieties; it is characterized by an "emerald green" colour emission (CIE coordinates = 0.19, 0.44), a maximun external efficiency (EQEmax) of 20%, and a luminance of ca 23.000 cd m-2 under a 12 V driving voltage. Besides, OLEDs prepared with PtL6Cl (6% wt) exhibit a "sky-blue" colour emission (CIE coordinates = 0.13, 0.26), an EQEmax of 8%, and a luminance of ca 27.000 cd m-2 under a 10 V driving voltage. The design strategy of adding steric bulk to the platinum complexes is a useful route for producing highly efficient OLEDs whereas the introduction of methyls ortho to the interannular C-C bond is a way to go towards blue OLEDs. Besides, red OLEDs are fabricated by using a film of the pure complexes as emitting layer.