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 Lambda N)(2)], in which N Lambda 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 Lambda N chelates adopt a trans configuration and show notable interligand C-(HN)-N-... hydrogen bonding within the complex. Interesting intermolecular interactions were observed in the solid state. Complex 2c formed a slipped-stack structure with a (PtPt)-Pt-... separation distance of 6.432 angstrom, 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 (PtPt)-Pt-... distance of 3.442 angstrom. The lowest absorption band for all complexes revealed strong state mixings between the singlet and triplet (MLCT and intraligand pi pi*) manifolds. Complexes 1 and 2 showed mixed (MLCT)-M-3 and (3)pi pi 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 (MMLCT)-M-3 in character. OLED devices with a multilayer configuration of ITO/NPB/CBP:2a/BCP/Alq(3)/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 lambda(max) at 556 nm and an external QE of similar to 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 similar to 2.1% was recorded. Aggregation of platinum(II) complexes in the solid state was proposed to account for the large red-shift in electroluminescence.