Driving a Liquid Crystal Phase Transition Using a Photochromic

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Driving a Liquid Crystal Phase Transition using a Photochromic Hydrazone Mark J Moran, mitchell magrini, David M. Walba, and Ivan Aprahamian J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09622 • Publication Date (Web): 06 Oct 2018 Downloaded from http://pubs.acs.org on October 7, 2018

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Journal of the American Chemical Society

Driving a Liquid Crystal Phase Transition using a Photochromic Hydrazone Mark J. Moran,† Mitchell Magrini,‡ David M. Walba,‡ and Ivan Aprahamian†,* † Department

‡ Department

of Chemistry, Dartmouth College, Hanover, New Hampshire, 03755, USA of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, CO 80309, USA

Supporting Information Placeholder ABSTRACT: The dynamic manipulation of the

properties of soft matter can lead to adaptive functional materials that can be used in advanced applications. Here we report on a new chiral dopant, built on an isosorbide scaffold attached to two bistable hydrazone-based light-switches, that can be used to control the self-assembly, and hence photophysical properties, of nematic liquid crystals (LCs). The bistability of the switch allows kinetic trapping of various helical assemblies as a function of the photostationary states, resulting in the reflection of different wavelengths of light. Surprisingly, doping 5CB with the chiral switch, and irradiation with blue light triggers an isothermal phase change from the helical cholesteric to the untwisted lamellar Smectic A* phase. This transition was used to modulate the transparency of a LC film resulting in a light-gated optical window.

Propagating the mesoscopic motion of molecular switches and motors to macroscopic events is one of the grand challenges in the field of artificial molecular machines.1 Efforts directed towards this goal led to the development of actuators (mainly polymeric) that can convert light and (electro)chemical energy into mechanical motion.2,3,4 The supramolecular self-assembly of liquid crystals5 (LCs) have made them a prime target of such studies as well, and the control over their long-range order has been harnessed in designing responsive reflectors, actuators, micromechanical systems, active smart surfaces, and sensors.6 Recently, there has been an uptick in using chiral photochromic dopants7 (far less so using chemically activated switches8), which organize achiral nematic LCs into optically active chiral (cholesteric) selfassembled structures, in controlling the

photophysical properties of LCs. In general, azobenzene is the light-trigger of choice in such applications.9 While numerous chiral azobenzene dopants have been developed throughout the decades, one drawback associated with their use is the production (in most cases)10 of a thermally unstable cis isomer upon photoisomerization. Hence, the obtained chiral LC assemblies, and accompanied properties, are transient and cannot be locked in place for extended periods. To address this issue bistable diarylethene switches have been used as chiral dopants. Initially, this strategy had limited success as the helical twisting power (HTP, i.e., their ability to twist an achiral nematic LC) of the dopants and/or the change in HTP (HTP) upon photoisomerization was low.11 This shortcoming was addressed by combining the large HTP transducing ability of BINOL with diarylethene, leading to systems that can be locked in place to give stable RGB color reflection,12 and even helicity inversion,13 among other properties.14 Given the limited structure space explored so far, and our interest in developing complementary approaches for control over LC assemblies, we initiated a study to determine whether the bistable photochromic hydrazone switches15 developed in our labs16 can be used as switchable chiral dopants. To achieve this goal, we combined the hydrazone switch with isosorbide (1), an inexpensive and relatively understudied chiral dopant, and introduced it into two commercially available achiral roomtemperature nematic LCs (5CB, and PCH5; Scheme 1). As shown here, this strategy was successful, as the chiral dopant resulted in good HTP and HTP values for both nematic hosts. The bistability of the switch was used to kinetically trap various selfassembled helical LC structures as a function of the wavelength dependent photostationary state (PSS), leading to the reflection of various wavelengths of

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near-infrared light. Most importantly, we report on the exceedingly rare light-induced unwinding of the helical self-assembly of the cholesteric phase into the untwisted lamellar chiral smectic A phase (SmA*). That is, the photoisomerization leads to increased ordering via layering of the LC phase,17 as opposed to the decrease in order observed with most chiral switchable dopants.9 This behavior, and the accompanying change from a semi-opaque (cholesteric) to transparent (SmA*) LC films, is exploited in the design of a light-gated optical window. Scheme 1. The structures of photochromic dopant 1, and the liquid crystalline hosts 5CB and PCH5.

comprising of 89% of 1-Z,Z.18 The quantum yields for the Z → E isomerization, and the reverse process were measured to be 2.6 ± 0.2% and 5.8 ± 0.6%, respectively (Figures S10-S11), while the thermal isomerization half-life was calculated to be 1080 ± 43 years (Table S2). The fatigue resistance of the switch was also studied, and minimal change in absorption intensity was observed after 10 consecutive switching cycles indicating that the system is robust (Figure S8). We took advantage of the bistability of the system to measure the Z/E isomer ratio of 1 at different PSSs by irradiating the system with various light wavelengths (Figure 1). Irradiating the mixture obtained at PSS410 with 394 nm lights yields 62% E,E and 37% E,Z at the PSS. Continuing irradiation with 375 nm light results in 13% E,E and 68% E,Z at PSS. Irradiation with 365 nm light results in 7% E,E and 62% E,Z at the PSS (Table S1 and Figures S9 and S10).

0.7 Pristine 410 nm 394 nm 375 nm 365 nm 340 nm

0.6 0.5

The photochromic switch 1, possesses a central isosorbide scaffold and is decorated at the periphery with 4-decyloxyphenyl benzoate moieties. The rationale behind incorporating the latter structural motif in the design of the dopant is two-fold. First, introduction of liquid crystalline like moieties was expected to enhance the solubility of the photochromic dopant in a LC matrix. Second, extending the rotor part (red/green) of the photoswitch was expected to amplify the geometrical change upon photoisomerization. The target molecule was synthesized in an efficient and straight forward manner (Scheme S1). Once in hand it was fully characterized using 1H and 13C NMR spectroscopies, and high-resolution mass spectrometry.

UV/Vis and 1H NMR spectroscopies were used to study the photophysical and photoisomerization properties of 1 (Figure 1, Figures S8-S11 and Tables S1 and S2). Irradiation of a pristine sample of 1 (max = 376,  = 51,000) in toluene with 410 nm light affords a PSS410 comprising of 97% of the E,E configuration (max = 340 nm,  = 54,000). The isomerization process can be reversed by irradiation with 340 nm light yielding a PSS340

0.4 0.3

A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.2 0.1 0 285

335

385

435

 / nm

Figure 1. The UV-Vis spectra of 1 in toluene in the pristine state and at the PSS when irradiated with 340, 365, 375, 394, and 410 nm light.

To study the effect of the photochromic switch on the LC properties of achiral nematic hosts it was doped (ca. 1 mol%; 5% by mass) into 5CB, and PCH5 (Scheme 1) yielding appropriate cholesteric phases. The HTP of dopant 1 in these hosts was measured using the Greandjean-Cano wedge method19 (Table 1). Both hosts show significant dopant-host interactions as evidenced by the good HTP values. Irradiation of the doped PCH5 mixture with 410 nm light induces a helical pitch of 2700 nm, which corresponds to an HTP410 of 37 m-1, while irradiation with 340 nm light lengthens the helical


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pitch to 5000 nm, which corresponds to an HTP340 value of 20 m-1, resulting in a difference (HTP) of 85%.20 The mixture of 1 in 5CB shows more dopanthost chirality transfer, as suggested by the larger HTP values (i.e., HTP410 of 57 m-1 and HTP340 of 35 m-1).

Table 1. Irradiation wavelength dependence of the helical pitch and HTP of 1 in 5CB and PCH5. 5CB

PCH5

Wavelength (nm)

Pitch (nm)

HTP (m-1)

Pitch (nm)

HTP (m-1)

340

3100

35

5000

20

365

2600

40

4200

24

375

2400

45

3600

28

394

2000

55

2900

34

410

1900

57

2700

37

Next, we studied the effect of the irradiation wavelength on the obtained HTP, as each PSS should have a different isomeric ratio of the switch, and thus different HTP value (Table 1).21 What is interesting here is that we can use the bistability of the system to lock each one of the PSS controlled helical pitches. Considering that the LC system is a self-assembled structure what we are achieving is the kinetic trapping of the supramolecular assembly as a function of wavelength. 105 100 95 90

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


85

340 nm

80

365 nm

75

375 nm 394 nm

70

We then set out to take advantage of this property and the relatively large HTP and HTP of these mixtures to make adaptive reflective films. A mixture of 1 (6.7%; 27% by mass) in 5CB was prepared and loaded into a 5 m homogeneously aligned LC cell (LC director parallel to the glass substrates) and the transmission was measured as a function of irradiation wavelength. Irradiation with 340 nm light gives rise to a film with a transmittance minimum at 920 nm (Figure 2). Irradiation with 365 nm light causes a hypsochromic shift to 890 nm, while irradiation with 375 nm light further shifts the transmittance minimum to 840 nm. Again, the bistability of the system allows us to lock all these states and associated light reflections. Surprisingly, irradiation with 394 nm light gave no peak in the transmission spectrum (i.e., 100% transmittance). Analysis by polarized optical microscopy suggested that the sample underwent an isothermal lightinduced phase transition from the helical cholesteric phase to the lamellar, non-twisted smectic A* phase (Figure 3). This is evidenced by the smooth ‘fan’ texture in a homogenous cell and the optically isotropic texture in a homeotropically aligned (director perpendicular to the substrates) LC cell. The cholesteric phase could be restored by irradiating with 394 or 410 nm light.22 To verify that the photo-induced LC phase change indeed leads to the SmA* phase, a sample of 1 in 5CB was prepared and analyzed by SAXS (Figure S16). The sample was divided into two portions. One was irradiated with 410 nm light and the other 340 nm light. The sample irradiated with 410 nm shows a halo at q = 0.23 Å-1, which corresponds to a layer spacing of ca. 27 Å. This spacing is longer than the molecular length of 5CB (18 Å),23 and can indicate the formation of a sub-class of the SmA phase known as the SmAd phase (Figure S17) in which the aromatic cores of 5CB phase segregate in a greater extent than in the traditional SmA phase leading to a larger layer spacing.24 SAXS measurements on the sample irradiated with 340 nm light were also conducted and no layering was observed suggesting the retention of the cholesteric phase (Figure S16). a)

65 700

800

900

1000

1100

 / nm

Figure 2. (a) The transmittance spectra of 1 in 5CB at various PSSs. The transmittance becomes 100% at PSS394, which coincides with an isothermal phase transition from the cholesteric to the SmA* phase.


b)

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Figure 3. Photomicrographs of the (a) cholesteric phase induced by irradiation with 340 nm light, and (b) the SmA* phase induced by irradiation with 410 nm light, of 1 in 5CB in a homogeneously aligned cell at 200x magnification.

Only a handful of systems that show photoinduced isothermal phase transitions to other mesophases (rather than a mesophase to isotropic liquid phase transition) are reported in the literature.16,25 In most cases specialized conditions are used to achieve such a transition, and the obtained SmA phase is an inherent property of the LC mixture, which is not the case here. There are two established mechanisms for a light induced cholesteric to SmA* phase transition. The first relies on nano-phase segregation of the dopant,26 which restricts the translational movement of the nematic phase. The second involves an increase in the LC host order via the shape change of the dopant (e.g., elongation).16d We speculate that the switching of 1 from Z to E in 5CB falls under the latter regime, as the change in shape anisotropy should be much subtler than the trans to cis switching in azobenzene which leads to phase seperation. 16a,b,c We decided to use the anomalous light-induced isothermal cholesteric-SmA* phase transition to modulate the transparency of LC films. A mixture of 1 in 5CB was loaded into a 20 m homeotropic aligned liquid crystal cell and significant light scattering was observed because of the poor alignment of the cholesteric phase (Figure 4). The homeotropically aligned LC cell was then irradiated with 410 nm light leading to the well aligned homeotropic SmA* phase, which is optically transparent, allowing for the exposure of the underlying hidden figure. The sample could be restored to the light-scattering cholesteric phase by irradiating the sample with 340 nm light.

isotropic melt, (b) the homeotropically aligned SmA* phase after irradiating with 410 nm light, and (c) restoration of the cholesteric phase after irradiation with 340 nm light. These alignments lead to optical windowing where (d) the cholesteric phase scatters light, (e) the homeotropically aligned SmA* phase is optically transparent, and (f) the reformed cholesteric phase scatters light again.

In conclusion, a new chiral photochromic dopant for nematic LCs with exceptionally long half-life has been developed. This dopant was used to modulate the near infrared light reflected from the LC films by controlling, and kinetically locking the PSS-defined helical self-assembly of the LC. The dopant also shows an unprecedented light-induced cholesteric to SmA* phase transition in 5CB, which was used to modulate the opacity of a thin LC film. This unexpected property shows how the development of new switchable chiral dopants that can control the morphology, helical structure, and photophysical properties of LCs can result in new and unexpected properties, opening the way for new frontiers in adaptive soft materials.27 ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: http://pubs.acs.org. General methods, experimental procedures, NMR spectra of key compounds, photoisomerization studies, kinetic studies, and details about doping experiments. (PDF) AUTHOR INFORMATION Corresponding Author

*[email protected] Notes

No competing financial interests have been declared.

ACKNOWLEDGMENT The authors would like to acknowledge the support of the Army Research Office (W911NF-15-1-0587), and the use of facilities and instrumentation supported by the NSF MRSC Grant DMR-1420736.

REFERENCE Figure 4. Photomicrographs of 1 in 5CB in a 20 m homeotropically aligned cell at 200x magnification, (a) upon slow cooling into the cholesteric phase from the

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Driving a Liquid Crystal Phase Transition Using a Photochromic

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