Kinetics of coloration in photochromic organoammonium polyoxomolybdates.

The excellent photochromic properties of (H2DABCO)2(HDMA)0.5Na0.75(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...