Molecular Packing Dependent Solid State Fluorescence Response of Supramolecular Metal-Organic Frameworks: Phenoxo-Bridged Trinuclear Zn(II) Centered Schiff Base Complexes with Halides and Pseudohalides

Abstract

Molecular packing plays a key role in determining the photophysical properties in the aggregated state, but fine controlling of molecular packing is a great challenge. Here we report a series of new phenoxo-bridged homometallic trinuclear Zn (II) metal complexes (BN3, BNCS, BN(CN)2, BCl, BBr, and BI [complexes 1-6] having different anions with an aim to vary the photophysical properties of the complexes, where B stands for the ligand, N,N′-bis(3-ethoxysalicylidenimino)-1,4-diaminobutane. All the complexes are structurally characterized by single crystal X-ray diffraction technique. The crystallographic investigation indicates that the complexes contain two types of Zn (II) centers: distorted square pyramidal and octahedral. The crystal structures are stabilized by intermolecular hydrogen bonding and C-H - -π interactions leading to distinct supramolecular frameworks. The ground state and excited state electronic properties of these metal complexes have been investigated. In solution state, all the metal complexes are found to be moderately fluorescent and emit at similar wavelengths with no significant effect of anions on fluorescence emission. However, interestingly, solid state emission properties have been found to be significantly dependent on molecular assembly. In this series, dicyanamide [2] and thiocyanate [3] complexes exhibit significantly red-shifted high fluorescence response in the solid state which is attributed to their distinct packing in comparison to others. To the best of our knowledge, these complexes are the first example of Schiff base complexes demonstrating intriguing molecular packing dependent fluorescence emission. Copyright © 2018 American Chemical Society.