Platinum(II) Complexes with Pyridyl Azolate-Based Chelates:  Synthesis, Structural Characterization, and Tuning of Photo- and Electrophosphorescence

A new series of luminescent platinum(II) azolate complexes with a formula of [Pt(N∧N)2], in which N∧N = mppz (1), bppz (2a), bzpz (2b), bmpz (2c), bqpz (2d), fppz (3a), hppz (3b), bptz (4), hptz (5), were synthesized, and their photophyscial properties were examined. Single-crystal X-ray diffraction studies of 2c and 3b revealed a planar molecular geometry, in which the N∧N chelates adopt a trans configuration and show notable interligand C−H···N hydrogen bonding within the complex. Interesting intermolecular interactions were observed in the solid state. Complex 2c formed a slipped-stack structure with a Pt···Pt separation distance of 6.432 Å, while complex 3b showed a columnar stacking with the molecules oriented in an alternating order in relation to the chain axis, giving a much reduced Pt···Pt distance of 3.442 Å. The lowest absorption band for all complexes revealed strong state mixings between the singlet and triplet (MLCT and intraligand ππ*) manifolds. Complexes 1 and 2 showed mixed 3MLCT and 3ππ phosphorescence in fluid solution. While radiationless deactivation was apparently dominant for complxes 3−5 in solution, resulting in rather weak emission, strong phosphorescence was observed in the room-temperature solid state with the peak wavelength being significantly red shifted compared to that in solution. The emission nature has been tentatively assigned to be 3MMLCT in character. OLED devices with a multilayer configuration of ITO/NPB/CBP:2a/BCP/Alq3/LiF/Al were successfully fabricated using a CBP layer doped with various amount of 2a, ranging from 6 to 100%, as the emitting layer. A substantial red shift with increasing doping concentrations was observed in electroluminescence. With a neat film of 2a, the device showed a green emission with λmax at 556 nm and an external QE of ∼1.6% at a driving current of 20 mA. Similarly, for the device using a neat film of 3a, an electroluminescence centered at 616 nm with a slightly reduced external QE of ∼2.1% was recorded. Aggregation of platinum(II) complexes in the solid state was proposed to account for the large red-shift in electroluminescence.