Spectroscopic\nand Crystallographic Investigations\nof Novel BODIPY-Derived Metal–Organic Frameworks

Abstract

To\nexplore new 4,4-difluoro-4-bora-3a,4a-diaza-<i>s</i>-indacene\n(BODIPY)-derived metal–organic frameworks (MOFs),\nwe employed 2,6-dicarboxyl-1,3,5,7-tetramethyl-8-phenyl-4,4-difluoroboradiazaindacene\n(H₂L) as a ligand to successfully synthesize five coordination\npolymers, namely, {[Zn₂(L)₂(bpp)]­·2H₂O­·2EtOH}_<i>n</i> (<b>1</b>), {[Cd₂(L)₂(bpp)]­·2H₂O­·EtOH}_<i>n</i> (<b>2</b>),\n{[Cd₂(L)­(bpe)₃(NO₃)₂]­·2H₂O­·DMF­·EtOH}_<i>n</i> (<b>3</b>), {[Cd­(L)­(bpe)_0.5­(DMF)­(H₂O)]}_<i>n</i> (<b>4</b>), and {[Cd­(L)­(bpe)_0.5]­·1.5H₂O­·DMF}_<i>n</i> (<b>5</b>) (bpp = 1,3-bi­(4-pyridyl)­propane, bpe = 1,2-bi­(4-pyridyl)­ethane).\nExcept for two 2D-layer coordination polymers <b>3</b> and <b>4</b>, the rest samples exhibit 3D metal–organic frameworks\nwith certain pore sizes, especially MOFs <b>1</b> and <b>5</b>. Spectroscopic and crystallographic investigations demonstrate\nthat the absorption and emission energies of the BODIPY chromophores\nare sensitive to the coordination modes. Moreover, in case <b>2</b>, the transition metal centers coordinated with the dicarboxylate\nligands L^2– are capable of forming the two BODIPY\nunits in coplanar arrangements (θ = 37.9°), simultaneously\nsuppressing the uncommon J-dimer absorption band centered at 705 nm\nwith a long tail into the near-infrared region at room temperature.\nOn the other hand, in comparison with the ligand H₂L, the\nemission of monomer-like BODIPY in case <b>3</b> is enhanced\nin the solid state by a considerably long distance between the parallel\nBODIPY planes (about 14.0 Å).