Tunable and Efficient White Light Phosphorescent Emission Based on Single Component N‑Heterocyclic Carbene Platinum(II) Complexes

A new class of cyclometalated
pyridine N-heterocyclic carbene (NHC) Pt­(II) complexes with electronically
different alkyne derivatives (CCR; R = C6H4C­(CH3)3 (1), C6H5 (2), C6H4F (3), C6H3(CF3)2 (4)) as ancillary ligands were synthesized, and the
consequences of the electronic properties of the different substituted
phenylacetylene ligands on the phosphorescent emission efficiencies
were studied, where CCC6H4C­(CH3)3 = 4-tert-butylphenylacetylene, CCC6H5 = phenylacetylene, CCC6H4F = 4-fluorophenylacetylene, and CCC6H3(CF3)2 = 3,5-bis­(trifluoromethyl)­phenylacetylene.
Structural characterization, electrochemistry, and photophysical investigations
were performed for all four compounds. Moreover, the emission quantum
efficiencies and wavelength emission intensities of the complexes
were also recorded in different weight percents in poly­(methyl methacrylate)
films (PMMA) and evaluated in the CIE-1931 chromaticity diagram. The
square planar coordination geometry with the alkynyl ligands was corroborated
for complexes 1, 2, and 3 by
single crystal X-ray diffraction studies. These complexes show tunable
monomeric high energy triplet emission and an additional concentration-dependent
low-energy excimer-based phosphorescence. While adopting weight percent
concentrations between 15 and 25%, the two emission bands covering
the entire visible spectrum were obtained with these particular complexes
displaying the properties of an efficient white light triplet emitter
with excellent CIE-1931 coordinates (0.31, 0.33). On the basis of
the high luminescent quantum efficiency of over 50% for white light
emission, these compounds could be potentially useful for white organic
light-emitting diodes (WOLEDs) based applications.