Alain Castellan

Photodimerization of anthracenes in fluid solution:structural aspects

Owing to their versatile photophysical and photochemical properties, anthracene and its derivatives are being employed in many systems, for instance as energy migration probes in polymers, triplet sensitizers, molecular fluorosensors, electron acceptor or donor chromophores in artificial photosynthesis, photochromic substrates in 3D memory materials, etc. One remarkable feature is their ability to photodimerize under UV irradiation which induces considerable changes in their physical properties. This account reports the preparative and structural aspects of this very useful and protean photocycloaddition.

Photodimerization of anthracenes in fluid solutions: (part 2) mechanistic aspects of the photocycloaddition and of the photochemical and thermal cleavage

One of the classics in photochemistry, the photodimerization of anthracenes can be considered as a paradigm of the photocycloaddition of non saturated hydrocarbons. The historical steps of the mechanistic studies are reviewed: based on fluorescence quenching, cyclization quantum yields measurement, the influence of dioxygen and solvents, they support a singlet state pathway; the dimerization rate constants are found to be generally high for reactions occurring within a few nanoseconds unless they are slowed down or inhibited by steric strain. In several cases, excimers have been demonstrated to be intermediates and it is shown that excimer fluorescence and cyclization are competitive processes. Another intermediate known as pericyclic minimum (or conical intersection) is postulated to form a sort of floppy cycloadduct where the reacting centres are at mutual distances shorter than in excimers and longer than in dimers. For intermolecular dimerizations, the triplet state is also reactive but through triplet–triplet annihilation in dilute solutions. Intramolecular photocycloadditions have also been carefully examined, for the role of multiple excimer formation, regioselectivity (9,10∶1′,4′ and 9,10∶1′,2′ cyclization) and solvent polarity. The triplet state reactivity is shown to lead to 4π + 2π or 4π + 4π cycloadducts, depending on geometric factors. In the latter case when intersystem crossing is favoured by the substituents, cyclization quantum yields as high as 0.65–0.72 have been observed. Photodissociation quantum yields are generally high and the reactions are partly adiabatic, leading to excimer and monomer fluorescence, but the major part follows another pathway not fully elucidated by flash photolysis. Thermodynamic and kinetic parameters for the thermal cleavage are given; they reveal a large gamut of stability for the photocycloadducts.

Biobased composites from glyoxal-phenolic resins and sisal fibers.

Lignocellulosic materials can significantly contribute to the development of biobased composites. In this work, glyoxal-phenolic resins for composites were prepared using glyoxal, which is a dialdehyde obtained from several natural resources. The resins were characterized by (1)H, (13)C, 2D, and (31)P NMR spectroscopies. Resorcinol (10%) was used as an accelerator for curing the glyoxal-phenol resins in order to obtain the thermosets. The impact-strength measurement showed that regardless of the cure cycle used, the reinforcement of thermosets by 30% (w/w) sisal fibers improved the impact strength by one order of magnitude. Curing with cycle 1 (150 degrees C) induced a high diffusion coefficient for water absorption in composites, due to less interaction between the sisal fibers and water. The composites cured with cycle 2 (180 degrees C) had less glyoxal resin coverage of the cellulosic fibers, as observed by images of the fractured interface observed by SEM. This study shows that biobased composites with good properties can be prepared using a high proportion of materials obtained from natural resources.

Photodegradation of lignin: a photophysical and photochemical study of a non-phenolic α-carbonyl β-O-4 lignin model dimer, 3,4-dimethoxy-α-(2′-methoxypenoxy)acetophenone

Abstract Photoyellowing of bleached high yield wood pulps induced by lignin was studied using the non-phenolic α-carbonyl β- O -4 lignin model compound 3,4-dimethoxy-α-(2′-methoxyphenoxy)acetophenone ( 1 ). Its photochemical reactivity was examined in both liquid (benzene and water-ethanol mixture) and solid (adsorbed on lignin free paper fibres) media; the influence of oxygen on the photolysis was considered. Efficient photodecomposition of 1 (φ R ≈ 0.3 in solution) yields monomers, dimers and a complex mixture of oligomers including coloured materials. The structures of the photoproducts are in accordance with a β-CO bond cleavage as the primary photochemical step, leading to phenacyl and guaiacoxy radicals. Laser flash photolysis experiments enables us to detect two transient absorptions attributed to two different triplet states of 1 : one with a fast decay (τ = 50 ns) from conformers where the β-phenyl ring and the carbonyl group interact, and another one with a slower decay (τ = 830 ns) attributed to conformers with no interchromophoric interactions. Whatever the experimental conditions (degassed or aerated, liquid or solid media), α-carbolyl β-1 structures isolated as photoproducts of compound 1 appear to also play a role in the formation of coloured oligomeric materials.

Photocatalytic degradation of lignin and lignin models, using titanium dioxide: the role of the hydroxyl radical.

The role of hydroxyl radicals on the degradation of lignins during a cellulosic pulp bleaching process including a photocatalytic stage, was assessed using peroxyformic acid lignins EL1 and REL1 and two phenolic niphenyl lignin models 1 and 2. The irradiations were performed in the absence of photocatalyst TiO2 and H2O2 (condition a), in the presence of TiO2 (condition b) and in the presence of H2O2 (condition c). The experiments were conducted in alkaline (pH approximately 11) aqueous ethanol solutions with oxygen bubbling. The relative phenolic content of the irradiated solutions, which is indicative of the involvement of hydroxyl radicals, was determined by ionization absorption spectroscopy. The results obtained show that the catalyzed reaction involves both degradation of the phenolate groups by electron transfer and hydroxylation of the lignin aromatic structure. Benzyl alcohol structural elements in sodium borohydride reduced lignin REL1 and compound 2 were also found as good trapping agents for the hydroxyl radicals. The degradation of EL1 was studied by measuring its fluorescence emission by comparison to the fluorescence of compound 2. The emission spectra indicate that some biphenyl phenolate anions in EL1 are reacting under UV/visible irradiation and some others, probably polyphenolic chromophores emitting less fluorescence, are formed.

Adding value to Alfa grass (Stipa tenacissima L.) soda lignin as phenolic resins 1. Lignin characterization

Abstract Soda lignin from black liquors of Alfa grass was precipitated with hydrochloric acid at pH 2 and 4.5, and with sulfuric acid at pH 4.5. The molecular structures of the lignins were analyzed for the first time. The phenolic structures were studied using UV-visible spectroscopy and 31 P-NMR spectrometry. The lignin fragments were also analyzed by size exclusion chromatography and β- O -4 bond content was determined by thioacidolysis. The total phenolic content was found to be low for annual plant lignin, with more phenolic guaiacyl units than syringyl ones, and with a low proportion of condensed units. The phenolic syringyl elements appear to include β- O -4 bonds in large proportion in contrast with guaiacyl and p- hydroxyphenyl units.

Discoloration of α-Carbonyl-Free Lignin Model Compounds Under UV Light Exposure

Abstract A series of α-carbonyl-free lignin model compounds was prepared and adsorbed onto bleached chemical pulp. The rates of discoloration of these materials, under simulated solar light exposure, were established and compared to that of peroxide bleached methylated (alkali and dimethyl sulfate) and nonmethylated chemithermomechanical pulps. Phenolic phenylcoumarone and phenolic stilbenes were found to be the most sensitive chromophores under UV irradiation, both inducing a strong yellowing. Also, monophenolic biphenyl and biphenylmethane entities were shown to be prone to discolouration in contrast to the biphenolic and dimethylated ones. Moreover, the catechol structures which are known to be easily oxidizable into ortho quinones, display significant yellowing only when their absorption spectra are shifted above 300 nm where the light source emits. The behaviour of the phenolic stilbenes and the biphenyl catechol is reminiscent of the behaviour of CTMP. This indicates the possible involvement of such...

Synthesis, cation binding, and photophysical properties of macrobicyclic anthraceno-cryptands

The new cryptands, (1a) and (1b), possess remarkable spectroscopic and photophysical properties which are markedly affected by cation binding and by protonation.

A photochemical study of an O-methylated α-carbonyl β-1 lignin model dimer: 1,2-di(3′,4′-dimethoxyphenyl)ethanone (deoxyveratroin)

Abstract The photochemistry of 1,2-di(3′,4′-dimethoxyphenyl)ethanone (deoxyveratroin) ( 1 ) (an O-methylated α-carbonyl β-1 lignin model dimer) was examined. Its photochemical reactivity was studied in both liquid (benzene and ethanol) and solid (adsorbed on lignin-free fibres) states in degassed or aerated media. In degassed benzene and fibres, 1 is photochemically transformed into several identified compounds (monomeric to tetrameric units) and a coloured mixture of higher molecular weight fractions. The structures and titration of the photoproducts are well explained by Norrish type 1 cleavage and photoreduction of the carbonyl group. The presence of oxygen inhibits the reductive mechanism and leads to veratric acid by trapping the ketyl and desyl radicals. The observation of a low disappearance quantum yield of 1 in degassed ethanol is attributed to the presence of a low-lying π,π* triplet state as shown by phosphorescence.

Valorization of an industrial organosolv–sugarcane bagasse lignin: Characterization and use as a matrix in biobased composites reinforced with sisal fibers

In the present study, the main focus was the characterization and application of the by‐product lignin isolated through an industrial organosolv acid hydrolysis process from sugarcane bagasse, aiming at the production of bioethanol. The sugarcane lignin was characterized and used to prepare phenolic‐type resins. The analysis confirmed that the industrial sugarcane lignin is of HGS type, with a high proportion of the less substituted aromatic ring p‐hydroxyphenyl units, which favors further reaction with formaldehyde. The lignin–formaldehyde resins were used to produce biobased composites reinforced with different proportions of randomly distributed sisal fibers. The presence of lignin moieties in both the fiber and matrix increases their mutual affinity, as confirmed by SEM images, which showed good adhesion at the biocomposite fiber/matrix interface. This in turn allowed good load transference from the matrix to the fiber, leading to biobased composites with good impact strength (near 500 J m−1 for a 40 wt% sisal fiber‐reinforced composite). The study demonstrates that sugarcane bagasse lignin obtained from a bioethanol plant can be used without excessive purification in the preparation of lignocellulosic fiber‐reinforced biobased composites displaying high mechanical properties. Biotechnol. Bioeng. 2010;107:612–621.