Revisiting 2-chloro-4-nitroaniline: analysis of intricate supramolecular ordering of a triclinic polymorph featuring a high value and strong second harmonic generation

Polymorphism represents an expanding area of crystal engineering and an opportunity for discovery of materials with interesting properties, thus giving new perspectives for "old", well-known compounds known by chemists for decades. In this work, we revisit 2-chloro-4-nitroaniline (2Cl4na) for which the crystal structure was first determined as early as 1965. Our failed trials to reproduce the original orthorhombic phase of 2Cl4na led to the determination of its new triclinic polymorph. Synthetic screening indicated that 2Cl4na can be obtained from several solvent mixtures as well as by slow cooling of the melted sample. This polymorph crystallizes in the non-centrosymmetric space group P 1 with a = 3.7755(2) Å, b = 13.5798(7) Å, c = 28.554(1) Å, α = 89.503(4)°, β = 88.612(4)° and γ = 86.402(4)° lattice parameters and Z = 8. This high number of crystallographically equivalent molecules in the independent part of the unit cell resulted in an unusual supramolecular arrangement of this compound. Energy vector diagrams indicate that the structure of the previously known orthorhombic polymorph is columnar, whereas the triclinic one is quasi-isotropic. Calculations of lattice energies by an ab initio method with periodic boundary conditions showed that the triclinic polymorph is more stable by 0.89 kcal mol −1 in comparison to the orthorhombic one. The cooperative character of weak hydrogen bonds and stacking interactions was suggested as a key governing factor in the crystal growth of needle-like crystals. With the use of the Kurtz-Perry test we also demonstrate that this new polymorphic form of 2Cl4na produces second harmonic generation (SHG) that is 2 times higher than that obtained from potassium dihydrogen phosphate (KDP) upon irradiation with an 800 nm laser beam. Alongside the strong SHG, greenish-yellow two-photon excited fluorescence is also observed. A new polymorph of 2-chloro-4-nitroaniline is more stable than its 55 year-old antecedent.