Hybrid organic-inorganic polyoxometalates (POMs) were synthesized in water by the reaction of a Mo VI precursor with bisphosphonate ligands functionalized by pyridine groups. The fully oxidized POM [(Mo VI 3O8)2(O)(O3PC(O)(C3H6NH2CH2C5H4NH)PO3)2] 4− has been isolated as water insoluble pure Na salt (NaMo 6 (Ale-4Py) 2) or mixed Na/K salt (NaKMo 6 (Ale-4Py) 2) and their structure solved using single-crystal X-ray diffraction. The mixed-valent complex [(Mo V 2O4)(Mo VI 2O6)2{O3PC(O)(C3H6N(CH2C5H4N)2(Mo VI O3))PO3}2] 8− was obtained as an ammonium salt (NH 4 Mo 6 (AlePy 2 Mo) 2), in the presence of a reducing agent (hydrazine). 31 P NMR spectroscopic studies in aqueous media have allowed determining the pH stability domain of NH 4 Mo 6 (AlePy 2 Mo) 2. NaMo 6 (Ale-4Py) 2 and NaKMo 6 (Ale-4Py)2 exhibit remarkable solid-state photochromic properties in ambient conditions. Under UV excitation, they develop a very fast color-change from white to deep purple and proved to be the fastest photochromic organoammonium/POM systems. The coloration kinetics has been fully quantified for both salts and is discussed in light of the hydrogen-bonding networks.

A cationic boron dipyrromethene (BODIPY) derivative (1 + ) has been successfully combined with two polyoxometalates (POMs), the Lindqvist-type [W6O19] 2and the β-[Mo8O26] 4units, into three new supramolecular fluorescent materials(1)2[W6O19]⋅2CH3CN, (1)2[W6O19] and (1)4[Mo8O26]⋅DMF⋅H2O. The resulting hybrid compounds have been fully characterized by a combination of single-crystal X-ray diffraction, IR and UV-Vis spectroscopies and photoluminescence analyses. This self-assembly approach prevents any π-π stacking interactions between the BODIPY units, responsible for aggregation-caused quenching (ACQ) effects, but also between the BODIPY and the POMs, avoiding intermolecular charge-transfer effects. Noticeably, the POM units do not only act as bulky spacers but their negative charge density drives the molecular arrangement of the 1 + luminophore, strongly modifying its fluorescence in the solid state. As a consequence, the 1 + cations are organized into dimers in (1)2[W6O19]⋅2CH3CN and (1)2[W6O19], which are weakly emissive at room temperature, and in a more compact layered assembly in (1)4[Mo8O26]⋅DMF⋅H2O which exhibits a redshifted and intense emission upon similar photoexcitation.