Reversible Circularly Polarized Reflection in a Self-organized Helical

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Reversible Circularly Polarized Reflection in a Selforganized Helical Superstructure Enabled by a Visible Light-Driven Axially Chiral Molecular Switch Hao Wang, Hari Krishna Bisoyi, Augustine Urbas, Timothy J. Bunning, and Quan Li J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 03 May 2019 Downloaded from http://pubs.acs.org on May 3, 2019

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Reversible Circularly Polarized Reflection in a Self-organized Helical Superstructure Enabled by a Visible Light-Driven Axially Chiral Molecular Switch Hao Wang,† Hari Krishna Bisoyi,† Augustine M. Urbas,‡ Timothy J. Bunning,‡ and Quan Li*,† †

Advanced Materials and Liquid Crystal Institute and Chemical Physics Interdisciplinary Program, Kent State University Kent, OH 44242 USA ‡

Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH, 45433 USA

Supporting Information Placeholder ABSTRACT: Development of light-driven functional materials capable of displaying reversible properties is currently a vibrant frontier from both scientific and technological points of view. Here a new visible light-driven chiral molecular switch is synthesized and characterized. To the best of our knowledge, this is the first example of a chiral molecular switch where the visible light-driven azobenzene motif is directly linked to an axially chiral scaffold through a C-C bond. The chiral molecular switch exhibits trans-to-cis photoisomerization upon 530 nm irradiation and cis-to-trans isomerization upon 450 nm irradiation. The switch can thus be photoisomerized in both directions using visible light of different wavelengths, a promising attribute for device applications. It was found that this relatively rigid molecular structure exhibited a high helical twisting power (HTP) in liquid crystal hosts and a large change of HTP value upon photoisomerization. We achieved dynamic reflection tuning across the visible spectrum through incorporation into a self-organized helical superstructure, i.e., cholesteric liquid crystal. We also demonstrated patterned photodisplays reflecting red, green and blue circularly polarized light using these cholesteric films. Phototunable color displays were fabricated by selective light irradiation where the information can be reversibly hidden by applying an electric field and restored by applying either a mechanical force or an electric field of higher voltage.

Stimuli-responsive functional soft materials capable of exhibiting reversibly tunable properties and dynamic performance are gaining tremendous attention for a variety of applications.1 Towards this end, the development of liquid crystalline dynamic helical super structures, i.e., cholesteric liquid crystals (CLCs), with tunable attributes is currently an intensely investigated area. The research and development of CLCs are fueled by their fascinating properties and potential in a myriad of applications such as polarizers, reflectors, optical filters, tunable lasers, one and two-dimensional beam steering devices, chiral photodisplays without drive electronics, and temperature sensors.1-7 The preferred approach in the fabrication of functional CLCs has been to dope an achiral nematic liquid crystal (LC) fluid with a miscible enantiopure chiral compound. Among different classes of chiral dopants, light-driven molecular

switches and motors have been extensively used to fabricate functional CLCs with tunable properties. A variety of lightdriven chiral molecular switches and motors have been developed,4-6 the majority of which are sensitive to UV radiation. Recently, the development of visible light-driven chiral molecular switches has gained attention as the detrimental effects of UV light can be avoided.8-16 This is particularly important for biological and biomedical applications. Previously, we reported different strategies to realize visible and near infrared light-driven CLCs.5 Extension of the conjugation within the chiral molecular switches by judicious molecular design and incorporation of upconversion nanoparticles into photoresponsive CLCs have been successful approaches in this endeavor.5 Here we report the synthesis and characterization of a new visible light-driven chiral molecular switch containing ortho-tetrafluoro azobenzene group that enables wide wavelength range, reversible reflection color tuning in CLCs (Figure 1). Ortho-tetrafluoro (electron-withdrawing groups) azobenzene has been chosen as the photo-isomerizing group due to the distinct band separation of the n-π* transitions of its trans and cis isomers. This unique characteristic enables trans-to-cis and cis-to-trans photoisomerization through n-π* transitions using visible light of different wavelengths. Moreover, the cis isomer of ortho-tetrafluoro azobenzene is known to exhibit very slow thermal relaxation which provides stability.8-15 In addition to broad wavelength reflection color tuning by visible light, we demonstrate color photodisplays simultaneously reflecting red, green and blue (RGB) colors in a single film. The information in the color photodisplays can be reversibly switched by applying either an electric field or mechanical force.

CLP Visible Light-Driven Chiral Switch

CLP

+ Vis 1 P Vis 2 Achiral Liquid Crystal

Figure 1. Schematic representation of fabrication and reversible tuning of circularly polarized light (CPL) reflection in a visible-light-driven CLC.

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The structure of the visible light-driven chiral molecular switch 5 is shown in Figure 2. The molecular design is such that ortho-tetrafluoro azobenzene motifs are directly linked to the central axially chiral binaphthyl unit through C-C bond. Ortho-tetrafluoro azobenzene derivatives have been recently investigated and found to exhibit efficient trans-cis photoisomerization in response to visible light irradiation involving n-π* transitions.9-15 In ortho-tetrafluoro substituted azobenzene derivatives, the fluorine atoms decrease the electron density around the N=N bond which leads to lowering of the energy of the nonbonding orbital. This electron withdrawing effect of the fluorine atoms increases the band gap between of n-orbital and π* orbital of the cis-isomer, resulting in the blue shift of the n-π* transition band and separated from that of trans-isomer.14 To date, nearly all reported binaphthyl based light-driven azobenzenes containing chiral molecular switches are based on the diazotisation of 2,2´-diaminebinaphthyl. Here we have linked the fluorinated azobenzene groups directly to the 3,3´-positions of the chiral binaphthyl moiety through a C-C bond by Suzuki cross-coupling reaction. From the literature, it is known that substitution of different groups through the 3,3´- positions of binaphthyl is one of the most effective ways to enhance the HTP of chiral dopants.16-18 Moreover, a relatively rigid molecular structure of chiral dopants also facilitates in improving their HTPs. The details of the synthetic procedure and characterization data of the intermediates and the target compound 5 are provided in the Supporting Information (SI).

transitions, respectively (Figure 3). The absorption profile of the molecular switch 5 is similar when tetrahydrofuran (THF) is used as the solvent (Figure S5).

Figure 3. (a): UV-vis absorption spectra of the chiral molecular switch 5 in hexane (10-5 M) in three different states (initial state, PSS 530 nm and PSS 450 nm). The irradiation time is 10 min for each wavelength; (b): The enlarged area of UV-vis absorption spectra in the range of 400-600 nm which clearly shows the distinct n-π* band separation between the trans and cis isomers of chiral compound 5. Upon irradiation with light of different wavelengths, the absorption spectra reveal different photo-stationary states (PSS) (Figure 3). Green light (530 nm) induces trans-to-cis photoisomerization whereas blue light (450 nm) drives cis-to-trans isomerization. From the PSS of 530 nm, blue light can drive the photoisomerization in the reverse direction till the PSS of 450 nm is reached. By looking at the enlarged UV-vis spectra (Figure 3b), we found that the n-π* band separation between the trans (peak maxima at 454 nm) and cis isomers (peak maxima at 432 nm) is about 22 nm. The n-π* absorption of cis isomer is blue shifted compared to the trans isomer. This band separation provides the opportunity of using different wavelengths of visible light located at both sides of the isobestic point to drive the photo-isomerization of the chiral molecular switch 5. The separation of n-π* bands of the trans and cis isomers also enhances the photo-isomerization yield. The photoisomerization process has also been monitored by 1H NMR spectroscopy using THF as the solvent (see SI, Figure S6).

Figure 2. Structure of the visible light-driven chiral molecular switch 5 and schematic representation of its reversible photoisomerization. Hydrogen atoms and the end groups –OC6H13 are omitted for clarity. The chemical structure of compound 5 has been characterized by proton (1H), carbon (13C), and fluorine (19F) nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectroscopy (HR-MS) (see Supporting Information). The photoresponsive behavior of the target molecular switch 5 was investigated in organic solvents. The UV-vis absorption spectra show that the chiral switch in hexane exhibits maxima of absorption around 350 nm and a relatively small absorption band around 460 nm, corresponding to the π -π* and n-π*

Figure 4. (a) Cell images (top) and the corresponding reflection spectra (bottom) of the CLC in a planar cell containing chiral molecular switch 5 (2.2 mol%) irradiated with 530 nm

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Journal of the American Chemical Society for different periods of time. (b) Cell images (top) and the corresponding reflection spectra (bottom) of the CLC in a planar cell containing chiral molecular switch 5 (2.2 mol%) irradiated with 450 nm for different periods of time. (c) Color photodisplays in planar and (d) homeotropic cells obtained using visible light irradiation through a photomask for different times. To study the properties of the chiral molecular switch in a LC system, we doped the chiral switch into the commercially available LC host E7 to induce a self-organized, phototunable helical superstructure. The Cano wedge cell method was used to measure the HTP and change of HTP of compound 5 upon light irradiation. A low concentration (0.27 mol%) of 5 was doped into E7 and the resulting CLC was capillary filled into the Cano wedge cell. Its HTP in the initial state is 168.5 µm-1 in molar ratio. Upon 530 nm light irradiation for 10 minutes, the HTP dropped to 98 µm-1. Upon subsequent shining with 450 nm light for 10 minutes, the HTP value recovered to 128 µm-1. Using the measured HTP value, we calculated and doped 2.2 mol% of 5 into the E7 host to fabricate a CLC mixture with a reflection wavelength below the visible region. Upon irradiation of the CLC mixture in a planar cell with 530 nm light for 10 sec, the cholesteric film reflects blue light and prolonged irradiation tunes the reflection from blue to red (Figure 4). Subsequent irradiation with 450 nm light, the reflection color of the cholesteric film can be tuned from red to near blue region (Figure 4). Figure 4c shows that using a photomask technique and varying the irradiation time enables different color reflecting information to be written in the cholesteric film using visible light. In Figure 5, the mixture is filled into a 15 µm gap homeotropic cell. Here, the short pitch CLC mixture in the homeotropic cell was heated to isotropic state and cooled to room temperature followed by mechanical pressure by hand to obtain the planar state. When the pitch of the CLC is on the order of hundreds of nanometer (P≪d) and the CLC has strong elastic energy, the CLC is able to overcome the anchoring effect of the homeotropic alignment when mechanically interrogated. It should be noted that long pitch CLC mixtures exhibit fingerprint texture in homeotropic cells (SI, Figure S7). The color patterns of “123” (Figure 4d) were achieved by shining the mechanically interrogated cell through a patterned photomask with visible light for different lengths of time. The red color of PSS530 nm can be maintained for 48 hours without noticeable change.

Figure 5. Black (reflection below visible region), blue, green and red reflection from the cholesteric film in a homeotropic cell containing molecular switch 5 (2.2 mol%) driven by visible light irradiation (530 nm, 2 mW/cm2) for different time (0 s, 10 s, 2 min, 5 min respectively). To demonstrate versatile stimuli-responsiveness of this CLC system, an AC electric field is applied to erase and restore information written on the CLC display cells with planar alignment. A green patterned “123” was generated on a black background (in black region, the CLC reflects UV light) between two electrodes and an AC electric pulse switched the states of the CLC display (Figure 6). If we apply a voltage of 30 V (100 Hz, square wave), the pattern disappears as the electric field

causes a transition of the CLC from the planar to focal conic state. Subsequently, a higher voltage AC electric pulse (60 V) restores the colored “123” pattern. Interestingly, if we apply mechanical force to the homeotropic cell (Figure S8) by pressure (see Supporting Information thumb press video) instead of an electric field, the green pattern hidden by applying 30 V can be restored. This is due to the fact that application of mechanical pressure to the CLC film facilitates the transition from high energy focal conic state to low energy planar state. The above described stimuli driven switching cycles can be repeated many times without noticeable degradation.

Figure 6. (a) Green “123” pattern on a black background in a 5 µm planar cell of the cholesteric film containing 5 (2.2 mol%). (b) 30 V electric pulse drives a focal conic state which “hides” the pattern. (c) 60 V AC electric pulse recovers the original green “123” pattern. Schematic representations of the transitions and corresponding POM textures are depicted in the middle and bottom row, respectively. In conclusion, we have designed and synthesized a novel ortho-fluorinated azobenzene containing binaphthyl-based axially chiral molecular switch. This is the first example of a chiral molecular switch where the visible light-driven ortho-tetrafluoro azobenzene motif is directly linked to the central chiral scaffold through a C-C bond. The relatively rigid chiral molecular switch exhibits a high HTP and a large change of HTP value upon isomerization with visible light. The chiral molecular switch can be driven by 530 nm and 450 nm visible light inducing trans-to-cis and cis-to-trans photo-isomerization, respectively. The induced photo-responsive helical superstructure obtained using this chiral molecular switch is capable of showing tunable circularly polarized reflection color across red, green and blue wavelengths in both planar and homeotropic cells. In a single cell, RGB color images were demonstrated against a black background. Color photodisplays driven by either an electric field in a planar cell or the combination of electric and mechanical fields in a homeotropic cell have been demonstrated. This visible light-driven chiral molecular switch could inspire further molecular design towards the fabrication of stimuli-responsive functional materials with tunable properties.

ASSOCIATED CONTENT Supporting Information

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Experimental procedures, analytical data, synthetic details and NMR spectra are included in the supporting information. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We thank the support from the Air Force Office of Scientific Research and the Air Force Research Laboratory.

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CPL Visible Light-Driven Chiral Switch

CPL

+ Vis1 Vis2 Achiral Liquid Crystal

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