Insights into the origin of solid-state photochromism and thermochromism of N-salicylideneanils: the intriguing case of aminopyridines.

The relationships between the crystal structure and optical properties of switchable N-salicylideneanils have been revised and discussed on the basis of new experimental results and a computational approach. N-salicylidene-3-aminopyridine (L(3)) is a versatile thermo- and photochromic molecule. It also exhibits an infinitely slow thermal back relaxation (k = 9.9x10(-8) s(-1)) after photoswitching that is suitable for optical memories. Contrary to reports in the literature, N-salicylidene-4-aminopyridine (L(4)) is exclusively thermochromic. To explain these unexpected optical properties in the solid state, crystallography combined with UV-visible spectroscopic data was exploited. L(3) was also used as a ligand in new thermochromic coordination complexes [M(CH(3)OH)(2)(L(3))(2)(NCX)(2)], in which M(II) = Fe, Co, Ni, Cu or Mn and X = S or Se (1-6), which allowed the fine-tuning of the electron density in the photochromic moiety. The influence of the coordination through the nitrogen of the pyridine ring is also fully discussed.