Structural chemistry and environment-dependent fluorescence of a tetratopic pyrrolo[3,2-b]pyrrole ligand

Abstract

Here we report a new tetratopic carboxylic acid ligand H₄L1 containing a fluorescent pyrrolo[3,2-b]pyrrole core, and examine its structural chemistry in two solvated forms with amide solvents and a porous strontium(II) MOF. The two solvates H₄L1·2DMF and H₄L1·4DMA exhibit similar hydrogen bonding characteristics at the N-aryl position, but are differentiated by the hydrogen bonding behaviour of the C-aryl position; while the DMF solvate forms a one-dimensional chain via carboxylic acid dimers, the DMA solvate is fully enclosed by solvent molecules occupying all hydrogen bonding sites. The tendency for the electron-rich core to avoid close π⋯π interactions in these solvates is further evident in the structure of poly-[Sr₂(L1)(DMF)₂(OH₂)]·DMF·H₂O 1, a 3-dimensional rod-packed MOF with narrow linear solvent channels. Exchange of the lattice solvent with methanol, with retention of single crystallinity, gives a framework which can be readily desolvated, and exhibits a maximum CO₂ uptake of 8 wt% at 283 K and 1 atm. The ligand is strongly fluorescent in DMSO in its protonated and deprotonated forms and exhibits characteristic solvatochromism, which is pronounced in solution and remains evident, to a lesser degree, in the fluorescent MOF 1.