Thermal, Photochromic and Dynamic Properties of Water‐Soluble Spiropyrans

Abstract 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.