Photochromic and thermochromic salicylidenamines: isomerization in the crystal, infrared identities and conformational influence

Abstract New spectroscopic data on a series of salicylidenamines C 6 H 4 (2-OH)CHNR have been collected from crystalline films of nine different derivatives (R = aryl, alkyl). For R = C 6 H 5 and R = C 6 H 4 -2-Cl, the photoisomerizations provide coloured species (λ max ≅ 480 and 540 nm) which subsequently evolve through thermal processes kinetically analysed as the consecutive reactions, X → Y → Z. The second reaction is faster than the first, but it may be observed, owing to the initial presence of Y which is a product of the photoisomerization. These species are respectively identified with the trans s-E quinoid structure (X), its cis s-Z isomer (Y) and the major cis phenolic tautomer (Z) cis and rans referring to rotation around the C 6 H 4 (2-OH)CH bond). The X → Y activation energy (∼ 23 kcal mol −1 ) is in qualitative agreement with the estimated intermolecular interactions occurring for the X → Y transition state which have been simulated from the crystallographic data of the R = C 6 H 4 -2-Cl derivative. From the FT-IR spectra of the nine derivatives, some of their deuterated isotopomers (OD) and a second crystalline form for R = C 6 H 4 -2-Cl, a series of group frequencies are well identified and assigned. Some specific IR absorption bands belonging to X and Y species are also observed; the Y bands are present as very minor components in the spectra before irradiation. For Z species, the frequencies and the band shapes of the γ(OH) out-of-plane vibrations, and the temperature effects, provide information on the influence of the R group conformation. Through conjugative (R = aryl) and hyperconjugative (R = -CH(CH 3 ) 2 ) effects, the R group modifies the availability of the nitrogen unshared electron pair which strongly influences the γ(OH) frequency.