Abstract
AbstractDonor–acceptor Stenhouse adducts (DASAs) are visible‐light‐responsive photoswitches with a variety of emerging applications in photoresponsive materials. Their two‐step modular synthesis, centered on the nucleophilic ring opening of an activated furan, makes DASAs readily accessible. However, the use of less reactive donors or acceptors renders the process slow and low yielding, which has limited their development. We demonstrate here that 1,1,1,3,3,3‐hexafluoro‐2‐propanol (HFIP) promotes the ring‐opening reaction and stabilizes the open isomer, allowing greatly reduced reaction times and increased yields for known derivatives. In addition, it provides access to previously unattainable DASA‐based photoswitches and DASA–polymer conjugates. The role of HFIP and the photochromic properties of a set of new DASAs is probed using a combination of 1H NMR and UV/Vis spectroscopy. The use of sterically hindered, electron‐poor amines enabled the dark equilibrium to be decoupled from closed‐isomer half‐lives for the first time.
Highlights
In recent years, photochromic molecules have found increased attention because of their ability to dynamically control physical and chemical properties with high spatial and temporal resolution.[1,2] The incorporation of these photoresponsive molecules into materials has led to a range of developments from molecular machines to life-science applications.[3,4,5,6] Critical to advancing these applications has been the ability to optimize the photochromic properties, such as absorption profile, quantum yield, and the thermal stability of the metastable isomers through synthetic structural modification
Initial experiments were conducted on the effect of HFIP as a cosolvent with dichloromethane for the reaction of 4,4’dimethoxydiphenylamine with the furan adduct derived from Meldrums acid. 4,4’-Dimethoxydiphenylamine is unreactive under the standard reported conditions for Donor–acceptor Stenhouse adducts (DASAs) formation with this acceptor.[13]
Rapid color formation was observed by the naked eye, supporting the formation of DASA-1 and the promising ability of HFIP to promote the synthesis of DASAs. 1H NMR and UV/Vis kinetic studies were used to confirm this qualitative evaluation
Summary
Photochromic molecules have found increased attention because of their ability to dynamically control physical and chemical properties with high spatial and temporal resolution.[1,2] The incorporation of these photoresponsive molecules into materials has led to a range of developments from molecular machines to life-science applications.[3,4,5,6] Critical to advancing these applications has been the ability to optimize the photochromic properties, such as absorption profile, quantum yield, and the thermal stability of the metastable isomers through synthetic structural modification. Reactions can take up to three weeks to reach full conversion, even with moderately reactive furan adducts derived from Meldrums acid.[17,18] Attempts to promote the ring-opening reaction by using excess of the amine component leads to partial degradation of the DASA This results from nucleophilic attack on the triene and/or 1,4-addition to the cyclopentenone closed form.[19,20] Besides long reaction times, purification can be a major challenge. Common purification methods such as column chromatography are often lowyielding, predominantly because of the conversion between the open and closed form that occurs during purification This challenge is highlighted by the low yields (typically < 50 %)[11,12] observed for the synthesis of second generation DASAs. Decreasing the required reaction time, simplifying the purification of DASAs, and expanding the design space to enable the use of unreactive donors or acceptors would, further expand the utility of this new class of photochromes.
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