Luminescent Platinum(II) Complexes Containing Isoquinolinyl Indazolate Ligands:  Synthetic Reaction Pathway and Photophysical Properties

New Pt(II) dichloride complexes [Pt(1-iqdzH)Cl<sub>2</sub>] (<b>2a</b>) and [Pt(3-iqdzH)Cl<sub>2</sub>] (<b>2b</b>), in which idqzH = 1- or 3-isoquinolinyl indazole, were prepared by treatment of the corresponding indazoles with K<sub>2</sub>PtCl<sub>4</sub> in aqueous HCl solution. Despite their nonemissive nature, these complexes could react with excess indazole, sodium picolinate, and 3-trifluoromethyl-5-(2-pyridyl) pyrazole [(fppz)H] to afford the respective <b>a</b> and <b>b</b> series of luminescent complexes [Pt(1-iqdz)(L<sup>∧</sup>X)] and [Pt(3-iqdz)(L<sup>∧</sup>X)], where L<sup>∧</sup>X = 1-iqdz (<b>1a</b>), 3-iqdz (<b>1b</b>), pic (<b>3a</b>, <b>3b</b>), and fppz (<b>4a</b>, <b>4b</b>). Single-crystal X-ray diffraction studies of <b>1b</b>, <b>2a</b>, and <b>3b</b> revealed a planar molecular geometry without notable intermolecular Pt···Pt contact in the solid crystal, a result of the steric repulsion imposed by the bulky indazole fragments. For coordination complexes <b>1</b>, <b>3</b>, and <b>4</b>, photoluminescence in degassed CH<sub>2</sub>Cl<sub>2</sub> revealed high quantum efficiency and short radiative lifetimes in the range of several microseconds. As supported by the spectral feature, the associated radiation lifetimes, and a computational approach based on time-dependent density function theory (TD-DFT), the origin of the emission is attributed to a mixed <sup>3</sup>MLCT/<sup>3</sup>ππ transition. The TD-DFT approach further confirmed that, except for the series <b>1</b> complexes, the HOMO of 3-iqdz complexes <b>3b</b> and <b>4b</b> is much less located at the central Pt(II) atom than the HOMO orbitals of the respective 1-iqdz complexes <b>3a</b> and <b>4a</b>, leading to a smaller degree of MLCT contribution. Consequently, there are a blue-shifted emission signal and an inferior emission quantum yield for the 3-iqdz derivatives. OLED devices with a multilayer configuration of ITO/NPB/CBP:<b>3a</b>/BCP/Alq<sub>3</sub>/LiF/Al were fabricated using a CBP layer doped with various concentrations of <b>3a</b>, ranging from 6% to 100%, within the emitting layer. The best device performance was realized using a 6% doping concentration, for which the external quantum yield of 4.93%, luminous efficiency of 12.19 cd/A, and power efficiency of 6.12 lm W<sup>-1</sup> were observed at 20 mA/cm<sup>2</sup>, while a maximum luminescence as high as 20296 cd/m<sup>2</sup> was also realized at 16 V, showing good prospect for the fabrication of Pt(II) based OLEDs.