Solid-state photochromism and thermochromism of N-salicylidene pyrene derivatives

N-Salicylidene pyrene derivatives of the common formula 1-pyrene-N = CH-(2-OH-aryl) [aryl = C6H4 (1), 5-Cl-C6H3 (2), 5-Br-C6H3 (3), 5-NO2-C6H3 (4), 3,5-Cl-2-C6H2 (5), 3,5-Br-2-C6H2 (6), and 3-CH3O-C6H3 (7)] have been synthesized and characterized by elemental analysis, NMR, diffuse reflectance and fluorescence spectroscopy. The crystal structures of 2, 3 and 5-7 are stabilized by an intramolecular hydrogen bond and a broad network of intermolecular pi...pi stacking interactions. All molecules, except 1, show the presence of a mixture of enol and cis- and trans-keto forms in the solid state at room temperature. Compounds 1 and 7 are exclusively thermochromic, while 5 displays negative trans-keto to cis-keto photochromism upon irradiation at lambda = 546 nm, which is photoreversible upon irradiation at lambda = 450 nm. The photogenerated cis-keto form of 5 is highly stable under ambient conditions and shows very negligible and slow thermal relaxation.