Water‐Soluble Spiropyrans with Inverse Photochromism and Their Photoresponsive Electrostatic Self‐Assembly

Abstract

A new type of light responsive nanoscale assemblies based on water-soluble spiropyrans is presented. We have synthesized four anionic spiropyrans bearing multiple sulfonate groups and investigated their photochromic behavior in aqueous solution. Depending on the pH, either inverse photochromism (acidic conditions) or normal photochromism (alkaline conditions) is found. Kinetic data for the interconversion of the spiropyran and merocyanine isomers including the subsequent slow hydrolysis have been obtained by UV/Vis spectroscopy. The results show that the spiropyrans undergo hydrolysis in both alkaline and acidic solution, while in the latter the rate is far slower than in the former. This prolonged hydrolytic stability together with the inverse photochromism under acidic conditions makes the sulfonated spiropyrans suitable to build photoresponsive nanostructures with cationic polyelectrolytes. We show how the self-assembly process is driven by electrostatic interactions and how the spiropyrans' photochromic property allows the size control of the supramolecular objects by visible light. The assembly size is characterized by dynamic light scattering and TEM. In addition, UV/Vis and fluorescence spectroscopy and ζ-potential measurements help to explain the size change upon visible light irradiation.

Photoresponsive Photoacid-Macroion Nano-Assemblies

“…A fundamental understanding regarding the interplaying interaction forces was developed [ 41 , 42 , 43 , 44 ]. Further, switchable structures based on electrostatic self-assembly were studied [ 45 ], in particular, the combination of a polyelectrolyte with a light-switchable molecule, such as azobenzenes [ 46 ] or spiropyrane [ 47 ], and—in one first concept study—a photoacid [ 37 ].…”

Tuning the Negative Photochromism of Water-Soluble Spiropyran Polymers

“…Much of the literature on spiropyran-containing materials has focused on photochromism in hydrophobic and organic media, especially of 6-nitrospiropyran derivatives, which are efficient in both UV-induced MC formation and thermal or photochemical reversion . Integration of SP photochromes into aqueous systems, which is important for biological applications, is comparatively rare. , A key difference in the behavior of spiropyrans in aqueous media is the increased importance of a third species: protonation of the MC phenoxide yields the cationic MCH + , which has an absorbance band between those of SP and MC. , Thus, three chemical speciesSP, MC, and MCH + , each with unique physical and chemical propertiesare potentially accessible, especially when spiropyran molecules are in protic media (Figure ).…”

Stimuli responsive dynamic transformations in supramolecular gels

“…9). [124][125][126][127][128] Dithienylethenes (DTE) undergo electrocyclic ring closing upon UV-irradiation giving rise to the products containing more sp 3 centres so tend to adopt a relatively rigid conformation (Fig. 9).…”