Supramolecular bisazopolymers exhibiting enhanced photoinduced birefringence and enhanced stability of birefringence for four-dimensional optical recording

Abstract

Extremely stable, high-density information storage media were prepared by the connection of two azobenzene groups via hydrogen bonding to form supramolecular bisazopolymers. The supramolecular bisazopolymers contain 4-((4-hydroxyphenyl)diazenyl)benzonitrile (AzoCN) as a hydrogen bonding donor and poly(6-(4-(pyridin-4-yldiazenyl)phenoxy)hexyl methacrylate) (pAzopy) as a hydrogen bonding acceptor. High quality films of the supramolecular bisazopolymers pAzopy/(AzoCN)_x (x = 0.25, 0.5, 0.75, 1.0) with different molar ratios of donor/acceptor were prepared by spin-casting. The supramolecular bisazopolymers spontaneously form lamellar structures with a periodic thickness of 7.1 nm. These samples exhibit optically induced birefringence. The birefringence and proportion of remnant birefringence increase from 0.0265 to 0.1 and 50.5% to 108%, respectively, as the content of AzoCN in the samples increases, which indicates a larger proportion of AzoCN enhances both the birefringence and its stability. We also use an azopolymer without pyridine groups (pAzoCH₃) and AzoCN to do a control experiment, which shows that pAzoCH₃/(AzoCN)_1.0 do not show significant enhanced birefringence and its stability. The enhancements in pAzopy/(AzoCN)_x are because of the unique structure of the supramolecular bisazopolymers. A new laser direct writing system has been developed for optical recording on the azopolymers. The pAzopy/(AzoCN)_1.0 film is significantly better at image recording than the pAzopy film, because the optically recorded images on the pAzopy/(AzoCN)_1.0 film are very clear after storage for four months whereas the optically recorded images on the pAzopy one disappear after just one day. Four-dimensional optical recording has been achieved by integrating the polarization and the intensity of the laser and the two dimensions of a plane. An information density of about 0.93 Gbit cm⁻² could be optically recorded on the pAzopy/(AzoCN)_1.0 film, which is about 20 times the information density of a normal DVD.

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Introduction

Supramolecular interactions such as hydrogen bonding, coordination, ionic interaction and π–π stacking are useful interactions in fabrication of functional materials.¹ Incorporating photoresponsive molecules into supramolecular systems leads to smart systems, the properties of which can be manipulated by light. Azopolymers (azobenzene-containing polymers) are photoresponsive materials, which show reversible trans–cis photoisomerization under the irradiation of light at different wavelengths.^2,3 More interesting, birefringence of azopolymers can be optically induced by irradiation of linearly polarized light and the optically induced birefringence can be erased by irradiation of non-polarized light or circularly polarized light.^2,3 A large number of research works on azopolymers have been done in the past 20 years because of the interesting properties and promising applications of azopolymers.^2,3

Recently, many groups fabricated various supramolecular azopolymers by supramolecular interactions and these materials show the following advantages.^4–31 (1) Compared with azo dyes doped polymers, supramolecular interactions in the supramolecular azopolymers can prevent the azobenzene groups from aggregation at a relatively high concentration and improve optical performances and stabilities of these materials;^8–10,25 (2) Compared with synthesis of azopolymers with azobenzene groups covalently linked to the polymer chain, fabrication of supramolecular azopolymers can avoid tedious synthetic work and allows for tuning the content and kind of the azobenzene derivatives easily.^4–31 (3) Furthermore, in the supramolecular azopolymer system, the azobenzene units can be selectively detached from the supramolecular system.⁴ There are mainly two types of supramolecular azopolymers reported in the literature. Their models are shown as type 1 and type 2 in Scheme 1. In the type 1 supramolecular azopolymer system, azobenzene derivatives are linked to polymer chains via supramolecular interactions. In the type 2 supramolecular azopolymer system, guest molecules are linked to azopolymer chains via supramolecular interactions. The supramolecular interactions in these two types of supramolecular azopolymers include hydrogen bonding,^4,6–18,26 coordination^18–20 and ionic interaction.^5,21–31 The host polymers in these two systems include linear homopolymers,^{7–11,16,17,19–21,25–30} linear random copolymers,^4,12,13 linear block copolymers,^5,18 crosslinked polymers,^22–24 hyperbranched polymers³¹ and dendrimers.³¹

Scheme 1 Schematic models of the three types of supramolecular azopolymers and the chemical structure of the supramolecular bisazopolymers pAzopy and AzoCN.

It is well known that azopolymers have been extensively explored for potential uses in various technological applications, such as optical information storage, optical switching, optically anisotropic materials, diffractive optical elements, and so on.^2,3 The frequent problems which prevent azopolymers from real applications are the optical performances, stability and processability. Bisazopolymers have better optical performance than the normal azopolymers, because a bisazobenzene unit with high length/diameter ratio has much larger birefringence than the normal azobenzene unit.^2,32–34 However, the solubility and film-forming property of the covalent bisazopolymers are very poor.^2,33 In this paper, we demonstrate a new strategy to obtain a new azopolymer system with good optical performances, stability and processability. Our concept of fabrication of such an azopolymer system is based on connection of two azobenzene groups via supramolecular interaction (hydrogen bonding) to form a new supramolecular bisazobenzene group. The model (type 3) and chemical structure of the supramolecular bisazopolymer are shown in Scheme 1. Different from the covalent bisazopolymers, the solubility and film-forming property of the supramolecular bisazopolymers are very good. Photoinduced birefringence and the temporal stability of the supramolecular bisazopolymer are largely enhanced. We record images on the supramolecular bisazopolymer by a newly built laser direct writing system. The polarization and the intensity of the laser and the two dimensions of a plane are integrated to achieve four-dimensional optical recording. The optical recording information density is about 0.93 Gbit cm⁻² and the optical recording images on the supramolecular bisazopolymers can be stored for at least 4 months.

Experimental

Materials

Synthesis of 4-((4-hydroxyphenyl)diazenyl)benzonitrile (AzoCN) is according to a previous work.³⁵ The syntheses of 6-(4-(pyridin-4-yldiazenyl)phenoxy)hexyl methacrylate (Azopy) and 6-(4-(p-tolyldiazenyl)phenoxy)hexyl methacrylate (AzoCH₃) are according to ref. 17 and 36.

Synthesis of poly(6-(4-(pyridin-4-yldiazenyl)phenoxy)hexyl methacrylate) (pAzopy). Azopy (0.552 g, 1.5 mmol) and 2,2′-azobisisobutyronitrile (24 mg, 0.015 mmol) were dissolved in anhydrous tetrahydrofuran (1.2 mL). After 3 freeze–thaw cycles, the flask was sealed in a vacuum. Polymerization was conducted at 55 °C for 72 h. The reaction mixture was finally precipitated in methanol (50 mL) and then NaOH aqueous solution (15 mL, 2 wt%) was added. The mixture was cooled in a fridge overnight. The resulting polymeric solid was filtered off and thoroughly washed with methanol and hexane. Finally, the polymer was dried in an oven under vacuum. The GPC measurement of average molecular weight and polydispersity are M_n = 4200 g mol⁻¹ and M_w/M_n = 1.3, respectively.

Synthesis of poly(6-(4-(p-tolyldiazenyl)phenoxy)hexyl methacrylate) (pAzoCH₃). AzoCH₃ (1.5 mmol) and 2,2′-azobisisobutyronitrile (24 mg, 0.015 mmol) were dissolved in anhydrous tetrahydrofuran (1.2 mL). After 3 freeze–thaw cycles, the flask was sealed in a vacuum. Polymerization was conducted at 55 °C for 72 h. The obtained polymer was precipitated in methanol, and the purification was repeated three times in a THF–methanol system. The GPC measurement of average molecular weight and polydispersity are M_n = 4800 g mol⁻¹ and M_w/M_n = 1.3, respectively.

Preparation of the supramolecular bisazopolymers of pAzopy and AzoCN. To prepare a supramolecular bisazopolymer, a calculated amount of pAzopy and AzoCN were dissolved in THF to obtain a clear solution. The solution was under sonication for 15 min and stirred for about 1 h and then most of the solvent was evaporated at room temperature under atmospheric pressure. The remainder of the solution was then dried in a vacuum oven at room temperature over night. Four supramolecular bisazopolymers pAzopy/(AzoCN)_x (x = 0.25, 0.5, 0.75, 1.0) with different concentration of AzoCN were prepared. The molar ratios of donor/acceptor were 0.25, 0.5, 0.75 and 1.0, respectively.

Preparation of thin films

The supramolecular bisazopolymers (30 mg) were dissolved in cyclopentane (0.5 mL). Thin films of the supramolecular bisazopolymers were prepared by spin-coating the solution on freshly cleaned glass or quartz substrates. The spin speed was 380–450 rpm. After spin-coating, the films were dried in an oven under vacuum at room temperature over night. The film thicknesses measured with a step-profiler were 380–500 nm.

Measurements

UV-vis absorption spectra were measured on a SHIMADZU UV-2550 PC spectrophotometer. FT-IR spectra were obtained using a Nicolet 8700 spectrometer. Polarized optical microscopy (POM) images were recorded on an Olympus BX51 microscope. X-Ray diffraction (XRD) patterns were measured on a PHILIPS X'PERT PRO X-ray diffract meter with Cu-Kα line (λ = 0.15418 nm). Optically induced birefringence was performed on a setup reported in our previous work.³⁷ Birefringence of the films was induced by linearly polarized light of a He–Cd laser at 442 nm with an intensity 317 mW cm⁻². The photoinduced orientation was detected by a probe light at the same time. The probe light, polarized at 45° with respect to the polarization direction of the writing light and passed through a pair of crossed polarizers, was a low power diode laser at 650 nm. The transmittance change of the probe light was recorded and the signals were transmitted to a computer. The birefringence Δn can be obtained from the transmission data:

I = I₀sin²(πΔnd/λ) (1)

where I₀ is the signal for parallel polarizer/analyzer orientation, d is the film thickness, and λ is the wavelength of the probe light (650 nm).

The optical recording experiments were using a lab-built laser direct writing system. The setup of the laser direct writing system as well as the energy distribution of the laser around the focal plane is shown in Scheme 2. The laser of this system was a semiconductor laser at 405 nm. The diameter of the laser at focal plane was about 3 μm. Computer software was used to control polarization and intensity of the laser, exposure time and the movement of the sample stage. The exposure time for every point was 2 ms and the intensity of the laser at the focal plane was 10 mw without a special statement. The software allows us to put a common image file into the computer and then the system can fabricate microstructures on azopolymer films according to the image file. Our concept of designing this system is to introduce the phase modulation technology into this system, so that the polarization direction of the laser can be precisely controlled, which is very important for the optical recording of azopolymers with polarization-dependent properties.

Scheme 2 Schematic model of the laser direct writing system for optical recording. The energy distribution of the laser around focal plane is shown in the upper left.

Results and discussion

Characterization of the supramolecular bisazopolymers

As shown in Scheme 1, the supramolecular bisazopolymers contain pAzopy as hydrogen bonding acceptors and AzoCN as hydrogen bonding donors. It is well-known that phenol groups and pyridine groups have a strong tendency to self-assemble and form hydrogen bonds.^9–11,38,39 Formation of hydrogen bonding between pAzopy and AzoCN is verified by infrared spectroscopy. As shown in Fig. 1, pure pAzopy has a symmetric ring stretching mode at 990 cm⁻¹. As the concentration of AzoCN increases, the band at 990 cm⁻¹ gradually decreases and a new band at 1007 cm⁻¹ gradually increases, which can be attributed to hydrogen bonding between the pyridine and phenol groups of pAzopy and AzoCN, respectively.^9,10,38

Fig. 1 FT-IR spectra of pAzopy, AzoCN and pAzopy/(AzoCN)_x (x = 0.25, 0.5, 0.75 and 1.0).

In general, the film-forming property of supramolecular azopolymers is better than that of the azo dyes doped polymers.^8–10 The photograph of spin-coating films of pAzopy, AzoCN and pAzopy/(AzoCN)_1.0 is shown in Fig. 2. PAzopy film is homogenous and highly transparent because polymers generally have good film-forming property. The AzoCN film is not homogenous and not highly transparent because the molecular weight of AzoCN is too low. It is well known, the performance of photoresponsive film materials is limited by the bad film-forming property, resulting in increased optical scattering losses and lowered optical response. In azo dyes doped polymers, the quality of the films is not good when the concentration of azo chromophores is higher than 10 wt%.⁸ However, as shown in Fig. 2, pAzopy/(AzoCN)_1.0 film with an azo chromophore concentration of 63.1 wt% is still homogenous and highly transparent, which shows the advantage of supramolecular azopolymer system.

Fig. 2 Photograph of films of pAzopy, AzoCN and pAzopy/(AzoCN)_1.0.

The UV-vis absorption spectra of spin-coating films of pAzopy and pAzopy/(AzoCN)_x are shown Fig. 3 (a). Strong π–π* transitions of trans azobenzene groups and weak n–π* transitions of cis azobenzene groups are located around 350 nm and 460 nm, respectively. The π–π* transition peaks (λ_max) and full widths at half-maximum (FWHM) of the films and THF solutions are calculated according to the UV-vis absorption spectra and shown in Fig. 3 (b) and (c). The shift of λ_max between the films and solutions are very small, only 1–4.5 nm, indicating the films are homogenous and no obvious phase separation happens in the thin films.^8,9 FWHM of the films increases as the molar ratio of AzoCN increases and FWHM of the films are larger than those of the solutions, indicating there are considerable π–π interactions between the azobenzene groups in films.^40–43

Fig. 3 (a) UV-vis absorption spectra of pAzopy and pAzopy/(AzoCN)_x (x = 0.25, 0.5, 0.75, 1.0) films. The curves of pAzopy/(AzoCN)_x are offset to avoid overlapping; (b) λ_max of pAzopy and pAzopy/(AzoCN)_x (x = 0.25, 0.5, 0.75, 1.0) films and THF solutions; (c) FWHM of pAzopy and pAzopy/(AzoCN)_x (x = 0.25, 0.5, 0.75, 1.0) films and THF solutions.

The morphologies of the films are studied by polarized optical microscopy (POM) and shown in Fig. 4. PAzopy is an amorphous polymer¹⁷ and the POM image of pAzopy is dark, which indicates that pAzopy does not form crystalline or liquid crystalline structures. AzoCN alone forms spherulite-like crystals whose sizes are from about 10 μm to 40 μm. PAzopy/(AzoCN)_0.25 seems to form some textures, but it is not clear under POM observation. The other supramolecular bisazopolymers pAzopy/(AzoCN)_x (x = 0.5, 0.75, 1.0) spontaneously form tiny wormlike textures which have typical sizes of 1 μm or less. The textures of pAzopy/(AzoCN)_x are different from the textures of both constituents alone. This observation is in line with earlier reports that the morphologies of hydrogen bonding complexes are quite different to those of the hydrogen bonding acceptors or donors alone.^{14,15,17,18,44,45}

Fig. 4 POM images of spin-coating films of pAzopy, pAzopy/(AzoCN)_x (x = 0.25, 0.5, 0.75, 1.0) and AzoCN.

To have a better understanding of the morphologies shown in Fig. 4, X-ray diffraction (XRD) patterns are measured with the same films already used for POM measurements. Fig. 5 (a) shows the XRD patterns of pAzopy, AzoCN and pAzopy/(AzoCN)_x. There is no diffraction peak in the pAzopy film. The XRD result and the dark POM image of pAzopy indicate that pAzopy is amorphous. There are diffraction peaks in the XRD patterns of AzoCN and pAzopy/(AzoCN)_x. We observe that a periodicity thickness of AzoCN film is 6.6 ± 0.1 nm and periodicity thicknesses of pAzopy/(AzoCN)_x films are 7.1 ± 0.1 nm, which are calculated by Bragg's law. Obviously, the periodicity thicknesses of AzoCN and pAzopy/(AzoCN)_x are different, which indicates the diffractions of pAzopy/(AzoCN)_x come from new periodic structures of the supramolecular bisazopolymers but not residual unbonded AzoCN. The lengths of AzoCN and the fully extended side chain of pAzopy are about 1.37 nm and 2.20 nm, respectively, which are calculated from the optimized MM2 model. By comparing these values to the periodicity thickness of the supramolecular bisazopolymers, one can assume a bilayer type of packing in the supramolecular bisazopolymers. Actually, many groups have reported that supramolecular azopolymers form lamellar structures.^{11,17,18,21,26–30} Considering the fully extended chain length is 7.14 nm, which is rather similar to the periodicity thickness of pAzopy/(AzoCN)_x, it seems that there is no interpenetration of the side groups of the supramolecular bisazopolymers. So, the supramolecular bisazopolymers most probably spontaneously assemble into lamellar structures and we propose two most possible models in Fig. 5 (b) and (c). In Fig. 5 (b), the side groups and the polymer backbone form alternate layers (side chain-backbone-side chain in every period). In Fig. 5 (c), the comb-like supramolecular bisazopolymers form an end-to-end bilayer structure (backbone-side chain-side chain-backbone in every period).

Fig. 5 (a) X-Ray diffraction (XRD) patterns of spin-coating films of pAzopy, pAzopy/(AzoCN)_x (x = 0.25, 0.5, 0.75, 1.0) and AzoCN; (b) and (c) schematic illustration of packing models of the supramolecular bisazopolymers.

Photoinduced birefringence of the supramolecular bisazopolymers

It is well known that covalent bisazobenzene groups can generate a larger birefringence per structural unit^2,32–34 and hydrogen bonding can improve the stability of birefringence.^8–10 So, the supramolecular bisazopolymers are expected to show improved photoinduced birefringence and stability. We use a linearly polarized laser at 442 nm to induce birefringence of the films. Photoinduced birefringence and the relaxation of birefringence of pAzopy and pAzopy/(AzoCN)_x films are shown in Fig. 6 (a) and the birefringence and the proportions of remnant birefringence as a function of the molar ratio of donor/acceptor are shown in Fig. 6 (b).

Fig. 6 (a) Photoinduced birefringence of spin-casting films of pAzopy and pAzopy/(AzoCN)_x (x = 0.25, 0.5, 0.75, 1.0). (b) Photoinduced birefringence and proportion of remnant birefringence of pAzopy and pAzopy/(AzoCN)_x as a function of molar ratio of donor/acceptor.

We observe that birefringence of the films increases from 0.0265 to 0.1 as the content of AzoCN increases. To exploring the reason why there is enhanced birefringence in the pAzopy/(AzoCN)_x films, we do a control experiment by using AzoCN and an azopolymer without pyridine groups (pAzoCH₃). In pAzoCH₃/(AzoCN)_1.0, the azobenzene groups of pAzoCH₃ cannot form hydrogen bonding with AzoCN because the azobenzene groups of pAzoCH₃ do not contain hydrogen bonding acceptors. Compared with the birefringence of pAzoCH₃, the birefringence of pAzoCH₃/(AzoCN)_1.0 shows an increment of 13%. According to Fig. 6 and the control experiment, there are 3 factors for the enhanced birefringence pAzopy/(AzoCN)_x:

(1) Concentration of azo chromophores is one factor which will affect photoinduced birefringence of azopolymers.⁸ The concentration of azo chromophores increases from 49.6 wt% in pAzopy to 63.1 wt% in pAzopy/(AzoCN)_1.0, so the birefringence should increase. In the control experiment, birefringence of pAzoCH₃/(AzoCN)_1.0 shows an increment of 13% compared with pAzoCH₃. However, birefringence of pAzopy/(AzoCN)_1.0 shows an increment of 277% compared with pAzopy, which is obvious larger than the increment of pAzoCH₃/(AzoCN)_1.0 system. So, there must be other reasons for the large increment of the birefringence in the supramolecular bisazopolymers.

(2) It is known that birefringence per unit of a covalent bisazopolymer is about 5 times that of a normal azopolymer with similar azo content because the length/diameter ratio of the covalent bisazobenzene group is bigger.³³ In the supramolecular bisazopolymers, two azobenzene groups form a supramolecular bisazobenzene group with higher length/diameter ratio. This way, an enhanced birefringence can result.

(3) There are both hydrogen bonding and π–π interactions in pAzopy/(AzoCN)_x. It is known that the interplay of hydrogen bonding and π–π interactions can enhance birefringence of supramolecular azopolymers.⁹ So, birefringence of the supramolecular bisazopolymers should increase.

We observe an interesting phenomenon in the relaxation process when the laser is turned off. Normally, after the laser is turned off, birefringence decreases.² We observe that birefringence of pAzopy and pAzopy/(AzoCN)_0.25 decreases after the laser is turned off, but birefringence of pAzopy/(AzoCN)_x (x = 0.5, 0.75, 1.0) increases after the laser is turned off. The stability of birefringence is often judged by the proportion of remnant birefringence after relaxation.^2,8,9,33 To the best of our knowledge, the best proportion of remnant birefringence of amorphous supramolecular azopolymer and liquid crystalline supramolecular azopolymer reported in the literature are 95 and 100%, respectively.^9,27,28 The proportion of remnant birefringence of side-chain covalent bisazopolymers and main-chain covalent bisazopolymers are 91–96% and 75–85%, respectively.^33,46 The proportion of remnant birefringence of pAzopy/(AzoCN)_1.0 is 108%, which shows excellent stability. We also studied the relaxation process in the control experiment, which shows that birefringence of pAzoCH₃/(AzoCN)_1.0 and pAzoCH₃ decrease after laser is turned off. So, the excellent stability of photoinduced birefringence of the supramolecular bisazoplymers must be caused by the effect of hydrogen bonding.

Firstly, the wormlike texture induced by hydrogen bonding is one reason for the excellent stability of supramolecular bisazoplymers. It is well known that birefringence of amorphous azopolymers with low glass transition temperatures (T_g) is not stable, whereas the liquid crystalline azopolymers with similar T_g is much stable.² PAzopy is an amorphous azopolymer and its T_g is about 35 °C. Fig. 4 shows that hydrogen bonding induces wormlike textures of pAzopy/(AzoCN)_x (x = 0.5, 0.75, 1.0) which may play the same role as liquid crystal phases to enhance the stability of birefringence.

Secondly, Priimagi et al. argues that the interplay of hydrogen bonding and chromophore–chromophore interactions is the reason for the enhanced stability of birefringence.⁹ And most recently, del Barrio et al. reported that photoinduced birefringence increases after laser is turned off in an azobenzene-containing linear-dendritic diblock copolymer at 50–70 °C.⁴⁷ They explain that this phenomenon can be attributed to cooperative interactions that enhance the photoinduced molecular order.⁴⁷ In our system, there are both hydrogen bonding and π–π interactions. So, cooperative interactions that enhance the photoinduced molecular order should also exist in our system.

Thirdly, a supramolecular bisazobenzene group contains two azobenzene groups. When the laser is turned off, there could be a case that only one azo chromophore of the supramolecular bisazobenzene group is oriented, i.e., the hydrogen bonding donor is oriented and the hydrogen bonding acceptor is not oriented, or the hydrogen bonding acceptor is oriented and the hydrogen bonding donor is not oriented. It is well known that hydrogen bonding is with directionality,¹i.e., the N, H and O atoms must be in a line in the supramolecular bisazobenzene group. The unoriented azo chromophore could be pulled (or pushed) by its oriented hydrogen bonding donor or acceptor to an optimized conformation, otherwise the hydrogen bonding is not stable. If this kind of cooperative interaction between the two hydrogen bonding-linked azobenzene groups happens, birefringence may increase spontaneously.

We conclude that the appearance of the hydrogen bonding plays an important role for the enhancement of birefringence and its stability.

Photoinduced birefringence of pAzopy/(AzoCN)_x can be optically erased under the irradiation of circularly polarized laser at 442 nm. The supramolecular bisazopolymers are suitable for grating fabrication and optical recording because of the good optical performances and good film-forming property. Optical recording of the supramolecular bisazopolymer films will be discussed in the next section.

Four-dimensional optical recording of the supramolecular bisazopolymer films

We use the laser direct writing system to fabricate microstructures on the supramolecular bisazopolymer films. The POM images of gratings on supramolecular bisazopolymer films are shown in Fig. 7. These gratings were fabricated on the first day and observed on the second day. The gratings on pAzopy/(AzoCN)_0.5 and pAzopy/(AzoCN)_1.0 are very clear after stored for one day. However, the gratings on pAzopy/(AzoCN)_0.25 become not clear and no grating can be observed on pAzopy after these two samples are stored for one day in summer time (Room temperature is about 30 °C). These results show that the stability of image recording is largely enhanced as the content of AzoCN increases, which agrees with the result of the stability of photoinduced birefringence shown in Fig. 6. It is well known that amorphous azopolymers are not stable for holographic storage.² The instability of pAzopy in image recording is because it is an amorphous azopolymer with low T_g. The gratings on pAzopy/(AzoCN)_1.0 are very stable and can be stored for at least 4 months at room temperature. These gratings are polarization-dependent. When we rotate the sample stage of the POM, the images change from light to dark or from dark to light in every 45°.

Fig. 7 POM images of gratings fabricated by the laser direct writing system. (a) pAzopy, (b) pAzopy/(AzoCN)_0.25, (c) pAzopy/(AzoCN)_0.5 and (d) pAzopy/(AzoCN)_1.0.

We could record images with different polarizations of the laser. We could also record images with the same polarization and different intensities of the laser. The details of these experiments are shown in the ESI.†

We could integrate the polarization and the intensity of the laser and the planar two dimensions to achieve four-dimensional optical recording. Fig. 8 (a) shows a schematic model of a microstructure with 4 different polarizations and 5 different intensities of the laser. Fig. 8 (b)–(g) are the images of the microstructure with different rotated angle under POM observations. Different parts of the microstructure with different polarizations and intensities of the laser can be easily distinguished under POM observation. Generally, a recording data point can only have two states as “0” and “1”. A recording data point on pAzopy/(AzoCN)_1.0 fabricated by the laser direct writing system is different. For a recording data point, our best experimental results are 14 different polarizations (every 3°) and 6 states of different intensities of laser, which is 42 times of the recording density of a normally recording data point. Considering the diameter of the laser at focal plane is about 3 μm, the information density on pAzopy/(AzoCN)_1.0 is about 0.93 Gbit cm⁻², which is about 20 times of the information density of a normal DVD. The optical recording images can be erased by heating and the erased samples can be reused for optical recording. It is worth noting that with enhanced birefringence, the number of states with different intensities and polarizations of laser can be increased. So, it is very important to enhance birefringence of materials for image recording. For further enhancement of the information density, we are currently using a hydrogen bonding donor of small molecular bisazobenzene to combine with pAzopy to form a supramolecular tri-azopolymer which could be with even larger photoinduced birefringence; and we are also trying to use two-photon technology for five-dimensional optical recording on the supramolecular bisazopolymers.

Fig. 8 (a) Schematic model of the four-dimensional patterns with 5 different writing powers and 4 different polarization directions. I₀ = 2 mw, I₁ = 4 mw, I₂ = 6 mw, I₃ = 8 mw and I₄ = 10 mw. (b)–(g) POM images of the four-dimensional patterns with different rotated angles.

Conclusions

We demonstrate the strategy of connection of two azo chromophores via hydrogen bonding to form supramolecular bisazopolymers for the enhancements of photoinduced birefringence, the stability of photoinduced birefringence and the stability of image recording. We integrate polarization and intensity of the laser and the two dimensions of a plane to achieve four-dimensional optical recording on the supramolecular bisazopolymer using a newly designed laser direct writing system. We believe the results of this paper open up new ways for designing photoresponsive polymers and can also inspire new ideas for designing of functional optoelectronic materials according to supramolecular interactions. The supramolecular bisazopolymers are promising materials for fabrication of devices because of the good optical performances, good stability, good solubility and good film-forming property. Our current effort is focused on designing new supramolecular azopolymers and fabrication of phase retardation plates and 3D microstructures by using these supramolecular azopolymers.

Acknowledgements

This work is supported by National Natural Science Foundation of China (no. 50703075, 50773075, 50533040 and 50875251), the Chinese Academy of Sciences (kjcx3.sywH02 and kjcx2-yw-m11), and National Basic Research Program of China (no. 2006cb302900). We thank Prof. C. Bubeck (Max Planck Institute for Polymer Research, Mainz) for helpful discussions.

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Footnote

† Electronic supplementary information (ESI) available: ¹H-NMR spectra, UV-vis absorption spectra, POM images, TGA, DSC, EOM performance and models. See DOI: 10.1039/c000073f

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