Photochemistry of a water-soluble coumarin-based photoswitch

A novel water-soluble stilbene-type molecular photoswitch (CP) has been synthesized by attaching an N-methylpyridinium unit to 7-diethylamino-4-methylcoumarin (coumarin 1) via an ethylene bridge. The photoisomerization cycle of CP and the spectroscopic properties of the two isomers were studied in acetonitrile and in water. In both solvents, the illumination of trans-CP with green light leads to a photostationary state (PSS) with 90% cis isomer and irradiating then this PSS mixture with near UV light, the trans isomer is almost fully recovered. In water, the quantum yield for the trans-cis photoisomerization is 0.042, for the cis-trans photoisomerization it is 0.23. In acetonitrile, the photoisomerization quantum yields are even higher, the respective values are 0.12 and 0.56. The trans isomer of CP is fluorescent with red emission and it has a large Stokes shift, whereas the cis isomer is non-fluorescent. The two isomers are thermally stable at room temperature. The efficient photoisomerization of CP in both direction, the thermal stability of both isomeric forms and the distinct fluorescence of the trans isomer suggest that some water-soluble coumarin-based photoswitches may be promising candidates for applications in fluorescence imaging of biological samples.