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.

Organic–inorganic hybrid rare earth complexes based on polymolybdates with intrinsic photosensitive properties

“…1a, atom distances of N1…O3 and N2… O56 being 2.959 and 3.031 Å, respectively), while no obvious counteractions between the crystal PO molecules and {Mo 36 } clusters. Therefore, it is supposed that the POM's backbone may 10 transfer its protons to the covalently grafted PO molecules under excitation, thus the N + -H…O hydrogen bonds at the organicinorganic interface are formed, which then leading to the photosensitive phenomenon [3,10,12] . In any case, this demonstrates that the organic-inorganic hybrid {Mo 36 } clusters 15 may be a class of perfect photosensitive materials under visible light irradiation, although the color change is irreversible.…”

“…One of the promising systems in this field are hybrid organic− inorganic materials based on polyoxomolybdates (POM's) and 35 organic cations (mainly organoammonium cations) [3][4][5][6][7]11] . To date, the photochromism mechanism of such materials has been investigated in detail.…”

“…To date, the photochromism mechanism of such materials has been investigated in detail. Namely, the physical process involved implies the photoreduction of Mo(Ⅵ) cations into Mo(Ⅴ) ones within the POM's block, with a concomitant displacement of a 40 labile hydrogen atom from the N + -H bond of the organic component to the mineral one [2][3][4]12]. Nevertheless, it must be underlined that the photogenerated hue of such hybrid materials depends on the chemical composition and the topology of the POM's, whatever the nature of the 45 associated organoammonium cations.…”

“…As a result, a large amount of POM's, such as {Mo 3 }, {Mo 6 }, {Mo 7 }, {Mo 8 } clusters [2,3,12] , as well as some phosphomolybdates [4][5][6][7] have been selected as the inorganic components of photochromic hybrids. Although such hybrid materials offer a wide range of UV-induced coloration 50 with remarkable coloration contrast, very few of them are sensitive to visible light [3,9] . Indeed, visible-light photochromism will be useful in compact storage devices design as well as solar energy conversion applications [9] .…”

Addition of Metalated 3‐Alkyl‐Substituted Alkoxyallenes to Imines: Preparation of Tetrasubstituted 2,5‐Dihydropyrroles, Pyrrolidin‐3‐ones, and Pyrroles

“…[1][2][3][4] Compounds based on polyoxometalates (POMs) of early transition metals, such as Mo or W, can be excellent candidates, due to their facile reduction and accompanying color changes. [11][12][13][14][15][16][17] Two mechanisms have been proposed, both of which involve UV-induced O→Mo/W ligand-to-metal charge-transfer (LMCT) transitions on the polyoxomolybdate or polyoxotungstate, affording a metastable charge-transfer state that contains Mo 5+ or W 5+ ions. [5][6][7][8] The conceptually simplest approach to photochromic POM-based compounds has been covalent grafting of organic photochromes, such as spiropyrans, to the POM framework, [9,10] which has not necessarily exploited the redox properties of the POM.…”