Formation of a biradical species from a monoradical with a photo- and thermo isomerizable imine group

The thermally induced trans-1/cis-1 isomerization process is studied here. The trans-1 → cis-1 isomerization takes place in three solvents with different polarities whereas cis-1 did not evidence the presence of a backward cis-1→trans-1 isomerization, independently of the polarity of the solvent used. This fact revealed that the cis-1 isomer is thermodynamically more stable probably due to the formation of a hydrogen bonded biradical species, as confirmed by ZINDO/1 semiempirical calculations.