Tuning the temperature dependence for switching in dithienylethene photochromic switches

Diarylethene photochromic switches use light to drive structural changes through reversible electrocyclization reactions. High efficiency in dynamic photoswitching is a prerequisite for applications, as is thermal stability and the selective addressability of both isomers ring-opened and -closed diarylethenes. These properties can be optimized readily through rational variation in molecular structure. The efficiency with regard to switching as a function of structural variation is much less understood, with the exception of geometric requirements placed on the reacting atoms. Ultimately, increasing the quantum efficiency of photochemical switching in diarylethenes requires a detailed understanding of the excited-state potential energy surface(s) and the mechanisms involved in switching. Through studies of the temperature dependence, photoswitching and theoretical studies demonstrate the occurrence or absence of thermal activation barriers in three constitutional isomers that bear distinct π-conjugated systems. We found that a decrease in the thermal barriers correlates with an increase in switching efficiency. The origin of the barriers is assigned to the decrease in π-conjugation that is concomitant with the progress of the photoreaction. Furthermore, we show that balanced molecular design can minimize the change in the extent of π-conjugation during switching and lead to optimal bidirectional switching efficiencies. Our findings hold implications for future structural design of diarylethene photochromic switches.