This paper demonstrates a metal–organic framework (MOF) containing photoswitches within the pores as a hybrid solar thermal fuel (STF) and solid–solid phase-change material (ss-PCM). A series of azobenzene-loaded MOFs were synthesized with the general formula Zn₂(BDC)₂(DABCO)(AB)_x (BDC = 1,4-benzenedicarboxylate, DABCO = 1,4-diazabicyclo[2.2.2]octane, AB = azobenzene, where x = 1.0, 0.9, 0.5, 0.3), herein named 1⊃AB_1.0, 1⊃AB_0.9, 1⊃AB_0.5, and 1⊃AB_0.3 respectively. X-ray powder diffraction, solid-state NMR, and density functional theory calculations were used to explore in detail the structural changes of the host framework that take place upon loading with the AB guest molecules. Differential scanning calorimetry measurements reveal a reversible phase change, which is absent from the evacuated framework. Upon irradiation with 365 nm light, 40% of the AB guests converted from the trans to the higher-energy cis isomeric form in 1⊃AB_1.0. The energy stored within the metastable cis isomers is released upon heating and balances the endotherm associated with the phase transition. However, the exotherm associated with the phase transition is retained upon cooling, resulting in a net energy release over a full heating–cooling cycle. The maximum energy density is observed for the fully loaded composite 1⊃AB_1.0, which releases 28.9 J g^–1. In addition, the cis-AB guests in this composite showed negligible thermal reconversion during 4 months at ambient temperature, with an estimated energy storage half-life of 4.5 years. Further development of MOF-based STF-ss-PCMs could lead to applications for solar energy conversion and storage, and thermal management.

中文翻译：

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基于MOF的固-固相变材料在环境条件下的长期太阳能存储

本文演示了一种金属-有机框架（MOF），其中包含光孔内的光电开关，作为混合太阳能热燃料（STF）和固-固相变材料（ss-PCM）。合成了一系列带有偶氮苯的MOF，其通式为Zn ₂（BDC）₂（DABCO）（AB）_x（BDC = 1,4-苯二甲酸，DABCO = 1,4-二氮杂双环[2.2.2]辛烷，AB =偶氮苯，其中X = 1.0，0.9，0.5，0.3），本文中命名1 ⊃AB _1.0，1 ⊃AB _0.9，1 ⊃AB _0.5，和1 ⊃AB _0.3分别。使用X射线粉末衍射，固态NMR和密度泛函理论计算来详细研究在装载AB客体分子时发生的主体构架的结构变化。差示扫描量热法测量显示可逆的相变，而真空框架没有这种相变。365 nm的光照射后，40％的AB客体以1 toAB _1.0从反式转化为高能顺式异构体。储存在亚稳态顺式中的能量异构体在加热时释放并平衡与相变有关的吸热。但是，与相变相关的放热在冷却时得以保留，从而导致在整个加热-冷却循环中释放出净能量。最大能量密度是观察满载复合1 ⊃AB _1.0，释放28.9Ĵ克^-1。此外，该复合物中的顺式-AB客体在环境温度下4个月内显示的热转化率可忽略不计，能量存储的半衰期估计为4.5年。基于MOF的STF-ss-PCM的进一步开发可能会导致太阳能转换和存储以及热管理的应用。