title: Kinetics of coloration in photochromic organoammonium polyoxomolybdates.
abstract: The excellent photochromic properties of (H(2)DABCO)(2)(HDMA)(0.5)Na(0.75)(H(3)O)(0.75)[Mo(8)O(27)] x 3 H(2)O (4), a new member of the (H(2)DABCO)(2)(A)(x)[Mo(8)O(27)] x n H(2)O series, are compared with those of (H(2)DABCO)(2)(NH(4))(2)[Mo(8)O(27)] x 4 H(2)O (1), (H(2)DABCO)(2)(H(2)pipz)[Mo(8)O(27)] (2), and (H(2)pipz)(3)[Mo(8)O(27)] (3). All these powdered materials turn from white to purple under illumination at 365 nm, which is associated with photoreduction of Mo(6+) cations into Mo(5+) cations. We show that the rates of coloration, which increase in the order 1 < 3, 2 < 4, are related to the decrease in the concentration of reducible Mo(6+) centers with irradiation time and follow a second-order reaction law because the event of light absorption at a reducible Mo(6+) site does not necessarily coincide with that of the N(+)-H bond breaking in the N(+)-H...O hydrogen bond associated with the Mo(6+) site. First-principles density functional electronic structure calculations were carried out to find that this trend correlates with the homolytic dissociation energies of the N(+)-H bonds in the organic cations HDMA(+), H(2)pipz(2+), H(2)DABCO(2+), and NH(4)(+). This observation is consistent with a photochromic mechanism based on the homolytic cleavage of N(+)-H bonds rather than on the heterolytic cleavage of N(+)-H bonds.
authors.affiliation: Institut des Materiaux Jean Rouxel, Universite de Nantes, CNRS, 2 rue de la Houssiniere, BP 32229, 44322 Nantes, France. remi.dessapt@cnrs-imn.fr
authors.forename: Rémi
authors.initials: R
authors.lastname: Dessapt
authors.forename: Mathieu
authors.initials: M
authors.lastname: Collet
authors.forename: Violaine
authors.initials: V
authors.lastname: Coué
authors.forename: Martine
authors.initials: M
authors.lastname: Bujoli-Doeuff
authors.forename: Stéphane
authors.initials: S
authors.lastname: Jobic
authors.forename: Changhoon
authors.initials: C
authors.lastname: Lee
authors.forename: Myung-Hwan
authors.initials: MH
authors.lastname: Whangbo
mesh_terms: D003116:Color; D006859:Hydrogen / Q000737:chemistry; D006860:Hydrogen Bonding; D007700:Kinetics; D008956:Models, Chemical; D008982:Molybdenum / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D055668:Photochemical Processes; D011789:Quantum Theory; D000644:Quaternary Ammonium Compounds / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet
chemical_list: D009942:Organometallic Compounds; D000644:Quaternary Ammonium Compounds; C044659:molybdate; D006859:Hydrogen; D008982:Molybdenum