ABSTRACT
The use of molecular photoswitches has spread to virtually every field of pure and applied chemistry because of the extraordinary level of control they provide over the behavior of matter at the smallest scales. Photoswitches possess at least two different states with distinct structures and/or electronics and further functionalization of their core chromophore structures is needed to tailor them for a specific application. In this work we present a different concept for the generation and use of molecular photoswitches. It allows not only simultaneous establishment of photochromism and functionalization, but also full recyclability of a non-photochromic precursor material. Using a high-yielding and reversible ammonium salt formation, a functional group is introduced into a symmetric precursor while at the same time a strong electronic push-pull character is established in the structure. The resulting desymmetrization leads to efficient photoswitching capacity and the functional group can be fully removed subsequently by a simple heating step recovering the precursor for another functionalization round. We finally demonstrate feasibility of this concept over two consecutive closed loop functionalization/photoswitching/recovery steps. This concept offers great potential in any chemical research and application driven area but especially for the creation of responsive reprogrammable materials, no-background photoswitch labeling, and sustainable chemistry.
ABSTRACT
Diaryl-hemithioindigos (diaryl-HTIs) are derivatives of a novel class of highly functionalized indigoid chromophores. In this work a systematic study of the electronic effects on their photoswitching reveals the design principles for achieving an excellent property profile. Two key elements need to be invoked for perfect diaryl-HTI performance, first introduction of strong electron donors and second establishment of cross-conjugation. The resulting photoswitches combine high thermal stability, large extinction coefficients, red-light responsiveness, pronounced photochromism, and strong isomer accumulation in the photostationary states with precise geometry changes. By using the inherent basicity of their strong electron donor moiety, diaryl-HTIs can be rendered into very potent tools for molecular logic applications. We demonstrate a variety of binary logic setups as well as sophisticated three- and four-input keypad locks for sequential logic operations. Three distinct states and up to four different stimuli are invoked for this multi-level molecular information processing. Diaryl-HTIs have thus entered the stage as very capable and promising photoswitch motives for anyone interested in reversible visible- and red-light as well as multi-stimuli responsive molecular behavior.
ABSTRACT
Introducing responsive elements into supramolecular recognition systems offers great advantages for the control of intermolecular interactions and represents an important stepping stone towards multi-purpose and reprogrammable synthetic systems. Of particular interest is implementation of light-responsiveness because of the unique ease and precision of this signal. Here we present visible light responsive hemithioindigo-based molecular tweezers that bear a highly polar sulfoxide function as an additional recognition unit inside their binding site. Sulfur oxidation allows to simultaneously enhance all crucial properties of this receptor type i.e. photoswitching capability, thermal stability of individual switching states, binding affinity, and binding modulation upon switching. With a novel titration method the thermodynamic binding parameters were determined using reduced sample amounts. Employing these strongly enhanced molecular tweezers allowed to demonstrate photocontrol of intermolecular charge transfer in a reversible manner.
