Photochemical properties of UV-absorbing chemicals in phase-controlled polymer microspheres

In this study, a different UV (ultra-violet) ray absorption system is presented in which butyl methoxydibenzoylmethane (BMDM, a model UV-A absorbent, 320–400 nm) is stabilized in phase-controlled poly(methyl methacrylate) (PMMA) microspheres. The photochemistry of BMDM in the microspheres was investigated considering its phase characteristics therein. Analysis by differential scanning calorimetry and X-ray diffraction showed that the BMDM in the microspheres was present in a non-crystalline state. The phase control of BMDM in the polymer microspheres has an excellent ability to protect against UV-A, whilst maintaining its photostability and thermal stability. The results obtained in this study illustrate well that the phase control of the UV absorbents in polymer microspheres is another key factor that determines their photochemistry and photostability in the final formulations.     

Photoinduced Aromatization of Unsymmetrically Substituted 1,4‐Dihydropyridines

Irradiation of unsymmetrically substituted 1,4‐dihydropyridines ( 1b ‐ 1j ) while bubbling oxygen or argon through the solution resulted in aromatization to the corresponding pyridine derivatives ( 3b ‐ 3j ). Compound 1a with 2‐nitrophenyl substituent in 4‐position undergoes elimination of water upon irradiation under both oxygen and argon atmospheres and formation of 3a with 2‐nitrosophenyl substituent. On the other hand, irradiation of the compounds 1e , 1k and 1l with 4‐hydroxy‐3‐methoxyphenyl, 5‐methyl‐2‐furyl and 2‐furyl substituent in this position, respectively, resulted in the expulsion of these substituents and formation of a pyridine derivative unsubstituted in position 4, namely compound 2 . Chloroform as a solvent causes the photo‐oxidation of these compounds by an electron transfer mechanism which is supported by the formation of dichloromethane according to GC analysis and presence of acid (HCl) in the solution after irradiation.     

Crystallization of photo-chain extended poly(ethylene glycol)

Coumarin-functionalized poly(ethylene glycol) (PEG) monols and diols were isothermally crystallized at temperatures between 20 and 35 °C before and after exposure to approximately 110 J cm⁻² of ultra-violet A (λ > 300 nm, UVA) irradiation. Irradiation dimerized the coumarin groups and chain-extended the coumarin-functionalized PEG oligomers. The higher molecular weights reduced the crystal growth rate by as much as 50% compared to the non-irradiated coumarin-functionalized PEG oligomers under ambient crystallization conditions. Hoffman’s kinetic nucleation theory was utilized to evaluate the types of nucleation that occurred for the coumarin-functionalized PEG diols (COU-PEG-COU). Crystallization regimes II and III were observed for the coumarin-modified PEG oligomers before and after exposure to UVA light.     

Photocatalytic Acetolysis of α‐Epoxyketones

Photocatalytic ring opening of α‐epoxyketones by 2,4,6‐triphenylpyrylium tetrafluoroborate in glacial acetic acid resulted in the formation of diastereomeric hydroxyester compounds. The presence of the additional substitutent on the parent molecule affects the rate of the ring opening.     

Significant solvent effects and unusual additions of p-chloranil in the photoinduced electron-transfer reaction of 2,2-dianisyl-4-isopropylidene-3,3-dimethylcyclobutanone

A photoinduced electron-transfer reaction of 2,2-dianisyl-4-isopropylidene-3,3-dimethylcyclobutanone (1) in benzene gave two unexpected CA-adducts, 5,5-dianisyl-4,4-dimethyl-3-[1-(4-hydroxy-2,3,5,6-tetrachlorophenyl)methylethylidene]dihydrofuran-2-one (7) and 2,2-dianisyl-4-(4-hydroxy-2,3,5,6-tetrachlorophenoxy)-4-isopropenyl-3,3-dimethylcyclobutanone (8), while the reaction in acetonitrile did not, suggesting significant solvent effects on the product distribution.     

Chromatographic investigations of the photolysis products of nitrobenzenes

Summary By using gas, liquid, column and thin-layer chromatography and spectroscopy the main products of the photolysis of nitrobenzene in ethereal solutions have been identified. These are: azobenzene, azoxybenzene, nitroazoxybenzene, 4-aminoazobenzene, 3- and 4-nitrobenzene, 2-, 3- and 4-nitrophenol, 1,4-quinone, phenol, aniline and 4-nitroaniline. The photolysis was carried out by irradiation with a mercury lamp. The removal of oxygen from the irradiated solution was found to affect both the qualitative and quantitative composition of the photolysis products. The mechanism of the photochemical processes occurring in the irradiated nitrobenzene solutions is discussed.     

Photomediated synthesis of β-alkylketones from cycloalkanes

β-Cycloalkylketones are prepared through a photomediated radical addition reaction onto enones starting from the corresponding alkanes (i.e., cyclopentane, -hexane, -heptane, -dodecane and adamantane). The alkyl radicals are generated via hydrogen abstraction by either an organic (benzophenone) or an inorganic (tetrabutylammonium decatungstate, TBADT) photomediator. Isolated yields vary in the range 30-80%. Benzophenone has to be considered as a reagent, since it is used in an equimolar amount with respect to enone and is completely consumed in the reaction. On the contrary, TBADT is shown to behave as a photocatalyst, which is active for at least 50 cycles. The potential of photomediated reactions for the generation of radicals from unusual precursors and the synthetic significance of this method are discussed.     

Direct observation and kinetic characterization of o-quinodimethane and its radical cation variant generated in a photoinduced electron-transfer reaction of 1,2-bis(α-styryl)benzene

Nanosecond time-resolved absorption spectroscopy on the laser flash photolysis of 1,2-bis(α-styryl)benzene (1) under N-methylquinolinium tetrafluoroborate-toluene-sensitized conditions in acetonitrile confirmed that an o-quinodimethane radical cation (2⁺, λ_max = 569 nm) decayed and the corresponding neutral prototype (2, λ_max = 444 nm) rose with rate constants of 5.6 and 5.9 × 10⁵ s⁻¹, respectively, showing the first agreement in kinetics between a reactive radical cation intermediate intervening in chemical reaction and the corresponding neutral species formed by back electron transfer.     

Room temperature syntheses of MnF3, MnF4 and hexafluoromanganete(IV) salts of alkali cations

Elemental fluorine oxidises MnF₂ in anhydrous hydrogen fluoride (aHF) as a solvent at room temperature yielding pure MnF₃. In the presence of a UV source, MnF₂ is completely oxidised by photodissociated F₂ in aHF to MnF₄. Photochemical reactions with elemental fluorine on 2AF (A=Li, Na, K, Rb, Cs)/MnF₂ mixtures in aHF as a solvent at room temperature yield pure A₂MnF₆ compounds. An attempt to prepare CsMn^VF₆ by oxidation of MnF₂ with UV-irradiated F₂ in the presence of an equimolar amount of CsF in aHF failed. The final products of these reactions were mixtures of MnF₄ and Cs₂MnF₆. Described syntheses represent milder approaches for the preparation of larger amounts of MnF₃, MnF₄, and A₂MnF₆ on a gram scale.     

Photochemistry of p-Nitroacetophenone in 2-Propanol

Upon irradiation in 2-propanol, p-nitroacetophenone 1 was reduced via the triplet state to p-hydroxyaminoacetophenone 5 which was further reduced to p-aminoacelophenone 2 and 4,4′-diacetylazobenzene 4 . Similar irradiation of 5 also gave 2 and 4 , and its oxidation by oxygen gave 4,4′-diacetylazoxybenzene 3 . Photolysis of monomeric p-nitrosoacetophenone 6 afforded acetophenone and 3 that were not produced during the irradiation of 1 . Possible photoreaction pathways were discussed on the basis of published mechanisms.