End-Functionalization of Diarylethene for Opto-Electronic Switching with High Fatigue Resistance

A molecular
and synthetic approach to strengthen the switching
performance of diarylethene (DAE)-based organic transistors is proposed.
By tuning the length of alkyl side chains of the biphenyl unit attached
to 1,2-bis­(5-biphenyl-2-methylthien-3-yl)­perfluorocyclopentene (DAE),
we show that the molecular environment for reversible photoisomerization
of DAEs can be optimized. Four different DAEs are synthesized with
different alkyl chains (DAE_C0, DAE_C1, DAE_C6, and DAE_C10), and
ITIC is chosen to construct a semiconductor matrix to maximize the
quantum yield of photoconversion considering the complementary absorption
range of both materials. From photophysical, structural, and morphological
analyses, the longer alkyl chains inhibit intermolecular aggregation
between DAEs and allow more hydrophobic surface properties of DAEs,
thus improving molecular miscibility with ITIC. The improved molecular
compatibility of DAEs with ITIC makes the overall bulk heterojunction
film amorphous, allowing more free volume for reversible photoisomerization.
Consequently, DAE_C6 exhibits the maximum quantum yield for both photocyclization
and photocycloreversion, enabling high light-controlled on/off ratios
in photoswitchable transistors. Furthermore, the exceptionally high
DAE_C6 quantum yield enables robust fatigue resistance under repeated
photoswitching with only a 30% decrease in the on/off ratio after
100 cycles. Overall, this work shows that not only the energy level
but also the molecular compatibility can endow significant switching
performances for molecular switches.