Azobenzene undergoes trans→cis isomerization when irradiated with light tuned to an appropriate wavelength. The reverse cis→trans isomerization can be driven by light or occurs thermally in the dark. Azobenzene's photochromatic properties make it an ideal component of numerous molecular devices and functional materials. Despite the abundance of application-driven research, azobenzene photochemistry and the isomerization mechanism remain topics of investigation. Additional substituents on the azobenzene ring system change the spectroscopic properties and isomerization mechanism. This critical review details the studies completed to date on the 3 main classes of azobenzene derivatives. Understanding the differences in photochemistry, which originate from substitution, is imperative in exploiting azobenzene in the desired applications.

Azobenzene undergoes reversible cis<-->trans photoisomerization upon irradiation. Substituents often change the isomerization behavior of azobenzene, but not always in a predictive manner. The synthesis and properties of three azobenzene derivatives, AzoAMP-1, -2, and -3, are reported. AzoAMP-1 (2,2'-bis[N-(2-pyridyl)methyl]diaminoazobenzene), which possesses two aminomethylpyridine groups ortho to the azo group, exhibits minimal trans-->cis photoisomerization and extremely rapid cis-->trans thermal recovery. AzoAMP-1 adopts a planar conformation in the solid state and is much more emissive (Phi(fl) = 0.003) than azobenzene when frozen in a matrix of 1:1 diethylether/ethanol at 77 K. Two strong intramolecular hydrogen bonds between anilino protons and pyridyl and azo nitrogen atoms are responsible for these unusual properties. Computational data predict AzoAMP-1 should not isomerize following S(2)<--S(0) excitation because of the presence of an energy barrier in the S(1) state. When potential energy curves are recalculated with methyl groups in place of anilino protons, the barrier to isomerization disappears. The dimethylated analogue AzoAMP-2 was independently synthesized, and the photoisomerization predicted by calculations was confirmed experimentally. AzoAMP-2, when irradiated at 460 nm, photoisomerizes with a quantum yield of 0.19 and has a much slower rate of thermal isomerization back to the trans form compared to that of AzoAMP-1. Its emission intensity at 77 K is comparable to that of azobenzene. Confirmation that the AzoAMP-1 and -2 retain excited state photochemistry analogous to azobenzene was provided by ultrafast transient absorption spectroscopy of both compounds in the visible spectral region. The isomerization of azobenzene occurs via a concerted inversion mechanism where both aryl rings must adopt a collinear arrangement prior to inversion. The hydrogen bonding in AzoAMP-1 prevents both aryl rings from adopting this conformation. To further probe the mechanism of isomerization, AzoAMP-3, which has only one anilinomethylpyridine substituent for hydrogen bonding, was prepared and characterized. AzoAMP-3 does not isomerize and exhibits emission (Phi(fl) = 0.0008) at 77 K. The hydrogen bonding motif in AzoAMP-1 and AzoAMP-3 provides the first example where inhibiting the concerted inversion pathway in an azobenzene prevents isomerization. These molecules provide important supporting evidence for the spectroscopic and computational studies aimed at elucidating the isomerization mechanism in azobenzene.

Caged complexes are metal ion chelators that release analytes when exposed to light of a specific wavelength. The synthesis and properties of ZinCleav-1, a cage for Zn 2þ that fragments upon photolysis, is reported. The general uncaging strategy involves integrating a nitrobenzyl group on the backbone of the ligand so that a carbon-heteroatom bond is cleaved by the photoreaction. The caged complex was obtained using a new synthetic strategy involving a Strecker synthesis to prepare a key aldehyde intermediate. ZinCleav-1 has a K d of 0.23 pM for Zn 2þ as measured by competitive titration with [Zn(PAR) 2 ] (PAR = 4-(2-pyridyl-2-azo) resorcinol). The quantum yield for ZinCleav-1 is 2.4% and 0.55% for the apo and Zn 2þ complex, respectively. The ability of ZinCleav-1 to increase free [Zn 2þ ] is calculated theoretically using the binding constants for the uncaged photoproducts, and demonstrated practically by using a fluorescent sensor to image the liberated Zn 2þ . Free Zn 2þ may function as a neurotransmitter and have a role in the pathology of several neurological diseases. Studying these physiological functions remains challenging because Zn 2þ is silent to most common spectroscopic techniques. We expect ZinCleav-1 to be the first in a class of caged complexes that will facilitate biological investigations.

In order to facilitate the development of recycling technologies for rare earth magnets from postconsumer products, we present herein an analysis of the neodymium (Nd) content in shredder scrap. This waste stream has been chosen on the basis of current business practices for the recycling of steel, aluminum, and copper from cars and household appliances, which contain significant amounts of rare earth magnets. Using approximations based on literature data, we have calculated the average Nd content in the ferrous shredder product stream to be between 0.13 and 0.29 kg per ton of ferrous scrap. A value analysis considering rare earth metal prices between 2002 and 2013 provides values between $1.32 and $145 per ton of ferrous scrap for this material, if recoverable as pure Nd metal. Furthermore, we present an analysis of the content and value of other rare earths (Pr, Dy, Tb).

This manuscript describes the development of an efficient process for the recovery of rare earth elements from materials mixtures such as in motors with a recovery rate of >80%. Selective dissolution enables efficient separation of steel and copper and selective precipitation of RE salts is the key for obtaining pure RE products.