Spirooxazine to merooxazine interconversion in the presence and absence of zinc: Approach to a bistable photochemical switch

A spironaphthoxazine (SO) photoswitch was synthesized, and its photochromic behaviors were investigated. SO underwent reversible ring-opening/closure isomerization between a spirocyclic isomer (closed form) and a merocyanine (MO isomer, open form) upon ultraviolet light irradiation. For the model SO in this work, the thermal equilibrium is substantially shifted toward the spirocyclic isomer even at -30.0° C. However, addition of zinc, as Zn(ClO ₄)₂, exerted an important effect on the thermal reversion process from the open (MO) to the closed form (SO). Kinetic analysis showed that thermal reversion with zinc is retarded more than 13-fold, significantly improving bistability. Moreover, introduction of zinc to the spirooxazine-merooxazine (SO-MO) system resulted in a new absorption band readily distinguishable from the bands arising from spirooxazine and merooxazine. For the first time, to the best of our knowledge, the microscopic rate constants for: MO photogeneration from SO (k₁), thermal reversion of MO to SO (k₂), complexation of MO with zinc (k ₃) and for dissociation of the complex, MO-Zn (k₄), as well as for the ionization equilibria of Zn(ClO₄)₂ have been evaluated. The preferred transoid structures of MO and those of MO-Zn derived from the preferred MO structures are considered. Although the kinetic study does not permit elucidation of the nature of zinc binding to MO to give MO-Zn, nor the precursor isomers of MO, a DFT calculational study in progress should shed light on the structure and relative stability of these essential intermediates.