Photochromism and thermochromism of solid trans-N,N′-bis-(salicylidene)-1,2-cyclohexanediamines and trans-N,N′-bis-(2-hydroxy-naphylidene)-1,2-cyclohexanediamine

The photochromic and thermochromic properties of trans-N,N′-bis(salicylidene)-1,2-cyclohexanediamine (1), trans-N,N′-bis(3,5-dichloro-salicylidene)-1,2-cyclohexanediamine (2), trans-N,N′-bis(3,5-di-t-butyl-salicylidene)-1,2-cyclohexanediamine (3) and trans-N,N′-bis(2-hydroxy-naphylidene)-1,2-cyclohexanediamine (4) were investigated by UV and fluorescence spectroscopies in the crystalline state at various temperatures and the molecular structures of 2 and 4 were determined by single-crystal X-ray diffraction. The existence of the two Schiff base groups on a single molecule does not seem to differentiate the chromobehavior of the present compounds (except possibly for 3) with respect to the usual Schiff bases of salicylaldehyde. It is suggested that for this class of compounds also, what determines the thermochromic behavior is the enhanced basicity of the nitrogen atom, due the absence of π,π- and n,π-conjugation with an aryl ring bound to it. The role of the crystal structure in this case is important only in so far as it affects the electron density on the nitrogen atom. Photochromism, however, is structure dependent and requires space for the generation of the photoproduct that involves cis to trans isomerization in the excited state.