Mechanochemical synthesis of a fluorenone-based metal organic framework with polarized fluorescence: an experimental and computational study

The regular assembly of organic chromophores into ordered supramolecular solids has received much attention for developing high-performance luminescent materials. Herein, a fluorenone-containing dicarboxylate (FDC) has been organized into a metal organic framework (MOF) with zinc as the metal cation by a facile mechanochemical synthesis method. The obtained Zn-FDC MOF features a pillared structure, in which the fluorenone chromophore units exhibit a highly ordered arrangement. Compared with the pristine FDC sample, the Zn-FDC MOF presents blue-shifted emission, as well as enhanced photoluminescence quantum yield and fluorescence lifetime, demonstrating the ordered arrangement of the FDC within the MOF is beneficial for the improvement of the photo-related properties of the organic chromophore. Moreover, the Zn-FDC MOF film exhibits well-defined polarized photoemission with a polarized anisotropy of 0.10-0.15. Periodic density functional theoretical (DFT) calculations illustrate that no energy/electronic transfer occurs between the chromophore and the metal cation, and the valence electrons localized in the FDC are confined effectively due to the energy blocking action of the metal cations. Combining experimental and theoretical studies on the luminescent MOF system, this work not only gives a rapid and environmentally friendly way to prepare a highly ordered inorganic-organic hybrid system with improved fluorescence properties, but also provides a deep understanding of the electronic structure of the 2D pillared MOF. It is expected that the Zn-FDC MOF can be potentially applied as a polarized fluorescence material.