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 = C<sub>6</sub>H<sub>4</sub>C­(CH<sub>3</sub>)<sub>3</sub> (<b>1</b>), C<sub>6</sub>H<sub>5</sub> (<b>2</b>), C<sub>6</sub>H<sub>4</sub>F (<b>3</b>), C<sub>6</sub>H<sub>3</sub>(CF<sub>3</sub>)<sub>2</sub> (<b>4</b>)) 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CC<sub>6</sub>H<sub>4</sub>C­(CH<sub>3</sub>)<sub>3</sub> = 4-<i>tert</i>-butylphenylacetylene, CCC<sub>6</sub>H<sub>5</sub> = phenylacetylene, CCC<sub>6</sub>H<sub>4</sub>F = 4-fluorophenylacetylene, and CCC<sub>6</sub>H<sub>3</sub>(CF<sub>3</sub>)<sub>2</sub> = 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 <b>1</b>, <b>2</b>, and <b>3</b> 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.