A new strategy for controlling the liquid crystalline and photophysical properties of supramolecular mesogens assembled via halogen bonding is reported. Changing the degree of fluorination at the halogen-bond donor of the supramolecular liquid crystal allows for the fine-tuning of the halogen bond strength and thereby provides control over the temperature range of the mesophase. At least three fluorine atoms have to be present to ensure efficient polarization of the halogen-bond donor and the formation of a mesophase. In addition, it was found that stilbazole acceptors are superior to their azopyridine counterparts in promoting stable liquid crystalline phases. The halogen-bonddriven supramolecular liquid crystals between fluorinated azobenzenes and stilbazole/azopyridine acceptors show a rich variety of photoinduced processes driven by azobenzene photoisomerization, dictated not only by the photochemical properties of the molecular components but also by the difference between the operation temperature and the clearing point.
Triplet energy transfer enables efficient Z-to-E photoswitching of azobenzenes even with near-infrared light. Ultrafast intersystem crossing of azobenzene makes the process entropy-driven and enables the use of endothermic sensitizer-azobenzene pairs.
The
response of soft actuators made of stimuli-responsive materials
can be phenomenologically described by a stimulus-deformation curve,
depicting the controllability and sensitivity of the actuator system.
Manipulating such stimulus-deformation curve allows fabricating soft
microrobots with reconfigurable actuation behavior, which is not easily
achievable using conventional materials. Here, we report a light-driven
actuator based on a liquid crystal polymer network containing diarylethene
(DAE) photoswitches as cross-links, in which the stimulus-deformation
curve under visible-light illumination is tuned with UV light. The
tuning is brought about by the reversible electrocyclization of the
DAE units. Because of the excellent thermal stability of the visible-absorbing
closed-form DAEs, the absorbance of the actuator can be optically
fixed to a desired value, which in turn dictates the efficiency of
photothermally induced deformation. We employ the controllability
in devising a logical AND gate with macroscopic output, i.e., an actuator
that bends negligibly under UV or visible light irradiation, but with
profound shape change when addressed to both simultaneously. The results
provide design tools for reconfigurable microrobotics and polymer-based
logic gating.
Thermally stable photoswitches that are driven with low-energy light are rare, yet crucial for extending the applicability of photoresponsive molecules and materials towards, e.g., living systems. Combined ortho-fluorination and -amination couples high visible light absorptivity of o-aminoazobenzenes with the extraordinary bistability of o-fluoroazobenzenes. Herein, we report a library of easily accessible o-aminofluoroazobenzenes and establish structure–property relationships regarding spectral qualities, visible light isomerization efficiency and thermal stability of the cis-isomer with respect to the degree of o-substitution and choice of amino substituent. We rationalize the experimental results with quantum chemical calculations, revealing the nature of low-lying excited states and providing insight into thermal isomerization. The synthesized azobenzenes absorb at up to 600 nm and their thermal cis-lifetimes range from milliseconds to months. The most unique example can be driven from trans to cis with any wavelength from UV up to 595 nm, while still exhibiting a thermal cis-lifetime of 81 days.
Graphical abstract
A tunable reflectance
filter based on a metal–hydrogel–metal
structure responsive to humidity and temperature is reported. The
filter employs a poly(N-isopropylacrylamide)–acrylamidobenzophenone
(PNIPAm–BP) hydrogel as an insulator layer in the metal–insulator–metal
(MIM) assembly. The optical resonance of the structure is tunable
by water immersion across the visible and near-infrared range. Swelling/deswelling
and the volume phase transition of the hydrogel allow continuous reversible
humidity- and/or temperature-induced tuning of the optical resonance.
This work paves the way toward low-cost large-area fabrication of
actively tunable reversible photonic devices.
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