Two tridentate pyridinyl-hydrazone zinc(II) complexes as fluorophores for blue emitting layers Rosita Diana a , Barbara Panunzi a , Angela Tuzi b , Ugo Caruso b, * a Department of Agriculture, University of Napoli Federico II, Portici, NA, Italy b Department of Chemical Sciences, University of Napoli Federico II, Napoli, Italy article info Article history: Received 18 June 2019 Received in revised form 19 July 2019 Accepted 26 July 2019 Available online 27 July 2019 Keywords: OLEDs Zinc complex PLQYs abstract Two new complexes were obtained by reaction of zinc (II) acetate with 4-fluoro-N'-(pyridin-2- ylmethylene) benzohydrazide or with 4-(hexyloxy)-N'-(pyridin-2-ylmethylene)benzohydrazide ligands in pyridine. Both ligands have a pyridinyl-hydrazone moiety able to act as a mono-negative tridentate ligand toward zinc ion in 2:1 stoichiometric ratio, producing an octahedral environment. The derived complexes are poor emitters in diluted solution. Instead, they exhibit intense blue fluorescence in the solid state due to AIE (aggregation induced emission) effect. The crystalline complexes are bright yellow in natural light and blue under UVevisible light with remarkable Stokes Shifts. Their photoluminescence quantum yields are ranging 20e30%, making them very promising as fluorophore dopants for blue emissive layers. One ligand and its complex were characterized by single crystal X-ray diffraction analysis that revealed an almost planar conformation of two ligands coordinated to the metal and the presence of significant p-p stacking of molecules at about 3.4 Å. © 2019 Published by Elsevier B.V. 1. Introduction Polydentate hydrazones are important scaffolds with a broad range of multidisciplinary applications in coordination chemistry. There are numerous reports dealing with aryl-hydrazone metal complexes as catalytic systems [1e 12], antimicrobial and antifungal molecules [13e16], chemospecific reactants [17], DNA binding and cleavage active tools [18, 19]. In addition to the biological activity, complexes from poly- dentate hydrazone has been investigated intensively in the last years for the wide possibility of coordination numbers, crystal structure and sometimes unusual symmetries [20e22]. In recent years, the optical and electro-optical properties of polydentate chelates were examined. The specific goal is to achieve light ab- sorption/emission performance for targeted technological appli- cations. In particular, their photoluminescence (PL) performances have been widely investigated as neat solids or as dopants in the construction of the emitting layers for LEDs and solar cells [23,24]. A lot of emissive heavy-metal complexes have been proposed to achieve high PL performance [4,25]. Despite recent progress, low device stability and high cost of rare metals still represent a limit in the industrialization of LEDs, especially for the blue emitters. From our laboratories a large number of articles have been published on metal complexes containing aroyl-hydrazones and/or pyridinyl-hydrazones [20,26], specially zinc (II) based. Zinc as a low cost and biocompatible metal [5,27e31], has been studied in complexes with polydentate ligands for the specific redox and co- ordination properties, and as a catalytic co-factor for many metal- loenzymes [6e9]. Our interest in this class of complexes was promoted by the ability to display photoluminescence activity in the crystal phase. This property results related to the structural features and intermolecular packing effects and highly tunable according to substituents of the ligand. Due to the closed-shell ef- fect, zinc ion can preserve or even enhance PL performances of the ligands. In particular, we demonstrated that tuning in emission wavelength of the zinc complexes can be obtained by insertion of an electron withdrawing and/or of a pyridine moiety into the main structure of hydrazone type ligands. This behavior is related to the change in the push-pull conjugation pattern [12,21 ,22,32e34]. Moreover, we found that the lowest unoccupied molecular orbital (LUMO) was often confined onto pyridine moiety which played an exclusive role in determining the PL response. In our previous contributions we prepared a number of polydentate hydrazones zinc-complexes promising as emissive components in PL devices * Corresponding author. E-mail address: ugo.caruso@unina.it (U. Caruso). Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc https://doi.org/10.1016/j.molstruc.2019.07.098 0022-2860/© 2019 Published by Elsevier B.V. Journal of Molecular Structure 1197 (2019) 672e680