Kinetics of Coloration in Photochromic Organoammonium Polyoxomolybdates

The excellent photochromic properties of (H(2)DABCO)(2)(HDMA)(0.5)Na-0.75(H3O)(0.75)[Mo8O27]center dot 3H(2)O (4), a new member of the (H(2)DABCO)(2)(A)(x)[Mo8O27]center dot nH(2)O series, are compared with those of (H(2)DABCO)(2)(NH4)(2)[Mo8O27]center dot 4H(2)O (1), (H(2)DABCO)(2)(H(2)pipz)[Mo8O27] (2), and (H(2)pipz)(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 Mos+ site does not necessarily coincide with that of the N+-H bond breaking in the N+-H center dot center dot center dot 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(+), H(2)pipz(2+), H(2)DABCO(2+), 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.