Tuning Molecular Solar Thermal Properties by Modification of a Promising Norbornadiene Photoswitch

Abstract

Carefully targeted modifications to the norbornadiene‐quadricyclane couple should give molecules with great potential for solar energy storage. Here we report the synthesis of derivatives of a well‐studied compound, 2‐cyano‐3‐anisyl norbornadiene (NBD5). The conjugation was extended by conversion of the nitrile into acrylonitrile and acrylate derivatives over two steps. Despite red‐shifting the absorbance properties, the metastable quadricyclanes exhibited extremely short lifetimes. Meanwhile, treatment of NBD5 with acetyl halides in the presence of aluminum trihalides led to the formation of 7‐acetyl norbornenes through a Wagner–Meerwein rearrangement. Subsequent transformations made it possible to synthesise norbornadienes with varying degrees of steric bulk at the 7‐position of the molecular scaffold. It was found that the bulkier this group was, the more stable the quadricyclane form. This reaction sequence allows for the synthesis of norbornadiene derivatives which are more suited to molecular solar thermal applications in domestic heating despite providing a slightly lower energy density.

Scalable Synthesis of Norbornadienes via in situ Cracking of Dicyclopentadiene Using Continuous Flow Chemistry

“…Data consistent with literature. [15] 2,2,2-trifluoro-3-(4-methoxyphenyl)bicyclo[2.2.1]hepta-2,5-dien-2-yl)ethan-1-one 16: Alkyne 25 (2.16 g, 9.47 mmol, 1.0 M), dicyclopentadiene (1.50 g, 1.52 mL, 11.4 mmol, 1.21 M) were mixed in 7.9 mL of toluene. The solution was pumped to a 5 mL reactor at 200 °C with a flow speed of 0.33 mL/min, a residence time of 15 min.…”

Molecular photoswitches in aqueous environments

“…108 The formed quadricyclane absorption is strongly hypsochromically shifted compared to the starting material. [387][388][389] The isomerization can be reverted photochemically, 390 thermally, or (electro)chemically, [391][392][393] for instance by adding a metal catalyst 394,395 or acid, 396 and (photo)switching is very robust over many cycles. 388,397,398 Norbornadiene and quadricyclane are similar in size and volume and therefore do not offer many possibilities for applications where the photoswitch should induce large geometrical changes in a system.…”

“…27B), the absorption profile of norbornadiene can be tuned up to the green region of the visible spectrum. 389,[404][405][406] The thermal lifetime of quadricyclane strongly depends on the substituents and the solvent or matrix. 398,402,405,406 Most studies naturally focus on characterizing the compounds' properties in film or toluene, however, photoisomerization was also reported in MeCN, revealing lower QYs F NB-QC (0.03 instead of 0.28) than in toluene, but longer thermal lifetimes (t 1/2 = 29 h instead of h) for the same compound.…”

Synthesis, characterization and computational evaluation of bicyclooctadienes towards molecular solar thermal energy storage

“…For our next series of donor/acceptor BODs, we want to introduce bulky substituents on the bridge with the aim of improving the storage times, which previously has been a successful strategy for prolonging the half-lives of QCs. 40,41 We envision that the established computational framework and the molecular engineering concepts presented here will be helpful in developing future functional MOST systems, based on the BOD/TCO based photoswitch system.…”