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Abstract
The reflection notch of cholesteric liquid crystals can be controlled using near infrared light (NIR) irradiation, and is demonstrated in this work. Opto-thermal tuning of a liquid crystal mixture near a SmA → CLC transition is achieved through use of a NIR absorbing dye, PBIBDF-BT, which can yield large changes in the spectral reflection. Compared to ultraviolet (UV) and visible light, employing NIR light is beneficial because of its invisibility, outstanding penetration for temporal and spatial remote activation of materials, and more. A simple fabrication method involving the addition of infrared absorbing dye into one of the alignment layers of a liquid crystal cell is presented. The tuning range and speed are dependent on the NIR light power, wavelength, and infrared absorbing dye concentration. Furthermore, RGB3-element color can be achieved by adjusting the NIR light power.
© 2017 Optical Society of America
1. Introduction
Cholesteric liquid crystals [1–3] (CLCs)—also known as chiral nematic liquid crystals (N*-LC)—have attracted much attention due to their spontaneous self-assembling periodic helical superstructures. The helical superstructures of CLCs are characterized by two structural parameters: handedness and pitch length [4–6]. The direction of director rotation about the helix axis defines its handedness, whereas the pitch length (p) corresponds to the distance over which the director rotation completes a full turn; the pitch length is inversely proportional to the concentration (c) and the helical twisting power (β) of the chiral dopant: p = 1/(c × β). It is well known that CLCs can selectively reflect a band of incident which circularly polarized light with the handedness as its helix due to the periodic variation in its refractive index, whereas the band with the opposite twist sense is transmitted. Outside the reflection band, both polarization states are simply transmitted. At normal incidence, the central wavelength, λ0, of the selective reflection is related to p via λ0 = n × p, where n is the average refractive index of the host liquid crystal (LC) [7]. Moreover, the weak intermolecular interaction in LCs suggests that this selective reflection can be adjusted by external stimuli, such as heat [8,9], light [10,11], electric field [12,13], and mechanical stress [14]. It has long been recognized that the ability to tune selective reflection of CLCs in real time could enhance their applications, such as in tunable dye lasers [15,16], optical switching [17], reflective displays [2,18], and information storage. Among the different readily accessible stimuli, light is particularly attractive because its ease of addressability, fast response time, and potential (i.e., remote, spatial, and temporal) controllability in a wide range of ambient environments. So far, photo-responsive CLCs are most commonly achieved by doping a small amount of photochromic chiral molecular switches or motors into an achiral nematic LC host. These chiral dopants can exist in at least two stable states with different helical twisting power; equilibrium of the transition between these two states can be achieved through irradiation [19].
Hitherto, there have been a large number of reports on the use of chiral photochromic molecules such as azobenzenes [20], overcrowded alkenes [21,22], and diarylethenes [23,24] as dopants for photo-responsive CLCs; primarily for ultraviolet (UV) or visible-light stimulus. The use of high-energy UV light in the above systems can result in material damage, environmental contamination, and poor penetration through container substrates. Compared to UV and visible light, near-infrared (NIR) light possesses much more benefits because of its invisibility, outstanding penetration for temporal and spatial remote activation of materials. Enabling remote, reversible, and dynamic control of helical superstructure via NIR light would improve many applications, especially in life sciences, drug delivery, materials science, and aerospace. Li et al. [25] reported on nanotransducer-impregnated, self-organized helical superstructures with unprecedented reversible handedness inversion upon irradiation with dual-wavelength near-infrared light; this method was based on upconversion nanoparticle technology. However, this technology still generates ultraviolet light and the conversion efficiency is low.
Heating through the smectic phase to the CLC phase transition leads to a very sensitive unwinding or tightening of the pitch, which can yield large changes in the spectral reflection at temperatures just above the smectic → CLC transition. In this paper, we demonstrate that the reflection notch of the cholesteric liquid crystals can be tuned by NIR light through the use of a NIR absorbing dye, PBIBDF-BT, in one of the alignment layers. Strong absorption by the PBIBDF-BT generates heat that is transferred to the surrounding LC host. The tuning range and speed are dependent on the NIR light power, wavelength, and dye concentration.
2. Experimental section
The cholesteric liquid crystal was formed by mixing commercially available chiral dopant S811 (Chengzhi Yonghua) and nematic LC E7 (HCCH, China) in differing ratios. The physical properties of E7 are as follows: Δε = 11.4 at 1 kHz and 25 °C, Δn = 0.223 at λ = 589 nm, and clearing temperature Tc = 59.2 °C. A previously reported donor–acceptor (D–A), conjugated, low bandgap polymer based on bithiophene and bis(2-oxoindolin-3-ylidene)-benzodifuran-dione (BIBDF) (i.e., PBIBDF-BT) was used as the near infrared absorbing material. The structures for S811 and PBIBDF-BT are shown in Fig. 1.
Fig. 1 The structure of: (a) the chiral dopant, S811, and (b) the infrared absorbing material, PBIBDF-BT.
The CLC mixtures were filled into 7 μm gap cells with anti-parallel rubbed alignment layers through capillary action, where cell thickness was controlled by a dispersion of spacer beads. The cells were fabricated with plain quartz glass substrates. Homogeneous alignment was achieved by pretreating the substrates with PMMA or polyimide and rubbing. To produce a NIR light responsive cholesteric liquid crystal, the infrared absorbing dye, PBIBDF-BT, was doped into one of the alignment layers.
The cell textures were observed under crossed polarizers using a polarized optical microscope (Leica DM2500M). The optical properties of the cells were measured using an ultraviolet spectrophotometer (Shimadzu UV2550). The thermotropic properties of the mixed CLCs were investigated using a differential scanning calorimetry (DSC) (TA Instruments Q2000) with a 1 °C/min cooling rate.
3. Results and discussion
The LC phases of the S811/E7 mixtures were characterized using DSC and polarized optical microscope (POM). The DSC results for all compositions are shown in Fig. 2(a). For compositions where S811 ≤ 20 wt%, only one exothermic peak is observed above 20 °C during the cooling process. When cooled from its isotropic state to room temperature, a cholesteric phase with a reflection notch located in the near-IR region was found (Fig. 7). For compositions where S811 > 20 wt%, two exothermic peaks were observed during the cooling process. An additional lower temperature smectic liquid crystal phase appears in the heavily-doped nematic systems due to the rod-like nature of the chiral dopant promotes packing of layers to form a smectic phase in large amounts. The Smectic A focal-conic fan texture and the cholesteric planar texture of the mixture (30 wt% S811) were identified by POM at 26 °C and 30 °C, as shown in Fig. 2(b), respectively. These systems exhibit a SmA → CLC transition just above room temperature. The onset of this transition temperature increases and the clearing temperature decreases as S811 content increases.
Fig. 2 (a) DSC curves of the S811/E7 mixtures at a 1 °C/min cooling rate. (b) LC textures for the S811 30 wt% mixture were recorded with crossed polarizers at 26 °C and 30 °C.
Prepared CLC mixtures were injected into anti-parallel rubbed cells, and the transmission spectra were measured as functions of temperature. Figure 3(a). shows the results of the sample with a 30 wt% S811 concentration. No reflection notch was observed at room temperature, but a notch appeared in the near-IR for temperatures exceeding 30 °C; this notch moved into the visible region as the temperature increased. Note that the reflection notch bandwidth narrows with the blue-shift, which is entirely consistent with the formula △λ = p × (ne-no). In order to more clearly describe the effects of temperature change on the reflection notch, the relationship between the central wavelength of the reflection notch and the temperature is shown in Fig. 3(b). The reflection notch first appeared near 700 nm at 30 °C. A very sensitive tuning range of 200 nm was observed over a temperature variation of △T = 6 °C; the maximum blue-shift (to 500 nm) occurred at 36 °C.
Fig. 3 (a) The CLC cell (S811 30 wt%) transmission spectra as functions of temperature. (b) A plot of the central wavelength of the cell versus the temperature.
The use of thermal tuning is rather awkward and difficult for actual device applications. Utilizing the same CLC mixture, we demonstrate that a similar wide range tuning can also be induced by NIR light. We added infrared absorbing dye, PBIBDF-BT, into one of the alignment layers (PBIBDF-BT:PMMA = 1:5). When the cell is exposed to 850 nm NIR light at a 30 mW cm−2 intensity, a reflection is first detected at 710 nm after 50 seconds of exposure; the reflectivity grows as it blue-shift upon continued irradiation (Fig. 4(a)). After 3600 s of irradiation, the reflection notch stabilizes at about 500 nm. We successfully tune the reflection notch more than 200 nm using NIR light. Strong absorption of the NIR light by the PBIBDF-BT dye leads to a nonradiative transition back to the ground state, which results in an efficient transfer of heat. The blue-shift is due to a pitch contraction that occurs as the cell temperature increases, which is typical for a SmA → CLC transition. Before irradiation, the cell is transparent at room temperature (25 °C) because the liquid crystal is in the smectic A phase at this state, as shown in the insert of Fig. 4(a). After 200 s of NIR irradiation, the reflection band blue-shifts to 650 nm and the cell appears red. Further irradiation (3600 s) tunes the reflection band to 500 nm, and the cell appears green. After removal of the light source, the cell returns to the original relatively transparent state.
Fig. 4 (a) The transmission spectra of the CLC cell as functions of 850 nm NIR light irradiation time. (b) Bragg wavelength as functions of irradiation time for different NIR wavelengths.
The tuning time and stabilized reflection notch depend on the NIR light wavelength and power. The change of the reflection notch under 940 nm NIR light (30 mW cm−2) was investigated, and is shown in Fig. 8 in the Appendix. Prolonged illumination shifts the reflection notch towards shorter wavelengths and reaches a stable state at 7000 s with the reflection band at 550 nm. Figure 9 shows the absorption spectrum of the PBIBDF-BT/PMMA film on quartz glass. In the work presented here, the PBIBDF-BT dye functions as a conversion agent to convert the NIR light into heat. The PBIBDF-BT dye is photochemically inert, has low emission quantum yields, and has a broad absorption spectrum with a maximum absorption at 850 nm. The strong absorption coefficient at 850 nm NIR light corresponds to a large opto-thermal tuning range in comparison to that for 940 nm NIR light.
Figure 5(a). shows the center of the reflection notch as a function of exposure time for different NIR light intensities (29.2 mW/cm2, 3.54 mW/cm2, 0.58 mW/cm2, and 0.14 mW/cm2). The time for the S811/E7 mixture to reach a stable reflection notch increased from 3600 s and 7000 s to 9500 s and 12000 s, respectively, for the reduced NIR light exposure powers. Simultaneously, the tuning range decreased due to the reduced NIR light intensities. These reduced intensities correspond to decreased thermal energy conversion, and CLC cell appears green (29.2 mW/cm2), yellow (3.54 mW/cm2), orange (0.58 mW/cm2), and red (0.14 mW/cm2) at their respective stabilized states. The NIR light intensity can be distinguished by the color of the CLC cell. The absorption of light by the PBIBDF-BT dye is followed by a loss of excitation energy via nonradiative thermal processes; as a result, the heat is transferred very rapidly and uniformly into the liquid crystal. Higher dye concentrations generate more heat and result in higher stabilized temperatures. Thus, the tuning range and stabilized reflection notch can be adjusted by controlling the dye concentration, as shown in Fig. 5(b). The stabilized reflection notch reached 700 nm for a long exposure time with pure PMMA because of the infrared light thermal effect. As the PBIBDF-BT concentration increased, the stabilized reflection notch reached 600 nm (PBIBDF-BT:PMMA = 1:10) and 500 nm (PBIBDF-BT:PMMA = 1:5). Furthermore, the tuning times decreased to 9000 s and 3600 s, respectively.
Fig. 5 (a) Bragg wavelength as a function of irradiation time for different light intensities. (b) Bragg wavelength as a function of NIR irradiation time for different PBIBDF-BT concentrations. (30 wt% S811 + E7).
The above results indicate that the CLC reflection notch can be tuned using NIR light, but the cell cannot produce a blue color, which limits their applications in the display field. In order to realize full RGB3-element color, we doped another chiral agent with high helical twisting power, S6N (0.5%), to blue-shift the stabilized reflection notch. The thermotropic and optical properties of the mixed CLCs (30% S811 + 0.5% S6N + E7) were investigated using DSC and transmission spectra measurements. The reflection notch can be tuned from 680 nm to 470 nm under 850 nm NIR light (30 mW cm−2), as shown in Fig. 10. Note that the tuning range can be varied over the entire visible range. Thus, we use diverse NIR light intensity values (29.2mW, 3.54mW, and 0.14mW) to produce different thermal energies generate various stabilized reflection notches (470 nm, 540 nm, and 640 nm). Finally, the CLC reflection RGB was demonstrated for different light intensity irradiation, as shown in Fig. 6.
Fig. 6 Bragg wavelength as a function of 850 nm NIR irradiation time for different light intensities. (30 wt% S811 + 0.5 wt% S6N + E7).
Conclusion
In conclusion, we have demonstrated that the reflection notch of cholesteric liquid crystals can be controlled by NIR irradiation. When chiral molecule S811 is doped into the positive nematic liquid crystal E7, a smectic A phase is formed in a heavily doped system. Furthermore, the optically generated increase in temperature serves to transition the phase from SmA to CLC, which can yield large changes in the spectral reflection. The tuning initiated by exposure to the NIR light is a relatively simple and convenient means to direct thermal tuning. We explored the effects of infrared light energy, wavelength, and PBIBDF-BT dye concentration on the tuning range, and have achieved RGB3-element color actuation by adjusting the NIR light intensity. This work may open a window to future applications in display and detection systems.
Appendix
1. The transmission spectra and LC textures of CLC cell (S811 20 wt %)
Fig. 7 (a) The transmission spectra of CLC cell (S811 20 wt %). (b) LC texture of the S811 20 wt% mixture was recorded using crossed polarizers at room temperature.
2. 940 nm NIR light tuning the reflection notch of CLC
Fig. 8 The CLC cell transmission spectra as functions of 940 nm light at different NIR irradiation times.
3. PBIBDF-BT Absorption spectra
4. Control of the CLC reflection notch with different NIR light intensities
Funding
Provincial Science Foundation of Anhui (1708085MF150); National Natural Science Foundation of China (grant No. 61107014, 51573036); Program for New Century Excellent Talents in University (Grant No. NCET-12-0839); Fundamental Research Funds for the Central Universities (Grant No.JD2017JGPY0006, JD2017JGPY0003).
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