Covalent Grafting of Eosin Y to the Giant Keplerate {Mo132} through an Organosilicon Linker in Homogeneous Regime

Abstract

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.

Keplerate {Mo132}–Stearic Acid Conjugates: Supramolecular Synthons for the Design of Dye-Loaded Nanovesicles, Langmuir–Schaefer Films, and Infochemical Applications

“…This constraint has been overcome through the recent development of a covalent grafting approach for giant POM with the Keplerate structure, which utilizes 3-aminopropyltrimethoxysilane (APTMS). Through this synthetic approach, Keplerate {Mo 132 } can be decorated with anionic dyes such as fluorescein (FL) and eosin Y (EY), leading to the emergence of a novel charge transfer band from POM to dye. , In the context of electrostatic association with cationic dyes, the photoinduced electron transfer (PET) process takes place from the excited state of the dye to the lowest unoccupied molecular orbital (LUMO) level of POM. During covalent conjugation, the charge transfer occurs from the ground state of POM to the LUMO of dye molecules.…”

A polyacrylamide–chitosan semi-interpenetrating self-healing network with embedded Keplerate {Mo132} for pH-controlled release of Eu-fluorescent tags

“…The Keplerate {Mo 132 } = (NH 4 ) 42 [Mo VI 72 Mo V 60 O 372 (CH 3 COO) 30 (H 2 O) 72 ]·(∼300H 2 O)·(∼10CH 3 COONH 4 ) readily dissolves in aqueous media, forming multicharged macroanions (maximal charge is 42-). 4 The Keplerate {Mo 132 }'s surface can be flexibly functionalized through covalent grafting, 5,6 electrostatic interactions, 7 hydrogen bonding, and van der Waals interactions. 8 Driven by the chaotropic effect, polyoxoanions can form dynamic supramolecular ensembles with cyclodextrins.…”