Thermal, Photochromic and Dynamic Properties of Water-Soluble Spiropyrans

Numerous studies in biological and material sciences have used nitro-BIPS, dinitro-BIPS as well as Py-BIPS as versatile photoswitches. Still, the photochemical picture of this class of compounds is far from complete. We present photometric steady-state and ultrafast time-resolved pump/probe spectroscopic measurements on water-soluble derivatives of these three spriopyrans. Our experiments reveal significant differences between the nitro-substituted spiropyrans and Py-BIPS. In contrast to the high resistance of Py-BIPS towards hydrolysis over weeks, the two nitro-BIPS derivatives decompose over hours. The fluorescence properties of Py-BIPS are unique in showing an emission of the spiro photoisomer. The ring-opening and -closing reaction of Py-BIPS is accomplished within picoseconds, whereas nitro-derivatives photoisomerize on longer time scales. These long-lived transients indicate either the contribution of triplet states or the involvement of multiple merocyanine isomers in the reaction pathway.