Introduction

Light-driven molecular switches are important technologies for future photonics applications [1]. They provide opportunities for the development of smart devices with the possibility of miniaturization [2, 3] and are designed by the incorporation of photoswitching into chiral molecules. To perform these functions, materials show reversible changes between two different states, which can be efficiently interchanged by incident light, where a high sensitivity and fast response are required [4].

A typical photoswitching molecule is azobenzene (AB), which is a well-known photochromic compound. AB exhibits reversible changes upon photoirradiation with UV/visible light (Vis) or heat [5]. These changes include changes in the molecular shape, length, orientation and dipole moment (δ) owing to trans-cis isomerization (Scheme 1(a)). AB possesses a rod-like shape in the trans form (δ = 0 Debye), while the cis isomer is bent (δ = 3 Debye). The distance between the 4- and 4′-carbons at the para positions decreases from 9.0 Å (trans form) to 5.5 Å (cis form) [6]. The photoinduced change in the molecular orientation results in the trans form changing its orientation to be perpendicular to the polarization direction of the actinic light after multiple cycles of isomerization, which is known as the Weigert effect [7]. For the chiral characteristic, the 1,1′-binaphthyl (BN) skeleton acts as an axially chiral segment [8]. The BN moiety undergoes a change in the dihedral angle (θ) between the two naphthalene rings owing to the restricted rotation around the naphthalene–naphthalene bond (Scheme 1(b)), which results in the two configurations of cisoid (θ < 90°) and transoid (θ > 90°) [9].

Scheme 1
scheme 1

a Photo-induced change in molecular structure of azobenzene moiety, (b) change in dihedral angle of binaphthyl moiety

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The combination of AB and BN units in a single molecule leads to novel optical properties. Montbach et al. introduced two AB units with ether linkages at the para positions to the 2,2′-positions of BN and fabricated fixable optically addressed photochiral displays with nematic liquid crystals for the first time [10]. Replacing the terminal ether groups with ester units possessing an extended π conjugation resulted in the self-organization of the AB-BN-AB into an optically tunable helical superstructure, which led to a reversible light-directed reflection in the near-infrared region [11]. When two AB units were incorporated at the 5,5′-positions of BN, distinct phase-induced chiral and photoswitching properties of the cholesteric liquid crystals were observed [12]. Enantiomeric chiral dopants composed of BN connected to AB through the 6,6′-positions in the liquid crystal showed reversible and fast red, green, and blue (RGB) reflections in the cell [13].

Kawamoto et al. introduced nitro AB segments into an ether BN unit, and the resulting compound was a novel chiral amorphous molecular material that exhibited highly efficient photoinduced polarization rotations [14]. They also designed non-destructive erasable molecular switches by connecting AB and BN moieties through an alkyl chain in the macrocycle structure and showed a uniquely chiroptical switching behavior with high fatigue resistance [15, 16]. However, the dynamic molecular motion was not efficient owing to relaxation because the AB and BN moieties were connected by a flexible alkyl chain. In addition, aggregation occurred in the case of small molecules owing to the phase separation, and it was difficult to form a homogeneous thin film. To overcome these problems, in this study, new atropisomeric polymers composed of AB and BN were designed and synthesized (Scheme 2). An AB moiety was directly connected to a BN moiety in the side chains. The rigid chromophore was expected to enable direct control over the molecular structure without relaxation. Additionally, to avoid the molecular aggregation that occurs in the case of small molecules, the AB and BN units were incorporated into the polymeric structure in the side chains, separated by ethylenic bridges. The polymeric material facilitated the formation of uniform and smooth thin films owing to the high molar mass and high solubility, which are useful for various device fabrications.

Scheme 2
scheme 2

Molecular structure of the light-driven atropisomeric azo polymer

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Experimental procedures

Materials

Unless otherwise noted, all chemicals and solvents were purchased from commercial suppliers and were used as received without further purification. Racemic 1,1′-bi-2-naphthol, 4-nitroaniline, nitrosobenzene, N-benzylcinchonidinium chloride, 6-bromo-1-hexanol, potassium peroxymonosulfate, methacryloyl chloride, and potassium carbonate were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Anhydrous N,N-dimethylformamide (DMF), anhydrous dimethyl sulfoxide (DMSO), anhydrous acetone, n-hexane, ethyl acetate, chloroform, methanol, tetrahydrofuran (THF, spectroscopic grade), and 1,4-dioxane (spectroscopic grade) were purchased from Wako Pure Chemicals Industries Ltd. (Osaka, Japan). 2,2′-Azobisisobutyronitrile (AIBN), triethylamine, hydroquinone, ammonium sulfite monohydrate, ammonia solution (NH3 aq., 28%), sodium hydroxide, acetic acid, hydrochloric acid (1 N), sodium sulfate, anhydrous magnesium sulfate, benzene, and isopropanol were purchased from Junsei Chemical Co., Ltd. (Tokyo, Japan). Deuterated chloroform and deuterated 1,4-dioxane were purchased from Cambridge Isotope Laboratories (Tewksbury, MA, USA). A fused silica substrate was purchased from Matsunami Glass Ind. Ltd. (45 mm × 12 mm, thickness: 1 mm, Osaka, Japan).

Synthesis of the atropisomeric azo monomers

Scheme 3 shows the synthetic route for the atropisomeric azo polymers. The atropisomeric unit, 2-amino-2′-hydroxy-1,1′-binaphthyl ((R)-NOBIN), was synthesized according to a previously reported method [17, 18]. (R)-NOBIN was separated by its optical resolution with >99% enantiomeric excess ([α]D25: +124, c = 1.0 in THF). The reaction of (R)-NOBIN with 1.2 equiv. of aryl nitroso species yielded the atropisomeric azo compounds: RAH when X = H and RANO 2 when X = NO2. After free radical polymerization of the methacrylated monomer under deoxygenated conditions, the colored powder of the polymer was obtained by precipitation in methanol.

Scheme 3
scheme 3

Synthetic pathway for PMRAH and PMRANO 2

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The methacrylated monomer of RAH (MRAH) was synthesized as follows. Triethylamine (0.4 g, 4.2 mmol) and hydroquinone (5 mg, 0.05 mmol) were added to a solution of RAH (1.0 g, 2.1 mmol) in THF (50 mL). After the reaction solution was stirred at 0 °C for 10 min, methacryloyl chloride (0.3 g, 2.3 mmol) in THF (10 mL) was slowly added to the solution. The reaction mixture was gradually warmed to 25 °C and then stirred for 24 h under argon. The resulting mixture was quenched by distilled water (250 mL). The crude product was extracted with dichloromethane (300 mL) before drying over MgSO4. The solvent was evaporated under reduced pressure, and the oily residue was purified by column chromatography on silica gel (eluents: hexane:ethyl acetate = 90:10 v/v) and by size exclusion chromatography (eluent: chloroform) to afford MRAH (0.7 g, 65%) as an orange-red oil. 1H-NMR (300 MHz, CDCl3): δ = 0.80–1.01 (m, 4 H, binaphthyl–O–CH2CH2CH2CH2CH2CH2OCOC(CH2CH3)), 1.25–1.37 (m, 4 H, binaphthyl-O–CH2CH2CH2CH2CH2CH2OCOC(CH2CH3)), 1.94 (s, 3 H, binaphthyl–O–CH2CH2CH2CH2CH2CH2OCOC(CH2CH3)), 3.85–3.97 (m, 4 H, binaphthyl–O–CH2CH2CH2CH2CH2CH2OCOC(CH2CH3)), 5.54 (s, 1 H, binaphthyl–O–CH2CH2CH2CH2CH2CH2OCOC(CH2CH3)), 6.07 (s, 1 H, binaphthyl–O–CH2CH2CH2CH2CH2CH2OCOC(CH2CH3)), 7.18–7.19 (m, 2 H, aromatic ring), 7.27–7.35 (m, 5 H, aromatic ring), 7.40–7.55 (m, 5 H, aromatic ring), 7.88–7.91 (d, 1 H, J = 8.1 Hz, aromatic ring), 7.95–8.03 (m, 3 H, aromatic ring), 8.15–8.18 (d, 1 H, J = 8.7 Hz, aromatic ring); 13C-NMR (300 MHz, CDCl3): δ = 18.31, 25.16, 25.24, 28.22, 29.08, 64.52, 69.24, 114.56, 115.02, 120.94, 122.81, 123.38, 125.09, 125.84, 126.28, 126.48, 127.26, 127.31, 127.72, 128.12, 128.75, 128.79, 129.44, 130.41, 133.72, 134.49, 134.89, 136.50, 136.89, 147.98, 152.78, 154.52, 167.41; FT-IR (ν cm−1): 3100, 3000, 2950, 2850, 1750, 1600, 1450, 1400, 1350, 1200, 1000, 900, 720, 675, 650; Anal. calcd. for C36H34N2O3: C, 79.68; H, 6.32; N, 5.16. Found: C, 79.52; H, 6.38; N, 5.02; HRMS calcd. for C36H34N2O3 [M + Na]+: 565.2462, found: 565.2462.

MRANO 2 was synthesized using a method similar to that used for the synthesis of MRHA. Yield: 67%; Anal. calcd. for C36H33N3O5: C, 73.58; H, 5.66; N, 7.15. Found: C, 73.58; H, 5.78; N, 6.87; HRMS calcd. for C36H33N3O5 [M + Na]+: 610.2312, found: 610.2312.

Polymerization

The polymer of MRAH (PMRAH) was synthesized as follows. AIBN (8.2 mg, 0.05 mmol) was added to a solution of MRAH (0.54 g, 1.0 mmol) in anhydrous DMF (10 mL) under argon. The reaction solution was degassed by freeze-pump-thaw cycles three times and then stirred for 72 h at 60 °C. The solution was poured into methanol (100 mL) after cooling to 25 °C. The precipitated product was collected by filtration. After the solid residue was dissolved in chloroform, the resulting solution was precipitated in methanol twice to afford PMRAH as an orange-red powder (0.3 g, 55%). The number-average molecular weight (Mn) was 7030; the polydispersity (weight-average molecular weight (Mw)/Mn) was 2.0.

Another polymer of MRANO 2 (PMRANO 2 ) was synthesized by free-radical polymerization with MRANO 2 in a similar manner. Yield: 52%; Mn: 7960; Mw/Mn: 1.5.

Thermal analysis

The thermal properties were evaluated using thermogravimetric analysis (TGA; TGA-50, heating rate: 10 °C min−1 under nitrogen; Shimadzu Co. Ltd., Kyoto, Japan) and differential scanning calorimetry (DSC; SII NanoTechnology EXSTAR 6220, heating and cooling rate: 10 °C min−1 under nitrogen; Seiko Instruments Inc., Tokyo, Japan).

Spectroscopic measurements

The photoresponsive behavior was studied using UV-Vis absorption spectroscopy (V-550, JASCO Co. Ltd., Tokyo, Japan). The thermal back-isomerization was examined on the same spectrometer equipped with a temperature controller system (EHC-477, JASCO). Photoluminescence spectra were recorded using a spectrofluorometer (FP-6500, JASCO). Circular dichroism (CD) spectra were obtained using a CD spectrophotometer (J-720, JASCO). Optical rotation was determined using a polarimeter (P-2200, path length: 10 cm, solvent: THF; JASCO). Samples were irradiated at 365 nm (light intensity: 5 mW/cm2) and 436 nm (light intensity: 3 mW/cm2) using glass filters (HQBP365 and HQBP436, respectively, Asahi Spectra, Tokyo, Japan) and a 120 W high-pressure mercury lamp (REX-120, Asahi Spectra).

Results and discussion

Synthesis and characterization of the atropisomeric azo polymers

Table 1 shows the characteristics of the azo polymers. From the Mn and Mw/Mn values, it was calculated that these polymers possessed 14–20 atropisomeric units in the side chain. Optical rotation measurements revealed that the dextrorotatory nature was maintained after the modification of (R)-NOBIN. These results suggested that the optical rotation of the atropisomeric unit arose from the axial chirality of the BN unit. The thermal behavior was investigated by TGA and DSC. TGA indicated that PMRAH and PMRANO 2 had relatively high thermal stabilities with a weight loss of 10% at 288 and 325 °C (Fig. S1). We also detected endothermic events for PMRAH corresponding to a glass transition temperature (Tg) of 77 °C for the third cycle in the DSC measurements (Fig. S2). PMRANO 2 exhibited a Tg of 112 °C, which indicated that the polymers were amorphous because of steric hindrance from the two non-planar naphthalene rings in the side chain. The thermal stability and amorphous nature are advantageous for solution-processed films.

Table 1 The characteristics of the atropisomeric azo polymers
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The polymers formed uniform and smooth films on a fused silica substrate via spin-coating or drop-casting techniques (Fig. S3). The resulting films showed good transparency without grain boundaries. The film thickness could be controlled over a range from 30 nm to 4.5 μm by varying the concentration of the polymer solution. This flexible processability contributed to the fabrication of the nano- to micrometer-scale films (Fig. S4).

Isomerization behavior of the atropisomeric azo polymers

Figure 1 shows the absorption and CD spectra of PMRAH and PMRANO 2 in neat films (film thickness: 135 and 120 nm, respectively). The maximum absorption wavelengths (λmaxs) were 234, 297, 341, and 350 nm. The λmaxs of the polymers were similar to those of the BN moiety [19] and were attributed to the 1Bb (234 nm), 1La (297 nm), and 1Lb (341 nm) transitions. The 1Bb and 1Lb bands were identified as the long axis of the naphthyl unit, while the 1La band was assigned to the short axis of the naphthyl unit. The λmax at 350 nm was expected to arise from the π–π* transitions of the azo (N = N) unit [20]. In the case of PMRANO 2 , a broad peak was observed at approximately 480 nm. This may contribute to the donor–acceptor effect caused by the 4-nitrophenyl group [21]. The CD spectra of the polymers exhibited exciton couplets at 240 nm and 320 nm, which were derived from the 1Bb and 1Lb transitions of the BN moiety [22]. We also detected exciton couplets of the azo unit at 340 and 450 nm, which corresponded to the π – π* and n – π* transitions, respectively [23].

Fig. 1
figure 1

Absorption and CD spectra for PMRAH and PMRANO 2 neat films (film thickness = 135 and 120 nm), respectively

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Irradiation of the polymers at 365 nm induced trans-cis photoisomerization of the azo unit (Fig. 2). After photoirradiation for 180 s at 25 °C, the polymers exhibited a photostationary state (PSS). The ratio of the trans:cis isomers in the PSS was determined to be 60:40 for PMRAH and 74:26 for PMRANO 2 using 1H-NMR spectroscopy (Fig. S9). However, recovery of the initial state was achieved after photoirradiation at 436 nm for 180 s or with heat owing to the cis-trans back-isomerization.

Fig. 2
figure 2

Isomerization behavior of polymers neat films; (a) and (b) trans to cis photoisomerization after UV irradiation at 365 nm, (c) and (d) cis to trans thermal back isomerization at 45 °C for PMRAH and PMRANO 2 (film thickness = 40 and 30 nm), respectively

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The temperature dependence of the isomerization behavior was also explored. The photoirradiation duration required to elicit the PSS changed from 180 s to 240 s when the temperature was raised from 25 to 65 °C. An acceleration of the cis-trans thermal back-isomerization occurred at high temperature, which led to the delayed PSS. The cis-trans thermal isomerization of the polymers was described according to the first-order kinetics, similar to that observed for conventional AB moieties [24]. We evaluated the kinetics and thermodynamic parameters for the cis-trans thermal back-isomerization at various temperatures, as shown in Fig. 3 and summarized in Table 2. The rate constant was determined by the first-order kinetics equation:

$${\mathrm{ln}}\frac{{A_\infty - A_t}}{{A_\infty - A_{\mathrm{o}}}} = - kt$$
(1)

where At, A0, and A∞ are the absorbance at 365 nm at time t, initially, and at infinity, respectively. The lifetime (τ: 1/k) of PMRAH was 13 h at 25 °C compared with 0.5 h at 65 °C. These results reveal that the cis-trans back-isomerization was accelerated by heat owing to the formation of the thermodynamically stable trans isomer. Furthermore, PMRANO 2 exhibited fast cis-trans back-isomerization, which was three times faster than that of PMRAH. The electron-withdrawing nitro group in PMRANO 2 was considered to accelerate the cis-trans thermal back-isomerization [25].

Fig. 3
figure 3

Kinetics studies of cis to trans thermal back-isomerization for polymers neat films; (a) and (b) first-order plots at 25 °C (squares), 35 °C (circles), 45 °C (up triangles), 55 °C (down triangles), and 65 °C (diamonds), (c) and (d) Eyring plots for PMRAH and PMRANO 2 (film thickness = 40 and 30 nm), respectively

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Table 2 Kinetic and thermodynamic parameters for the cis-trans thermal back-isomerization of the atropisomeric azo polymers
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The thermodynamic parameters of the enthalpy of activation (ΔΗ≠) and entropy of activation (ΔS≠) were determined according to the Eyring equation: [26]

$$\ln \left( {\frac{{kh}}{{k_BT}}} \right) = - \frac{{\Delta H^ \ne }}{{RT}} + \frac{{\Delta S^ \ne }}{R}$$
(2)

where R, k B , and h are the gas, Boltzmann, and Planck constants, respectively. The values of ΔS≠ and ΔH≠ were obtained from the intersect and slope, respectively, of the linear plot of ln(kh/k B T) versus 1/T extrapolated to T → ∞. ΔS≠ reflects the difference in the degree of freedom between the ground and transition states. The negative values of ΔS≠ indicated that the cis-trans thermal back-isomerization occurred through the inversion mechanism [27].

Chiroptical switching of the atropisomeric azo polymers

As described above, the polymers exhibited reversible trans-cis photoisomerization in thin films. Figure 4 shows the reversible change of the polymer films that occurred when alternating photoirradiation at 365 and 436 nm at room temperature. Because the absorbance recovered when the film was irradiated at 436 nm, this photoswitching was ascribed to the trans-cis isomerization of the atropisomeric AB unit.

Fig. 4
figure 4

Photoresponsive behavior of polymers neat films; (a) and (b) reversible trans-cis photoisomerization after UV/Vis light irradiation; (c) and (d) Switchable behavior of absorption after UV/Vis repetitions cycle of irradiation (180 s, 5 cycles, monitoring wavelengths: 336 and 345 nm) for PMRAH and PMRANO 2 (film thickness = 40 and 30 nm), respectively

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After photoirradiation at 365 nm, the PMRAH film showed a change in ellipticity at 240 nm, which corresponded to the 1Bb transition. This observation arose from the photoinduced molecular twisting motion [28, 29] of the BN moiety. Photomodulation of the dihedral angle was estimated to be 21% with an efficient switching behavior of the ellipticities in the case of the PMRAH thin film (Fig. 5). The coordinated action of the absorbance and elasticity suggested that the light-driven molecular motion resulted from the change in ellipticity.

Fig. 5
figure 5

Chiroptical switching behavior of polymers neat films; (a) and (b) photo-induced modulation in ellipticity after UV/Vis light irradiation, (c) and (d) switchable behavior of ellipticity after UV/Vis repetitions cycle of irradiation (180 s, 5 cycles) for PMRAH and PMRANO 2 (film thickness = 135 and 120 nm), respectively

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Furthermore, the specific optical rotation (\([\alpha ]_{\mathrm{D}}^{25}\), c 0.02, THF) of PMRAH and PMRANO 2 significantly changed from 255° to 967° and from 138° to 344°, respectively, after irradiation at 365 nm (Fig. S21). A recovery of the initial state could be achieved by irradiation at 436 nm. The same molecular response was investigated in the polymer films, as shown in Fig. 6. The optical rotation of the neat polymer films (α, degree cm−1) switched from 96° to 155° and from 158° to 216° after alternating photoirradiation at 365 and 436 nm for PMRAH and PMRANO 2 , respectively.

Fig. 6
figure 6

Chiroptical switching in optical rotations of polymers neat films for PMRAH (a) and PMRANO 2 (b) after UV/Vis repetitions cycle of irradiation (500 s, 5 cycles, monitoring wavelength: 589 nm, film thickness = 135 and 120 nm, respectively)

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The \([\alpha ]_{\mathrm{D}}^{25}\), α (degree cm−1), ellipticity, and dihedral angle of the atropisomeric PMRAH underwent large changes upon UV/Vis irradiation, with efficient reversibility in a neat film, as well as in solutions. Because of these favorable properties, PMRAH is regarded as a potential light-driven atropisomeric material for chiroptical switches.

Fluorescence switching induced by photoisomerization

The fluorescence spectra of the polymers show λmax at 409 nm (Fig. S13). Considering the non-radiative relaxation process of trans-cis isomerization, AB compounds are non-fluorescent [5, 30]. Based on the molecular design, the BN moiety was determined to act as a fluorescent segment [31], which led to the fluorescence of the investigated polymers. We observed that the fluorescence spectra of (R)-NOBIN under the same conditions exhibited a λmax at 404 nm, which was almost the same λmax as that of the polymers (Fig. S13). Connecting the BN moiety directly to the azo group is considered one of the exceptional methods [32,33,34] to induce fluorescence emission from AB compounds. The fluorescence quantum yields (Φf) were 2.3 × 10−1, 3.3 × 10−4, and 1.2 × 10−4 for (R)-NOBIN, PMRAH, and PMRANO 2 , respectively. These Φf values of PMRAH and PMRANO 2 were still better than those of the simple azobenzene (10−7) [35].

Interestingly, photoswitchable fluorescence was also obtained; an increase in the fluorescence intensity after irradiation at 365 nm and recovery to the initial state after irradiation at 436 nm was achieved (Fig. S14). A PSS was obtained after irradiation for 180 s for the two states. A switchable fluorescence intensity was obtained after repeated cycles of UV/Vis irradiation, with efficient reversibility (Fig. S15). The Φf values of PMRAH and PMRANO 2 were also enhanced from 3.3 × 10−4 and 1.2 × 10−4 of the initial state to 4.1 × 10−4 and 1.8 × 10−4, respectively, after irradiation at 365 nm.

Han et al. [36] also reported an unusual fluorescence enhancement of AB and attributed this enhancement to the light-driven self-assembly of cis-AB, which exhibited a sufficient lifetime and a large dipole moment. In our case, the presence of BN is considered to contribute to the enhancement of the dipole moment of cis-AB.

Photo-induced change in molecular orientation due to linearly polarized light

The photocontrol of the molecular orientation of the polymers was explored in the film using our four-wave-mixing setup (Scheme S5). A refractive index grating was generated by the irradiation of two linearly polarized pump beams (both in s-polarization). The power of diffraction was measured simultaneously in two polarization states of the probe beam: parallel (s-polarization) and normal (p-polarization) to the direction of the pump beam polarization. We used a near-infrared wavelength (830 nm) for the probe beam, which showed no absorption by our polymer samples. Raman–Nath diffraction, which would lead to a small diffraction, was expected because the sample was thin (several microns). In our system, the insensitive probe beam enabled the detection of a very small diffraction efficiency below 10−4.

We found that the s-polarized probe beam showed high diffraction. After photoirradiation of the neat polymer films (thicknesses of 4.5 and 3.2 µm) using the pump beams, the diffraction power reached approximately 2 and 6 V for PMRAH and PMRANO 2 , respectively (Fig. S16). After turning off the laser beam, the diffraction power decreased significantly to approximately 1 and 4 V for PMRAH and PMRANO 2 , respectively. These values represent decays of almost 50 and 35% in the diffraction power for PMRAH and PMRANO 2 , respectively, after turning off the laser. The photoinduced change in the refractive index (Δn) was evaluated from the diffraction efficiency of the grating according to the following equation [37]:

$${\mathrm{\Delta }}n \approx \frac{\lambda }{{\pi L}}\sqrt \eta$$
(3)

where λ, L, and η are the monitoring wavelength, film thickness, and diffraction efficiency, respectively. The photomodulation of Δn for PMRAH and PMRANO 2 was 0.003 and 0.006, respectively, as shown in Fig. 7. The value of Δn for PMRANO 2 was two times higher than that of PMRAH. This difference arose from the effects of the NO2 chromophore: a) the donor-acceptor effect, which led to the polarizability of the molecules, and b) the photochromic effect, which led to an increase in the absorbance at 532 nm; the corresponding absorption at 532 nm was 0.16 and 0.4 for PMRAH and PMRANO 2 , respectively.

Fig. 7
figure 7

Photoinduced change in the refractive index of polymers neat films for PMRAH (a) and PMRANO 2 (b). Detection wavelength: 830 nm; film thickness: 4.5 and 3.2 μm, respectively

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However, much lower diffraction was measured by the p-polarization state of the probe beam (Fig. S16). As a result, the diffraction power was approximately 0.5 and 1.0 V for the neat PMRAH and PMRANO 2 films, respectively. Such different powers in the two polarization states could be explained by the selective photochromic excitation of AB–BN molecules by the linearly polarized pump beam [38]. The polymer exhibited an initial state in a trans form, which has a dominant molecular absorption along the charge-transfer (CT) axis involving an N = N bond. After photoexcitation, the pump beam selectively excited the trans form, of which the CT axis was aligned in parallel to the polarization of the pump, which led to selective trans-to-cis photoisomerization. As a result, the excited trans form changed its original molecular orientation, which resulted in optical anisotropy.

Conclusions

We successfully synthesized and characterized light-driven atropisomeric polymers containing AB and BN units in the side chains. The polymers displayed reversible changes in the fluorescence intensities by photoisomerization. The polymers formed a uniform and smooth film and showed reversible trans-cis photoisomerization. Additionally, a photoinduced refractive change was observed after photoirradiation by linearly polarized light. These results suggested that the polymers are potential photoresponsive materials for light-driven molecular switches.