Generating Stable Nitrogen Bubble Layers on Poly(methyl methacrylate) Films by Photolysis of 2-Azidoanthracene

2-Azidoanthracene (2N3-AN) can act as a photochemical source of N2 gas when dissolved in an optically transparent polymer such as poly(methyl methacrylate) (PMMA). Irradiation at 365 or 405 nm of a 150 μm-thick polymer film submerged in water causes the rapid appearance of a surface layer of bubbles. The rapid appearance of surface bubbles cannot be explained by normal diffusion of N2 through the polymer and likely results from internal gas pressure buildup during the reaction. For an azide concentration of 0.1 M and a light intensity of 140 mW/cm2, the yield of gas bubbles is calculated to be approximately 40%. The dynamics of bubble growth depend on the surface morphology, light intensity, and 2N3-AN concentration. A combination of nanoscale surface roughness, high azide concentration, and high light intensity is required to attain the threshold N2 gas density necessary for rapid, high-yield bubble formation. The N2 bubbles adhered to the PMMA surface and survived for days under water. The ability to generate stable gas bubbles "on demand" using light permits the demonstration of photoinduced flotation and patterned bubble arrays.

Wavelength-Dependent UV-LED Photolysis of Fluorinated Pesticides and Pharmaceuticals

The wavelength dependence of photoproduct formation and quantum yields was evaluated for fluorinated pesticides and pharmaceuticals using UV-light emitting diodes (LEDs) with 255, 275, 308, 365, and 405 nm peak wavelengths. The fluorinated compounds chosen were saflufenacil, penoxsulam, sulfoxaflor, fluoxetine, 4-nitro-3-trifluoromethylphenol (TFM), florasulam, voriconazole, and favipiravir, covering key fluorine motifs (benzylic-CF₃, heteroaromatic-CF₃, aryl-F, and heteroaromatic-F). Quantum yields for the compounds were consistently higher for UV-C as compared to UV-A wavelengths and did not show the same trend as molar absorptivity. For all compounds except favipiravir and TFM, the fastest degradation was observed using 255 or 275 nm light, despite the low power of the LEDs. Using quantitative ¹⁹F NMR, fluoride, trifluoroacetate, and additional fluorinated byproducts were tracked and quantified. Trifluoroacetate was observed for both Ar-CF₃ and Het-CF₃ motifs and increased at longer wavelengths for Het-CF₃. Fluoride formation from Het-CF₃ was significantly lower as compared to other motifs. Ar-F and Het-F motifs readily formed fluoride at all wavelengths. For Het-CF₃ and some Ar-CF₃ motifs, 365 nm light produced either a greater number of or different major products. Aliphatic-CF₂/CF₃ products were stable under all wavelengths. These results assist in selecting the most efficient wavelengths for UV-LED degradation and informing future design of fluorinated compounds.

Visible-light-driven photocyclization reaction: photocatalyst-free mediated intramolecular N–N coupling for the synthesis of 2H-indazole-3-carboxamides from aryl azides

A visible-light-driven photocyclization reaction of aryl azides to access 2H-indazole-3-carboxamides in moderate to excellent yields has been realized efficiently under photocatalyst-free and external additive-free conditions.

High-spin intermediates of the photolysis of 2,4,6-triazido-3-chloro-5-fluoropyridine

In contrast to theoretical expectations, the photolysis of 2,4,6-triazido-3-chloro-5-fluoropyridine in argon at 5 K gives rise to EPR peaks of just two triplet mononitrenes, two quintet dinitrenes, and a septet trinitrene. EPR spectral simulations in combination with DFT calculations show that observable nitrenes can be assigned to triplet 2,4-diazido-3-chloro-5-fluoropyridyl-6-nitrene (D T = 1.026 cm(-1), E T = 0), triplet 2,6-diazido-3-chloro-5-fluoropyridyl-4-nitrene (D T = 1.122 cm(-1), E T = 0.0018 cm(-1)), quintet 4-azido-3-chloro-5-fluoropyridyl-2,6-dinitrene (D Q = 0.215 cm(-1), E Q = 0.0545 cm(-1)), quintet 2-azido-3-chloro-5-fluoropyridyl-4,6-dinitrene (D Q = 0.209 cm(-1), E Q = 0.039 cm(-1)) and septet 3-chloro-5-fluoropyridyl-2,4,6-trinitrene (D S = -0.1021 cm(-1), E S = -0.0034 cm(-1)). Preferential photodissociation of the azido groups located in ortho-positions to the fluorine atom of pyridines is associated with strong π-conjugation of these groups with the pyridine ring. On photoexcitation, such azido groups are more efficiently involved in reorganization of the molecular electronic system and more easily adopt geometries of the locally excited predissociation states.

A fluorogenic probe for the catalyst-free detection of azide-tagged molecules

Fluorogenic reactions in which non- or weakly fluorescent reagents produce highly fluorescent products can be exploited to detect a broad range of compounds including biomolecules and materials. We describe a modified dibenzocyclooctyne that under catalyst-free conditions undergoes fast strain-promoted cycloadditions with azides to yield strongly fluorescent triazoles. The cycloaddition products are more than 1000-fold brighter compared to the starting cyclooctyne, exhibit large Stokes shift, and can be excited above 350 nm, which is required for many applications. Quantum mechanical calculations indicate that the fluorescence increase upon triazole formation is due to large differences in oscillator strengths of the S(0) ↔ S(1) transitions in the planar C(2v)-symmetric starting material compared to the symmetry-broken and nonplanar cycloaddition products. The new fluorogenic probe was successfully employed for labeling of proteins modified by an azide moiety.

EPR studies on branched high-spin arylnitrenes

The UV (λ>305 nm) photolysis of triazide 3 in 2-methyl-tetrahydrofuran glass at 7 K selectively produces triplet mononitrene 4 (g=2.003, D(T)=0.92 cm(-1), E(T)=0 cm(-1)), quintet dinitrene 6 (g=2.003, D(Q)=0.204 cm(-1), E(Q)=0.035 cm(-1)), and septet trinitrene 8 (g=2.003, D(S)=-0.0904 cm(-1), E(S) =-0.0102 cm(-1)). After 45 min of irradiation, the major products are dinitrene 6 and trinitrene 8 in a ratio of ∼1:2, respectively. These nitrenes are formed as mixtures of rotational isomers each of which has slightly different magnetic parameters D and E. The best agreement between the line-shape spectral simulations and the experimental electron paramagnetic resonance (EPR) spectrum is obtained with the line-broadening parameters Γ(E(Q))=180 MHz for dinitrene 6 and Γ(E(S))=330 MHz for trinitrene 8. According to these line-broadening parameters, the variations of the angles Θ in rotational isomers of 6 and 8 are expected to be about ±1 and ±3°, respectively. Theoretical estimations of the magnetic parameters obtained from PBE/DZ(COSMO)//UB3LYP/6-311+G(d,p) calculations overestimate the E and D values by 1 and 8 %, respectively. Despite the large distances between the nitrene units and the extended π systems, the zero field splitting (zfs) parameters D are found to be close to those in quintet dinitrenes and septet trinitrenes, where the nitrene centers are attached to the same aryl ring. The large D values of branched septet nitrenes are due to strong negative one-center spin-spin interactions in combination with weak positive two-center spin-spin interactions, as predicted by theoretical considerations.

Photografting of albumin onto dimethyldichlorosilane-coated glass

4-Azido-2-nitrophenyl albumin (ANP-albumin) was prepared by displacing the fluoro group of 4-fluoro-3-nitrophenyl azide (FNPA) by an amino group of albumin. Photolysis of phenyl azides of ANP-albumin was studied by Fourier-transform infrared (FTIR) spectroscopy. The band of phenyl azide disappeared completely after a 12-min exposure to long wave UV light (366 nm), and the photolysis was first-order. Albumin was grafted onto dimethyldichlorosilane-coated glass (DDS-glass) by photolysis of the azido groups of ANP-albumin without any premodification of the surface. The albumin-grafted DDS-glass was characterized by determining the relative amount of nitrogen resulting from the grafted albumin on the surface using electron spectroscopy for chemical analysis (ESCA). The amount of nitrogen increased when the concentration of ANP-albumin in the adsorption solution increased up to 0.1 mg/ml. As the solution concentration increased above this value, the amount of nitrogen decreased. The platelet resistance of the albumin-grafted surfaces was evaluated by measuring the number of adherent platelets and the extent of activation that was quantitated by the area of platelets spread on the surfaces. The maximum platelet-resistant effect was observed when the ANP-albumin was adsorbed for more than 50 min at the solution concentration ranging from 0.05 to 10 mg/ml.

Photoaffinity labelling of mitochondrial NADH: ubiquinone reductase with pethidine analogues

1. Chemically reactive derivatives of pethidine analogues--novel potent inhibitors of the mitochondrial NADH: ubiquinone reductase (complex I)--were synthesized. 2. Dose-response curves of these components revealed that the photoactivatable aryl azido derivative has retained most of the inhibitory activity displayed by the parent substance. After introduction of a radioactive iodine isotope into the molecule, it was used as a probe for the localization of the inhibitor binding polypeptides within complex I. 3. Photolysis of the radiolabelled derivative bound to isolated complex I both from Neurospora crassa and beef heart resulted in a covalent incorporation of the inhibitor into 6-7 individual subunits of the enzyme. Essentially the same labelling patterns were obtained, when whole mitochondrial membranes were incubated with the reactive derivative. 4. Applying a double isotope labelling technique, the inhibitor-binding polypeptides in N. crassa were identified as mitochondrially synthesized constituents of complex I (ND gene products). In the beef heart enzyme the ND-1 product was detected to be among the polypeptides reacting with the inhibitor. 5. Competition experiments employing either NADH or decylbenzoquinone (DB), together with the pethidine analogue, showed that both enzyme substrates interfere specifically with the inhibitor binding to complex I.

P-Azidophenacyl bromide, a versatile photolabile bifunctional reagent. Reaction with glyceraldehyde-3-phosphate dehydrogenase

The synthesis of the photochemically labile bifunctional reagent p-azidophenacyl bromide (1) is described. This compound may be covalently attached to a known site of an enzyme or other macromolecule by nucleophilic displacement at the alpha-bromo ketone moiety. Subsequent irradiation of the bound reagent gives a nitrene which may insert into a second portion of the enzyme forming a cross-link. Regeant 1 proved to be an excellent inhibitor of rabbit muscle glyceraldehyde-3-phosphate dehydrogenase.