Donor-Acceptor chromophores are active non-linear optic (NLO) species, if their crystal packing is non-centrosymmetric. External pressure could enhance the stereo-electronic factors inducing NLO signals. For example, above 1.0 GPa 4-amminobenzophenone becomes more planar, increasing the hyper-polarizability hence the second harmonic generation.

This work was carried out in the group of PD Dr. Piero Macchi, in collaboration with the Paul Scherrer Institute. Other studies within this SNF project "Chemical bonding in molecular crystals at high pressure" are dedicated to pressure induced transformations in porous metal organic frameworks.
See http://www.chemcrys.unibe.ch for more information.

References:

• E. Marelli, N. Casati, F. Gozzo, P. Macchi, P. Simoncic, A. Sironi;
  "High pressure modification of organic NLO materials: large conformational re-arrangement of 4-amminobenzophenone"
  CrystEngComm, 13, 6845-6849, (2011); doi:10.1039/C1CE05742A.
• T. D. Bennett, P. Simoncic, S. A. Moggach, F. Gozzo, P. Macchi, D. A. Keen, J. C. Tan, A. K. Cheetham;
  "Reversible Pressure-Induced Amorphization of a Zeolitic Imidazolate Framework (ZIF-4)"
  Chem. Comm., 47, 7983-7985, (2011); doi:10.1039/C1CC11985K.