Kinetics of Coloration in Photochromic Organoammonium Polyoxomolybdates

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• 作者：Ré ; mi Dessapt ; Mathieu Collet ; Violaine Coué ; Martine Bujoli-Doeuff ; Sté ; phane Jobic ; Changhoon Lee ; Myung-Hwan Whangbo
• 刊名：Inorganic Chemistry
• 出版年：2009
• 出版时间：January 19, 2009
• 年：2009
• 卷：48
• 期：2
• 页码：574-580
• 全文大小：224K
• 年卷期：v.48,no.2(January 19, 2009)
• ISSN：1520-510X

文摘

The excellent photochromic properties of (H₂DABCO)₂(HDMA)_0.5Na_0.75(H₃O)_0.75[Mo₈O₂₇]·3H₂O (4), a new member of the (H₂DABCO)₂(A)_x[Mo₈O₂₇]·nH₂O series, are compared with those of (H₂DABCO)₂(NH₄)₂[Mo₈O₂₇]·4H₂O (1), (H₂DABCO)₂(H₂pipz)[Mo₈O₂₇] (2), and (H₂pipz)₃[Mo₈O₂₇] (3). All these powdered materials turn from white to purple under illumination at 365 nm, which is associated with photoreduction of Mo⁶⁺ cations into Mo⁵⁺ 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 Mo⁶⁺ centers with irradiation time and follow a second-order reaction law because the event of light absorption at a reducible Mo⁶⁺ site does not necessarily coincide with that of the N⁺−H bond breaking in the N⁺−H···O hydrogen bond associated with the Mo⁶⁺ 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⁺, H₂pipz²⁺, H₂DABCO²⁺, and NH₄⁺. 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.