Photochemical reactions of lignocellulose derived aromatic compounds

Cette thèse est à la fois une contribution au domaine de la recherche fondamentale en photochimie organique et une participation à la valorisation des composés aromatiques issus de la biomasse végétale. Les résultats sont divisés en deux parties.La première partie montre que les composés aromatiques dérivés de la lignine donnent une diversité de molécules complexes grâce à une réaction photochimique, après seulement quelques ajustements des groupements fonctionnels. Tout d’abord, une étude sur la compétition entre les photocycloaddition [2+2] et [2+3] et sur le rôle des états excités singulet (S1) et triplet (T1) est entreprise à partir de la simple vanilline. Ensuite, un nouveau dérivé de la bis-vanilline est préparé grâce à un couplage enzymatique. La réactivité photochimique de ce composé mène à un nouveau type de réarrangement qui ouvre de nombreuses perspectives théoriques et expérimentales. Enfin, l’irradiation de complexes d’inclusion de substrats aromatiques simples dans des cyclodextrines a permis d’obtenir pour la première fois l’induction d’un excès énantiomérique (34 %) dans la photocycloaddition [2+2] intramoléculaire entre un composé aromatique et un alcène.La deuxième partie repose sur une réaction de photooxygénation faite à l’échelle de 100 grammes qui permet de transformer le furfural dérivé de l’hémicellulose en 5 hydroxyfuranone. Une synthèse de nouveaux colorants polyméthines est mise en place à partir de ce composé plateforme. Des études physico-chimiques et théoriques effectuées en collaboration investiguent leurs propriétés spectroscopiques et montrent des applications potentielles comme photosensibilisateurs ou colorants responsifs.This thesis is a contribution to fundamental research in organic photochemistry on one hand and an effort to promote the use of aromatic compounds derived from biomass on the other hand. The results are divided into two parts.The first part shows that lignin-derived aromatic compounds yield a diversity of complex molecules thanks to a photochemical reaction preceded by few functional groups adjustments. First, a study of the competition between [2+2] and [2+3] modes of photocycloadditions and the role of the singlet (S1) and triplet (T1) excited states is carried out on vanillin. Then, a new compound derived from bis-vanillin is prepared thanks to an enzymatic coupling. The photochemical reactivity of this compound leads to a new type of rearrangement that opens many perspectives. Finally, the irradiation of inclusion complexes of simple aromatic compounds in cyclodextrins gave the first induction of an enantiomeric excess (34 %) in an intramolecular [2+2] photocycloaddition between an arene and an alkene.The second part is based on a photooxygenation reaction performed at the 100 grams scale that transforms hemicellulose-derived furfural into 5 hydroxyfuranone. The chemical synthesis of new polymethines dyes is established from this platform chemical. Their spectroscopic properties are investigated by collaborative physicochemical and computational studies that show potential applications as photosensitizers or responsive dyes

Synthesis and characterization of polymethine dyes carrying thiobarbituric and carboxylic acid moieties

International audienceAn efficient synthesis of polymethine dyes carrying thiobarbituric and a carboxylic acid moiety has been developed. Such compounds play a key role in many photometric detections and quantifications of enzyme activities. In such tests, the metabolite of the enzyme activities is transformed into a β-dicarbonyl derivative. In the present study, this compound was prepared from furfural through organic synthesis. Its in situ transformation with thiobarbituric acid derivatives yields the target compounds on a gram-scale (0.4 to 0.6 g). A combined experimental and theoretical study of the photophysical properties of the synthesized compounds was carried out. Absorption and emission spectroscopy measurements highlighted a slight solvatochromism effect. The luminescence was quenched by molecular oxygen, indicating the partial triplet multiplicity character of the lowest excited state. Density Functional Theory (DFT) calculations have been applied for the evaluation of favoured conformations for these new compounds and the study of their optical properties. Within the Franck Condon principle, vibrationally resolved electronic one-photon absorption spectrum has been simulated. This simulation shows the presence of a major band followed by a vibronic sideband, typical of organic chromophores in solution. The performed computational study revealed that the transition from the ground to the first excited electronic state has a π-π* character. Finally, TD-DFT energy level diagram calculations highlighted the presence of triplet states very close to the first singlet excited one, suggesting probable access to the triplet-excited state