Tuning the Photochromic Properties of Molybdenum Bisphosphonate Polyoxometalates

Seven hybrid organic–inorganic bisphosphonate
molybdenum­(VI)
polyoxometalate complexes with the general formula [(Mo3O8)4(O3PC­(CmH2mNRR′R″)­(O)­PO3)4]8– (m = 3;
R, R′, and R″ = H or CH3) and [(Mo3O8)2(O)­(O3PC­(CmH2mNRR′R″)­(O)­PO3)2]6– (m = 3
or 4; R, R′, and R″ = H or CH3) have been
synthesized and their structures solved using single-crystal X-ray
diffraction. These compounds are made of a {Mo12} or a
{Mo6} inorganic core functionalized by various alkylammonium
bisphosphonates, with these ligands differing by the length of their
alkyl chains and the number of methyl groups grafted on the N atom.
The nature of the counter-cations (Na+, K+,
Rb+, Cs+, and/or NH4+)
constituting these materials has also been modulated. 31P NMR spectroscopic studies in aqueous media have shown that all
the dodecanuclear complexes reported here are stable in solution,
whereas for the hexanuclear compounds, a dynamic equilibrium between
two isomers has been evidenced, and the corresponding standard thermodynamic
parameters determined for one of them. The electrochemical properties
of six representative compounds of this family have been investigated.
It has been found that the Mo6+/Mo5+ reduction
potential is similar for all the polyoxometalates studied. Besides,
it is shown that electrochemical cycling is an efficient method for
the deposition of these compounds on a surface. The photochromic properties
of all the complexes reported herein have been studied in the solid
state. Under irradiation in the near ultraviolet (UV), the {Mo12} systems shift from white to reddish-brown, while the {Mo6} compounds develop a purple coloration. The coloration kinetics
has been systematically quantified and the optical band gaps, the salient coloration kinetic parameters and the coloration kinetic
half-life times have been determined. This has evidenced that several
of these materials develop very strong and rapid UV-induced color
changes, with remarkable coloration contrasts. Finally, the optical
properties of these systems are discussed in light of several salient
parameters as the POM topology, the nature of the grafted bisphosphonate
ligand, and the design of the hydrogen-bonding network at the organic–inorganic
interface.