The template effect of giant polyoxometalates (POM) shows promising results towards the supramolecular design of hybrid materials suitable for photocatalytic reactions. Here, we demonstrate a novel synthetic approach for covalently grafting the xanthene dye eosin Y (EY) to the nanoscale Keplerate POM {Mo132} via an organosilicon linker (3-aminopropyltrimethoxysilane, APTMS) in a homogeneous regime. Using a phase transfer agent, tetrabutylammonium bromide, we solubilize the Keplerate POM modified with six {Si(CH2)3NH2} groups, {Mo132}@Si6, in a series of organic solvents—acetonitrile, acetone, tetrahydrofuran, and dichloromethane—to perform post-functionalization by using an NHS-ester of EY. Both IR and Raman spectroscopy affirm the preservation of the POM’s structure and showcase an amide bond formation between POM and EY in the obtained conjugate {Mo132}@Si6@EY@TBA. Grafting’s success is observed through significant downfield shifting of EY’s aromatic protons’ signals on the 1H NMR spectrum as compared to the spectra of EY and EY-NHS. The current synthetic approach enables us to exercise precise control of the stoichiometry in the POM-dye conjugates—1:1 for the POM-EY system—as confirmed by elemental analysis. Comprehensive photophysical analysis of {Mo132}@Si6@EY@TBA by means of UV-Vis and steady-state and time-resolved fluorescence measurements points to an existing strong interaction between molecular orbitals of EY and {Mo132}, leading to a photoinduced electron transfer, partial fluorescence quenching, and elongation of the excited state’s lifetime. These findings demonstrate that using APTMS as an organosilicon linker in tandem with the Keplerate POM as a nanoscale template can be readily applied as a routine synthetic procedure for grafting various organic dyes or other organic molecules bearing a carboxylic group in their structure to the giant POM surface in a variety of aprotic organic solvents.

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