Mixed-Monolayer-Protected Au₂₅ Clusters with Bulky Calix[4]arene Functionalities

Abstract

Although various complex, bulky ligands have been used to functionalize plasmonic gold nanoparticles, introducing them to small, atomically precise gold clusters is not trivial. Here we demonstrate a simple one-pot procedure to synthesize fluorescent magic number Au25 clusters carrying controlled amounts of bulky calix [4]arene functionalities. These clusters are obtained from a synthesis feed containing binary mixtures of tetrathiolated calix [4]arene and 1-butanethiol. By systematic variation of the molar ratio of ligands, clusters carrying one to eight calixarene moieties were obtained. Structural characterization reveals unexpected binding of the calix [4]arenes to the Au25 cluster surface with two or four thiolates per moiety. TOC GRAPHICS KEYWORDS: Chart 1. Structures of Thiol LigandsAu25 clusters passivated with mixtures of Calix-4SH and BuSH were synthesized by modifying the method by Qian et al. 10 Details of the synthetic procedure can be found in the Supporting Information. Briefly, a mixture of Calix-4SH and BuSH was added to a solution containing HAuCl4 and tetraoctylammonium bromide (TOAB) in tetrahydrofuran (THF). After the formation of colorless Au-thiolates, NaBH4 was added in aqueous solution to obtain a polydisperse cluster mixture. This polydisperse cluster mixture was size-focused by allowing it to react until Au25 cluster absorption features became prominent. The clusters were purified by repeated centrifugal washing with methanol followed by size-exclusion chromatography (SEC) to ensure removal of free Calix-4SH from the final cluster product. SEC served well to separate free Calix-4SH as well as a minor portion of larger cluster species from the final product ( Figure 1, inset). The Au25 cluster yields were typically 12−20 %.6 Figure 1. UV-vis absorption spectra of Au25 clusters with varying Calix-4SH concentration (spectra are vertically shifted for clarity). The inset shows spectra before and after size-exclusion chromatography.Absorption spectroscopy was initially used to get information about the core size of the clusters. Prominent absorption features were observed from all cluster samples at 680 nm, 442 nm, 400 nm and 320 nm which are well-known features of Au25(SR)18 cluster core (Figure 1). In addition, details in the absorption spectrum revealed the charge state of the cluster. It has beenshown that a broad absorption at 800 nm is an indication of a negatively charged, reduced gold core as well as a ratio of 1.2 between ~400 nm and ~440 nm absorptions. 29,30 All Calix-4S functionalized cluster samples showed these features thereby indicating a majority of negatively charged cluster cores as observed also in previous reports from similar syntheses. 10,29,30 The counter-ion of these negatively charged clusters was TOA + which was detected in the nuclear magnetic resonance (NMR) spectra of all Au25(Calix-4S)x(BuS)y clusters.To further analyze the structure of the clusters, the ligand compositions were probed by electrospray ionization mass spectrometry (ESI-MS) and N...

Selected applications of calixarene derivatives

“…It is noteworthy that the applications of calixarenes are diversified; one may mention here the preparation of calixarene-based ionic liquids, 101,102 and formation of metal NPs, 103,104 today intensively studied. Although due to the large amount of works dealing with calixarenes, [105][106][107][108][109][110] in this review only selected examples are presented, one might hope that the recent reports here cited will help towards a better understanding of this interesting field of chemistry and related sciences.…”

Simple and Efficient Separation of Atomically Precise Noble Metal Clusters

“…Tetrathiolated calix [4]arene (25,26,27,28-tetrakis(4-mercapton-butoxy)calix [4]arene) was synthesized according to a reported method. 32 All the chemicals were used as received without further purification. All solvents (dichloromethane (DCM), hexane, tetrahydrofuran (THF) and methanol) were purchased from Sigma or Rankem and were of analytical grade.…”

Deepening the Understanding of Thiolate-Protected Metal Clusters Using High-Performance Liquid Chromatography

“…Since the beginning of research on SR-protected metal clusters, structural analysis methods, such as transmission electron microscopy (TEM), 2237,119461 optical absorption spectroscopy, 2236,3963,6571,118,120407,462846 photoluminescence spectroscopy, 2228,3947,72,120,121,123196,370,387,388,394,395,408413,462533,847858 infrared absorption spectroscopy, 22,39,48,49,119146,197245,408410,414417,462470,534566,859869 nuclear magnetic resonance spectroscopy, 22,23,3942,5052,73,120122,147154,197218,246283,408,414,418426,462465,471487,534543,567622,847,859865,870894 thermogravimetric analysis, 23,24,…”

“…For example, MALDI mass spectrometry and ESI mass spectrometry have been used to evaluate the products of hydrophobic metal cluster synthesis; however, in both cases, the resulting mass spectrum is comparatively simple. 23,24,29,3235,39,43,46,49,50,5356,5963,6567,72,74,76,120122,126130,132,133,135137,147150,152,155,158160,162164,170,171,174180,182,183,202,203,216,219223,225,227,231,235,236,238,239,250252,254,260,262,263,275,276,278,286,287,291294,…”