Carboranethiol-Protected Propeller-Shaped Photoresponsive Silver Nanomolecule

Abstract

We report the synthesis, structural characterization, and photophysical properties of a propeller-shaped Ag 21 nanomolecule with six rotary arms, protected with m-carborane-9-thiol (MCT) and triphenylphosphine (TPP) ligands. Structural analysis reveals that the nanomolecule has an Ag 13 central icosahedral core with six directly connected silver atoms and two more silver atoms connected through three Ag−S−Ag bridging motifs. While 12 MCT ligands protect the core through metal−thiolate bonds in a 3−6−3-layered fashion, two TPP ligands solely protect the two bridging silver atoms. Interestingly, the rotational orientation of a silver sulfide staple motif is opposite to the orientation of carborane ligands, resembling the existence of a bidirectional rotational orientation in the nanomolecule. Careful analysis reveals that the orientation of carborane ligands on the cluster's surface resembles an assembly of double rotors. The zero circular dichroism signal indicates its achiral nature in solution. There are multiple absorption peaks in its UV−vis absorption spectrum, characteristic of a quantized electronic structure. The spectrum appears as a fingerprint for the cluster. High-resolution electrospray ionization mass spectrometry proves the structure and composition of the nanocluster in solution, and systematic fragmentation of the molecular ion starts with the loss of surface-bound ligands with increasing collision energy. Its multiple optical absorption features are in good agreement with the theoretically calculated spectrum. The cluster shows a narrow near-IR emission at 814 nm. The Ag 21 nanomolecule is thermally stable at ambient conditions up to 100 °C. However, white-light illumination (lamp power = 120−160 W) shows photosensitivity, and this induces structural distortion, as confirmed by changes in the Raman and electronic absorption spectra. Femtosecond and nanosecond transient absorption studies reveal an exceptionally stable excited state having a lifetime of 3.26 ± 0.02 μs for the carriers, spread over a broad wavelength region of 520−650 nm. The formation of core-centered long-lived carriers in the excited state is responsible for the observed light-activated structural distortion.

Atomically Precise Clusters: Chemical Evolution of Molecular Matter at the Nanoscale

“…Another report from our group showed a propeller-shaped structure with six rotary arms in a Ag 21 nanocluster, primarily protected by meta-carborane 9-thiol ligands. 46 The staggered arrangements of six peripheral carboranes determine the rotary nature of the cluster. Therefore, efficient tuning of the ligand shell will result in various structural frameworks.…”

“…Figure 2: Illustration of the structural model of monolayer protected metal nanoclusters having different types of multilayered core-shell geometry, interfacial motifs and surface ligands. Reproduced with permission from Ref27,32,46,50,51,65 …”

Stepwise structural evolution toward robust carboranealkynyl-protected copper nanocluster catalysts for nitrate electroreduction

“…Bulky ligands strategy is effective to give accessible or preorganized open metal sites by taking the advantage of the steric effect (22,23). Pradeep group and we reported a series of coinage metal NCs passivated by bulky carborane derivatives with labile coprotective molecules in the ligand shell, which favors the posttreatment to expose open metal atoms and the accurate identification of the active sites (24)(25)(26)(27)(28).…”

Photoconversion of Ag31 to Ag42 Initiated by Solvated Electrons

“…Atomically precise metal nanoclusters (NCs) with a kernel of metal atoms, protected by various ligands, connecting the gap between traditional nanoparticles and single atoms, have emerged as a new class of functional nanomaterials with unique optical and physicochemical properties. − Such type of ultrasmall nanoparticles with a core dimension <3 nm displayed exotic optical properties, such as molecule-like multiple absorption and emission due to the electronic transitions between their discrete energy levels. − A wide range of silver NCs with varying nuclearities, such as Ag 6 , Ag 14 , Ag 16 , Ag 21 , Ag 25 , Ag 29 , Ag 35 , Ag 40 –Ag 46 , Ag 61 , Ag 67 , Ag 146 , Ag 374 , and so forth, with protecting ligands were synthesized, and some of them have been structurally characterized using single-crystal X-ray diffraction studies. − ,− Atomic arrangements of such nanomaterials, associated energy levels, and surface reactivity promote their potential applications in the fields of catalysis, molecular recognition, biomedical imaging, and optoelectronic devices. − The stability of silver NCs mainly depends on the outer coverage of surface ligands. The bare NCs without ligands are highly reactive, and they react to themselves or with species in the surrounding environments and form larger particles or adduct species .…”