Anomalous compression of a weakly CH⋯O bonded nonlinear optical molecular crystal

The organic nonlinear optical crystal, 3-methyl-4-nitropyridine N-oxide (POM), exhibits a negative-linear-compressibility (NLC) region as well as exceptionally large positive thermal expansion. High-pressure single crystal X-ray diffraction measurements have revealed an anomalous reversal of NLC at 0.12 GPa, induced by the collapse of the CH⋯O bonded supramolecular framework and subtle rotations of the nitro group. The initial compression of the weak supramolecular network in the molecular POM crystal is analogous to the hydrostatic responses of the framework crystals with much stronger cohesion forces. Density functional theory (DFT) calculations show that both the subtle conformational distortions and the crystal compression modify the second-harmonic generation (SHG) efficiency of POM.