Dual emissions and thermochromic luminescences of isomorphic chiral twofold interpenetrated 3-D nets built from I1O2 type hybrid inorganic–organic frameworks of [NH2(CH3)2]3[Pb2X3(BDC)2] (X = Br, I)

Abstract

Two pairs of MOF-based hybrid enantiomorphs, [NH₂(CH₃)₂]₃[Pb₂X₃(BDC)₂] (H₂BDC = 1,4-benzenedicarboxylic acid, X = Br or I), have been synthesized using the solvothermal reaction and then manually separated, which are labeled as 1a/1b (X = Br) and 2a/2b (X = I). The isomorphic 1a and 2a crystallize in tetragonal space group P4₃2₁2, and the isomorphic 1b and 2b in the enantiomorphic space group P4₁2₁2. Twofold interpenetrated three-dimensional (3-D) networks were built from two sets of equivalent I¹O² type hybrid inorganic–organic frameworks in 1a/1b and 2a/2b. Each I¹O² type hybrid inorganic–organic framework constructs by the inorganic pentagonal bipyramid-shape PbX₃O₄ (X = Br or I) polyhedral chains along the c-axis, which are further connected though bridged BDC²⁻ ligands in the directions perpendicular to the c-axis. Hybrids 1a/1b and 2a/2b have been characterized by elemental analysis (C, H and N elements), thermogravimetric and powder X-ray diffraction techniques, and UV-visible absorption spectroscopy in the solid state. These hybrids show dual emissions at ambient conditions, which arise from the π–π* electron transition within the aromatic rings in the BDC²⁻ ligands and the electron transition in the inorganic polyhedral semiconducting chains, as well as thermochromic luminescence behavior from 10 to 300 K owing to two emission bands displaying different responses to the temperature change.