title: Photoinduced proton transfer promoted by peripheral subunits for some Hantzsch esters.
abstract: It is noted that, for a small series of 3,5-diacetyl-1,4-dihydrolutidine (DDL) derivatives and the corresponding Hantzsch esters, the presence of methyl groups at the 2,6-positions serves to extinguish fluorescence in solution but not in the solid state. Emission is weakly activated and affected by changes in solvent polarity. The latter situation arises because the optical transition involves intramolecular charge transfer. Calculations, both semiempirical and DFT, indicate that, in all cases, rotation of the carbonyl function is facile and that the dihydropyridine ring is planar. These calculations also indicate that the 2,6-methyl groups do not affect the generic structure of the molecule. It is proposed that illumination increases the molecular dipole moment and pushes electron density toward the carbonyl oxygen atom. Proton transfer can now occur from one of the methyl groups, leading to formation of a relatively low-energy, neutral intermediate, followed by a second proton transfer step that forms the enol. Reaction profiles computed for the ground-state species indicate that this route is highly favored relative to hydrogen transfer from the 4-position. The barriers for light-induced proton transfer are greatly reduced relative to the ground-state process but such large-scale structural transformations are hindered in the solid state. A rigid analogue that cannot form an enol is highly emissive in solution, supporting the conclusion that proton transfer is in competition to fluorescence in solution.
authors.affiliation: Institut de Chimie et Procédés pour l'Energie, l'Environnement et la Santé (ICPEES), Laboratoire de Chimie Moléculaire et Spectroscopies Avancées (ICPEES-LCOSA), UMR 7515 au CNRS, Ecole Européenne de Chimie, Polymères et Matériaux , 25 rue Becquerel, 67087 Strasbourg Cedex 02, France.
authors.forename: Sébastien
authors.initials: S
authors.lastname: Azizi
authors.forename: Gilles
authors.initials: G
authors.lastname: Ulrich
authors.forename: Maud
authors.initials: M
authors.lastname: Guglielmino
authors.forename: Stéphane
authors.initials: S
authors.lastname: le Calvé
authors.forename: Jerry P
authors.initials: JP
authors.lastname: Hagon
authors.forename: Anthony
authors.initials: A
authors.lastname: Harriman
authors.forename: Raymond
authors.initials: R
authors.lastname: Ziessel
mesh_terms: D004095:Dihydropyridines / Q000737:chemistry*; D004952:Esters / Q000737:chemistry*; D015394:Molecular Structure; D055668:Photochemical Processes; D011522:Protons; D011789:Quantum Theory
chemical_list: D004095:Dihydropyridines; D004952:Esters; D011522:Protons; C003953:3,5-diacetyl-1,4-dihydrolutidine