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

New Pt(II) dichloride complexes [Pt(1-iqdzH)Cl₂] (2a) and [Pt(3-iqdzH)Cl₂] (2b), in which idqzH = 1- or 3-isoquinolinyl indazole, were prepared by treatment of the corresponding indazoles with K₂PtCl₄ 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 a and b series of luminescent complexes [Pt(1-iqdz)(L^∧X)] and [Pt(3-iqdz)(L^∧X)], where L^∧X = 1-iqdz (1a), 3-iqdz (1b), pic (3a, 3b), and fppz (4a, 4b). Single-crystal X-ray diffraction studies of 1b, 2a, and 3b 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 1, 3, and 4, photoluminescence in degassed CH₂Cl₂ 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 ³MLCT/³ππ transition. The TD-DFT approach further confirmed that, except for the series 1 complexes, the HOMO of 3-iqdz complexes 3b and 4b is much less located at the central Pt(II) atom than the HOMO orbitals of the respective 1-iqdz complexes 3a and 4a, 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:3a/BCP/Alq₃/LiF/Al were fabricated using a CBP layer doped with various concentrations of 3a, 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^-1 were observed at 20 mA/cm², while a maximum luminescence as high as 20296 cd/m² was also realized at 16 V, showing good prospect for the fabrication of Pt(II) based OLEDs.