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Abstract
This work investigates the effect of ultrasound on switching of cis azobenzene isomers to their trans counterparts in solid films of methyl methacrylate and methacryloyloxyazobenzene copolymers [P(MMA/MOAB)]. Ultraviolet–visible and 1H nuclear magnetic resonance spectroscopies demonstrate that 46% of the cis isomer converts to the trans form purely by ultrasonic agitation and 46% converts to the trans isomer by localized ultrasound-induced heating effects. Comparative studies of isomerization by ultrasound wave, heat, and visible irradiation show that ultrasound exposure requires a longer time to switch the cis-to-trans conformation. The estimated activation energy for the cis-to-trans conversion in solid polymer films is shown to be comparable to previous values of azobenzene isomerization, indicating that incorporation of the chromophore in a polymeric system affects the kinetics of transition but not the barriers to conformational change.
Highlights
The compositions of the polymers used in this investigation are shown in Table 1; a broader table of characteristics are shown in Table S1, and the AB loading in each polymer was established by 1H nuclear magnetic resonance (NMR)
The spectrum has two distinct absorption bands: 318 nm which is attributed to the trans isomers due to the π−π* transition state and 440 nm corresponding to the cis isomers due to the forbidden n−π* transition.[25]
Our findings show that c-AB → t-AB isomerization in solid polymer films can be triggered ultrasonically, but it increases the temperature of the sample, which can enhance both c-AB → t-AB and t-AB → c-AB
Summary
It is well-established that stimuli-responsive molecules can change their molecular shape under appropriate stimuli such as light, temperature, pH, and mechanical force, allowing the control of physical, optical, and even mechanical properties of the host materials.[1,2] The use of mechanical energy to trigger molecules into different forms is of great interest because of the possible applications in optical switching and data storage, liquid crystal displays, molecular machines, surface relief gratings, nanodevices, and nonlinear optics.[3−10] One group of such materials are the azobenzene (AB)-based chromophores, which can reversibly alter their isomeric forms between trans (planar, t-AB) and cis (twisted, c-AB) geometries in response to external stimuli such as light and heat.[11−15] In glassy polymers such as methacrylic polymers, isomerization occurs through the inversion of the azo (−N N−) bond because this mechanism requires a smaller free volume than rotation.[16] In the case of AB-based polymers, the distance between the two benzene rings of the AB chromophore is altered reversibly by 39% from 9 to 5.5 Å,17,18 resulting in associated changes in the mechanical properties of a polymer matrix (Figure 1). For everyday usage of these types of materials, for example, in opto-mechanical devices[21,22] and solar thermal energy storage,[23] these materials need to be in a solid form; it is important to identify the nature of the cis → trans (c-AB → t-AB) isomerization by ultrasound in solid polymers rather than in liquid
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