7-Trifluoromethylquinoline-Functionalized Luminescent Photochromic Spiropyran with the Stable Merocyanine Species Both in Solution and in the Solid State

A new spiropyran (SP2) with the stable merocyanine form (MC2) both in solution and in the solid
state at room temperature was designed and synthesized. The stability of MC2 is believed to be
due to the electron-withdrawing effect of both the quinoline and the trifluoromethyl groups. 1H
NMR spectra indicate that the ratio of the open form vs the closed form of SP2 is dependent on the
polarity of solvents. Single crystals composed of only the open form (MC2) were successfully
obtained. X-ray structural analysis indicates that except trifluoromethyl and two methyl groups
MC2 is completely planar with an s-trans,s-cis conformation. It should be noted that this is the
first report of the X-ray crystal structure of the pure open form of spiropyran. MC2 can be slowly
transformed into SP2 at −30 °C or lower temperature, and the process is accelerated by visible
light irradiation. This special photochromic behavior can be explained by the calculated thermodynamic data. The spectral properties of SP2/MC2 in the presence of different metal ions are also
studied, and the results show the potential application of SP2/MC2 in sensing metal ions.