Donor-Acceptor Stenhouse Adducts – Opportunity in Complexity
Our ability to visually perceive our environment is driven by small molecules – retinal – in our eyes. Upon exposure to light retinal undergoes a change in shape which is translated to the very image of this text. Synthetic versions undergoing similar transformations are called photoswitches. These small molecular machines have been used in a range of applications to control by example pharmacological activity, polarity, conductivity and mechanical properties. Over the years several classes of photoswitches have been developed with different photophysical properties. One key design principle which drives photoswitch development is to combine a simple synthetic strategy with visible light response.
In 2014 our group has introduced a novel class of photochromic molecules called donor-acceptor Stenhouse adducts (DASA). DASAs combine a number of promising properties such as a straightforward synthesis, visible light response as well as a large molecular shape and polarity change. From 2014 significant effort has been made to understand their switching mechanism and structure-property relationships. Since then their multi-step mechanism has been identified while their donor and acceptor design space has been explored. However, the design space of DASA is limited by the reactivity of both donor and acceptor groups. Furthermore while the multi-step mechanism is understood little efforts have been made to take advantage of the unique and promising characteristics of multiple steps photoswitches.To address some of these problems both the synthetic access to DASA photoswitches as well as the understanding on how structural changes relates to energy landscape have to be improved. If these conditions can be met, DASAs multi-step reaction pathway promises unique photoswitching properties and responses. To widen the synthetic access to DASA we showcase a catalytic method to accelerated the DASA forming reaction and widen the design space of DASA through the use of 1,1,1,3,3,3-hexafluoroisopropanol. By using solvatochromatic shift analysis we were able to link the ground state charge separation of the open form of DASA to the kinetics as well as the effect of concentration and environnment on DASA switching properties. We were able to understand the effect of polarity on the switching pathway and by targeting key barriers in the switching pathway we demonstrate how the same two stimuli can provide a multitude of outcomes in a complex DASA mixture through pathway selectivity. By intercepting intermediates along the reaction pathway we introduced DASA as a negative multi-stage photoswitch. The multi-stage character of the mechanism allows for dual-wavelength control of DASA as well as unique switching properties. To employ the desirable photophysical properties of DASA in applications such as photomechanical work a new chemistry has to developed for main chain incorporation due to DASA susceptibility to nucleophile and radical chemistry. Through Diels–Alder chemistry we demonstrated main chain incorporation of DASA in liquid crystalline networks.
Through this work we hope to open the door to widespread use of DASA by realizing the full potential of DASA multi-step switching mechanism in solution as well as in solid materials.