![]() ![]() Previous reports revealed that some spiropyrans ( Scheme 2) containing a phenol moiety with electron-withdrawing substituents such as −COOH ( 2), (16) −COOCH 3 ( 3), (17) −SO 3H ( 4), (18) and −NO 2 ( 5), (19) when dissolved in polar solvents, are isomerized to the MC form under dark conditions, even though they exist in the SP form in less polar solvents. Polarity of the solvent is one of the most familiar and easy-to-handle stimuli for promoting isomerization. The SP ⇌ MC isomerization terminates within barely 30 s even at room temperature because the naphthol moiety with high π-electron density lowers the activation energy for the rate-determining rotational step. ![]() Solvation stabilizes the negative charge of the zwitterionic form and decreases its ground state energy, thereby enhancing SP → MC isomerization. In polar media, the zwitterionic form dominates mainly owing to solvation by polar molecules. The MC form involves two resonance structures─the quinoidal and zwitterionic forms. The equilibrium amount of the MC form increases with an increase in the polarity of solvents. It exists as a colorless spirocyclic (SP) form in less polar solvents but is isomerized to a colored merocyanine (MC) form in polar solvents. In this study, we describe that a spiropyran containing a hydroxynaphthalimide unit ( 1) exhibits a polarity-driven isomerization at room temperature. Although some spiropyrans undergo polarity-driven isomerization, they must be heated owing to the high activation energy required for isomerization. Design of spiropyrans showing spontaneous isomerization driven by the polarity of solvents is an important consideration for the synthesis of optical sensory materials. ![]()
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