Spectroscopic and Crystallographic Investigations of Novel BODIPY-Derived Metal–Organic Frameworks

To
explore new 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene
(BODIPY)-derived metal–organic frameworks (MOFs),
we employed 2,6-dicarboxyl-1,3,5,7-tetramethyl-8-phenyl-4,4-difluoroboradiazaindacene
(H2L) as a ligand to successfully synthesize five coordination
polymers, namely, {[Zn2(L)2(bpp)]­·2H2O­·2EtOH}n (1), {[Cd2(L)2(bpp)]­·2H2O­·EtOH}n (2),
{[Cd2(L)­(bpe)3(NO3)2]­·2H2O­·DMF­·EtOH}n (3), {[Cd­(L)­(bpe)0.5­(DMF)­(H2O)]}n (4), and {[Cd­(L)­(bpe)0.5]­·1.5H2O­·DMF}n (5) (bpp = 1,3-bi­(4-pyridyl)­propane, bpe = 1,2-bi­(4-pyridyl)­ethane).
Except for two 2D-layer coordination polymers 3 and 4, the rest samples exhibit 3D metal–organic frameworks
with certain pore sizes, especially MOFs 1 and 5. Spectroscopic and crystallographic investigations demonstrate
that the absorption and emission energies of the BODIPY chromophores
are sensitive to the coordination modes. Moreover, in case 2, the transition metal centers coordinated with the dicarboxylate
ligands L2– are capable of forming the two BODIPY
units in coplanar arrangements (θ = 37.9°), simultaneously
suppressing the uncommon J-dimer absorption band centered at 705 nm
with a long tail into the near-infrared region at room temperature.
On the other hand, in comparison with the ligand H2L, the
emission of monomer-like BODIPY in case 3 is enhanced
in the solid state by a considerably long distance between the parallel
BODIPY planes (about 14.0 Å).