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.