Insights into the Origin of Solid‐State Photochromism and Thermochromism of <i>N</i>‐Salicylideneanils: The Intriguing Case of Aminopyridines

Abstract

Abstract All light and no heat? Structure–property relationships for photochromic and thermochromic N ‐salicylideneanils have been revised (see picture). One derivative has a packed crystal structure, is photochromic and exhibits an infinitely slow thermal back relaxation of technological interest. Complexation of the anils to transition metals has also been studied. magnified image 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 =9.9×10 −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.