Unravelling the Mechano-Responsive Solvent Inclusion Behaviour of a Luminescent Organic Solid

Organic solids typically adopt close-packed structures to maximize the weak intermolecular interactions. From this perspective, unraveling the conditions in which a seemingly close-packed molecular solid undergoes guest-induced phase transition and shows reversible guest inclusion/desolvation behavior remains a challenge. An organic luminogen, 4-hydroxy-benzoic acid (2-hydroxy-naphthalen-1-ylmethylene)-hydrazide (2N-4 OH), produced two types of solids, viz., the yellow emissive C-Y form and the green emissive C-G form, of which the latter form was isolated as N.N-dimethylformamide (DMF) solvate. The green emissive C-G phase exhibited distinctive responses under conditions of mechanical grinding and compression, which was evident from spectroscopic, thermal, and X-ray diffraction measurements. Further, when infused with DMF vapor, the C-Y form transformed into the green emissive metastable C-M form, which was structurally similar to the metastable phase obtained from the C-G form upon hydraulic compression. Subsequently, we explain the relation between the C-Y and C-G forms, obtained directly upon crystallization, and the metastable C-M form and the C-G(M) form obtained after mechanical stimulation.