Purification and crystal growth of NPB via imidazolium based ionic liquids

Here we report the production of high purity and crystallinity organic electronic material of NPB (N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl-4,4′-diamine (C44H32N2) through solution recrystallization within imidazolium based ionic liquids. When low purity NPB was recrystallized at 170 °C within C8MIM[TFSI], its purity was drastically improved from 82% to 99.92%. These recrystallized NPB crystals showed 0.040° FWHM (Full Width Half Maximum) of X-ray (1 1 1) diffraction peak. Such small FWHM angle indicates single-crystal like crystallinity. Initial NPB powder was dissolved at 100 °C and recrystallized at temperature above 110 °C. At higher temperature of 170 °C, a small number of bigger crystals were formed compared to those at 110 °C. This can be well explained by the classical nucleation and growth theory. Therefore, solution recrystallization process using ionic liquid might be promising for mass production of organic electronic materials by replacing the widely-used sublimation purification method.