Tailoring the Solid-State Fluorescence of BODIPY by Supramolecular Assembly with Polyoxometalates

A cationic boron dipyrromethene (BODIPY) derivative (1(+)) has been successfully combined with two polyoxometalates (POMs), the Lindqvist-type [W6O19](2-) and the beta-[Mo8O26](4-) units, into three new supramolecular fluorescent materials (1)(2)[W6O19]center dot 2CH(3)CN, (1)(2)[W6O19], and (1)(4)[Mo8O26]center dot DMF center dot 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 pi-pi stacking interactions not only 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 luminophore, strongly modifying its fluorescence in the solid state. As a consequence, the 1(+) cations are organized into dimers in (1)(2)[W6O19]center dot 2CH(3)CN and (1)(2)[W6O19], which are weakly emissive at room temperature, and in a more compact layered assembly in (1)(4)[Mo8O26]center dot DMF center dot H2O, which exhibits a red-shifted and intense emission upon similar photoexcitation.