Tetra- ortho-substituted azobenzenes emerged as privileged light-responsive molecular photoswitches, with good absorption band separation and half-lives of the metastable cis isomer in the range that enables multiple applications. Here we present a systematic spectroscopic and theoretical investigation into the photochemistry of tetra- ortho-chloro-azobenzenes, with the aim to provide a guide for their design. 31- 33 It ultimately allows both the tuning of these properties, and the effective choice of substituents determining the function of the photoresponsive unit in a biological system, material, or a molecular machine. This understanding is enabled through synthesis, spectroscopic studies, and theoretical investigations. The successful application of the new visible-light-responsive photoswitches depends on establishing their design principles, based on the understanding of the interplay between the nature of the substituents and the key photochemical properties. 27- 29 This is relevant especially in biological applications, where red/NIR light enables deep (1 cm) tissue penetration without the toxic effects induced by higher energy light. 26 The development of new molecular photoswitches is largely driven by the challenge of enabling the use of visible, and red or even near-IR (NIR) light for operation in both directions. Furthermore, various novel designs 18 have appeared during the last decade, including donor–acceptor Stenhouse adducts (DASAs), 19, 20 hydrazone 21- and acylhydrazone 22-based switches, BF 2-coordinated azo compounds, 23 diazocines, 24 indigos, 15, 25 and iminothioindoxyls. ![]() The available panel of molecular photoswitches features many established architectures that mainly rely on double bond isomerisation (azobenzenes, 12 azoheteroarenes, 13 stilbenes, 14 hemithioindigos 15), electrocyclisation (diarylethenes 16), or mixed mechanisms (spiropyrans 17). 6, 7 In particular, their potential in biomedical context, along the principles of photopharmacology, 8- 11 evoked considerable interest in recent years. 1 They have found application in remotely manipulating biological systems, 2, 3 smart materials, 4, 5 and molecular machines. Molecular photoswitches form the basis of light-responsive systems that are designed to enable reversible control of function with high spatiotemporal resolution. A set of guidelines is presented that enables tuning of properties to the desired application through informed photochrome engineering. This is achieved through joint photochemical and theoretical analyses of a representative library of molecules featuring substituents of varying electronic nature. Here we provide a design rulebook for tetra-ortho-chloroazobenzenes, an emerging class of visible-light-responsive photochromes, by elucidating the role that substituents play in defining their key characteristics: absorption spectra, band overlap, photoswitching efficiencies, and half-lives of the unstable cis isomers. The lack of clear design principles for the adaptation and optimization of such systems limits further applications. Their development is driven by the need for low energy (green-red-NIR) light switching, to allow non-invasive operation with deep tissue penetration. In some sources it also has an NFPA 704 rating of 4 for health, 4 for flammability, 4 for reactivity and is a potent oxidant, however other sources claim lower ratings of 3-2-2 or 1-4-4.Molecular photoswitches enable reversible external control of biological systems, nanomachines, and smart materials. Ī solution of tert-butyl hydroperoxide and water with a concentration of greater than 90% is forbidden to be shipped according to US Department of Transportation Hazardous Materials Table 49 CFR 172.101. Tert-butyl hydroperoxide is potentially dangerous, but explosions are rare. Many synthetic routes are available, e.g. On a much smaller scale, tert-butyl hydroperoxide is used to produce some fine chemicals by the Sharpless epoxidation. The byproduct t-butanol, which can be dehydrated to isobutene and convert to MTBE. (CH 3) 3COOH + CH 2=CHCH 3 → (CH 3) 3COH + CH 2OCHCH 3 ![]() In the Halcon process, molybdenum-based catalysts are used for this reaction: Industrially, tert-butyl hydroperoxide is used to prepare propylene oxide.
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