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Hydroxy azo‐benzenes are very well known for their rapid trans⇆cis photoisomerization under polar solvents. In contrast, we synthesized two hydroxymethylated‐ hydroxyazobenzene derivatives and investigated their light‐triggered photoisomerization and stability under polar solvents. The result showed just opposite behavior and very slow trans⇆cis isomerization was observed (minutes‐days). Thus, the UV‐vis spectrum revealed a very low decrease in the absorbance of π–π* transition. Surprisingly, the isomerization process became faster with the addition of ionic species. Here, we attempted to understand the underlying cause of the unusual photo‐switching behavior. The presence of hydroxymethyl and fluorine substituents was found to have a significant effect on the stability of the trans and photo‐isomerized cis products. Here, it was confirmed with NMR and DLS studies that the unusual photostability of trans compounds was caused by polar solvent‐assisted aggregation (hydrodynamic radius, RH 5660–1720 nm) which underwent dispersion (RH, 220–68 nm) and formed a significant stable solvated photoproduct under photoirradiation. Furthermore, this aggregation‐dispersion was found to be very slowly reversible. Further, the fluorescence emission demonstrated a characteristic dispersion‐induced quenching. Regular photoswitching was observed under non‐polar conditions in benzene where an expected blue shift of π–π* transition and an increase in the intensity of n–π* transitions. [ABSTRACT FROM AUTHOR]

Here we present the synthesis of a new fluorinated pyridyl‐azo dye and demonstrate an effective way to convert monomer fluorinated pyridyl‐azo dye to H‐aggregated form through UV‐light induced trans‐cis‐trans photoisomerization. And upto a 12 : 88 ratio between monomer : aggregation was achieved through a small pulse of UV irradiation. The H‐aggregation shows a unique high extinction co‐efficient in UV‐visible absorbance and unexpected fluorescence emission with a remarkably large Stokes shift. The solvent DMF and fluorine substituent were found to have a critical role in aggregation. 1H, 19F NMR, and 19F‐DOSY NMR experiments were used to confirm the aggregation and also confirm the absence of π‐π stacking. [ABSTRACT FROM AUTHOR]

Herein, we report a comparative study on the interactions of different metal ions with fluorinated and non-fluorinated azobenzene-Schiff bases. Three new varieties of fluorinated Schiff bases were synthesized in high yields and changes in their UV-vis absorbance and fluorescence emission when they interacted with various metal ions in EtOH-H2O were studied. It was observed that alkali metals in the presence of F-substituent significantly altered UV-vis absorbance, causing red shifts, whereas transition metal ions caused blue shifts. In all cases, metal interactions increased the fluorescence emission. Interestingly, alkali metal ions intensified the color of parent compounds, whereas transition metal ions turned the solution colorless. We observed that the presence and position of fluorinated substituent had a significant impact on the interaction of metal ions. Interestingly, in the presence of a polar solvent such as DMSO, the interactions increased significantly, the C-F bond appeared to be activated in the presence of alkali metal ions, and the F-substituent was removed very quickly at room temperature as evidenced by the appearance of a very interesting azobenzene-dibenzoazepine compound in ¹H-NMR spectra. To gain deep insights into the nature of interactions, time-dependent ¹H- and 19F-NMR spectroscopy were used, which confirmed the participation of M-F interactions and the formation of new (N and) O...M+...F complexes due to significant changes in the chemical shift of 19F resonance (shielding effect) with respect to free ligands. It was also confirmed that alkali metals interacted at the site containing F-substituents, whereas transition metals preferred a non-fluorinated site. The appearance of a new shielded peak of the F-substituent in 19F NMR was found to be in the decreasing order of the ionic size of metal ions. Interestingly, in the absence of weak bases, electron-withdrawing groups, and other activators and under the action of polar solvents and alkali metal salts, the C-F bond exhibited σ bond donor ability with alkali metal ions, which could be activated. Thus, this method allows for the modulation of a Schiff base to create interesting functional and useful materials. [ABSTRACT FROM AUTHOR]

Parasite Plasmodium falciparum is continuously giving a challenge to human beings by changing itself against most of the antimalarial drugs and its consequences can be seen in the form of a huge number of deaths each year especially in the poor and developing country. Due to its drug resistance ability, new drugs are regularly needed to kill the organism. Many new drugs have been developed based on different mechanisms. One of the potential mechanisms is to hamper protein synthesis by blocking the gene expression. S-Adenosyl-L-homocysteine (SAH) hydrolase is a NAD+ dependent tetrameric enzyme, which is responsible for the reversible hydrolysis of AdoHcy to adenosine and L-homocysteine, has been recognized as a new target for antimalarial agents since the parasite has a specific SAH hydrolase. The inhibition of SAH hydrolase causes the intracellular accumulation of S-Adenosyl-L-homocysteine, elevating the ratio of SAH to S-adenosylmethionine (SAM) and inhibiting SAM-dependent methyltransferase that catalyzes methylation of the capped structure at the 5'-terminus of mRNA, and other methylation reaction which is essential for parasite proliferation. In other words, S-Adenosyl-Lhomocysteine hydrolase regulates methyltransferase reactions. In this way, SAH hydrolase inhibitors can be used for the treatment of different diseases like malaria, cancer, viral infection, etc. by ultimately stopping the synthesis of protein. Many antiviral drugs have been synthesized and marketed which are based on the inhibition of SAH hydrolase. This review summarises the development of SAH inhibitors developed over the last 20 years and their potentiality for the treatment of malaria., (Copyright© Bentham Science Publishers; For any queries, please email at epub@benthamscience.net.)