Flexible Porous Coordination Polymers from Divergent Photoluminescent 4-Oxo-1,8-naphthalimide Ligands

Two new luminescent ditopic naphthalimide-derived ligands, N-(4-cyanophenylmethylene)-4-(4-cyanophenoxy)-1,8-naphthalimide (L3) and N-(4-carboxyphenylmethylene)-4-(4-carboxyphenoxy)-1,8-naphthalimide (H₂L4), have been prepared, and their coordination chemistry has been explored in the synthesis of three new coordination polymer materials. Complex poly-[Ag(L3)₂]BF₄·4.5H₂O·0.5THF (1) is a 3-fold 2D → 2D parallel interpenetrated coordination polymer in which three interwoven sheets define inter- and intralayer channels containing anions and solvent molecules. Molecules of L3 interact in 1 through dominant head-to-head π-π stacking interactions, in an opposite aggregation mode to that observed in the free ligand in the crystalline phase. Complexes poly-[Cu(L4)(OH₂)]·2DMF·0.5H₂O (2) and poly-[Cd₂(L4)₂(OH₂)₂]·1.5DMF·3H₂O (3) are related noninterpenetrated two-dimensional coordination polymers defined by one-dimensional metal-carboxylate chains, forming layers that interdigitate with adjacent networks through naphthalimide π-π interactions. Both materials undergo structural rearrangements on solvent exchange with acetonitrile; in the case of 3, this transformation can be followed by single-crystal X-ray diffraction, revealing the structure of the acetonitrile solvate poly-[Cd₂(OH₂)₂(L4)₂]·2MeCN (4), which shows a significant compression of the primary channels to accommodate the solvent guest molecules. Both materials display modest CO₂ adsorption after complete evacuation, and the original expanded phases can be regenerated by reimmersion in DMF. The photophysical properties of each ligand and complex were also explored, which revealed variations in emission wavelength, based on solid-state interactions, including a notable shift in the fluorescence emission band of 3 upon structural rearrangement to 4.