ConspectusRecently, there has been a great deal of interest in using the photoisomerization of azobenzene compounds to control specific biological targets in vivo. These azo compounds can be used as research tools or, in principle, could act as optically controlled drugs. Such “photopharmaceuticals” offer the prospect of targeted drug action and an unprecedented degree of temporal control.A key feature of azo compounds designed to photoswitch in vivo is the wavelength of light required to cause the photoisomerization. To pass through tissue such as the human hand, wavelengths in the red, far-red, or ideally near infrared region are required. This Account describes our attempts to produce such azo compounds.Introducing electron-donating or push/pull substituents at the para positions delocalizes the azobenzene chromophore and leads to long wavelength absorption but usually also lowers the thermal barrier to interconversion of the isomers. Fast thermal relaxation means it is difficult to produce a large ste...

Red-Shifting Azobenzene Photoswitches for in Vivo Use.

The photoisomerization of azobenzenes provides a general means for the photocontrol of molecular structure and function. For applications in vivo, however, the wavelength of irradiation required for trans-to-cis isomerization of azobenzenes is critical since UV and most visible wavelengths are strongly scattered by cells and tissues. We report here that azobenzene compounds in which all four positions ortho to the azo group are substituted with bulky electron-rich substituents can be effectively isomerized with red light (630–660 nm), a wavelength range that is orders of magnitude more penetrating through tissue than other parts of the visible spectrum. When the ortho substituent is chloro, the compounds also exhibit stability to reduction by glutathione, enabling their use in intracellular environments in vivo.

Photoswitching Azo Compounds in Vivo with Red Light

Molecular systems encompassing more than one photochromic entity can be used to build highly functional materials, thanks to their potential multi-addressability and/or multi-response properties. Over the last decade, the synthesis and spectroscopic and kinetic characterisation as well as the modeling of a wide range of multiphotochromes have been achieved in a field that is emerging as a distinct branch of photochemistry. In this review, we provide an overview of the available multiphotochromic compounds which use a variety of photoactive building blocks, e.g., diarylethene, azobenzene, spiropyran, naphthopyran or fulgimide derivatives. Their efficiency in terms of multi-responsiveness is discussed and several strategies to circumvent the most common limitation (i.e., the loss of photochromism of one part) are described.

Multiphotochromic molecular systems

Development of renewable energy technologies has been a significant area of research amongst scientists with the aim of attaining a sustainable world society Solar thermal fuels that can capture, convert, store, and release solar energy in the form of heat through reversible photoisomerization of molecular photoswitches such as azobenzene derivatives are currently in the limelight of research Herein, we provide a state-of-the-art account on the recent advancements in solar thermal fuels based on azobenzene photoswitches We begin with an overview on the importance of azobenzene-based solar thermal fuels and their fundamentals Then, we highlight the recent advances in diverse azobenzene materials for solar thermal fuels such as pure azobenzene derivatives, nanocarbon-templated azobenzene, and polymer-templated azobenzene The basic design concepts of these advanced solar energy storage materials are discussed, and their promising applications are highlighted We then introduce the recent endeavors in the molecular design of azobenzene derivatives toward efficient solar thermal fuels, and conclude with new perspectives on the future scope, opportunities and challenges It is expected that continuous pioneering research involving scientists and engineers from diverse technological backgrounds could trigger the rapid advancement of this important interdisciplinary field, which embraces chemistry, physics, engineering, nanoscience, nanotechnology, materials science, polymer science, etc

Azobenzene-based solar thermal fuels: design, properties, and applications

Life at Low Reynolds Number

Reaction of Co(NCS)2 with 4-ethylpyridine leads to the formation of three new compounds of composition Co(NCS)2(4-ethylpyridine)4 (1), [(Co(NCS)2]2(4-ethylpyridine)6 (2), and [Co(NCS)2(4-ethylpyridine)2]n (3). In all compounds the coordination of the Co(II) ions is distorted octahedral. 1 consists of discrete monomeric complexes and in 2 two Co(II) cations are linked by pairs of μ-1,3-bridging thiocyanato ligands into dimers. In the crystal structure of 3 the Co(II) cations are connected into chains by the same bridge as in 2. Magnetic studies show that 1 and 2 are paramagnets down to a temperature of 2 K, while compound 3, which is the main object of this study, is an antiferromagnet with the Neel temperature TN = 3.4 K. Its magnetic structure is built from ferromagnetic chains, which are weakly antiferromagnetically coupled. With increasing magnetic field a metamagnetic transition starts at ∼175 Oe, as observed for a polycrystalline sample. Magnetic relaxations, which were observed in the antiferromagne...

Structural and Magnetic Studies of a New Co(II) Thiocyanato Coordination Polymer Showing Slow Magnetic Relaxations and a Metamagnetic Transition

Azobenzenes are robust, reliable, and easy to synthesize photochromic switches However, their high conformational flexibility is a disadvantage in machine-like applications The almost free rotation of the phenyl groups can be restricted by bridging two ortho positions with a CH2CH2 group, as realized in the dihydrodibenzo diazocine framework We present the synthesis and properties of 3,3’-amino- and 3,3’-acetamido substituted diazocines Upon irradiation with light of 405 and 530 nm they isomerize from the cis to the trans configuration and back, and thereby perform a pincer-like motion In the thermodynamically more stable cis isomer the lone pairs of the amino nitrogen atoms point towards each other, and in the trans form they point in opposite directions The distance between the amino nitrogen atoms changes between 8 A (cis) and 11 A (trans isomer)

https://www.beilstein-journals.org/bjoc/content/pdf/1860-5397-9-1.pdf

Amino-substituted diazocines as pincer-type photochromic switches

A systematic study is reported of the photochemical properties of the multi-azobenzene compounds bis[4-(phenylazo)phenyl]amine (BPAPA) and tris[4-(phenylazo)phenyl]amine (TPAPA) compared to the parent molecule 4-aminoazobenzene (AAB). The bis- and tris-azobenzenes were synthesised by a variant of the Ullmann reaction and exist in their stable all-E forms at room temperature. Striking changes in the spectral positions and intensities of their first ππ* absorption bands compared to AAB reveal strong electronic coupling between the AB units. The nature of the excited states was explored by quantum chemical calculations at the approximate coupled-cluster (CC2) level. Upon UV/VIS irradiation, the molecules isomerise to the Z-isomer (AAB), ZE- and ZZ-isomers (BPAPA), and ZEE-, ZZE- and ZZZ-isomers (TPAPA), respectively. The photoswitching behaviours were investigated by UV/VIS and NMR spectroscopies. All individual isomers were detected by one-dimensional (1D) 1H NMR spectroscopy (BPAPA) and two-dimensional (2D) HSQC NMR spectroscopy (TPAPA). A kinetic analysis provided the isomer-specific thermal lifetimes. The variance of the thermal lifetimes demonstrates a dependence of the Z–E isomerisation on the chromophore size and number of AB units.

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Photochemical properties of multi-azobenzene compounds.

Reaction of Fe(NCS)2 and Co(NCS)2 with 2-methylpyrazine in different molar ratios and solvents at room temperature leads to the formation of five new coordination compounds of composition M(NCS)2(2-methylpyrazine)2(H2O)2 (M = Fe (1-Fe) , Co (1-Co)), Co(NCS)2(2-methylpyrazine)2(CH3OH)2 (2-Co) and Co(NCS)2(2-methylpyrazine)4·2-methylpyrazine solvate (3-Co). In all of these compounds, discrete complexes are found in which the metal cations are octahedrally coordinated by two terminal N-bonded thiocyanato anions and four N- or O-donor co-ligands. On heating compounds 1–3 in a thermobalance new coordination polymers of composition M(NCS)2(2-methylpyrazine)2 (M = Co (4-Co) , Fe (4-Fe)) are obtained in the first step, which transform into M(NCS)2(2-methylpyrazine) (M = Co (5-Co) , Fe (5-Fe)) in the second. Because of the low chalcophilicity of these cations, compounds 4 and 5 are not accessible from solution. Further investigations prove that 4-Co and 4-Fe obtained by thermal decomposition are of low crystallinity, might be isotypic and might consist of metal cations, in which the anionic ligands are only terminal N-bonded. In contrast, 5-Co and 5-Fe are of good crystallinity but their structure cannot be solved from X-ray powder data. However, a compound of the same composition (5-Cd) based on the more chalcophilic cadmium can easily be crystallized from solution and characterized by single crystal X-ray diffraction. This compound is isotypic to 5-Co and 5-Fe and therefore their structures were determined by Rietveld refinements. In their crystal structures the metal atoms are linked by μ-1,3-bridging thiocyanato anions into a 2D network. Magnetic measurements reveal that compounds 1–4 show only Curie–Weiss paramagnetism and that for 5-Fe antiferromagnetic ordering is observed. In contrast, 5-Co shows metamagnetic behavior with a very large critical field. Finally, it was shown that 4-Co and 4-Fe are hygroscopic and transform within minutes into the hydrates 1-Co and 1-Fe.

/pdf/synthesis-structures-and-magnetic-properties-of-fe-ii-and-co-1vt8ysu0nr.pdf

Synthesis, structures and magnetic properties of Fe(II) and Co(II) thiocyanato coordination compounds: on the importance of the diamagnetic counterparts for structure determination

The isolation and structure elucidation of the novel calcaripeptides A (1), B (2), and C (3) and studies on their biosynthetic origin are described. The calcaripeptides were identified from Calcarisporium sp. strain KF525, which was isolated from the German Wadden Sea. Compounds 1–3 are macrocyclic structures composed of a proline and a phenylalanine residue as well as a nonpeptidic substructure. Structure elucidation was achieved by applying one- and two-dimensional NMR spectroscopy supported by high-resolution mass spectrometry. X-ray crystallography was performed to determine the relative configuration of 1. The absolute configuration of 1 was assigned by HPLC of the amino acids after hydrolysis of the molecule and derivatization with chiral agents. Studies on the biosynthesis by feeding 13C-labeled substrates revealed that the nonpeptidic part of 1 originates from acetate and l-methionine. The involvement of a hybrid between a polyketide synthase and a nonribosomal peptide synthetase in the biosynthes...

Christian Näther

Papers

Structural and Magnetic Studies of a New Co(II) Thiocyanato Coordination Polymer Showing Slow Magnetic Relaxations and a Metamagnetic Transition

Amino-substituted diazocines as pincer-type photochromic switches

Photochemical properties of multi-azobenzene compounds.

Synthesis, structures and magnetic properties of Fe(II) and Co(II) thiocyanato coordination compounds: on the importance of the diamagnetic counterparts for structure determination

Calcaripeptides A-C, cyclodepsipeptides from a Calcarisporium strain.