Electric-field-dictated phase diagram and accelerated dynamics of a reentrant nematic liquid crystal under photostimulation

In a system consisting of photoactive molecules that exhibit light-driven isomerization transformations, actinic light can diminish or enhance ordering to the extent that transitions from the equilibrium to a more disordered phase can be brought about isothermally. This feature enables light to be used as a thermodynamiclike parameter to investigate phase behavior and adds another dimension to the studies owing to the nonequilibrium character of the isothermal transitions. We have carried out experiments which exploit the combination of two recent findings, viz., an electric field can accelerate the return to the nematic liquid crystalline phase from a photodriven isotropic phase; and in a reentrant mesogen, the photoinduced phase can be more ordered. To photostimulate the nonequilibrium transitions a low power uv radiation $\left(0.1\text{mW}{\text{cm}}^{-2}\right)$ has been used. Unique temperature–electric-field phase diagrams of a liquid crystal exhibiting isotropic–nematic–smectic-$A$ –reentrant nematic sequence, mapped using light transmission as probe reveal that the electric field influences all the transitions, but the effect is maximum on the equilibrium reentrant nematic to the photoinduced smectic-$A$ transition. Temporal measurements have been performed under nonequilibrium conditions to study the dynamics of both the photochemical and the back relaxation processes across the different transitions. The electric field is indeed observed to accelerate the thermal back relaxation in each case, and especially the recovery of the reentrant phase is hastened by three orders of magnitude in time. We explore possible causes for the acceleration and present a finding which can be associated with one of the predictions of density-functional calculations for isomerization of azobenzenes.

Figure 1

(Color online) Scans of the laser transmitted intensity as a function of temperature (a) in the absence of and (b) upon shining uv light $\left(0.1\text{mW}{\text{cm}}^{-2}\right)$. Panels (c), (d), and (e) show the effect of different magnitudes of voltage when the uv light is simultaneously present.

Figure 2

(Color online) Diagram to show that clear signatures of the transitions are observed in the (a) capacitance as well as (b) transmitted light intensity measurements also.

Figure 3

(Color online) Temperature-voltage phase diagram. The dashed lines represent the transition temperatures obtained in the absence of photostimulation. The filled circles are the transition temperatures obtained when the sample is illuminated with uv radiation. The lines through the data points are only guides to the eye. Notice that the electric field drastically affects the Sm-$A\text{−}{N}_{\text{re}}$ transition and that with about 20 V the photoinduced diminution in the transition temperature is nearly annulled.

Figure 4

The time-resolved variation in the transmitted intensity when the uv light is turned on at $\text{time}=0$ instant, keeping the sample temperature $2°\text{C}$ below the respective equilibrium transition temperature. The notations used to define the delay $\left({\tau }_{\text{d}1}\right)$ and the response $\left({\tau }_{\text{on}}\right)$ times are shown. Particularly, notice that the $N_{\text{re}}$-Sm-$A$ transition has the sharpest response.

Figure 5

The time-resolved variation in the transmitted intensity after the uv illumination is turned off, while the sample temperature is maintained $2°\text{C}$ below the respective equilibrium transition temperature. The notations used to define the delay $\left({\tau }_{\text{d}2}\right)$ and the response $\left({\tau }_{\text{off}}\right)$ times are shown. Particularly, notice that the thermal back relaxation process for the recovery of the $N_{\text{re}}$ phase (Sm-$A\text{−}{N}_{\text{re}}$ transition) takes much longer than in the case of the other two phases.

Figure 6

(Color online) (a)–(c) Schematic representation of the molecular arrangements in the equilibrium (left panels) and photodriven (right panels) states. Notice especially the nanophase segregation in the photostimulated Sm-$A$ phase (panel c). (d) and (e) Configuration of triplet of molecules, with the cyan (gray) colored circles representing host polar LC molecules and the red (dark) colored circles representing the photoactive EPH molecules. The dot and the cross in the center indicate dipoles pointing into or out of the plane. The red (solid) and the blue (dashed) lines indicate the interactions. In (d) the EPH molecule is in its $E$ state with negligible dipole moment and therefore does not cause a frustration in the triplet, whereas in the $Z$ form it does [indicated by the question mark in (e)] due to the large value of the dipole moment.

Figure 7

(Color online) Temporal behavior of the transmitted intensity as a function of the time elapsed since the uv is turned off and simultaneously the voltage of different magnitudes is turned on during the thermally back relaxation of the (a) $I\text{−}N$, (b) $N$–Sm-$A$, and (c) Sm-$A\text{−}{N}_{\text{re}}$ transitions. Notice that generally with increasing voltage the process is hastened, with the effect being large for the Sm-$A\text{−}{N}_{\text{re}}$ transition. However, the 1 V data for the latter two transitions deviate from this behavior.

Figure 8

Dependence of the delay time ${\tau }_{\text{d}2}$, for the thermal back relaxation process on the applied voltage for the $I\text{−}N$, $N$–Sm-$A$, and Sm-$A\text{−}{N}_{\text{re}}$ transitions. The dashed lines merely connect the data points to illustrate the nonmonotonic behavior around 1 V. The solid lines are fits obtained to an exponential decay function performed by excluding the data for 1 V.

Figure 9

Dependence of ${\tau }_{\text{off}}$, the response time for the thermal back relaxation process on the applied voltage for the $I\text{−}N$, $N$–Sm-$A$, and Sm-$A\text{−}{N}_{\text{re}}$ transitions. The dashed lines merely connect the data points to illustrate the nonmonotonic behavior around 1 V. The solid lines are fits obtained to an exponential decay function performed by excluding the data for 1 V.

Figure 10

Temporal dependence of the back relaxation process associated with the recovery of the $N_{\text{re}}$ phase from the photostimulated Sm-$A$ phase, exhibiting asymmetric behavior for the opposite signs of the applied voltage, with the delay time $\left({\tau }_{\text{d}2}\right)$ and response time $\left({\tau }_{\text{off}}\right)$ being shorter for the negative voltage.