Polymorphism and crystal-crystal transformations of the highly optically nonlinear organic compound α-[(4′-methoxyphenyl)methylene]-4-nitro-benzeneacetonitrile

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Crystals of α-[(4′-methoxyphenyl)methylene]-4-nitro-benzeneacetonitrile exhibit powder second-harmonic generation (SHG) efficiencies under 1.06 μm irradiation several times that of 2-methyl-4-nitroaniline. We have identified three distinct, solution-grown polymorphs which show marked variation in SHG efficiency. Controlled transformation between these polymorphs can be made either solvent assisted or thermally. One polymorph shows a 25-fold increase in SHG efficiency when the irradiation wavelength is changed to 1.32 μm while the others show a small or no increase.