<p></p><p>Azobenzene-based
bent-core liquid crystals demonstrate a variety of mesomorphic behaviors and photochromic
properties which are desirable for optical switching. Nowadays azobenzene-based
bent-core liquid crystal (ABLC) compounds usually exhibit at least one of the
following traits which are unfavorable for practical applications: (1) narrow
temperature windows of nematic phases, (2) high phase transition temperature, and
(3) long period of light stimulation to reach photostationary states. In this
study, a series of ABLC compounds <b>4a–4g</b> were synthesized by adding azo functional
groups and chlorine substituent to the central bent-cores to form 4-chloro-1,3-dizaophenylene
bent-cores. These ABLC compounds were characterized by i. fourier-transform infrared
spectroscopy (FTIR), <sup>1</sup>H and <sup>13</sup>C nuclear magnetic resonance
(NMR), and mass spectrometry (MS) for their structures, ii. differential scanning
calorimetry (DSC) and polarized optical microscopy (POM) for their mesogenic properties,
and iii. ultraviolet–visible spectroscopy (UV-Vis) and POM for their photosensitivity.
The experimental results show that all compounds exhibited broad temperature windows
of mesogenic phases. In particular, compound <b>4c</b> showed a broad temperature
window of 63.8 °C for nematic phase. Molecular simulations indicate that the
molecular dipole moments of compounds <b>4a–4g</b> are closely associated with the
temperatures of Sm – N phase transition and temperature ranges of nematic
phases. In addition, simulation results reveal that the terminal alkyl chains
exhibit a diphasic effect on the molecular polarity: extending the terminal
chain can initially reduce and then increase the molecular dipole moments due
to the severe structural disorder of overly extended terminal chain. These
findings indicate that the intermolecular forces play a vital role in shaping
the mesogenic behavior of ABLCs. Comprehensive characterizations of
photochromatic properties show that <b>4c</b> was highly photosensitive and displayed
rapid photoisomerization processes. At room temperature, compound <b>4c </b>dissolved
in ethyl acetate solution can reach photostationary state in 10 seconds. At 95
°C,
compound <b>4c</b> in nematic phase became isotropic liquid under
UV-irradiation in 3 seconds due to the forward <i>trans – cis</i>
photoisomerization and can be restored to be nematic under natural visible
light in 5 seconds because of the backward <i>cis – trans</i> photoisomerization.
This study linking the mechanistic details with mesogenic properties provides
valuable insights to improve future design of azobenzene bent-core liquid
crystals for practical applications especially in photonic applications.</p><p><br></p><p>Pertinent molecular structure files (mol2) can be downloaded from</p><p><a href="https://github.com/er1czz/ABLC">https://github.com/er1czz/ABLC</a><br></p><p></p>
Emergence and stabilization of transient twisted defect structures in confined achiral liquid crystals at a phase transition.
