We report the X-ray crystal structure of the Au30–x Ag x (S-tBu)18 alloy and the effect of the ligand on alloying site preferences. Gold–silver nanoalloys prepared by co-reduction of metal salts are known to have only partial Ag occupancies. Interestingly, Au30–x Ag x (S-tBu)18 has 100% Ag occupancy at two sites on the core surface as well as partial Ag occupancies on the surface, capping, and staples sites. The Au30–x Ag x (S-tBu)18 (x = 1–5) composition was confirmed by X-ray diffraction and electrospray ionization mass spectrometry studies. Thiolate ligands can be categorized into three classes on the basis of the groups at the α-position as aliphatic, aromatic, and bulky thiols. The effect of the ligand on Ag doping can be clearly seen in the crystal structures of Au36–x Ag x (SPh-tBu)24 and Au38–x Ag x (SCH2CH2Ph)24 when compared with that of Au30–x Ag x (S-tBu)18. Ag is preferentially doped onto the core surface when the ligand is aliphatic, and Ag is doped in both core surface and staple metal sites when the ligand is aromatic or bulky.
Transformation brought about by ligand exchange is one of the effective methods for the synthesis of gold-thiolate nanomolecules (AuNMs). In this method, the AuNMs are treated with an excess exogenous thiol at an elevated temperature. It has been found that the ligand exchange is often accompanied by conversion of the metal core from a larger size to a smaller size, depending on the type of exogenous capping ligand employed. In this work, we present the transformation of a smaller-size AuNM (133 Au atoms) to a larger-size AuNM (279 Au atoms). Here, we observe that the Au 144 (SCH 2 CH 2 Ph) 60 AuNM in the presence of 4-tert-butylbenzenethiol under ref luxing conditions first transforms to Au 133 (SPh-tBu) 52 , and then with the transformation reaction proceeding to form larger-sized AuNMs, Au 191 (SPh-tBu) 66 and Au 279 (SPh-tBu) 84 . The reaction progress was monitored with matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) and UV−vis spectroscopy, and the intermediates and AuNMs were identified with electrospray ionization (ESI) MS. In conjunction with the above experiments, theoretical explorations using density functional theory calculations have been carried out, probing the energetics and thermodynamic stabilities underlying the observed size-changing transformations. It also elucidates the systematic size-dependent trends in the electronic structure of the original 144-gold-atoms-capped AuNM and the transformation products, including analysis of formation of superatom shells through the use of the core-cluster-shell model.
Highly monodisperse and pure samples of atomically precise gold nanomolecules (AuNMs) are essential to understand their properties and to develop applications using them. Unfortunately, the synthetic protocols that yield a single-sized nanomolecule in a single-step reaction are unavailable. Instead, we observe a polydisperse product with a mixture of core sizes. This product requires post-synthetic reactions and separation techniques to isolate pure nanomolecules. Solvent fractionation based on the varying solubility of different sizes serves well to a certain extent in isolating pure compounds. It becomes tedious and offers less control while separating AuNMs that are very similar in size. Here, we report the versatile and the indispensable nature of using size exclusion chromatography (SEC) as a tool for separating nanomolecules and nanoparticles. We have demonstrated the following: (1) the ease of separation offered by SEC over solvent fractionation; (2) the separation of a wider size range (∼5–200 kDa or ∼1–3 nm) and larger-scale separation (20–100 mg per load); (3) the separation of closely sized AuNMs, demonstrated by purifying Au137(SR)56 from a mixture of Au329(SR)84, Au144(SR)60, Au137(SR)56, and Au130(SR)50, which could not be achieved using solvent fractionation; (4) the separation of AuNMs protected by different thiolate ligands (aliphatic, aromatic, and bulky); and (5) the separation can be improved by increasing the column length. Mass spectrometry was used for analyzing the SEC fractions.
Strong photoinduced oxidants are important to organic synthesis and solar energy conversion, to chemical fuels or electric. For these applications, visible light absorption is important to solar energy conversion and long‐lived excited states are needed to drive catalysis. With respect to these desirable qualities, a series of five 5,6‐dicyano[2,1,3]benzothiadiazole (DCBT) dyes are examined as organic chromophores that can serve as strong photooxidants in catalytic systems. The series utilizes a DCBT core with aryl groups on the periphery with varying electron donation strengths relative to the core. The dyes are studied via both steady‐state and transient absorption and emission studies. Additionally, computational analysis, voltammetry, crystallography, and absorption spectroelectrochemistry are also used to better understand the behavior of these dyes. Ultimately, a strong photooxidant is arrived at with an exceptionally long excited state lifetime for an organic chromophore of 16 µs. The long‐lived excited state photosensitizer is well‐suited for use in catalysis, and visible light driven photosensitized water oxidation is demonstrated using a water‐soluble photosensitizer.
The design of shortwave infrared (SWIR) emissive small molecules with good stability in water remains an important challenge for fluorescence biological imaging applications. A series of four SWIR emissive rhodindolizine (RI) dyes were rationally designed and synthesized to probe the effects of nonconjugated substituents, conjugated donor groups, and nanoencapsulation in a water-soluble polymer on the stability and optical properties of the dyes. Steric protecting groups were added at the site of a significant LUMO presence to probe the effects on stability. Indolizine donor groups with added dimethylaniline groups were added to reduce the electrophilicity of the dyes toward nucleophiles such as water. All of the dyes were found to absorb (920–1096 nm peak values) and emit (1082–1256 nm peak values) within the SWIR region. Among xanthene-based emissive dyes, emission values >1200 nm are exceptional with 1256 nm peak emission being a longer emission than the recent record setting VIX-4 xanthene-based dye. Half-lives were improved from ∼5 to >24 h through the incorporation of either steric-based core protection groups or donors with increased donation strength. Importantly, the nanoencapsulation of the dyes in a water-soluble surfactant (Triton-X) allows for the use of these dyes in biological imaging applications.
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