Role of Solvent and Effect of Substituent on Azobenzene Isomerization by Using Room-Temperature Ionic Liquids as Reaction Media

Bistable Aryl Azopyrazolium Ionic Photoswitches in Water

We report a new class of arylazopyrazolium-based ionic photoswitches (AAPIPs). These AAPIPs with different counter ions have been accessed through a modular synthetic approach in high yields. More importantly, the AAPIPs exhibit excellent reversible photoswitching and exceptional thermal stability in water. The effects of solvents, counter ions, substitutions, concentration, pH, and glutathione (GSH) have been evaluated using spectroscopic investigations. The results revealed that the bistability of studied AAPIPs is robust and near quantitative. The thermal half-life of Z isomers is extremely high in water (up to years), and it can be lowered electronically by the electron-withdrawing groups or highly basic pH.

Catalytic synthesis of azoarenes via metal-mediated nitrene coupling

Various valuable properties of azoarenes (“azo dyes”), including their vivid colors and their facile cis-trans photoisomerization, lead to their wide use in the chemical industry. As a result, ~700,000 metric...

Novel pyrazolyazoindole derivatives as photoswitches: design, synthesis, and photoswitching behavior research combined with theoretical methods

Novel pyrazolyazoindole photoswitches with high to near-complete photoconversion, adjustable thermal half-lives, photochromic properties, and potential application value are developed.

Design of phase-transition molecular solar thermal energy storage compounds: compact molecules with high energy densities

A series of compact azobenzene derivatives were investigated as phase-transition molecular solar thermal energy storage compounds that exhibit maximum energy storage densities around 300 J g^-1. The relative size and polarity of the functional groups on azobenzene were manifested to significantly influence the phase of isomers and their energy storage capacity.

Fluorescent Azobenzene-Containing Compounds: From Structure to Mechanism

The reversible photoisomerization of azobenzenes has been extensively studied to construct systems with optical responsiveness; however, this process limits the luminescence of these compounds. Recently, there have been many efforts to design and synthesize fluorescent azobenzene compounds, such as inhibition of electron transfer, inducing aggregation, and metal-enhancement, which make the materials ideal for application in fluorescence probes, light-emitting devices, molecular detection, etc. Herein, we review the recently reported progress in the development of various fluorescent azobenzenes and summarize the possible mechanism of their fluorescence emission. The potential applications of these materials are also discussed. Finally, in order to guide research in this field, the existing problems and future development prospects are discussed.

The Nature of Azo-Substituted Carbocations: N-N π-Electron Stabilization versus Nitrogen Nonbonding Electron Stabilization

Computational and experimental studies reveal two different modes of cation stabilization by the phenylazo group. The first mode involves a relatively weak conjugative interaction with the azo π-bond, while the second mode involves an interaction with the nitrogen nonbonding electrons. The 4-phenylazo group is slightly rate-retarding in the solvolysis of cumyl chloride and benzyl mesylate derivatives but rate-enhancing in the solvolysis of α-CF₃ benzylic analogs. The phenylazo group can become a potent electron-donating group in cations such as [Me₂C-N═N-Ph]⁺. Nonbonding electron stabilization can be strong enough to offset the very powerful γ-silyl stabilization. In aromatic cyclopropenium and tropylium cations, the demand for stabilization is quite low, and the mode of phenylazo stabilization reverts back to the less-effective π-stabilization. The solvolysis of cis-4-phenylazo benzyl mesylate is faster than that of trans-4-phenylazo benzyl mesylate. Products formed suggest a stepwise ionization, cation isomerization, and nucleophile capture mechanism. Computational studies indicate a vanishingly small barrier for the isomerization of the cis-cation intermediate to the trans-cation.

Microwave-assisted simple synthesis of 2-anilinopyrimidines by the reaction of 2-chloro-4,6-dimethylpyrimidine with aniline derivatives

A series of 2-anilinopyrimidines including novel derivatives has been obtained from 2-chloro-4,6-dimethylpyrimidine by aromatic nucleophilic substitution with differently substituted anilines under microwave conditions. The substituents had a significant impact on the course and efficiency of the reaction. The results reported herein demonstrate the efficacy of microwaves in the synthesis of the title heterocyclic compounds as compared to the results obtained with conventional heating. The 2-anilinopyrimidines described are of potential bioactivity.

A rapid, one pot, green approach by microwave heating in ethanol provides a new method to obtain active and new potentially active compounds in high yields.

Synthesis of Bis-β-Diketonate Lanthanide Complexes with an Azobenzene Bridge and Studies of Their Reversible Photo/Thermal Isomerization Properties

The ligand, bis-β-diketone with an azobenzene bridge (4,4'-(4,4,4-trifluoro-1,3-butanedione)azobenzene, H 2 L), was prepared for the synthesis of a series of dinuclear lanthanide complexes with the formula [Ln 2 L 3 (DMSO) 4 ] (Ln = Eu3+, Gd3+, Tb3+, and DMSO = dimethyl sulfoxide). X-ray crystallographic analysis reveals that the three complexes are triple-stranded dinuclear structures formed by three bis-β-diketonate ligands with two lanthanide ions (Ln3+). The trans-to-cis photoisomerization rates of the azobenzene group of the three [Ln 2 L 3 (DMSO) 4 ] complexes in ethanol and acetonitrile solutions are similar to those of the pure H 2 L ligand and other azobenzene-containing mononuclear lanthanide complexes, but the trans-to-cis quantum yields (Φt→c = 10-3) are 1 order of magnitude smaller. The first-order rate constant for the cis-to-trans thermal isomerization at 50 °C of the H 2 L ligand is similar to those of azobenzene derivatives, while those for the [Ln 2 L 3 (DMSO) 4 ] complexes (k iso = 10-4 s-1) are higher than those of the mononuclear azobenzene-containing lanthanide complexes. Furthermore, as the lanthanide ionic radius becomes smaller from Eu3+ to Gd3+ to Tb3+, the thermal isomerization rate constant decreases and the half-life increases. All these results are proposed to arise from the rigidity at both ends of the azo group by coordination to the dinuclear lanthanide ions and the different isomerization mechanisms. These are the first examples of bis-β-diketonate dinuclear lanthanide complexes with an azobenzene bridge and help illustrate the mechanism of azobenzene isomerization.

Learning organic chemistry day by day: The best choice of the best pharmacy students

BACKGROUND AND PURPOSE

During over ten years of experience in teaching organic chemistry at the Department of Pharmacy we have tried to answer the following question: why do most students tend to take the exam one, two, or more years after the end of the course they have attended? Several reasons could justify this delay, but three seem to be the most common drawbacks for our students: a) time needed for the comprehension of the arguments; b) the number of mandatory exams to pass before organic chemistry; c) lack of a self-evaluation method.

EDUCATIONAL ACTIVITY AND SETTING

To increase the number of students in the exam sessions of the semester just after the course we have proposed two strategies: 1) a systematic, but stressless, approach by which homework and everyday life examples concerning organic chemistry are used to increase the sense of responsibility in studying; 2) the modification of the number of mandatory exams required for organic chemistry.

FINDINGS

The rate of successful students in the exam sessions at the end of the course increased from 38.3 up to 61.3%. Interestingly, the highest scores tend to be obtained by students in the first session available just after the conclusion of the course.

DISCUSSION AND SUMMARY

The combined effect of Strategy 1 and Strategy 2 seems to be effective in promoting the learning of organic chemistry and points out that the best performances tend to be associated with students which follow organized studying.

The trans/cis photoisomerization in hydrogen bonded complexes with stability controlled by substituent effects: 3-(6-aminopyridin-3-yl)acrylate case study

The association of aminopyridine-based acrylic acid and its salt was studied by NMR titration experiments. The AA (acceptor, acceptor) hydrogen-bonding pattern present in the salt forms a complex readily with a DD (donor, donor) hydrogen-bonding pattern of the substituted ureas even in polar and competitive environment. The double carbon–carbon bond in the acrylic acid derivative is subjected to photoisomerization. This is dependent on the association with substituted urea derivatives. The substituent in ureas influences the trans/cis isomerization kinetics and position of the photostationary state. Two mechanisms that influence the photoisomerization were proposed. To the best of our knowledge, the trans/cis photoisomerization influenced by the substituent in such a hydrogen-bonding pattern has not observed previously. It was shown that interaction with urea derivatives causes lowering of the trans-to-cis photoreaction rates.

The association of aminopyridine-based acrylic acid and its salt was studied by NMR titration experiments.

Monitoring cis-to-trans isomerization of azobenzene using terahertz time-domain spectroscopy

We present terahertz time-domain spectroscopy (THz-TDS) to explore the conformational dynamics of thermally induced and photoinduced isomerization of azobenzene.

Kinetics and Energetics of Thermal Cis-Trans Isomerization of a Resonance-Activated Azobenzene in BMIM-Based Ionic Liquids for PF₆-/Tf₂N- Comparison

BMIM PF₆ (1-butyl-3-methylimidazolium hexafluorophosphate) and BMIM Tf₂N (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) are two conventional room-temperature ionic liquids widely employed and investigated as reaction media. Despite the presence of the same imidazolium ring in their structure they are different in many chemical and physical properties due to the nature of the anions. The thermal cis-trans isomerization of an electronically activated azobenzene have been used as reaction model to compare the behavior of PF₆- and Tf₂N-. Rotation is the mechanism by which the investigated azobenzene is converted into the trans isomer spontaneously in the dark both in BMIM PF₆ and in BMIM Tf₂N. The kinetic rate constants of the process have been determined at different temperatures and the activation energies of the reaction have been calculated according to the Arrhenius and Eyring equations. The results presented herein highlight different solute-solvent interactions involving the PF₆- and Tf₂N- anions during the cis-trans isomerization.

Solvent dependent isomerization of photochromic dithienylethenes: synthesis, photochromism, and self-assembly

The self-assembly of photochromic dithienylethene molecules into circular nanostructures is reported. The formation of nanostructures is dependent on factors such as photoisomerization, solvent, and hydrogen bonding.

Synthesis and characterization of photoresponsive POSS-based polymers and their switchable water and oil wettability on cotton fabric

Photoresponsive POSS-based polymers were successfully prepared via radical polymerization. The surface wettability of both water and oil on the polymers coated fabrics can be intelligently controlled by applying irradiation with UV or visible light.