Luminescence Enhancement of an Indium(III) Complex Through Introducing an Electron-Donating Group in (1H-Pyrazol-1-yl)pyridazine.

New deep-blue-emitting materials are crucial for Organic Light-Emitting Diode (OLED) technology, as iridium-based blue emitters often suffer from degradation and inadequate colour purity. Indium(III) complexes offer an alternative, since the d10 configuration of In3+ precludes metal-centred transitions, and emission arises from ligand-centred states via the chelation-enhanced fluorescence (CHEF) effect. We previously reported complexes 1 ([In(LH)(H2O)Cl3]) and 2 ([In(LMe)2Cl2][InCl4]), which exhibited excitation-dependent emission, and the electronic transitions were assigned as ILCT for 1 and mixed ILCT/LL'CT for 2. Herein, we present a new complex, complex 3, i.e., [In(LMorph)2Cl2][InCl4], where LMorph contains electron-donating morpholine substituents, in contrast to the acceptor chloride group in LH and LMe. Complex 3 was characterised by elemental CHN analysis, infrared spectroscopy (IR), nuclear magnetic resonance spectroscopy (NMR), ultraviolet-visible spectroscopy (UV-Vis), single-crystal X-ray diffraction analysis (SC-XRD), and photoluminescence. The ionic structure of 3 is analogous to 2, with a cis-octahedral [In(LMorph)2Cl2]+ cation and tetrahedral [InCl4]- anion. The UV-Vis spectrum of 3 shows a significant bathochromic shift relative to 1 and 2 which is attributable to the morpholine group. TD-DFT calculations were employed to assign the electronic transitions. The introduction of a morpholino group into the pyridazine ring resulted in significant changes to the solid-state photoluminescence properties of the indium complex: (i) the emission maximum shifted to the red region (CIE 1931 coordinates: 0.1725, 0.2487), (ii) the lifetime increased by an order of magnitude, and (iii) the quantum yield rose to 15%. This work highlights that substituent variation on the pyrazolyl-pyridazine scaffold provides a versatile route to tune the structural and photophysical properties of indium(III) complexes.