Solid-state thermo- and photochromism in N,N′-bis(5-X-salicylidene)diamines (X = H, Br)

Abstract

Sixteen N,N′-bis(salicylidene)diamines of common formula (o-OHC₆H₄CN–)₂Z [Z = o-C₆H₄ (1a), m-C₆H₄ (2a), p-C₆H₄ (3a), Ø (4a), CH₂(p-C₆H₄)₂ (5a), O(p-C₆H₄)₂ (6a), S(p-C₆H₄)₂ (7a), SO₂(p-C₆H₄)₂ (8a)] and N,N′-bis(5-bromosalicylidene)diamines of common formula (2-OH-(5-Br)C₆H₃CN–)₂Z [Z = o-C₆H₄ (1b), m-C₆H₄ (2b), p-C₆H₄ (3), Ø (4b), CH₂(p-C₆H₄)₂ (5b), O(p-C₆H₄)₂ (6b), S(p-C₆H₄)₂ (7b), SO₂(p-C₆H₄)₂ (8b)] have been synthesized and characterized by elemental analysis, X-ray powder diffraction, NMR, diffuse reflectance, Raman, IR and fluorescence spectroscopy. Molecular structures of 2a, 6a, 7a, 8a and 4b were elucidated by single crystal X-ray diffraction. Their structures are stabilized by two intramolecular hydrogen bonds and weak intermolecular π⋯π stacking interactions. All molecules are thermochromic, while molecules 2a and 2b exclusively display photochromism upon irradiation at 450 and 365 nm, respectively, which is reversible and exhibits relatively slow thermal relaxation.