Triboluminescence and Metal Phosphor for Organic Light-Emitting Diodes: Functional Pt(II) Complexes with Both 2‑Pyridylimidazol-2-ylidene and Bipyrazolate Chelates

We
report the utilization of both pyrid-2-yl-imidazolylidene and
dianionic bipz chelates as constituents in syntheses of a new series
of charge-neutral Pt­(II) complexes 1–4, among which complex 4 revealed remarkable triboluminescence,
i.e., generation of photoemission upon grinding or cracking of the
solid sample. The triboluminescence is found to be sensitive to the
subtle changes of the associated substituents of pyrid-2-yl-imidazolylidene
chelate, as verified by the disappearance of the triboluminescence
for complexes 1–3. Alternatively,
the well-ordered solid packing of 3, as indicated by
the grazing incidence X-ray scattering experiment, serves as an ideal
emitter for the fabrication of highly efficient OLEDs, rendering high
external quantum efficienciy (25.9%) and luminesce efficiency (90
cd A–1) at the practical brightness
of 100 cd m–2. The rather low roll-off
in efficiency (24.4%, 85 cd A–1 at
high brightness of 1000 cd m–2) is attributed to
the short excited-state lifetime of 3 (∼800 ns)
in the solid state, which in turn is associated with the MMLCT transition
character.