7 research outputs found
Asymmetric Dimers of Chiral Azobenzene Dopants Exhibiting Unusual Helical Twisting Power upon Photoswitching in Cholesteric Liquid Crystals
In this study, we synthesized asymmetric
dimeric chiral molecules as photon-mode chiral switches for reversible
tuning of self-assembled helical superstructures. The chiral switches
bearing two mesogen unitsî—¸cholesterol and azobenzene moieties
connected through flexible alkylenedioxy bridgesî—¸were doped
into nematic liquid crystals, resulting in a chiral nematic (cholesteric)
phase. Under irradiation with UV light, photoisomerization of the
azobenzene units led to unprecedented switching of the cholesteric
pitch and helical twisting power (HTP, β), with a higher HTP
found in the cis-rich state (bent-form) than in the trans-state (rod-form).
We attribute this behavior to the elongated cybotactic smectic clusters
disrupting the helical orientation of the molecules in the cholesteric
liquid crystals; their reversible decay and reassembly was evidenced
upon sequential irradiation with UV and visible light, respectively.
In addition to the photoisomerization of the azobenzene units, the
odd/even parity of the alkylenedioxy linkers of the dimeric dopants
also had a dramatic effect on the transitions of the cybotactic smectic
domains. On the basis of the large rotational reorganization of the
cholesteric helix and HTP switching (Δβ/β<sub>ini</sub> of up to 50%), we could control the macroscopic rotational motion
of microsized glass rods upon irradiating the surface of a cholesteric
liquid crystal film featuring a polygonal fingerprint texture using
UV and visible light
Asymmetric Cyclophanes Permit Access to Supercooled Nematic Liquid Crystals with Stimulus-Responsive Luminescence
A novel
material with stimulus-responsive luminescence was created
by integrating a chromophore with assembly-dependent emission properties
into a liquid crystalline compound that can be kinetically trapped
in a supercooled liquid crystalline state. This was achieved by synthesizing
an asymmetric cyclophane containing one 1,6-bisÂ(phenylethynyl)Âpyrene
group. The new compound displays a broad nematic phase above ∼110
°C upon being heated. Quenching to room temperature allows one
to avoid crystallization, and the supercooled nematic phase is stable
for at least 1 h. When the compound is heated, a phase transition
from the kinetically trapped state to a crystalline state occurs,
concomitant with a pronounced change in photoluminescence. The crystalline
phase thus accessed shows mechanoresponsive luminescence behavior
Complete ON/OFF Photoswitching of the Motility of a Nanobiomolecular Machine
To apply motor proteins as natural nanomolecular machines to transporting systems in nanotechnology, complete temporal control over ON/OFF switching of the motility is necessary. We have studied the photoresponsive inhibition properties of azobenzene-tethered peptides for regulation of kinesin-microtubule motility. Although a compound containing a peptide having an amino acid sequence derived from the kinesin’s C-terminus (a known inhibitor of kinesin’s motor domain) and also featuring a terminal azobenzene unit exhibited an inhibition effect, the phototunability of this behavior upon irradiation with UV or visible light was only moderate. Unexpectedly, newly synthesized peptides featuring the reverse sequence of amino acids of the C-terminus of kinesin exhibited excellent photoresponsive inhibition. In particular, azobenzene-CONH-IPKAIQASHGR completely stopped and started the motility of kinesin-microtubules in its <i>trans</i>- and <i>cis</i>-rich states, respectively, obtained after irradiation with visible and UV light, respectively. A gliding motility system containing this photoresponsive inhibitor allowed <i>in situ</i> control of the motion of microtubules on a kinesin-coated glass substrate. It is expected that the present results on the photoresponsive nanomotor system open up new opportunities to design nanotransportation systems
Targeted Activation of Molecular Transportation by Visible Light
Regulated
transportation of nanoscale objects with a high degree
of spatiotemporal precision is a prerequisite for the development
of targeted molecular delivery. <i>In vitro</i> integration
of the kinesin-microtubule motor system with synthetic molecules offers
opportunities to develop controllable molecular shuttles for lab-on-a-chip
applications. We attempted a combination of the kinesin-microtubule
motor system with push–pull type azobenzene tethered inhibitory
peptides (azo-peptides) through which reversible, spatiotemporal control
over the kinesin motor activity was achieved locally by a single,
visible wavelength. The fast thermal relaxation of the <i>cis</i>-isomers of azo-peptides offered us quick and complete resetting
of the <i>trans</i>-state in the dark, circumventing the
requirement of two distinct wavelengths for two-way switching of kinesin-driven
microtubule motility. Herein, we report the manipulation of selected,
single microtubule movement while keeping other microtubules at complete
rest. The photoresponsive inhibitors discussed herein would help in
realizing complex bionanodevices
Rotaxanes as Mechanochromic Fluorescent Force Transducers in Polymers
The
integration of mechanophores, motifs that transduce mechanical
forces into chemical reactions, allows creating materials with stress-dependent
properties. Typical mechanophores are activated by cleaving weak covalent
bonds, but these reactions can also be triggered by other stimuli,
and this renders the behavior unspecific. Here we show that this problem
can be overcome by extending the molecular-shuttle function of rotaxanes
to mechanical activation. A mechanically interlocked mechanophore
composed of a fluorophore-carrying macrocycle and a dumbbell-shaped
molecule containing a matching quencher was integrated into a polyurethane
elastomer. Deformation of this polymer causes a fluorescence turn-on,
due to the spatial separation of fluorophore and quencher. This process
is specific, efficient, instantly reversible, and elicits an easily
detectable optical signal that correlates with the applied force
Reversible Photogeneration of a Stable Chiral Radical-Pair from a Fast Photochromic Molecule
The photochromic naphthalene-bridged imidazole dimer containing a naphthyl moiety that tethers two triarylimidazole units shows instantaneous coloration upon exposure to UV light and rapid fading in the dark. In this work, we demonstrate the formation of a stable chiral radical-pair that exhibits no photoracemization even by repeated photochromic cycles. The photogenerated radical-pair from the imidazole dimer exhibits the Cotton effect in the visible light region, indicating the retention of the enantiomeric conformation of the radical-pair. This result suggests that the chirality resulting from the binaphthyl moiety induces exciton coupling between the two radical chromophores by through-space interaction
Phenylazothiazoles as Visible-Light Photoswitches
A novel class of photoswitches based on a phenylazothiazole
scaffold
that undergoes reversible isomerization under visible-light irradiation
is reported. The photoswitch, which comprises a thiazole heteroaryl
segment directly connected to a phenyl azo chromophore, has very different
spectral characteristics, such as a redshifted absorption maximum
wavelength and well-separated absorption bands of the trans and cis isomers, than conventional azobenzene and
other heteroaryl azo compounds. Substituents at the ortho and para positions of the phenyl ring of the photoswitch
resulted in a further shift to longer wavelengths up to 525 nm at
the absorption maximum with a small thermal stability compensation.
These photoswitches showed excellent photostationary distributions
of the trans and cis isomers, thermal
half-lives of up to 7.2 h, and excellent reductant stability. The
X-ray crystal structure analysis revealed that the trans isomers exhibited a planar geometry and the cis isomers exhibited a T-shaped orthogonal geometry. Detailed ab initio
calculations further demonstrated the plausible electronic transitions
and isomerization energy barriers, which were consistent with the
experimental observations. The fundamental design principles elucidated
in this study will aid in the development of a wide variety of visible-light
photoswitches for photopharmacological applications