Color-tunable triple state ‘smart’ window

through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/adpr.202100134. This article is protected by copyright. All rights reserved Color-tunable triple state ‘smart’ window Gilles H. Timmermans, Jiajun Wu, Albert P.H.J. Schenning, Jianbin Lin*, and Michael G. Debije*


Introduction
Materials that can alter their optical properties in reaction to external stimuli have received considerable attention as they can be used for a variety of applications including displays, [1][2][3][4] 'smart' windows [5][6][7] and luminescent solar concentrators (LSCs). [8][9][10][11] More recent applications under consideration include greenhouses, [12][13][14] camouflage [15,16] and sensors. [17,18] In 'smart' windows, the switchable devices have advantages over standard static systems, allowing different appearances which may change in time. A fast response is generally desired for windows used in buildings, although the rate of response really depends on the application, where changes in months, days, hours, minutes, seconds or even milliseconds are all demanded. Switchable properties that are of particular interest are changes in color, transparency, scattering and/or reflectivity. [19,20] Potential stimuli of interest as triggers include electrical, [20][21][22] thermal [23][24][25] and optical, [26][27][28] although there are many other options. [17,29,30] A potential deployment that has not been exploited for such 'smart' systems is signage.
The printed signage market has a multimillion dollar turnover [31] and is still growing, expected to have a compound annual growth rate of 0.52% for the period 2020-2025. [32] A highly visible, fluorescence based sign that may be electrically switched and thermally erased and re-written with new messages could be quite attractive.
In this work, we propose a device that can alter both its color and transparency in response to both heat and electrical potential, making for a very flexible design. In earlier work, we demonstrated a thermally reversible perylene-core dye in the liquid crystal (LC) blend E7. [33] While this dye produced reversible color variation upon heating, returning to the transparent state took quite a long time (>45 minutes) which makes it less viable for use in

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This article is protected by copyright. All rights reserved devices where rapid reversibility are desired. We conjectured that the addition of cyanobiphenyl groups [34] at the end of the alkyl tails of a perylene core to better match the chemical nature of the host LC E7 [35] would allow both enhanced interaction between the dye and the host LC, but also promote more rapid association of the dyes upon cooling. Indeed, we show the cyano-biphenyl groups result in faster dye reaction speeds, while maintaining excellent alignment and provide an improved electrical response. The new dye combined with a second dichroic dye and a chiral dopant is used to fabricate a color tunable triple-state 'smart' window that is electrically switchable between a dark, 'absorbing' state and a transparent state, with an intermediate scattering state, with a thermal override to a bright red color. The synthesis of the novel cyano-biphenyl dye, which we will refer to as CB-perylene (3), was done in three steps as shown in Figure 1. The key compound 1 was synthesized via Suzuki coupling between in situ formed bis-boron derivative with both 4'-bromo-4-cyano-

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This article is protected by copyright. All rights reserved biphenyl [36] and 4-bromoaniline sequentially in the same pot. 1 was purified by silica gel column chromatography using DCM/hexane as eluent. Following a modified literature protocol, [37] compound 2 and 3 were obtained in good yields. The detailed procedures are shown in the Supporting Information. The target compound CB-perylene was characterized in detail with 1 H and 13 C nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectrometry (HRMS) (see supporting information, Figure S3).
Similar to its predecessor C12-PBI (see Figure 2), [33] most of dye aggregates at 0.25 wt% in the nematic host LC E7 at room temperature (see Figure S4), resulting a relatively colorless cell. Upon increasing the temperature to >55 °C, the dye mostly dissolves, aligning parallel to the planar LC host. The order parameter (S) of CB-perylene in E7 was measured in the dissolved state using polarized absorption and calculated using: where A par and A per are the peak absorbances of the sample when incident light is polarized parallel and perpendicular to the alignment direction of the host LCs, respectively. We determine a high degree of alignment (S = 0.6, see Figure S5), indicating a good interaction between the dye and the E7 host; this value is greater than that found for the coumarin derivative dye labeled 6 in Figure 2, which has been one of the highest reported in E7, [38,39] and approximately the degree of alignment of the E7 host itself. [40] Accepted Article This article is protected by copyright. All rights reserved  [33] , 5) chiral dopant S1011 and 6) the coumarin derivative.
While at room temperature the system is mostly transparent, application of 7 V RMS rotates the E7 host to the homeotropic alignment, realigning the minor amount of nonaggregated dye, further reducing absorption since the dye is dichroic, and so absorbs little light when orientated with its molecular long axis directed towards to viewer (see Figure 3).
Increasing the temperature allows dye to de-aggregate and promote light absorption. As more dye dissolves, more dye can realign when an electrical potential is applied across the cell, reaching a maximum difference between the 'on' and 'off' states of the window at 55 °C (see

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This article is protected by copyright. All rights reserved  The recovery of the system to colorless upon cooling from its colored state was measured by first heating the sample to 80 °C and measuring the absorption spectra every 5 minutes as the sample cooled at ambient conditions. The results for CB-perylene show that within 5 minutes the absorption has returned to 88% of its initial value and completely recovers to its initial state at ~25 minutes (see Figure S6). In contrast, C12-PBI, which does not boast the cyano biphenyl groups, takes much longer to recrystallize: the 88% recovery Absorbance This article is protected by copyright. All rights reserved takes 45 min and still has not reached its minimum value even after 90 minutes. This rapid recrystallization in CB-perylene is promising for future applications where quick switching is desired, such as rewriteable message boards (preliminary efforts are shown as Figure S7).

Color tunable triple state 'smart' window
A method to create a scattering state in a switchable liquid crystal (LC) system without polymerization is using a 'supertwist' system. [3,10,41,42]  increased absorption as a result of multiple alignments throughout the thickness of the samples. [10] To fabricate the 'smart' window we used 0.25 wt% of CB-perylene 3, 0.25 wt% of a fluorescent coumarin derivative 6, and 1 wt% S1011 chiral dopant 5 (see Figure 2) dissolved in the nematic LC host. When heating the 'smart' window, more CB-perylene dye dissolves and the window becomes increasingly red colored (Figure 5a). The dissolution of the red dye clusters was monitored using UV-vis spectroscopy (see Figure 5b). Initially, only the 450 nm absorption peak from the coumarin derivative is visible with some scattering at longer wavelengths caused by the CB-perylene agglomerates. Increasing the temperature results in the dissolving of the CB-perylene clusters and the appearance of the perylene absorption peak, with corresponding reduction in scattering, with scattering reaching a minimum around 60 °C, the isotropic temperature of the mixture (see Figure S8 for the DSC). Application of a voltage (Figure 5c and Figure 5d shown at RT) below the temperature of the isotropic transition increases the scattering, with a maximum appearing around 9 V RMS as a focal conic LC state is achieved. Further increasing the potential decreases both the absorption and

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This article is protected by copyright. All rights reserved scattering of the cell as a homeotropic LC state with both the two dichroic dyes aligned perpendicular to the cell surface (see Figure S9). All electrically-driven state transitions, including reverting to the rest state upon turning off the applied voltage, occur in ~1 sec (see Video 1 in the Supplemental Information). Not only can the 'smart' window be switched independently with temperature or applied voltage, but the two stimuli can also be applied simultaneously (see Figure 6a).
Below ~60 °C the system is responsive to both temperature and electrical fields, after which the LC becomes isotropic and further application of a voltage has no effect. The effect of intermediate temperatures and voltage on the absorption of CB-perylene at 575 nm (corrected for scattering) is plotted in Figure 6b. The original data can be seen in Figures S10.

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This article is protected by copyright. All rights reserved  Figure S8). Lines are added between points as an aid for the eye. The window could be repeatedly switched electrically with no loss in performance after twenty cycles. While no significant changes in the bulk absorbance was obvious after ten heating cycles to 80 °C, there was some additional clustering of the CB-perylene visible, suggesting the cycling needs to be improved with some modification of structure, although it is likely the extreme temperatures used may have played a role (see Figure S11).

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This article is protected by copyright. All rights reserved Heating the sample to a higher temperature below the isotropic phase transition leads to (middle row, left to right) a red absorbing state with the CB-Perylene accepting this energy transfer from the coumarin. Upon application of intermediate and higher voltages, the focal conic and homeotropic states are attained. Finally, (bottom row) heating the sample to above the isotropic phase transition leads to a random alignment and a red color that is not affected by the application of an electric field.
To investigate the performance of the 'smart' window for electricity generation in luminescent solar concentrator-like applications, [8] [43] the edge emissions were measured as a function of temperature and applied voltage when exposed to an AM 1.5 solar spectrum ( Figure S12). At the application of 20 V RMS there is a decrease in emission at every wavelength as the sample becomes homeotropic. The emissions of the coumarin derivative

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This article is protected by copyright. All rights reserved and S14 for details).

Conclusion
A fluorescent dye was synthesized to have interaction with its host LC by attaching cyano-biphenyl groups at the end of the alkyl tails on a perylene core. The dye aligns well in the nematic LC host E7, responding to an electrical field and rapidly aggregating at lower temperatures and dissolving at higher temperatures.
Using this dye in combination with a second fluorescent dichroic dye in a supertwist LC system, a color tunable triple state 'smart' window was fabricated. The 'smart' window can change its optical properties both through temperature but also through application of an electrical field. A dramatic color change from green/yellow to brilliant red with a corresponding increase of red edge emission via enhanced fluorescence transfer was achieved by heating the device. A scattering state is generated due to the application of an intermediate voltage; it is possible to override the bright yellow/green or red colors by application of a higher field to generate a transparent system. This system has potential for application as 'smart' windows, and possibly as visually stunning, rewritable signage.

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This article is protected by copyright. All rights reserved

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Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.