Asymmetric Dimers of Chiral Azobenzene Dopants Exhibiting Unusual

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Asymmetric dimers of chiral azobenzene dopants exhibiting unusual helical twisting power upon photoswitching in cholesteric liquid crystals Yuna Kim, and Nobuyuki Tamaoki ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11888 • Publication Date (Web): 27 Jan 2016 Downloaded from http://pubs.acs.org on February 1, 2016

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ACS Applied Materials & Interfaces

Asymmetric dimers of chiral azobenzene dopants exhibiting unusual helical twisting power upon photoswitching in cholesteric liquid crystals

Yuna Kim and Nobuyuki Tamaoki* Research Institute for Electronic Science, Hokkaido University, Kita20, Nishi10, Kita-ku, Sapporo, Hokkaido 001-0020, Japan E-mail: [email protected]

Abstract In this study we synthesized asymmetric dimeric chiral molecules as photon-mode chiral switches for reversible tuning of self-assembled helical superstructures. The chiral switches bearing two mesogen units—cholesterol and azobenzene moieties connected through flexible alkylenedioxy bridges—were doped into nematic liquid crystals, resulting in a chiral nematic (cholesteric) phase. Under irradiation with UV light, photoisomerization of the azobenzene units led to unprecedented switching of the cholesteric pitch and helical twisting power (HTP,

β), with a higher HTP found in the cis-rich state (bent-form) than in the trans-state (rod-form). We attribute this behavior to the elongated cybotactic smectic clusters disrupting the helical orientation of the molecules in the cholesteric liquid crystals; their reversible decay and reassembly was evidenced upon sequential irradiation with UV and visible light, respectively. In addition to the photoisomerization of the azobenzene units, the odd/even parity of the alkylenedioxy linkers of the dimeric dopants also had a dramatic effect on the transitions of the cybotactic smectic domains. Based on the large rotational reorganization of the cholesteric helix and HTP switching (∆β/βini of up to 50 %), we could control the macroscopic rotational motion of microsized glass rods upon irradiating, with UV and visible light, the surface of a cholesteric liquid crystal film featuring a polygonal fingerprint texture.

Keywords: cholesteric liquid crystal, azobenzene, chiral dopant, photoswitching, helical twisting power 1 ACS Paragon Plus Environment


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1. Introduction Cholesteric liquid crystals (CLCs) have attracted great interest because the self-organization of these superstructures can be controlled under external stimuli, including temperature,1–3 pressure,4 electric field,5,6 dopants,7–11 and light.3,7–14 Superstructural switching of CLCs has been applied in recent years to optical switching, displays, information storage, soft actuators, and molecular motor applications.15–29 Among established dopants, photoisomerizable azobenzene units are particularly promising because they can induce a photoresponsive actuating mechanism through isomerization between trans (E, rodlike) and cis (Z, bent) forms upon irradiation with UV and visible light, respectively. Such light-induced conformational changes can lead to macroscopic self-reorganization of the host system. When a chiral azobenzene is dissolved in an achiral nematic liquid crystal (NLC), its molecular chirality can be transferred to the NLC medium, resulting in photoresponsive chiral nematic (N* or cholesteric) liquid crystal phase.21,22 The ability of a chiral molecule to induce a helical structure in an achiral NLC can be quantified in terms of the helical twisting power (HTP, β), expressed as

β = (PC)–1 where C is the concentration of the chiral dopant and P is the pitch (the distance over which the director of molecules undergoes one full turn).23,24 In general, rodlike trans isomers are more efficient at chiral induction than are their corresponding bent-form cis isomers; thus, the compatibility of a rodlike LC host can dramatically affect its chirality transfer properties. Accordingly, irradiation with UV light of a CLC containing a chiral azobenzene will typically disturb its fluidic macroscopic structure and result in a lower HTP and longer helical pitch.30 The opposite switching phenomenon of an increase in the HTP, occurring when a chiral azobenzene undergoes a conformational transition from its trans form to a cis-rich state in a CLC medium, has been relatively unexplored, despite its potential for application in

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information processing or molecular machinery, because light stimulus inducing a higherthan-original HTP could provide efficient mechanical work. To the best of our knowledge, only two such models have been explored previously with valid mechanisms of action.31,32 The first, demonstrated by the Ichimura group,31 involved azobenzenes substituted with spacers at the 3,3´-positions and at the ortho-position; the cis isomers associated with higher HTPs which were more stabilized and exhibited elongated rodlike conformations relative to their trans isomers. In the other, the Kurihara group revealed that the molecular aspect ratio (molecular length divided by diameter) affects the intermolecular interactions between LCs and chiral azobenzene dopants, with a higher molecular aspect ratio in the cis-rich state tending to induce a higher HTP than that in the trans state.32 In this study we observed a unique effect of stronger helical twisting induction in the cis-rich state, differing from the two cases mentioned above, based on a much more generalized azobenzene molecular structure and modulation of the cybotactic clusters surrounding the dopant moieties in the cholesteric phase. Here, we exploited the cooperative effects of chirality and photoisomerization from cholesterol and simple azobenzene units, respectively, in the form of asymmetric dimeric dopant (Scheme 1) incorporating two mesogens connected with a flexible linker. This approach involves simple syntheses from inexpensive starting materials; variability in molecular design (e.g., spacer length and parity) for mesomorphic reorganization; and the ability to manipulate the resulting cybotactic clusters in a cholesteric phase. The most interesting aspect of the design of our new photoresponsive chiral liquid crystals was the steric configuration. A rigid, long shape can induce anisotropic intermolecular interactions through van der Waals forces that stabilize parallel molecular stacking,33 thereby assisting the formation of cybotactic domains in a helical superstructure. Other attractive features of our approach are the facile one-pot synthesis and the absence of 3 ACS Paragon Plus Environment


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any need to use HPLC to separate enantiomers: the eight chiral centers in the cholesterol moiety can readily realize chirality in mesophases.33,34 Furthermore, we demonstrate herein the first example of the application of chiral azobenzene dopants, providing an increased sense of HTP upon UV light irradiation, that induce large rotational motion of micro glass rods on the surface of a CLC mixture film as a result of rotary reorganization of the cholesteric helix.

2. Results and discussion 2.1. Photophysical and mesomorphic properties of the chiral dopants Photoresponsive dimeric chiral dopants (Scheme 1) were synthesized through one-pot syntheses, performed according to previous reports;35,36 they comprised cholesterol and azobenzene units connected through flexible alkylenedioxy bridges, with various spacer and bridge lengths of the dopants. Their synthesis and structural analysis are described in the Experimental section and Supporting Information.

Scheme 1. Molecular structures of cholesteryl azobenzene derivatives.


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First, we used UV–Vis and 1H NMR spectroscopy to investigate the photochromic properties of the dopants. In THF solution, all of the compounds exhibited reversible photoisomerization characteristics upon irradiation initially with UV light and subsequently with visible light (see Figure S3). In each case, the spectrum recorded prior to photoirradiation featured an absorption maximum near 322 nm, representing the π–π* transition of the azobenzene unit. A weak absorption band was also observed near 450 nm, corresponding to the n–π* transition of the azobenzene unit. Upon UV irradiation at 365 nm to its photostationary state (PSS365nm), the absorption of the band for the π–π* transition decreased in intensity, accompanied by an increase in the absorption intensity for the n–π* transition. Subsequent irradiation with visible light at 436 nm to its photostationary state (PSS436 nm) resulted in the recovery to the original spectrum. The ratio of E and Z isomers at each PSS (Table 1), analyzed using NMR spectroscopy, did not change significant upon varying the molecular structure. We obtained average values of 13:87 and 77:23 at PSS365nm and PSS436nm, respectively, that were stable over several cycles without any sign of fatigue. We also determined the thermal back rate constants by monitoring the change in absorbance at the absorption maximum of each compound from its cis-rich (PSS365nm) to trans state in the dark at room temperature (Table 1 and Figure S4); each value was on the same order of magnitude (k = ca. 10–4 min–1). Table 1. Thermal phase transition temperatures and photophysical properties of cholesteryl azobenzene derivatives. Phase transition temperaturea,b) Conversion ratioc) Thermal back rate Compou d) PSS365nm PSS436nm constant nd Heating (°C) Cooling (°C) trans:cis trans:cis (k × 10–4, min–1) N2 Cr149Ch195Sm197Iso Iso198Sm130Ch114Cr 14:86 78:22 3.2 N3 Cr127Iso Iso119Cr 23:77 81:19 3.2 N8 Cr103Ch141Iso Iso146 Ch87Cr 10:90 80:20 4.3 N20 Cr97Ch104Iso Iso101Ch81Cr 10:90 70:30 4.2 N20a Cr70 Sm76Ch82Iso Iso80Ch&Sm36Cr 12:88 74:26 4.2 5 ACS Paragon Plus Environment


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N20b Cr89Sm112Iso Iso110Sm30Cr 12:88 81:19 6.5 a) Phase transition temperatures recorded during the second heating and cooling cycle; those from N20, N20a, and N20b were obtained from ref. 36. b) Phase assignments: Cr, crystalline phase; Ch, cholesteric phase; Sm, smectic phase; Iso, isotropic liquid. c) Determined using 1H NMR spectroscopy (400 MHz, CDCl3); see Supporting Information. d) Measured from a solution in THF; see Supporting Information. We used a polarized microscope equipped with a heating stage to investigate the mesomorphic phase transition characteristics of the compounds (Table 1). We observed different thermal phase transitions for the various dimeric cholesteryl azobenzene derivatives, indicating that their spacers influenced their thermal molecular alignment. Interestingly, we did not observe a liquid crystalline phase for odd-parity N3, which also provided an isotropic temperature lower than those of even-parity N2 and N8. This behavior is consistent with that reported for other well-known LCs as well as for dimesogens containing cholesterol moieties.23,35–37 Figure 1 displays the observed optical texture of the smectic phase for N2. In case of dimesogens with highly extended (n = 20) aliphatic linkages, the phase transition temperatures were relatively low during both heating and cooling when compared with those of dimesogens bearing shorter linkers (n ≤ 8), presumably because of the lower density of self-organized mesogenic domains. Although smectic phases appeared for both the diacetylene-containing compounds N20a and N20b, no cholesteric phase was evident for the terminal alkyl–substituted compound N20b, suggesting that its mesogenic orientation was influenced by the intermolecular interactions of its terminal alkyl group.


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Figure 1. Smectic liquid crystalline texture of N2, observed using a polarizing optical microscope. The black scale bar corresponds to 50 µm. 2.2 Unprecedented switching of HTPs: Larger values for cis forms

We examined the chirality transfer characteristics of each dopant in a nematic liquid crystal medium. Each chiral nematic liquid crystal composite was prepared by mixing the host nematic liquid crystal and the cholesteryl azobenzene dopant homogeneously, followed by injection into a Grandjean–Cano wedge cell under capillary force at room temperature. We used Cano’s wedge method to determine the induced helical pitch at various concentrations of the nematic host and dopant.38 The photoinduced variations in the pitch length and the HTP (β) were evaluated using a polarized optical microscope upon illumination of the wedge cell with UV (365 nm) and visible (436 nm) light. Reversible shortening and lengthening of the distance between the Cano lines occurred upon irradiation with UV and visible light, respectively (Figure 2). The helical pitch length of 5 wt% N8 in 5CB in each photophysical state was 12.04 µm for the initial state, 9.07 µm for PSS365nm, and 11.82 µm for PSS436nm. The residual Z-form at PSS436nm prevented the recovery to the original (i.e., prior to photoirradiation) pitch length. Table 2 lists the HTPs of the cholesteryl azobenzene dimers in the nematic liquid crystal hosts 5CB, E-7, and JC-1041XX before (initial) and after irradiation with UV (365 nm) and visible (436 nm) light to each PSS. Remarkably, every dopant in different nematic LC host showed a tendency of increasing manner of HTP upon UV light irradiation, and decreasing upon subsequent visible light irradiation. The largest HTP photoswitching ratio was 50% (∆β/βini), achieved in the presence of N2, the dopant having the shortest linkage, where the initial value of 0.66 µm–1 prior to photoirradiation changed to 0.99 µm–1 after irradiation at 365 nm, then recovered to 0.69 µm–1 after subsequent irradiation at 436 nm. All of the dimeric dopants induced larger HTPs in their cis-rich forms than in their trans forms. 7 ACS Paragon Plus Environment


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In general,24,30 because a bent-form dopant tends to be incompatible with a rodlike NLC host, photoisomerization of a doped azobenzene to its cis form will diminish chirality transfer and decrease the HTP. Thus, such a reversed mode of photoswitching of HTPs has rarely been achieved31,32 previously, due to the intrinsic structural configuration of azobenzene units.

Figure 2. Pitch lengths of 5 wt% N8 in the LC host 5CB after sequential irradiation with UV and visible light, observed from a wedge cell. (a) 12.04 µm; (b) 9.07 µm; (c) 11.82 µm. Table 2. HTPs of cholesteryl azobenzene derivatives in various states in NLC hostsa) β (µm–1) ∆β/|βini|c) Dopant Host NLC ∆βb) (%) Initial PSS365nm PSS436nm N2 5CB 1.52 1.96 1.62 0.44 29 E-7 1.55 1.97 1.66 0.42 27 JC-1041XX 0.66 0.99 0.69 0.33 50 N3 5CB 2.37 2.37 2.37 0 0 E-7 2.25 2.56 2.43 0.31 14 JC-1041XX 1.01 1.17 1.06 0.16 16 N8 5CB 1.56 2.06 1.62 0.50 32 E-7 0.44 0.48 0.46 0.04 9 JC-1041XX 1.22 1.52 1.27 0.27 22 N20 5CB 1.45 1.50 1.46 0.05 3 E-7 0.90 0.92 0.90 0.02 2 JC-1041XX 1.67 1.72 1.68 0.05 3 N20a 5CB 1.23 1.33 1.24 0.10 8 E-7 0.93 1.03 0.98 0.10 11 JC-1041XX 0.48 0.49 0.49 0.01 3 N20b 5CB 1.47 1.61 1.53 0.14 10 E-7 1.36 1.48 1.43 0.12 7 JC-1041XX 0.72 0.80 0.77 0.08 11 a) Determined using Cano’s wedge method and the change in HTP in each photoirradiation condition. b) Change in HTP between the initial and final Z-rich states (βfin – βini). c) Percentage change in HTP observed between the initial state and PSS365nm. To date, strategies that achieve the opposite mode of HTP switching have been focused on stabilizing elongated rodlike conformations and stereochemically restricting conformational mobility in cis-rich states; for example, by substituting azobenzene derivatives 8 ACS Paragon Plus Environment

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with linkages at their 3,3´-positions and methyl groups at their ortho positions,31 or by increasing the molecular aspect ratio to enhance intermolecular interactions between the NLC host and the azobenzene dopant in its cis form.32 2.3 Effect of spacer unit on HTP photoswitching The mesogenic properties of the trigger (dopant) will promote chirality transfer to the NLC host as a result of its affecting molecular-scale solvent–solute interactions; HTP is a measure of the transfer effectiveness.23,24 In addition, it is generally accepted that triggers having overall structures that more resemble those of their NLC hosts will give larger HTPs.7 The compounds examined in this study feature the same mesogen units (azobenzene and cholesterol), but different aliphatic linkers or terminal units that varied the overall molecular rigidity and length. To our surprise, all of the tested dopants exhibited similar initial HTPs, except for the odd-numbered dopant N3, when dissolved in 5CB or E7. Although compound N3 provided the highest initial HTP (2.37 µm–1) among all of the dopants, irradiation with UV and visible light resulted in only subtle switching of the cholesteric pitch—indeed, we observed no change in HTP when N3 was introduced in 5CB. In contrast, N2, which is similar to N3 in shape and size, provided a lower initial HTP of 1.52 µm–1, but exhibited a larger photoswitching ratio of 29 % in 5CB. Thus, the only difference between N2 and N3, the odd/even parity of their methylene spacer units, affected the chirality transfer characteristics. Meanwhile, photoisomerization of the N20 series between trans and cis states of their azobenzene units did not induce the drastic changes in HTPs observed when the dopants contained shorter linkers. Variations in the linker structure (introducing central diacetylene motifs) or terminal unit had little effect on the chirality transfer (cf. the HTP photoswitching results for N20, N20a, and N20b in Table 2). We suspect that the long distance between the chiral and photoisomerizable moieties prevented any cooperative effects or efficient chirality transfer to the neighboring nematic host molecules, resulting in lower degrees of switching through photoinduced helical reorientation. 9 ACS Paragon Plus Environment


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Odd/even linker effects of dimeric chiral dopant molecules on HTP in host NLC molecules are rare; indeed, we are aware of only one previous literature report, for twin mesogen units connected through a chiral methylene chain.39 In that study, however, the phenomena were opposite from ours; that is, the dimeric dopant containing the odd-numbered chiral linkage induced a longer helical pitch, and barely affected the dynamics of the host molecules in each layer. Thus, it did not cause a strong correlation between the motion and direction of the cores of the host molecules in the adjacent layers. On the other hand, the parallel conformer of that even-numbered spacer did affect the dynamics of the host molecules, and brought about a strong interlayer correlation. 2.4 In situ XRD To reveal the mechanism of the unprecedented mesomorphic photoswitching behavior of the cholesteric helix in our system, we recorded in situ X-ray diffraction (XRD) patterns of the CLC mixtures in their various PSSs. Light was directed onto the film surfaces of the CLC mixtures, which were coated on glass holders. The liquid crystalline phase was maintained during the cis-isomerization upon UV light irradiation, avoiding any unexpected transition to an isotropic phase as a result of heat from the light source or a high concentration of dopant.


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Figure 3. Intensity profiles in the XRD patterns of (a) N2, (b) N3, and (c) N8 in 5CB (0.1 wt%) before photoirradiation (initial, black line) and after irradiation with UV (red line) and visible (blue line) light at each PSS. Insets: Magnified patterns corresponding to the smectic fluctuation region. Figure 3 presents XRD patterns of CLC films containing 0.1 wt% of N2, N3, and N8 in 5CB before and after irradiation with UV and visible light at room temperature. In each pristine state prior to irradiation, we observe two common peaks, at 20° (d = 4.44 Å) and near 3.5° (d = 25.22 Å), in the diffraction pattern of each film. The peak at 4.44 Å originated from the liquid-like correlation of the molecules, while the peak at 25.22 Å can be ascribed to smectic fluctuations in the CLC. 35,36,40,41 Because the shape and size of a molecule can affect its ability to induce cybotactic smectic clusters, photoinduced trans and cis isomerizations can manipulate the number and size of smectic fluctuations reversibly, thereby leading to photoswitching of the cholesteric pitch length. Elongated rigid trans isomers are likely to form cybotactic smectic-like clusters, with the molecules within them aligning parallel to each other without twisting, leading to anomalous lengthening of the cholesteric pitch (small HTP). Upon irradiation with UV light of the pristine films of CLC mixtures based on N2, N3, and N8, the pronounced peak at 3.5° in the initial state broadened and its intensity decreased. We 12 ACS Paragon Plus Environment

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attribute this behavior to the configurational change of the chiral dopants from trans to cis forms upon irradiation with UV light, destabilizing the smectic clusters and leading to decreases in both their sizes and amounts. Subsequent irradiation with visible light to the CLC at PSS436nm resulted in the recovery of the peaks at 3.5°, nearly to their original shapes, reflecting the regeneration and re-expansion of the smectic clusters after cis-to-trans photoisomerization of the dopants. Table 3. Half-widths and changes in intensity of the small-angle peak in X-ray diffractograms of CLCs at a dopant concentration of 0.1 wt% in 5CB. Dopant Photoirradiation Imaxa) FWHMb) (°) ∆I/Iinic) (%) N2 Initial 127 1.09 67 PSS365nm 41 1.18 PSS436nm 114 1.00 N3 Initial 139 1.31 39 PSS365nm 85 1.36 PSS436nm 135 1.21 N8 Initial 168 1.23 72 47 1.40 PSS365nm PSS436nm 152 1.26 a) b) Maximum intensity of the small-angle peak. Full width at half maximum. c) Percentage change in intensity of the small-angle peak (Imax) between initial state and PSS365 nm. Although the three CLC films exhibited the same tendencies upon photoirradiation, the ratio of the change in the peak width and intensity varied depending on the length of the methylene spacers. Table 3 presents the quantified photoinduced changes in half-widths and intensities of the low-angle peaks (ca. 3.5°); the data are consistent with the smectic fluctuations determined from each diffraction pattern. Interestingly, a relationship existed between the photoisomerization-induced HTP contrast (∆β/|βini|) and the difference in lowangle peak intensity (∆I/Iini) in the XRD patterns of N2, N3, and N8 in the same NLC host, 5CB. The photoswitching ratios were similar for the even-numbered N8 and N2 for HTP and XRD: 32 and 29 %, respectively, for ∆β/|βini| and 72 and 67 %, respectively, for ∆I/Iini. In contrast, the even-numbered N3 in 5CB provided lower changes of 0 and 39 % for both ∆β/|βini| and ∆I/Iini, respectively. Thus, there was less chance of forming cybotactic smectic domains because the slightly distorted conformation of N3 contributed to the initial shortest 13 ACS Paragon Plus Environment


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cholesteric pitch (largest HTP) and minimized the photoinduced dynamic reorganization of the CLC. Although thermal smectic domain fluctuations in the chiral nematic phase are wellestablished, 35,36,40-43 to the best of our knowledge this paper reports the first example of XRD characterization of the photoisomerization of a chiral dopant inducing reversible smectic domain transitions in a cholesteric LC. Typically, the reversible nature of the photoisomerization of the dopant makes it difficult to use X-ray analysis to measure changes in the smectic clusters in cholesteric films upon UV light irradiation. In a previous study,40 we found that photoisomerization of butadiene dopants from E to Z forms in a dimeric CLC (dicholesteryl-10,12-docosadiynedioate) under UV light resulted in broadening of the lowangle XRD peak in the cholesteric mixtures, indicating a smectic cybotactic decrease, as a result of their bent structures leading to shortening of the pitch in the CLC phase. The achiral photoresponsive dopants employed in that previous study exhibit thermally irreversible PSSs; therefore, the reversible and simultaneous photoswitching of cybotactic domains could not be detected. A compound bearing an even number of methylene units in the spacer can form a molecular shape in which the cholesteryl moiety lies approximately parallel to the azobenzene unit.35 This conformation favors the formation of smectic clusters,35,36 due to its rodlike shape. A compound having an odd number of carbon atoms in the spacer will have a slightly distorted rodlike shape, which destabilizes the formation of smectic cybotactic groups.35-37 The thermal mesomorphic transition for N3 also reveals the absence of a liquid crystalline phase and phase transition temperatures lower than those observed for the other compounds. Therefore, it is possible that the lower probability of formation and transition of the smectic clusters in CLCs containing N3 can be ascribed to the shorter cholesteric pitch, resulting in initially high HTPs and lower HTP photoswitching ratios relative to those of N2.


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Scheme 2. Cooperative effect of trans–cis photoisomerization and even (N2)-/odd (N3)numbered methylene linkers of the dopant on the smectic clusters, resulting in changes in helical pitch lengths and HTPs. Scheme 2 provides a cartoon representation of the interplay between the dopant and host molecules in stabilizing/disrupting cybotactic mesophases, with resultant differences arising from cooperative effects in the trans–cis photoisomerization of dopants containing even (N2)- and odd (N3)-numbered methylene linkers. Because cis isomers (bent forms) in CLCs disrupt the elongated cybotactic clusters, the extended helicity shrinks and leads to the unusual behavior of an increase in HTP after UV irradiation. The CLC containing N2 underwent the dynamic change of smectic clusters upon photoisomerization between trans (long pitch) and cis (short pitch) forms, resulting in a large change of the helical pitch length and HTP, whereas N3 provided a rather static and minimal transition in the mesophase because there was less chance of forming intrinsic cybotatic clusters. Photoresponsive cholesteric liquid crystals comprising a nematic host and a chiral azobenzene dopant can be categorized into two types in terms of their HTP photoswitching behavior (cholesteric pitch length): the azobenzene unit’s transition from trans to cis form can induce either a decrease in HTP (lengthening the pitch length or red-shifting the reflection band) or an increase in HTP (shortening the pitch length or blue-shifting the reflection band). 15 ACS Paragon Plus Environment


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As mentioned above, most reported CLCs based on chiral azobenzene dopants (e.g., axially chiral, planar chiral, or para-substituted) fall into the former case.1–3,23,24,30 In contrast, only two models—spacer substitution position31 or molecular aspect ratio modification32—have been proposed for the latter case. Herein, we observed increases in HTP for cis-rich states for dopants that are cholesteryl azobenzene derivatives—a phenomenon rarely noted for photoisomerizable chiral dopants in nematic LC hosts. In general, the change in cholesteric pitch in a CLC originates from the integrated effect of the difference in the configurational chirality transfer efficiency and the fluctuation of the cybotactic smectic domain upon transformation between trans and cis states of the dopant. For CLC mixtures comprising a cholesteric LC host and a small amount of an azobenzene derivative, the contribution of the smectic cybotactic domain to the change in helical pitch has typically been enhanced by a trans-azobenzene and destroyed by a cisazobenzene. This behavior occurs for cholesteryl ester hosts that originally form smectic clusters and undergo cholesteric pitch shortening upon increasing the temperature (dp/dT < 0), due to destruction of smectic clusters.8,10,11,13 Because the formation of cybotactic smectic domains from cholesteryl azobenzenes has previously been evidenced only in a highly mesomorphic single-component CLC (nematic host free),35,36 it is remarkable that such cybotactic smectic fluctuations can also be induced by small amounts of cholesteryl azobenzenes (dopant concentration: ≤5 wt%) in nematic LC hosts. The thermal coefficients of CLCs incorporating our chiral dopants (Figure S5) were negative (dp/dT < 0), consistent with those observed from CLCs based on cholesteryl ester hosts and achiral azobenzene dopants.8,10,11,13

2.5 Photoregulation of rotational motion of microsized glass rods Although our initial HTPs are lower than those reported for axial- or planar-chiral azobenzene dopant systems,7,44–46 the maximum HTP switching ratio of 50 % [(βfin – βini)/|βini|], obtained 16 ACS Paragon Plus Environment

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from N2 dissolved in JC-1041XX, is comparable with those from reported chiral azobenzene derivatives exhibiting large HTP switching.3,7,30 Thus, we could use the photoisomerization of dopant N2 to examine the light-induced rotational motion of micro-objects on its cholesteric mixture. Feringa and co-workers18–20 demonstrated that the collective action of molecular motors in a CLC can be translated into macroscale rotational motion. They reported forward rotation induced by the photoisomerization of sterically overcrowded alkene motors and reverse rotation achieved through thermally induced reverse isomerization. In addition, fully photocontrolled rotary and translational motion of glass microrods has been reported based on photoisomerization of centrally chiral azobenzene molecules.16 Furthermore, in a previous study15 we found that, for planar-chiral azobenzene dopants, the speed and the degree of rotation of micro-objects are determined by the intensity of irradiation and by the ratio of the difference in HTP before and after irradiation against the initial HTP. Recently, we reported the lowest dopant concentration (ca. 0.3 wt%) required for a planar-chiral azobenzene dopant to induce one full cycle of rotation of microsized glass rods.7 For our present system, the largest change in HTP was that obtained when using JC1041XX as a nematic host for N2; thus, we employed JC-1041XX to evaluate the rotational motion induced by the chiral dopant. The doped liquid crystalline mixture was drop-cast onto a glass substrate coated with a unidirectionally rubbed polyimide alignment film,7 then observed under an optical microscope equipped with a polarizer. The films of the doped liquid crystals exhibited a polygonal fingerprint texture (Figure S6). Fingerprint textures are a direct consequence of the cholesteric geometry, by aligning the axis of the cholesteric helix parallel to the substrate at the interface between the liquid crystal and air. The line width from a polygonal fingerprint texture corresponds to half of the cholesteric pitch (p) length.7 At concentrations of N2 between 3 and 20 wt%, the film displayed a clear fingerprint texture; increasing the concentration caused the line gap to become narrower. Having confirmed the formation of polygonal textures, we freckled glass rods (average length: 25 µm; average 17 ACS Paragon Plus Environment


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diameter: 5 µm) onto the film surface and then irradiated the films with light of 365 nm to attain the Z-rich state of the chiral dopants, as displayed in Figure S6(e).

Figure 4. Rotational motion of a microsize glass rod on the film of a chiral nematic LC mixture (N2 and JC-1041XX; dopant concentration: 5 wt%), starting from (a) the initial state to (c) PSS365 nm, followed by irradiation with visible light (d and e) to (f) PSS436 nm. Images were taken in a timely manner. White scale bar: 50 µm. Upon irradiation with UV light, the films underwent reorganization, with the distance between the lines decreasing in the textured structure. This behavior is consistent with the increase in the value of β upon UV irradiation, resulting in shortening of the helical pitch. The films that featured polygonal fingerprint textures led to rotational motion of the glass rods upon photoirradiation. Because the thickness of the LC film had a great effect on the angle of rotation, we employed a uniform thickness for each film of approximately 30 µm, based on our previous report.7,15 The rotational speed of a rod was fast initially, but decreased as irradiation continued, eventually stopping once the PSS of the dopant was reached. The angle of rotation was determined by measuring the change in angle of the glass rod during irradiation. When the concentration of N2 was 5 wt%, the rotation angles were maximized under irradiation with both UV and visible light over one full cycle. Figures 4a–c reveal a clockwise rotational reorganization of the polygonal fingerprint texture, with a rotational 18 ACS Paragon Plus Environment

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angle of 430° induced after irradiation of the film in its initial state with 365-nm light, resulting from photoconversion of the azobenzene dopant from its trans to cis-rich form. On the other hand, subsequent photoisomerization from the (Z)- to (E)-rich LC film resulted in an anticlockwise rotation, with the rotational angle of 370° (Figures 4d–f). It is remarkable that such a large reorganization could be achieved from such a small HTP. When we repeated the experiment using the dopants N3 and N8, the largest rotational angles upon irradiation with UV light were 185° (at 15 wt%) and 180° (at 10 wt%), respectively (Table S1). Even at doping levels higher than those of N2, we observed lower degrees of rotational reorganization, presumably because the photoswitching ratios of the HTPs for N3 and N8 were lower than that of N2. Moreover, a high dopant concentration can result in unfavorable phase separation and a disordered inhomogeneous fingerprint texture that fails to induce macroscale movement. We observed such behavior when increasing the concentration of N2 above 5 wt%: at dopant concentrations of 10 and 15 wt%, the maximum rotation was 350°, while at 20 wt% there was no rotational motion of a glass rod.

3. Conclusion We have synthesized new asymmetric dimeric chiral azobenzene dopants and investigated their photoresponsive properties in nematic LC media after forming cholesteric LC superstructures. When introduced as chiral dopants, we observed an unusual photoswitching behavior in which the HTP increased after irradiation with UV light formed the cis-rich state. We attribute this phenomenon to decreases in the number and size of the elongated cybotactic smectic-like clusters upon photoisomerization of the doped chiral azobenzene molecules from trans to cis form. The length and parity of the spacer of the dopants had dramatic effects on the self-organization and HTP switching characteristics of the CLCs. This photochemical switching also induced reorganization of the polygonal fingerprint texture of CLC films, with a low level of doping leading to greater control over the rotational motion of microsized 19 ACS Paragon Plus Environment


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objects on the surface of CLCs. Therefore, such systems have great potential for application as light-driven molecular motors.

4. Experimental Section The nematic liquid crystal host 5CB was purchased from Tokyo Chemical Industry (Tokyo, Japan). E7 and JC-1041XX were obtained from Daily Polymer and Chisso Petrochemical, respectively. All other solvents and chemicals were purchased from commercial sources and used without further purification. Micro glass rods were purchased from Nippon Electric Glass with an average length of 25 µm and average diameter of 5 µm (model PF-50s). 1H and 12

C NMR spectra were recorded using a JEOL ECX 400 MHz spectrometer. Molecular

weights were measured using a MALDI-TOF mass spectrometer (Voyager-DE STR-H, Applied Biosystems). Absorption spectra were recorded using an Agilent 8453 spectrophotometer. Photoisomerization was conducted using 365-nm (Hamamatsu LED controller model c11924-101) and 436-nm [(Hg lamp equipped with an Optima QB4 optical filter (USHIO)] light sources. Microscopic analyses were performed using an Olympus BX60 optical microscope equipped with a Sony DXC-950 3CCD camera. The phase transition characteristics of the compounds were investigated from the micrographic optical textures obtained from an optical microscope equipped with a polarizer and a hot stage (Mettler Toledo F82HT). XRD patterns were recorded using a Rigaku (RINT 2000) diffractometer and monochromated Cu Kα (λ = 1.5406 Å) X-ray radiation (40 kV, 40 mA). The samples were uniformly coated on a glass holder. Measurement of helical pitch: CLC mixtures were formed from an achiral nematic liquid

crystal host and a photoresponsive chiral dopant. The homogeneously mixed composite was injected into a wedge cell (EHC, KCRK-07; tanθ = 0.01858) by capillary action at room temperature. The pitch was determined by measuring the distance between the


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Cano lines on the surfaces of the wedge cells. The relationship between the distances between the Cano lines and the helical pitch is given by P = 2Rtanθ where R is the distance between the Cano lines and θ is the angle between the substrates. N8, N20, N20a, and N20b were synthesized using previously reported methods. 35,36 Synthesis of N2 and N3: A solution of N,N-dimethyl-4-aminopyridine (0.370 g, 3.02 mmol) and N,N´-dicyclohexylcarbodiimide (0.690 g, 3.02 mmol) in CH2Cl2 (30 mL) was added to a solution of 4-(phenylazo)phenol (0.400 g, 2.01 mmol), cholesterol (0.810 g, 2.01 mmol), and either succinic acid or glutaric acid (2.01 mmol) in CH2Cl2 (6 mL) and then the mixture was stirred at room temperature for 3.5 h. After filtration, the solution was washed with brine and dried (MgSO4). The solvent was evaporated and the residue purified through column chromatography (SiO2; CH2Cl2).

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. 1

H NMR and

12

C NMR spectra, absorption spectra of compounds, a linear fit using the

absorption data for each compound at λmax to calculate the thermal back rate constant, helical pitch change according to increasing the temperature of wedge-cells, CLC mixture film surface images containing N2 in JC-1041XX, schematic of the light induced rotational motion experiment, and a table of rotational angles of microsized glass rods upon UV and subsequent visible light irradiation. (PDF)

Acknowledgements Financial support from Toyo Gosei Memorial Foundation is gratefully acknowledged.



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Asymmetric Dimers of Chiral Azobenzene Dopants Exhibiting Unusual

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