Evgeniia Sergeevna Sedykh, Nikita Vasilievich Naletov, Veronika Igorevna Komlyagina, Yulia Sergeevna Spiridonova, Elvira Ilgizovna Musina, Marianna Ivanovna Rakhmanova, Nikolay Filippovich Romashev, Katerina Aleksandrovna Vinogradova, Marat Damirovich Nafikov, Alexey Yuryevich Vorob'ev, Iakov Sergeevich Fomenko, Artem Leonidovich Gushchin
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.