FULL PAPER DOI:10.1002/ejic.201402717 Color Tuning and Noteworthy Photoluminescence Quantum Yields in Crystalline Mono-/Dinuclear Zn II Complexes Mario Argeri, [a] Fabio Borbone, [b] Ugo Caruso, [b] Mauro Causà, [c] Sandra Fusco, [b] Barbara Panunzi,* [d] Antonio Roviello, [b] Rafi Shikler, [e] and Angela Tuzi [b] Keywords: Zinc / Schiff bases / Tridentate ligands / Luminescence A series of Zn II complexes with an O,N,O tridentate core is reported. Four species of the general formula ZnL i Py 2 were obtained from pyridine, where L i is a substituted 4-[(aroyl- hydrazono)methyl]-3-hydroxyphenyl 4-(octyloxy)benzoate group. Crystal structures were solved by single-crystal X-ray diffraction. Nematogenic phase behavior was detected only for the ligands. Photoluminescence spectra were recorded both in solution and in the solid state, and the complexes showed noteworthy photoluminescence in the solid state Introduction The complexation of zinc(II) ions with polydentate O,N ligands bearing functional groups is a well-studied area in coordination chemistry. [1] In recent years our research group has focused interest on using these types of ligands for coordination to transition metal atoms to form com- plexes with stable structures with the aim of achieving spe- cific properties. [2] Zinc chelate complexes attracted our interest [3] for their optical properties, particularly their pho- toluminescence performance. [4,5] Our starting point was the study of coordination of Zn II with N'-(2-hydroxy-4-octyl- oxybenzylidene)aroylhydrazide O,N,O ligands published in a recent article, [6] which led to a widely characterized group [a] Department of Sciences and Technological Innovation, Università del Piemonte Orientale “Amedeo Avogadro”, Viale Teresa Michel 11, 15121 Alessandria, Italy [b] Dipartimento di Scienze Chimiche, Università di Napoli Federico II, via Cintia, 80126 Napoli, Italy E-mail: barbara.panunzi@unina.it www.international.unina.it [c] Dipartimento di Ingegneria Chimica, dei Materiali e delle Produzioni Industriali, Università di Napoli Federico II, Piazzale V. Tecchio 80, 80125 Napoli, Italy [d] Dipartimento di Agraria, Università di Napoli Federico II, via Università 100, 80055 Portici (Napoli), Italy http://www.international.unina.it/ [e] Department of Electrical and Computer Engineering, Ben-Gurion University of the Negev, P. O. B. 653, Beer-Sheva 84105, Israel Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.201402717. Eur. J. Inorg. Chem. 2014, 5916–5924 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 5916 with wavelengths tunable by varying the aroyl group. Me- dium to very high photoluminescence quantum yields (PLQY) were recorded on the crystalline complexes obtained in their mononuclear coordination environments. In the case of a 4-nitrobenzoyl substituent on the ligand, a dinuclear complex with formula Zn 2 L 4 2 Py 2 showed an unprecedented and exceptionally high PLQY value for a crystalline zinc complex. Molecular and periodic calculations were per- formed by using Density Functional Theory (DFT). of dinuclear complexes in which the metal atom coordina- tion sphere was completed by pyridine molecules. Unlike the free ligands, the complexes were highly photolumines- cent in the solid state. This paper is the result of the attempt to improve the photoluminescence performances through modifications of the ligand’s chemical structure without any changes to the coordination core. A bulky substituent was added to the structure of the ligands, and interesting effects were revealed, particularly on the coordination environment and photophysical performance. As demonstrated in the lit- erature, [7] the addition of bulky groups to ligands can im- prove the photoluminescence quantum yield (PLQY) by in- creasing the distance between the emitting species in the solid state and decreasing self-quenching. In this work mononuclear neutral complexes with the general formula ZnL i Py 2 were obtained from pyridine. The ligand L i was a substituted N'-{2-hydroxy-4-[(4'-octyloxy)benzoyloxy]- benzylidene}aroylhydrazide in which a bulky benzoyloxy moiety was inserted. Tuning of the emission wavelength was still achievable by varying the aroyl electron-acceptor group with significant differences compared with the previous series. [6] Single-crystal X-ray analysis gave essential infor- mation about the structures of the complexes and added support to the interpretation of the photophysical proper- ties observed in the solid state. The crystalline complexes revealed medium to very high PLQY values, which is ap- pealing to any related fields of application. When the aroyl group was 4-nitrobenzoyl, specific crystallization conditions afforded a further complex with an O-bridged dinuclear