Photothermal effect of azopyridine compounds and their applications

Abstract

Photothermal effect of azopyridine compounds and their azobenzene analogs were studied systematically. Under the same condition, the azopyridine compounds showed higher photothermal conversion efficiency than their analogs, and both of their photoresponsiveness were greatly accelerated when the intensity of irradiated UV light was higher than 400 mW cm⁻². Two possible applications were proposed for the photothermal effect of azopyridine compounds. One was intelligent stickup technology, and the tensile strength between two glass sides was photocontrolled to be as high as 0.661 MPa. The other was photo-driven movement with bilayer films composed of one A12AzPy/PVA composite film, and photoinduced bending was successfully observed due to volume change caused by the photothermal effect.

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Introduction

Light irradiation is a fascinating tool to adjust material properties, which can be controlled remotely, instantly and precisely.^1–5 When matter absorbs light, thermal energy is often produced due to the energy conversion from photons of the actinic light. This is so-called photothermal effect, which includes a wide range of techniques and phenomena through the conversion of absorbed light into thermal energy.⁶ Recently, divers of nanomaterials such as carbon nanotubes (CNTs),^7–9 gold nanoparticles¹⁰ and graphene¹¹ represent a class of photothermal materials. They are often adopted to convert near-infrared (NIR) radiation to vibrational energy, thus generating heat sufficient for therapeutic applications or soft robotics.^7–11

Azobenzene compounds are another class of photothermal materials. Upon photoirradiation of azobenzene-containing materials, two unique photoresponsive features are often obtained. One is the increase in temperature due to the photothermal effect; the other is the geometric change in molecular configuration owing to the trans–cis isomerization. The combination of these two effect has been utilized for diverse photoactuators.^12–17 Lee and White pointed out that the contribution of absorptive light based on the intensity can be ascribed into two regimes: photochemical (<100 mW cm⁻²) and photochemical/photothermal (>100 mW cm⁻²).^18,19 Compared to inorganic nanomaterials, one of the most fascinating properties of azobenzene compounds is that they can be easily miscible with organic materials by simple blending or chemical bonding. This enables them to be superior to inorganic nanomaterials in organic systems.

Azopyridine derivatives show similar molecular structures to azobenzene compounds except that only one carbon atom at the para-position of one benzene ring is replaced by nitrogen atom. This small modification of chemical composition seldom changes the topological space, enabling azopyridine derivatives to exhibit similar light-activity related to azobenzene compounds. On the other hand, the introduction of the nitrogen atom endows azopyridine derivatives with extraordinary features such as self-assembly through pyridyl groups,^20–29 Recently, supramolecularly self-assembled nanofibres showing conductivity and hydrogen-bonded or halogen-bonded liquid crystals with photoinduced phase transition properties have been fabricated based on hierarchical assembly of azopyridine derivatives.^20–23 However, there are few reports about the photothermal effect of azopyridine compounds till now. In this paper, we report the comparison of photothermal effect between azopyridine compounds and their analogous azobenzene compounds. Meanwhile, we try to explore potential applications for the photothermal effect of azopyridine derivatives, e.g., intelligent stickup technology and photo-driven movement with azopyridine/polymer composite films.

Experimental section

Materials and characterization

The synthesis and characterization of azobenzene and azopyridine compounds are shown in the Schemes 1 and S1.† The fabrication process of the bilayer film is given in the ESI.† Photothermal effect was studied using light source with 365 nm (BANGWO, Guangzhou Co., Ltd, FUV-6BK NO: 322015).

Scheme 1 Chemical structures and their abbreviations of the azobenzene compounds (AzBz and A12AzBz) and azopyridine derivatives (AzPy and A12AzPy) used in this study.

¹H-NMR spectra of the compounds were recorded on a Bruker Avance III 400. Differential scanning calorimetry (DSC) examination was carried out on a Perkin-Elmer DSC 8000 with a heating and a cooling rate of 10 °C min⁻¹ under a dry nitrogen purge. Polarizing optical microscope (POM) observations were performed with Zeiss Axio Scope A1 microscope equipped with a hot stage (Linkam LTSE420). Fluorescence quantum yield was measured using a Perkin-Elmer LS55 luminescence spectrometer. The infrared images were captured with FLIR A615 (Fluke).

The photothermal effect of all the samples with different amount was quantitatively tested at room temperature. In all the cases, the samples in the powder form were filled into one container with a diameter of 7 mm to make sure that they are fixed at the same area but different thickness. The thickness was adjusted by changing the sample amount. The temperature was taken by measuring the middle area of the sample. For example, the difference between the 5 mg sample and 15 mg sample is the change in thickness, which means the 15 mg sample is much thicker than 5 mg sample in the same area.

The process of the two examples of envisioned applications as follows

The first application is photocontrollable adhesion. A certain amount of A12AzPy powder was put on the glass slide and then irradiated with UV light. Next, put another glass slide on the melted samples, and then the two glass slides were sticky together. Tensile testing of A12AzPy samples was carried out using rectangular specimens with a size of 100 mm × 25 mm × 1.6 mm using WDW-1 testing machine at a crosshead speed of 5 mm min⁻¹. The tensile modulus and elongation at the break of the sample were calculated from the load–displacement curve. At least five specimens were tested for each set of samples and the mean values were reported.

The second application is photoactuators. The poly(vinyl alcohol) (PVA) aqueous solution was obtained by dissolving 10 g PVA in 90 g water. Another A12AzPy/PVA solution was obtained by dispersion of a THF solution of A12AzPy in the obtained PVA aqueous solution and then evaporation of the THF to make sure that A12AzPy has 0.3 wt% comparing to PVA. Then the substrate layer was achieved by casting the PVA solution on a glass substrate and evaporation of the water. The second layer was obtained by casting the A12AzPy/PVA solution on the first PVA layer. After evaporating the residual water at room temperature, the A12AzPy/PVA bilayer film was obtained by being peeled off from the glass substrate. The thickness of the film was measured using a micrometer. Typical thickness of the A12AzPy/PVA bilayer was approximately 150 μm.

Results and discussion

To elucidate the underlying photothermal effect of the azobenzene and azopyridine compounds, we used an infrared camera to in situ measure the real-time change in temperature of the samples upon UV irradiation at 365 nm. Fig. 1a shows the change in temperature of the azobenzene and azopyridine compounds with the same amount (5 mg) as a function of exposure time to UV light. For irradiation with intensity of 400 mW cm⁻², the temperature of the azobenzene and azopyridine compounds increased rapidly at the initial 5 s and gradually reached a plateau after 10 s. The highest temperatures of the AzPy, AzBz, A12AzPy, A12AzBz and empty sample were 71.9, 66.6, 66.3, 61.9 and 29.8 °C, respectively. Obviously, the azopyridine compounds exhibited a higher light-to-thermal conversion efficiency compared with the corresponding azobenzene compounds in the same condition, which should be ascribed to the difference between the pyridine ring and the benzene ring.

Fig. 1 Photothermal effect of azobenzene and azopyridine derivatives at room temperature. (a) Plots of temperature change for different compounds with the same quality of 5 mg as a function of photoirradiation time (365 nm, 400 mW cm⁻²). (b) Plots of temperature change for A12AzPy as a function of exposure time (365 nm, 400 mW cm⁻²) for different amount, and the samples were 5 mg, 10 mg and 15 mg, respectively. (c) Plots of temperature increase for different UV intensity (365 nm) with the same amount of 5 mg as a function of UV irradiation time.

To further study the photothermal effect of A12AzPy, the influence of sample amount on the induced temperature was investigated in detail. Three samples of 5, 10 and 15 mg were in situ measured and the results are given in Fig. 1b. The highest temperature of the irradiated sample increased with the more used amount (more thickness) of A12AzPy, indicating that the sample quantity had effect on the photothermal efficiency. For 15 mg A12AzPy, 69 °C was obtained with irradiation of 10 s. The heat produced from photothermal effect can be divided into two parts. One part was used for increasing the temperature of the samples; the other part was released to the external environment. The more quantity samples used, the more slowly for the latter part released to the external environment. Thus, more quantity of sample produced much higher thermal energy, leading to a higher temperature of irradiated samples.

Since it has been reported that the intensity of the actinic light is very important for the photothermal effect,^18,19 the change in temperature of the samples as a function of photoirradiation time was investigated by changing the intensity of UV light (365 nm). Fig. 1c shows the experimental results of UV light with different intensity using the same amount of 5 mg. Obviously, the temperature of A12AzPy samples increased with a stronger intensity. The highest temperature of the irradiated sample with 100, 200, 300, 400, 500 and 1000 mW cm⁻² were 38.4, 51.4, 59.5, 66.3, 71.3 and 82.3 °C, respectively. This indicates that the intensity of the actinic light shows great influence on the photothermal effect of A12AzPy samples.

Fig. 2 shows the UV-Vis absorption spectra of the azobenzene or azopyridine compounds in THF solutions and as-cast films, and the maximum absorption peaks of these samples are summarized in Table 1. In THF solutions, the maximum absorption wavelengths of AzPy, AzBz, A12AzPy and A12AzBz were 357, 350, 356 and 349 nm (Fig. 2a), respectively. Due to the existence of the π–π* transitions of azobenzene or azopyridine groups,^20–29 these maximum absorption wavelengths of the as-cast films shifted to 419, 354, 308 and 302 nm, respectively (Fig. 2b). Obviously, the azopyridine compounds had certain red shift with regard to that of the azobenzene compounds. This is the result of the introduction of the nitrogen atom, which has the lone pair electrons in the pyridine ring, participating in the conjugate of the benzene. Thus, this may be helpful for promotion to generate thermal energy more efficiently than the azobenzene ring. Meanwhile, both reflectance and transmittance of the film samples were measured (see Fig. S2†), because only absorbed light will be converted into heat. The absorptivity of the AzBz, A12AzBz, AzPy and A12AzPy are 12.5%, 40.1%, 11.6% and 21.1%, respectively. Because the sample film for measurement was very thin and the photothermal effect were not obvious, their temperatures were a little higher than room temperature (24.6 °C) under UV light with intensity of 200 mW cm⁻². The temperature at the irradiated spot on AzPy, AzBz, A12AzPy, A12AzBz films and the empty sample were 32.0 °C, 30.4 °C, 30.2 °C, 29.8 °C, and 28.2 °C, respectively (see Fig. S2†). In general, the photothermal effect of materials is related with both the absorptivity and the maximum absorption band. Although A12AzBz and A12AzPy exhibited higher absorptivity at the maximum absorption band, they still showed lower light-to-thermal conversion efficiency because the 365 nm UV light was used. Furthermore, in order to study the efficiency of generation of thermal energy, the fluorescence quantum yields were measured and shown in Fig. S3 and Table S1.† The azobenzene compounds and azopyridine compounds were almost non-fluorescent in solution at room temperature, which is in accordance with the reported in the literature.^30,31

Fig. 2 UV-Vis absorption spectra of the azobenzene compounds (AzBz and A12AzBz) and azopyridine derivatives (AzPy and A12AzPy) in THF solutions (a) and in film states (b). (c) is the UV-Vis absorption spectra of A12AzPy casting films before and after UV irradiation.

Table 1 Summary of the maximum absorption of azobenzene and azopyridine compounds in THF solutions and in films, respectively

Sample	AzPy	AzBz	A12AzPy	A12AzBz
In THF (nm)	357	350	356	349
In film (nm)	419	354	308	302

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Fig. 2c shows the change in the UV-vis absorption spectra of A12AzPy films when exposed to UV light (365 nm, 1000 mW cm⁻²). The absorption of the maximum peak at 308 nm attributed to the π–π* transition of A12AzPy decreased from 0.45 to 0.22 after 10 s exposure. At the same time, the absorption peak at about 460 nm due to the n–π* transition of A12AzPy increased a little. Obviously, trans–cis isomerization still occurred in this UV irradiation process.^32–34 The spectral change indicates that both the photothermal effect and the photoisomerization process existed simultaneously upon UV irradiation with a high intensity.

After confirming the high photothermal efficiency of azopyridine compounds, we attempted to explore the applications of the photothermal effect of azopyridine compounds. First of all, A12AzPy was applied in intelligent stickup technology due to its immersional wetting of the long alkyl chain length. Moreover, they can be repeated for multiple cycles.

Fig. 3 shows the photothermally induced adhesive of A12AzPy under UV irradiation (365 nm, 1000 mW cm⁻²). The A12AzPy powder was first put on one glass slide and then irradiated. The temperature of the powder was 26 °C without UV irradiation and it rapidly increased to 99 °C upon UV irradiation, which is higher than the melting point of A12AzPy (72 °C). Next, put another glass slide on the melting powder, and then the two glass slides were stick together. The photographs and infrared images of the A12AzPy as adhesive before and after UV irradiation are shown in Fig. 3 and 4, respectively. ESI Video S1† shows the cycles of intelligent stickup technology based on the photothermal A12AzPy samples.

Fig. 3 Application of A12AzPy as photocontrolled adhesive upon UV irradiation (365 nm, 1000 mW cm⁻²). Onset the curve are the photos of two glass slides connected by the A12AzPy powder.

Fig. 4 The infrared images of the A12AzPy as photocontrolled adhesive before (a) and (b) after UV irradiation.

The strength of the adhesive joint can be characterized by the work of detachment per unit area of interface, which is given by the equation:

where P is the tensile strength (MPa) and F is the mean value of tensile force (N); S is the shear area (mm²). The tensile strength of the A12AzPy was evaluated as 0.661 MPa, which was comparable to that of conventional double-stick tape.^35,36 So it has the promising of using as intelligent stickup adhesive. The mechanical force produced by the adhesive A12AzPy is shown in Fig. S4 and Table S2.† Recently, Akiyama et al. have used multi-azobenzene sugar-alcohol derivatives as reworkable adhesives, and it exhibited somewhat stronger adhesion than A12AzPy, which is comparable to conventional adhesives.³⁵ Meanwhile, the storage modulus of the azobenzene microparticle/liquid-crystal composite gels has also been investigated by Yamamoto and Yoshida, and it could reached over 104 Pa.³⁶ This work shows that the crystalline A12AzPy samples may exhibit similarly photocontrollable behaviour to that of liquid-crystal samples.³⁶

As the second application of the photothermal effect of azopyridine compounds, we tried to fabricate light-driven actuators using A12AzPy and PVA composite films because of the highly miscible property of the small molecular A12AzPy with organic polymer materials. The idea of converting the input light energy into output mechanical work in a polymer matrix was first discussed in 1967.³⁷ Light can drive the movement of microscopic objects based on the photo-manipulated materials, making them promising for possible photoactuator applications.^37–39

The distribution of A12AzPy in PVA of the A12AzPy/PVA layer is shown in Fig. S5.† Fig. 5 shows photoresponsive results of A12AzPy/PVA bilayer before and after UV irradiation. One end of the bilayer was fixed on one substrate and then the other end was irradiated from the left side. Closing to the light source was the pure PVA layer, and the contrary side was the A12AzPy/PVA composite layer containing 0.3 wt% A12AzPy. Since the PVA layer is transparent, UV light can pass it through and irradiate the A12AzPy/PVA layer. Thus, photothermal effect occurred only in the A12AzPy/PVA layer. The temperature was 27.2 °C before irradiation and increased to 77.3 °C upon UV irradiation. Then the film slowly bent towards the light source as shown in Video S2,† which is far difference from the liquid-crystalline polymer films showing an anisotropic ordering of mesogens with a cooperative effect.^{1–5,12–17,40} This change in shape of the bilayer was due to the volume expansion of the A12AzPy/PVA layer caused by the light-to-thermal energy conversion.^1–5,41,42 Since little change was produced in the pure PVA layer because of no photoresponsvie chromophores existed inside, the bilayer film bent towards to the actinic light source following the bimetal mechanism, as shown in Fig. 5c. When pure PVA films without A12AzPy were used, the increase of the temperature was not clearly observed and this photoinduced bending movement was not achieved certainly.

Fig. 5 Application of the A12AzPy/PVA composite films as photoactuator. (a) Scheme of the A12AzPy/PVA bilayer film (25 × 5 × 0.1 mm) undergoing bending toward the light upon the UV irradiation. (b) Infrared images of the A12AzPy/PVA bilayer before and under UV illumination (365 nm, 1000 mW cm⁻²). (c) Optical images of A12AzPy/PVA bilayer before and under UV irradiation (365 nm, 1000 mW cm⁻²).

In both cases of the applications of azopyridine compounds, they showed good photothermal effect upon UV irradiation. It is the thermal energy produced due to the transformation from the actinic light that results in the two possible applications of photocontrolled adhesion or photomechanical behaviours.

Conclusions

The photothermal effect of azopyridine compounds and their azobenzene analogs was studied upon UV irradiation with a high intensity. Both the amount of samples and the intensity of the actinic light showed great influence on the photothermal behaviours. In our experimental system, the azopyridine derivatives had higher light-to-thermal conversion efficiency than azobenzene compounds. Then, two possible applications for the photothermal effect of azopyridine compounds were tentatively carried out. One was the photocontrolled adhesive using A12AzPy powder and the tensile strength between two glass slides reached to 0.661 MPa. The other possible application was photomechanical work of A12AzPy/PVA bilayer film, and the photoinduced bending was successfully obtained because of the volume change caused by the photothermal energy conversion.

Acknowledgements

Yang thanks for the support from the Major Project of International Cooperation of the Ministry of Science and Technology (Grant no. 2013DFB50340), the Key Project of the National Natural Science Foundation of China (Grant no. 51333001), the Doctoral Fund of Chinese Ministry of Education (Grant no. 20120001130005) and the Major Program of Chinese Ministry of Education (Grant no. 313002). Yu acknowledges the National Natural Science Foundation of China (Grant no. 51322301) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry for the support.

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Footnote

† Electronic supplementary information (ESI) available: The synthesis of compounds (A12AzBz, AzPy and A12AzPy), fabrication of A12AzPy/PVA bilayer and the mechanical properties of A12AzPy as adhesive. See DOI: 10.1039/c4ra11844h

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