Conformational analysis of some pyridinium phenolates and synthetic precursors based on X-Ray and IR characterisations

7 pagesInternational audienceThis manuscript reports X-Ray and IR characterizations of representative pyridinium phenolates, model compounds for nonlinear optics. These analyses reveal the close dependence existing between molecular structure and the contribution of quinone and zwitterionic limiting forms. The bond length alternation (BLA) values, the well-known parameter correlated to hyperpolarisability β, are also discussed and compared with literature data

Synthèse de phénolates de pyridinium, composés modèles pour l optique non linéaire quadratique: étude de la relation entre la conformation de ces chromophores et leurs propriétés optiques et physico-chimiques

Les biaryles de type phénolates de pyridinium constituent, à cause de leur caractère zwitterionique, des composés modèles pour l optique non linéaire quadratique (ONL). Des simulations numériques ont mis en évidence que les propriétés ONL de ces chromophores dépendent de l angle de torsion existant entre les deux cycles aromatiques. Afin de vérifier expérimentalement cette relation structure/activité, des phénolates de pyridinium substitués en positions ortho de la liaison intercycle ont été préparés, l étape clé de ces synthèses étant un couplage de Suzuki. Plus les groupements alkyle introduits sont stériquement encombrants, plus l angle dièdre est important. Par ailleurs, ce changement conformationnel se répercute également sur les propriétés photo-, électro- et physico-chimiques du chromophore (pKa de la fonction phénol, solvatochromie ).Pyridinium phenoxides, a family of zwitterionic biaryl-like molecules, are model compounds for quadratic non linear optics (NLO). Numerical simulations indicate that the quadratic NLO properties of these chromophores depend on the twist angle existing between the two aromatic rings. In order to experimentally verify this structure/activity relationship, pyridinium phenoxides substituted at the ortho positions of the intercyclic bond were synthesized using a Suzuki cross-coupling reaction as key step. The bulkier the introduced alkyl groups are, the higher the twist angle is. Furthermore, this conformational change also affects the photo-, electro- and physicochemical properties of the biaryl (pKa of the phenol function, solvatochromism )MULHOUSE-SCD Sciences (682242102) / SudocSudocFranceF

Syntheses of extreme sterically hindered 4-methoxyboronic acids

International audience4-Iodoanisoles 3a,b, 3d and 4-bromoanisoles 4a–d were readily obtained. An extreme steric hindrance precluded obtaining 3c. Catalytic borylation of 3a,b, 3d followed by hydrolysis of boronic ester 26a,b, 26d easily provided the boronic acids 5a,b, 5d. Compounds 5a and 5d were also synthesised, starting from 4a and 4d, by halogen/metal exchange. Because of a too important steric hindrance, this last reaction failed with 4c and 4b led to the unexpected but stable boronic ester 6. The final obtaining of 5b required a strongly basic hydrolysis with heating

Influence du stage actif chez le praticien sur le sentiment de compétence professionnelle de l’étudiant en chirurgie dentaire

Contexte : Un stage actif d’initiation à la vie professionnelle a été mis en place dans le cursus des étudiants en odontologie au cours de l’année universitaire 2007–2008. À l’UFR de Clermont-Ferrand, les étudiants en dernière année d’études exercent pendant deux mois à temps complet au sein d’un cabinet libéral sous la responsabilité d’un maître de stage. Buts: L’objectif de cette étude est de mettre en évidence l’influence de ce stage sur le sentiment de compétence ou d’efficacité ressenti par les étudiants. Collectifs et méthodes : Une promotion d’étudiants (n = 42) a évalué, grâce à une échelle visuelle analogique non graduée, son niveau de performance concernant 52 capacités professionnelles définies par le réseau thématique européen DentEd. Cette auto-évaluation s’est faite à trois reprises : en fin de cinquième année (D3) et au cours de la dernière année d’études (T1) avant puis après le stage actif. Les maîtres de stage qui ont accueilli les étudiants ont à leur tour évalué les capacités des étudiants en utilisant le même procédé. Résultats : En comparant les résultats des trois auto-évaluations par des tests statistiques, notre étude a montré un réel impact positif du stage actif sur le sentiment de compétence perçu par nos étudiants concernant des capacités professionnelles telles que la gestion financière et administrative, la comptabilité, la stérilisation, la gestion des personnels et l’élaboration d’un plan de traitement multidisciplinaire. Pour la plupart des capacités ou habiletés évaluées, il n’existe pas de différence significative entre l’évaluation des étudiants et celle des maîtres de stage. Conclusion : Le stage actif donne à l’étudiant le sentiment de mieux maîtriser certaines capacités ou habiletés qui sont difficilement acquises en milieu hospitalier

A 13C NMR Study of pyridinium phenoxide series with increasing sterical hindrance reveals the dramatic influence of torsion on their structure

International audienceThe 13C resonance signals of five twisted pyridinium phenoxides has been assigned in two different solvents (CD3OD and D6-DMSO), while the torsion angle was varied by changing the pyridinium substituents at ortho positions of the intercyclic bond. The experimental 13C chemical shifts of these compounds were adjusted using calculating shift parameters evaluated from reference compounds, revealing the changes of 13C signals due to the different interplanar angles. A dramatic modification of the structure was observed as the angle increases (transition from quinone form to zwitterion one), adding a piece of information on the still debated question: the relative contributions of the two limit forms (quinone, zwitterion) in a pyridinium phenoxide series. Then the ability of four other twist compounds, bearing no ‘‘protected’’ groups at ortho position of the phenoxide function, to rapid deuteration was studied. This property is once more related to the twist structure of pyridinium phenolates

Negative solvatochromism of a series of pyridinium phenolate betaine dyes with increasing steric hindrance

International audienceThe negative solvatochromism of a series of pyridinium phenolates betaine dyes is investigated using three approaches: (a) empirical, based on the empirical solvent polarity parameter ET(30); (b) semi-empirical, based on the p⁄, a, b set of parameters used in Kamlet-Taft's Solvatochromic Comparison Method (SCM); and (c) physical, by means of the Onsager reaction-field model of solvents. Comparison of the obtained correlations underscored the roles of both the two tert-butyl groups ortho to the oxygen atom and the interplanar angle between the phenyl rings on the very blue shifts observed as the polarity increases

Photoinduced intramolecular electron transfer in a 2,7-diaminofluorene chromophore decorated with two benzophenone subunits

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Photoinduced coupled charge and proton transfers in gradually twisted phenol-pyridinium biaryl series

International audienceA detailed photophysical analysis of a phenol−pyridinium biphenyl series with gradual twisted geometry is presented in this paper. The low-energy CT absorption band of the compounds undergoes a decrease of intensity with a progressive blue shift by increasing the twist angle of the central bond (ΘAD). These effects are well described and quantified within the framework of the Mulliken−Murrel approach, which allows us to extend such a model to the charge-shift process. The biaryl compounds exhibit broadened fluorescence bands assigned to the radiative deactivation of a charge shift (CSh) species generated by an intramolecular twisting relaxation of the locally excited (LE) state. Parallel to the rotamerism of the central single bond, excited-state proton-transfer (ESPT) processes are occurring from both excited states and lead to the nonemissive phenolate forms. Solvatochromic shifts of the emission bands are correlated by the Kamlet−Taft parameters (π*, α, and β). The correlation first confirms the π* dependence of the CSh band shift but also demonstrates a clear β dependence. The contribution of the latter parameter to the band hypsochromy is markedly increasing with ΘAD. Such an unusual effect was ascribed to a much higher ESPT rate relative to the highly twisted conformation with respect to that of more planar geometry. Despite the suppression of the geometrical relaxation in ethanol glass at 77 K, the fluorescence of the phenolate species produced by ESPT from LE state is detected. The relative increase of its fluorescence band intensity with ΘAD confirms the gradual enhancement of the excited states acidity