Heteroaryl-linked norbornadiene dimers with redshifted absorptions

Abstract

Development of Molecular Solar Thermal (MOST) systems for harvesting and storing solar energy is based on molecular photoswitches that undergo photoisomerizations to metastable isomers. One challenge is to achieve low-molecular weight molecules that absorb at sufficiently long wavelengths to match the solar spectrum. Here we show that this can be achieved by linking two norbornadiene (NBD) photoswitches to a central heterocycle, thiophene or carbazole, via alkyne appendages. In this approach, the same heteroaryl is used to tune the properties of two photoswitches at the same time, thereby keeping the molecular weight as low as possible. A series of NBD dimers was prepared by Sonogashira coupling reactions, and these compounds showed remarkable redshifted absorptions, with onsets of absorption as high as 468 nm, and thermal half-lives ranging from 44 seconds to 16 hours.

Molecular photoswitches in aqueous environments

“…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.…”

“…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. 407 To the best of our knowledge, the compound's photochromism was never studied in aqueous environment and the acid-sensitivity 396 of the quadricyclane as well as small geometrical changes upon isomerization 387 limit its potential applications.…”

Smart Photochromic Materials Triggered with Visible Light

“…Diaryl‐substituted NBDs, also switchable with visible light (<462 nm), have the thermal stability of the respective QCs in the range of days‐to‐months [103] . Heteroaryl‐linked NBD dimers switch below 468 nm (QC lifetime up to 16 h), but due to lower MW are better suited for solar thermal energy storage (increased energy storage capacity per mol) [104] . Dithiafulvalene linker can raise the absorption onset as high as 556 nm, but these compounds are more sensitive on photodegradation, and photoisomerization reversibility is in some cases compromised [105] …”

Investigation of the Structural and Thermochemical Properties of [2.2.2]-Bicyclooctadiene Photoswitches

“… In these systems, the energy is stored in the photoisomer, and when triggered, the energy is released and the parent compound is recovered, ready to be recharged by light (Figure ). Several molecular systems, including azobenzene, − anthracene, tetracarbonyl-fulvalene-diruthenium, , dihydroazulene/vinylheptafulvene (DHA/VHF), − and norbornadiene/quadricyclane, − have been studied for MOST applications. Among these, one of the top candidates is the norbornadiene/quadricyclane (NBD/QC) system, where the NBD undergoes a photoinduced [2 + 2] cycloaddition to the photoisomer QC.…”