Mixed Organic Counterion Strategy Modulates the Self-Assembly of Polyoxometalate Hybrids into Toroids and Affects Their Photochromic and Photocatalytic Properties

Abstract

A series of five mixed-counterion polyoxometalate (POM) hybrids 1−5 have been developed starting from the [SiW 12 O 40 ] 4− cluster and two different aryl sulfonium counterions (ASCIs), (4-hydroxyphenyl)dimethylsulfonium (HPDS) and (4-formylphenyl)dimethylsulfonium (FPDS), bearing different functional groups −OH and −CHO, respectively. The HPDS/FPDS counterion ratio in hybrids 1−5 varied as 4:0 (1), 3:1 (2), 2:2 (3), 1:3 (4), and 0:4 (5), and it resulted in a morphological transition of the self-assembly of these hybrids. Hybrid 1 exhibited spherical self-assembly, while hybrid 5 exhibited large toroidal self-assembly predominantly. The hybrids 2−4 gave a mixture of spherical, dimpled spherical, and toroidal self-assemblies in varying amounts, indicating a gradual transition of self-assembly from spherical to toroidal as the percentage of FPDS increases in the system. A plausible explanation for the differences in the self-assembly properties of these hybrids has been proposed based on the differences in the H-bonding interactions of the two counterions with the dispersion medium. The HPDS and FPDS counterions exhibited different HOMO− LUMO gaps, and their varying percentages in hybrids 1−5 led to differences in optical band gaps and photochromic activities. Hybrid 2 showed the lowest band gap value and the best photochromic activity (t 1/2 = 0.72 min) in the series. The influence of the mixed-counterion strategy in catalysis has been demonstrated by evaluating the photocatalytic activity of these hybrids toward rhodamine B dye degradation. The hybrid 4, exhibiting stable toroidal self-assembly, showed the best catalytic activity in the series, probably due to a combination of reasons such as the enhanced surface area due to toroid formation and also the presence of Hbonding −OH functionality on the counterion. Therefore, the present study suggests a new method for altering the self-assembly and the materials and catalytic properties of POM hybrids through a mixed-counterion strategy.

Self-Sulfuration and Carbonization of a Mixed-Metal Aryl Sulfonium Polyoxometalate Hybrid: A Path to Electrocatalytically Active Ternary Composite

“…The W 4f scans (Figure a–d) showed the first set of doublets in the range of 35.5–36 and 37.7–38.2 eV owing to W 6+ 4f 7/2 and W 6+ 4f 5/2 . The next set of doublets were observed at 34.5–34.9 and 36.3–37.2 eV, which are attributed to W 5+ 4f 7/2 and W 5+ 4f 5/2 . It was also noticed that the amount of mixed valent species of W (6+ and 5+) gradually increases with increasing calcination temperature.…”

“…However, a few broad peak-like features could be seen near 8–10, 15–40, and 45–70°. The presence of these peaks, particularly the broad peak in the 8–10° region, indicates the presence of Keggin clusters in the sample . In the case of AS-CoPOM700 and AS-CoPOM900 , a significant improvement in the crystallinity of the materials could be observed.…”

“…Thus, from the TEM analysis, it was confirmed that the best arrangement of the CoWO 4 /WS 2 /WO 3 @C nanocomposite was obtained after calcination of AS-CoPOM at 900 °C, where the rod and sphere-like nanostructures of CoWO 2 eV owing to W 6+ 4f 7/2 and W 6+ 4f 5/2 . 7 The next set of doublets were observed at 34.5−34.9 and 36.3−37.2 eV, which are attributed to W 5+ 4f 7/2 and W 5+ 4f 5/2 . 7 It was also noticed that the amount of mixed valent species of W (6+ and 5+) gradually increases with increasing calcination temperature.…”

“…29 The synthesis of DPDST from DPMS was also achieved using a reported protocol (Scheme S1, Supporting Information (SI)). 7 The formation of DPDST was confirmed by positive mode electrospray ionization-mass spectrometry (ESI-MS) and 1 H NMR spectroscopy. The positive mode ESI-MS spectra showed a peak at m/z = 295.2406, corresponding to the molecular ion peak of DPDS.…”

“…The properties of a POM cluster, such as its solubility, stability, and redox behavior, can be modulated by the charge balancing counterions . Among the various organic counterion motifs used in POM chemistry, the alkylammonium and phosphonium counterions are the most widely explored for preparing POM-based ionic liquids (POM-ILs). − On the other hand, rigid aromatic cations serve as structure-directing units, forming ordered organic–inorganic hybrid materials. − Such hybrids can be subjected to thermal treatment, where the neighboring POM clusters undergo condensation to produce nanostructured bulk metal oxides. The topography and nanostructure of the resulting oxide depend on the nature of the precursor .…”