Diarylethene photoswitches for super-resolution microscopy in polymer science

Super-resolution fluorescence microscopy allows for in situ visualization of biological structures, but its application to materials science has so far been comparatively limited. One of the main reasons is the lack of powerful dyes that allow for labeling and photoswitching in materials science systems. As it provides information on dynamics and allows for a visualization of materials beyond their surface, super-resolution fluorescence microscopy has the potential to give new insights that complement those obtained by well-known and recognized techniques in materials science. Diarylethenes are popular photochromic compounds with potential applications in data storage and optical switching. However, their emission in the visible region vanishes and, additionally, efficient photochromism results in low fluorescence quantum yields, since most of the excitation energy is used for switching. In contrast to most other diarylethenes, sulfone derivatives of the benzothiophene diarylethenes exhibit strong fluorescence in the closed form. The photoswitchable diarylethene derivatives under investigation show suitable properties for super-resolution microscopy applications: their on- and off-states are spectrally well-separated, they possess high absorption coefficients and high fluorescence quantum yield of the bright on-state, as well as high photostability (more than one million photons can be emitted before photobleaching). One of the key properties of diarylethenes is the well tunable off-switching quantum yield via corresponding substitution. Many thousands of photons can be collected for the derivatives which contain strong electron donating substitutes as the off-switching occurs very slowly. This property is essential for PALM imaging since the localization accuracy and, thus, the reachable resolution directly depends on the number of collected photons. On the other hand, introducing electron-withdrawing substitutes and, thus, increasing the off-switching rates is very important for the deterministic super-resolution techniques such as RESOLFT. In this thesis it is shown that appropriate substitution of diarylethenes bearing a fluorescent closed and dark open form opens the way for imaging nanostructured materials with three of the most popular super-resolution fluorescence microscopy methods such as PALM, SOFI and RESOLFT, that are based on different concepts to achieve imaging beyond the diffraction limit of light. The key to obtain optimal resolution lies in proper control over the photochemistry of the photoswitches via proper derivative choice and its adaption to the system to be imaged. The possibility of the PALM imaging with only one laser wavelength used for the control of activation, excitation and deactivation was shown. Apart from localization microscopy, diarylethene dyes are remarkably interesting dyes for RESOLFT microscopy, a technique analogous to STED, but with the advantage that the off-switching process and not the stimulated emission is utilized for creating the depletion zone around the center. Therefore, such switchable dyes can be used without the need of pulsed lasers and at low powers, drastically reducing photobleaching