Luminescent Pt(II) complexes bearing dual isoquinolinyl pyrazolates: fundamentals and applications

A series of four Pt(II) metal complexes with trans-arranged isoquinolinyl azolates have been prepared, [Pt(Lx)2], x = 1-4, (1-4). The associated chelates possess various substituents; namely: one t-butyl (Bu(t)) at the 6-position (L1), two Bu(t) groups at the 5,7-positions (L2), one dip (2,6-di-isopropylphenyl) group at the 6-position (L3), and a single dip group at the 4-position of the 1-isoquinolinyl fragment (L4), respectively. Crystal structures of 1 and 4 were determined to shed light on the relationship of photophysics and packing arrangements. Their photophysical properties were measured and compared, for which the solid-state emission spectra of 2 and 4 are nearly identical to the solution spectra of all the Pt(II) complexes, showing the formation of isolated molecular entities. In contrast, the Pt(II) complexes 1 and 3 are found to be sensitive to their morphological states and external stimulus. This is confirmed by the gradual red-shifting of the emission with increasing concentration in the PMMA matrix, and the eventual formation of the broadened, metal-metal-to-ligand charge transfer (MMLCT) emission, by (i) wetting with acetone and drying in air, or (ii) grinding with a mortar and pestle, respectively. Organic light-emitting diodes (OLEDs) were also fabricated using multiple layered architecture and lowered doping concentration (e.g. 8 wt%), the latter is for avoiding dopant aggregation in the emitting layer. The associated OLED performances (i.e. η(max) = 11.5%, 8.5%, and 11.2% for 1, 2 and 3) confirmed their suitability and potential as dopants for phosphorescent OLEDs.