Polymorphs, ionic cocrystal and inclusion complex of N-amino-1,8-naphthalimide

Abstract

Self-assemblies of small molecules provide impetus to analyze the constituent building blocks contributing to the overall physical properties. In this study, two polymorphs of N-amino-1,8-naphthalimide having or not having symmetry independent molecules in their respective unit cell are described. The two polymorphs have distinguishable hydrogen bonded assemblies; one has a zig-zag chain of symmetry non-equivalent molecules. A nitrate salt and an ionic cocrystal of the nitrate salt of the N-amino-1,8-naphthalimide were prepared and structurally characterized. The ionic cocrystal has structural similarities to L-2 as both have symmetry independent species in their structure related by the 2₁ screw axis. The self-assemblies of the nitrate salt as well as of the ionic cocrystal of the nitrate salt of L have distinguishable π-stacks among the naphthalimide rings. The nitrate salt has a water assisted hydrogen bonded double chain formed between the nitrate and HL⁺. It had shown higher molar conductance than the expected value of a 1 : 1 electrolyte, and this has been attributed to proton conductivity over the ionic cocrystal which has a single chain of such a kind. The structural features of a tetra L-ligated copper(II) complex as well as an eight coordinate cadmium(II) self-inclusion complex with L are presented to elucidate the modulation of the contents of L in the copper and cadmium complexes. The molar conductance values of the complexes in solution are lower than the expected values of the respective ionic composition found in the crystal structure. This is possibly due to the lower ionic mobility of the large size of the cationic part present in these complexes. Due to extensive π-stacking among naphthalimide rings, each form presented here is non-fluorescent in the solid state, but the emission of L could be turned-ON upon addition of nitric acid or cadmium nitrate to a solution of L. The isothermal calorimetric titrations suggest the difference in the binding abilities of L with zinc(II) nitrate and copper(II) nitrate.