Multistep Thermal Relaxation of Photoisomers in Polyphotochromic Molecules

Abstract

UV irradiation of a polyphotochromic molecule bearing a spironaphthoxazine unit linked to a naphthopyran by a Z-ethenic bridge produces several interconverting nonequilibrium dihydrophenanthrenic photocyclized isomers. A quantitative kinetic analysis of the thermal evolution after photocyclization has been performed using 1H and 19F NMR spectroscopy at different temperatures. Spirooxazine (Spo) ring opening and TTC/CTC (i.e., trans−transoid−cis/cis−transoid−cis) isomerization occur reversibly, but only when the naphthopyran ring is closed. In contrast, naphthopyran ring opening is irreversible and is observed when the spirooxazine ring is either closed or open in the TTC configuration, but not from the CTC configuration. Activation parameters of the main isomerization processes have been determined. Relative stability of the various isomers (TTC > CTC > Spo) has been established. The use of model compounds bearing either a spirooxazine or a naphthopyran in a similar dihydrophenanthrenic environment confirms these specific reactivities and relative stabilities in accordance with PM-3 calculations.