Improving the Fatigue Resistance of Diarylethene Switches

When applying photochromic switches
as functional units in light-responsive
materials or devices, an often disregarded yet crucial property is
their resistance to fatigue during photoisomerization. In the large
family of diaryl­ethene photoswitches, formation of an annulated
isomer as a byproduct of the photochromic reaction turns out to prevent
the desired high reversibility for many different derivatives. To
overcome this general problem, we have synthesized and thoroughly
investigated the fatigue behavior of a series of diaryl­ethenes,
varying the nature of the hetaryl moieties, the bridging units, and
the substituents. By analysis of photokinetic data, a quantification
of the tendency for byproduct formation in terms of quantum yields
could be achieved, and a strong dependency on the electronic properties
of the substituents was observed. In particular, substitution with
3,5-bis­(trifluoro­methyl)­phenyl or 3,5-bis­(pentafluoro­sulfanyl)­phenyl
groups strongly suppresses the byproduct formation and opens up a
general strategy to construct highly fatigue-resistant diaryl­ethene
photochromic systems with a large structural flexibility.