Photo-induced isomerization of organic compounds has shown an excellent approach for utilizing solar energy. In this category, azobenzene can undergo a photo-induced change from low energy trans-isomer to high-energy cis-isomer through the absorption of photons at a particular wavelength. The resulting cis-isomer can reverse back trans-isomer owing to its low thermodynamic stability of when exposed to external stimulus, for example, heat and light.
The reversible photo isomerization attributes makes Azo-based element to be a crucial building block for photo-thermal fuels in view of its capacity to store light energy in chemical bonds and release it later as heat energy. However, the implementation of the photochromic azo compounds as photo-thermal fuels is greatly hindered by their low exothermicity as well as low activation barrier to thermal reversion. For this reason, most research works have been based on ways to enhance isomerization enthalpy and half-life through the design of several substituents.
Multi-branched Azo molecules have exhibited potential for application in photo-thermal fuels thanks to their intermolecular interactions. The isomerization is limited by large steric hindrance, which in turn increases the half-life. The intermolecular hydrogen bonds lead to an increase in isomerization enthalpy counting on the decreased energy of the trans-isomer. For this reason, tuning the steric configuration of the multi-branched Azo molecules is fundamental in the occurrence of a number of molecular interactions.
Researchers led by Professor Wei Feng at the Tianjin University in China presented a template assembly of bisazobenzene that was grafted covalently onto reduced graphene oxide. The two Azo assemblages with varying branched structures were synthesized to analyze the impact of the molecular interactions on the photo-thermal attributes. Their research work is published in ChemSusChem.
The authors prepared the reduced graphene oxide-bisazobenzene solution where it was then irradiated with Ultraviolet light in order to induce trans-to-cis isomerization. This was continuously done until the photo-stationary was noted. The absorbed energy was stored in metastable cis-isomer of the azo benzene on the reduced graphene oxide.
The resulting graphene oxide-bisazobenzene films were then irradiated with the same UV light until a photo stationary state was realized. Long duration irradiation was implemented to initiate trans-to-cis isomerization of azobenzene in the film owing to steric hindrance.
The research team successively prepared uniform photo-thermal fuel implementing a close packed graphic oxide-bisazobenzene. The grafting density was set at 1/23. Reduced Graphene oxide-bisazobenzene-2 posted high energy density of approximately 131Whkg-1, a power density of 2517Wkg-1. The compound also posted a long half-life of about 37days with good cyclic performance for about 50 cycles reference to inter- and intramolecular hydrogen bonding and steric performance.
The low isomerization in the solid-state graphene-based bisAzobenzene films led to energy density decrease of about 25% from what was reported for powder sample reference to steric hindrance. The authors also investigated a closed cycle of UV radiation, storage and heat release of the resulting photo-thermal. Graphene oxide based Azobenzene films were able to release and accumulate heat to realize a maximum temperature difference of 15°C. However, the films were observed to retain a temperature difference of more than 10° for about 30 minutes when the temperature difference on the environment was over 100° C.
From the results of their study it was concluded that molecular engineering for high-energy storage as well as an optimized microstructure for high degree isomerization are necessary for high-performance photo thermal fuels. The ability to tune heat release in the solid-state assembly bears a groundbreaking mechanism for developing photo thermal fuels into functional gadgets.
Xiaoze Zhao, Yiyu Feng, Chengqun Qin, Weixiang Yang, Qianyu Si, and Wei Feng. Controlling Heat Release from a Close-Packed Bisazobenzene–Reduced-Graphene-Oxide Assembly Film for High-Energy Solid-State Photothermal Fuels. ChemSusChem 2017, 10, 1395 – 1404.Go To ChemSusChem