Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark.
Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Kemivägen 10, 412 96 Gothenburg, Sweden.
Org Biomol Chem. 2019 Sep 7;17(33):7735-7746. doi: 10.1039/c9ob01545k. Epub 2019 Aug 6.
The introduction of various photochromic units into the same molecule is an attractive approach for the development of novel molecular solar thermal (MOST) energy storage systems. Here, we present the synthesis and characterisation of a series of covalently linked norbornadiene/dihydroazulene (NBD/DHA) conjugates, using the Sonogashira coupling as the key synthetic step. Generation of the fully photoisomerized quadricyclane/vinylheptafulvene (QC/VHF) isomer was found to depend strongly on how the two units are connected - by linear conjugation (a para-phenylene bridge) or cross-conjugation (a meta-phenylene bridge) or by linking to the five- or seven-membered ring of DHA - as well as on the electronic character of another substituent group on the NBD unit. When the QC-VHF system could be reached, the QC-to-NBD back-reaction occurred faster than the VHF-to-DHA back-reaction, while the latter could be promoted simply by the addition of Cu(i) ions. The absence or presence of Cu(i) can thus be used to control whether heat releases should occur on different or identical time scales. The experimental findings were rationalized in a computational study by comparing natural transition orbitals (NTOs). Moreover, the calculations revealed an energy storage capacity of 106-110 kJ mol of the QC-VHF isomers, which is higher than the sum of the capacities of the individual, separate units. The major contribution to the energy storage relates to the energetic QC form, while the major contribution to the absorption of visible light originates from the DHA photochrome; some of the NBD-DHA conjugates had absorption onsets at 450 nm or beyond.
将各种光致变色单元引入同一个分子中是开发新型分子太阳能热（MOST）储能系统的一种有吸引力的方法。在这里，我们通过 Sonogashira 偶联作为关键的合成步骤，展示了一系列共价连接的降冰片二烯/二氢苊（NBD/DHA） 化合物的合成和表征。发现完全光异构化的四环烷/乙烯庚富烯（QC/VHF）异构体的生成强烈依赖于两个单元的连接方式——通过线性共轭（对苯撑桥）或交叉共轭（间苯撑桥），或者与 DHA 的五环或七元环连接——以及 NBD 单元上另一个取代基的电子特性。当可以达到 QC-VHF 系统时，QC 到 NBD 的反向反应比 VHF 到 DHA 的反向反应更快，而后者可以通过添加 Cu(i) 离子来促进。因此，Cu(i) 的存在或不存在可以用来控制热量是否应该在不同或相同的时间尺度上释放。实验结果通过比较自然跃迁轨道（NTOs）在计算研究中得到了合理化。此外，计算结果表明，QC-VHF 异构体的能量存储容量为 106-110 kJ mol，高于单个分离单元的容量之和。能量存储的主要贡献来自于能量丰富的 QC 形式，而可见光吸收的主要贡献则来自于 DHA 光致变色体；一些 NBD-DHA 化合物的吸收起始波长在 450nm 或更远处。
