First evidence of thermo- and two-step photochromism of tris-anils

Abstract

Seven N,N′,N′′-tris(salicylidene)triamines of common formula N(CH₂CH₂NCH–aryl) [aryl = 2-OH-C₆H₄ (1), 2-OH-(5-Cl)C₆H₃ (2), 2-OH-(5-Br)C₆H₃ (3), 2,5-(OH)₂C₆H₃ (4), 2,4-(OH)₂C₆H₃ (5), 2,3,4-(OH)₃C₆H₂ (6) and 2-OH-C₁₀H₆ (7)] have been synthesized and characterized by elemental analysis, X-ray powder diffraction, NMR, diffuse reflectance, Raman and fluorescence spectroscopy. All the tris-anils are thermochromic. 4 exclusively displays photochromism upon irradiation at 365 nm with a unique two-step back thermal relaxation, with kinetic constants k_I = 1.0 × 10⁻⁵ s⁻¹ and k_II = 4.0 × 10⁻⁵ s⁻¹ for the first and second steps, respectively. The origin of the observed photochromism is due to a cis/trans-keto isomerization.