Gold Nanoparticles Functionalized with Fully Conjugated Fullerene

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C: Physical Processes in Nanomaterials and Nanostructures 60

Gold Nanoparticles Functionalized with Fully Conjugated Fullerene C Derivatives as a Material with Exceptional Capability of Absorbing Electrons Bartlomiej Bonczak, Wojciech Lisowski, Agnieszka Kaminska, Marcin Holdynski, and Marcin Fialkowski J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10842 • Publication Date (Web): 18 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

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The Journal of Physical Chemistry

Gold Nanoparticles Functionalized with Fully Conjugated Fullerene C 60 Derivatives as a Material with Exceptional Capability of Absorbing Electrons Bartłomiej Bończaka, Wojciech Lisowskia, Agnieszka Kamińskaa, Marcin Hołdyńskia, Marcin Fiałkowski*a a

Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw (Poland).

ABSTRACT: We report a highly electron-absorbing material (“electron sponge”) obtained by coupling gold nanoparticles (AuNP, 8 nm in diameter) with fullerene C 60 spheres by fully conjugated linker molecules. This goal has been achieved through synthesis of a new class of conducting thionoester-substituted and thioketone-substituted azahomo-[60]fullerenes. The obtained C 60 derivatives have been found to possess the capability of binding covalently to the surface of gold. Thereby, for the first time, we show that thioketone and thionoester moieties can be successfully employed as new anchoring groups for conjugated fullerene derivatives. High resolution X-ray Photoelectron Spectroscopy studies have revealed that the ligands are attached to the gold surface of AuNPs through strong Au-S bonding. One of the synthesized C 60 derivatives, O-Butyl 4-(azahomo(C 60 -I h )fullerene)benzothioate (C 60 -BCT-OBu), has been investigated in detail. The C 60 -BCT-OBu-coated AuNPs form highly insoluble precipitate, which can be easily dissolved in toluene into individual AuNPs using cationic surfactants. The precipitate has been found to exhibit an extraordinary capability of absorbing electrons. A single AuNP can absorb on average about 4,500 electrons in an experimental conditions in which THF suspension of the precipitate of the C 60 -BCT-OBu-coated AuNPs is charged in a lithium naphtalide-mediated process. The electrical properties of the AuNPs are successfully explained by a model in which the C 60 -BCT-OBu-coated AuNP is represented by an equipotential heterostructure composed of C 60 spheres connected with the gold AuNP core by conducting linkers.

INTRODUCTION Due to unique electronic and photophysical1–4 properties fullerene has become an irreplaceable compound in design and building of many devices, like solar cells, photosynthetic antennas, and almost all devices where cascade charge transfer is employed.5–9 Many approaches for derivatization of fullerene pristine leading to produce materials with desired features like cation complexation,10,11 light harvesting,12 electron transfer,13,14 or junction abilities15 have been reported in literature. Most popular strategies of synthesis lead to introduction of sp3 hybridized carbon atom in the fullerene sphere.17 This drawback disallows full conjugation of the fullerene C 60 sphere with attached functional part of fullerene derivatives that is utterly important for devices in which quick charge separation is a desired property.18, 19 Combination of the exceptional ability of C 60 for accepting electrons with excellent electrical conductivity of metallic gold is thus an appealing yet challenging idea. Such nanostructure could facilitate quick charge transfer between the fullerene sphere and the Au core, making it potentially useful in engineering components in electronic or optoelectronic systems. In particular, it could be employed in advanced light harvesting devices,9 enabling fast transport of the induced charges to the electrode. When designing conjugated molecules with high conductance properties, which are meant to be organized on a metal surface, proper selection of anchoring group20-22 is of great importance. In case of decorating gold surfaces the most popular anchoring group of choice are thiol derivatives. Thanks to the p electrons of sulfur atom, the conjugation of gold and ligand can be achieved.23, 24 Unfortunately, introduction of the thiol group

is not an easy task because of obvious synthetic problems related to this moiety.25 The most important difficulties are due to high reactivity, strong nucleophility, and ease for oxidation of the thiol group. Given these disadvantages, a variety of protecting groups, like thioacetates or ethyl TMS group,16 have been proposed. However, using of these groups requires protectingdeprotecting steps that prolong the synthesis, and decrease yields of final compounds. Moreover, using thioacetates as the protecting groups limits usage of coupling reactions in which strong bases and/or aqueous environment are required. Furthermore, it has been suggested by theoretical and experimental studies26 that thiol group anchored to a gold surface can exhibit slight hybridization change that leads to stochastic junction break between the conductive ligand and gold. Employing thiocarboxylic groups that exhibit this behavior has been proposed as potential solution to this issue.27 Thiocarboxylic groups, including thioketones, thioesters, thiolactones, and thioamides, possess fixed hybridization and two electron pairs that are known to create bonds with noble metals,28, 29 especially with Au atoms30-32 and cations.33 Different approaches to decorate gold nanoparticles (AuNP) with C 60 derivatives have recently been tested.34, 35 However, none of synthesized previously C 60 derivatives exhibited full conjugation between fullerene sphere and gold anchoring group. The C 60 derivative synthesized in Ref. 34 was not able to bind chemically to AuNP, and it lacked conjugation. In ref. 35 C 60 was attached to AuNP through linker that was only partially conjugated. Here we present a novel approach to obtain AuNPs functionalized with C 60 spheres that are connected to the gold surface through fully conjugated linkers. For the first time, we demonstrate that thioketone and thionoesters form Au-S bonds with

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the surface of AuNP, and thus can be successfully applied as anchoring groups. We show that coupling of metallic gold cores of AuNPs with highly electron-accepting fullerene spheres gives rise to a heterostructure possessing unusual electronic properties. As the main result, we demonstrate that AuNPs functionalized with the fully conjugated C 60 derivatives exhibit extraordinary ability to accept electrons and act as a highly efficient electron absorber referred in the literature as the “electron-sponge”.36, 37 EXPERIMENTAL SECTION Materials. p-aminobenzoic acid, p-acetyloaniline, sodium azide, Lawesson’s reagent, tetrachloroauric(III) acid, sodium citrate, naphtalene, lithium, sodium dodecyl sulfate (SDS), hexadecyltrimethylammonium bromide (CTAB), decaethylene glycol monododecyl ether (C 12 E 10 ), SOCl 2 , dodecane were purchased from Sigma-Aldrich. Hexadecanol, didodecyldimethylammonium bromide (DDAC), and benzylcetyldimethylammonium chloride (BDAC) was purchased from TCI. Fullerene C 60 (99.5%) was purchased from SAS Research. Carbon disulphide (low in benzene content) and hexadecane was purchased from Across Organics The reagents were used as received. Solvents (diethyl ether, methanol, chloroform, methylene chloride) were pro analysis grade (ChemPur, POCh) and were used without purification. Hexane was distilled prior to use. Xylene (mixtures of isomers) and toluene were dried over CaH 2 , distilled and stored under argon atmosphere. Tetrahydrofuran (THF) was dried with potassium and benzophenone, distilled and stored under argon atmosphere. Reactions were monitored by thin layer chromatography (TLC) on silica gel precoated aluminum sheets (Merck). Visualization was accomplished by irradiation with UV light at 254 nm and/or Ceric Ammonium Molybdate (CAM) stain. Column chromatography was performed on Merck silica gel (60, particle size 0.040-0.063 mm). Synthesis of gold nanoparticles. Gold nanoparticles (AuNPs) coated with dodecylamine (DDA) were synthesized according to the known literature procedure,38 yielding AuNPs solution with gold atoms concentration of 7 mM, and average size of AuNP about 8 nm. AuNPs coated with undecanotiol (UDT) were synthesized according to the known protocol.38 N,N,N-trimetylo-11-mercaptoundecyloamonium chloride (TMA) coated AuNPs were prepared by following the published protocol.39 Citrate stabilized AuNPs were synthesized according to literature procedure,40 yielding AuNPs of average size about 9 nm. Ligand exchange in toluene: DDA- and UDT-coated AuNPs. In standard procedure employed for ligand exchange, 5 mL of the AuNP suspension was firstly treated with ca. 25 mL of methanol in order to remove excess of surfactants. Precipitated AuNPs were allowed to settle on the bottom of flask for a few (3-4) h. Liquid was then carefully removed and the AuNPs were redispersed in fresh toluene (5 mL). Next, 0.044 mmol of a chosen thione ligand was solubilized in 5 mL of toluene and injected in one portion into vigorously stirred AuNP solution after which immediate solution color change was observed from purple to blue. Reaction was allowed to run for 10 h, after which the mixture was centrifuged (10 min, 4000 rpm), supernatant was discarded, and residue solid was sonicated for 10 min in ultrasonic bath with 5 mL of fresh toluene. This cycle of cen-

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trifugation and redispersing was repeated at least 6 times or until no traces of free ligand was observed in supernatant. The obtained composite was finally dried under vacuum overnight. Ligand exchange at the toluene-water interface: TMAand citrate-coated AuNPs. In typical experiment, 5 mL of aqueous solution of AuNPs were poured into a vial and 5 mL of thionofullerene solution in toluene were added. Next, the content of the vial was either mixed via magnetic stirrer or sonicated (for 24 and 1 h, respectively). In each case subsequent color change of water phase was observed. In case of sonicated samples the emulsification of water phase occurred. The size distribution profile of the AuNPs, measured by DLS, increased in all ligand exchange experiments in contrary to the reference sample (toluene phase without thione ligand). Ligand exchange on the TMA-coated AuNPs immobilized at the solid substrate. Prior to the ligand exchange reaction, a hydrophilic plate (quartz or silica) of the size 1 × 1 cm2 was immersed in about 5 mL of aqueous solution of TMA-coated AuNPs. After 1 h the plate was removed from the reaction vessel, thoroughly rinsed with ethanol, and was immersed in about 5 mL of thionofullerene solution in toluene. After 1 h the plate was removed from the reaction vessel, thoroughly rinsed with toluene, and dried under vacuum overnight. Determination of the amount of electrons absorbed by pristine C 60 , UDT-coated AuNPs, and C 60 -BCT-OBucoated AuNP. In experiments either 2.6 mg of fullerene C 60 or 5 mL of toluene solution of UDT-coated AuNPs (2.23∙10-6 mmol), or the precipitate of C 60 -BCT-OBu-coated AuNPs (2.23∙10-6 mmol) were used. The latter was suspended by sonication in dry THF prior to use. Chosen substances were transferred to dried Schlenk type flask under Ar atmosphere. Then the volatiles were removed by evaporation under reduced pressure. The flask was then purged with Ar. This step was repeated three times. Next, the obtained dry material was redispersed in 5.0 mL of freshly dried THF. Next, predefined amount of naphthalene solution (10.45 mg/50.0 mL dry THF) was added to the suspension. Then a piece of cleaned Li metal (~150 mg) was introduced, and the mixture was tightly sealed and agitated in ultrasonic bath, which was cooled with ice, for 60 min obtaining purple colored sample. Finally, 0.10 mL of such prepared suspension was collected and quenched with 5.0 mL of distilled water, and pH of the resulting supernatant was measured. Preparation of lithium naphtalenide (LiNaph) solution. Schlenk type flask was dried in the flow of hot air, connected to vacuum, allowed to cool down to rt, and pressurized with Ar. The flask was purged with Ar three times, then 148.50 mg of Naph was added to the flask, followed by adding 25.0 mL of dry THF. Cleaned piece of lithium metal was cut under protecting layer of paraffin oil, immersed in a mixture of hexane and toluene, then in hexane, then swiftly dried in Ar flow, and finally introduced to naphthalene solution (Li was added in excess, ~150 mg). The as-prepared mixture was stirred with glass coated magnetic bar in the dark overnight. The resulting solution had a deep green color. Determination of the mean amount of electrons carried by naphthalene molecule. Prescribed volume of LiNaph THF solution (46.48 mmol dm-3) was quenched with 10 mL of distilled water, and the resulting pH was measured. Four different volumes of LiNaph solution were employed: 1, 2, 3, and 5 mL. The mean number of electrons carried by a single Naph molecule was determined as the intercept using linear regression on data pairs pH vs. volume of LiNaph solution.


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Dissolution of the precipitate by cationic surfactants. Suspension of a chosen surfactant was prepared by sonicating 1 mmol of surfactant with 10 mL of toluene for 20 min. As prepared suspension was added to the precipitate, and the obtained mixture was sonicated for 30 min. Then the sample was allowed to settle for 5 min, and clear supernatant was collected for measurements. Instrumentation. Proton and carbon NMR spectra were recorded on Bruker (400 MHz) spectrometers in deuterated solvents at the temperature 303 K. Proton chemical shifts are reported in ppm (δ) relative to tetramethylsilane (TMS) with the solvent resonance employed as the internal standard (CDCl 3 δ 7.26 ppm; C 6 D 6 δ 7.16 ppm). The solvents were purchased from ABCR (dried with A4 molecular sieves). 13C chemical shifts are reported in ppm from tetramethylsilane (TMS) with the solvent resonance as the internal standard (CDCl 3 δ 77.16 ppm; C 6 D 6 δ 128.08 ppm). UV-Vis absorption spectra were recorded with a Thermo Scientific Evolution 220 instrument with temperature controller. The measurements were carried out using 1 cm quartz cuvettes (Hellma). Sizes and Z-potential of the NPs were determined using Dynamic Light Scattering (DLS) technique. The DLS studies were conducted with the Malvern ZetaSizer Nano-ZS instrument using 1 cm quartz cuvettes (Hellma) and Universal dip cell. Prior to introducing samples that were sensitive to air and water, the cuvette was flushed with Ar and small piece of cleaned lithium (~5 mg) was added. The cuvette was sealed with tight PTFE cap and protected with Parafilm. All measurements were conducted at 25 °C, except of samples of toluene solutions of BDAC which were conducted at 60 °C. pH measurements were conducted on Hanna Instruments HI3220 equipped with pH In Lab Micro electrode from Mettler Toledo. Surface Enhanced Raman Spectroscopy (SERS) measurements were performed with SERS measurements were performed using a Renishaw inVia Raman system equipped with a 300 mW diode laser emitting a 785 nm line used as the excitation source. The light from the laser was passed through a line filter and focused on a sample mounted on an X-Y-Z translation stage with a ×10 microscope objective. The Raman-scattered light was collected by the same objective through a holographic notch filter to block out Rayleigh scattering. A 1800 groove·mm–1 grating was used to provide a spectral resolution of 5 cm–1. The Raman scattering signal was recorded by a 1024 × 256 pixel RenCam CCD detector. The beam diameter was approximately 5 µm. Typically, the SERS spectra were acquired for 30–60 seconds with the laser power measured at the sample being 2.5 mW. X-ray photoelectron spectroscopic (XPS) measurements were performed using the a PHI 5000 VersaProbe (ULVACPHI) spectrometer with monochromatic Al Kα radiation (hν = 1486.6 eV) from an X-ray source operating at 100 µm spot size, 25 W and 15 kV. The high-resolution (HR) XPS spectra were collected with the hemispherical analyzer at the pass energy of 23.5 eV, the energy step size of 0.1 eV and the photoelectron take off angle 45° with respect to the surface plane. Shirley background subtraction and peak fitting with Gaussian–Lorentzian-shaped profiles was performed for the high-resolution XPS spectra analysis. Charge compensation was achieved using a low energy electron flood gun.

Scanning Electron Microscopy (SEM) and Scanning Transmission Electron Microscopy (STEM) studies were performed with Nova NanoSEM 450 instrument under high vacuum (10-7 mbar). For the SEM analysis the precipitate obtained in the ligand exchange reaction was deposited on a silicon or graphite substrate, and mounted onto the typical SEM stub. SEM images were collected using the Through Lens Detector (TLD) of secondary electrons at primary beam energy of 10 kV and at 4.8 mm working distance from the pole piece. The STEM studies were performed on samples deposited on a TEM grid (Quantifoil R2/2, 300 Cu mesh) with high acceleration voltage of 30 kV and at working distance optimized to 6.7 mm from the pole piece. The presented STEM images were obtained using bright field (BF) contrast mode of detector (two segment planar solidstate p–n junction) attached under the grid holder. Both SEM and STEM images were obtained at long scan acquisition time (20 μs) of typically 30 seconds per frame after choosing the inspection region. Thermogravimetric Analysis (TGA) was carried out using TGA Q50 instrument (TA Instruments) under a nitrogen atmosphere. The samples were placed in a platinum cell and heated from 25 to 1000 °C at a rate of 10 °C/min. RESULTS Synthesis and characterization of the fully conjugated C 60 derivatives. A series of novel ligands were synthesized according to the synthetic paths shown in Scheme 1. In our approach, p-azidobenzoic acid (1) was firstly synthesized by diazonization of p-aminobenzoic acid, followed by quenching of the formed diazonium salt with sodium azide. Similar method was used to synthesize p-azideacetophenone (2a). Compound 1 was subsequently esterified with methanol (2b) (reaction with SOCl 2 ), butanol, and hexadecanol, following esterification procedure with DCC (2c and 2d). Next, the azides were reacted by the 1,3-dipolar cycloaddition with C 60 in optimized conditions that provided maximal yield of the desired [5,6]-open product and minimalized [6,6]-closed isomer output (3a-d). After purification, the obtained C 60 derivatives were introduced into reaction with Lawesson’s reagent41 generating one thioketone (4a), and three thionoesters (4b-d). Details of the ligand synthesis are provided in Supporting Information. The corresponding 1 H and 13C NMR spectra are presented in Fig. S1.

Scheme 1. Synthetic strategies for the preparation of the fully conjugated C 60 derivatives.



We performed preliminary investigation of the obtained ligands aimed at characterizing their ability to bind to the gold core of the AuNPs. For this purpose, we applied these compounds in ligand exchange reactions on AuNPs coated with dodecylamine (DDA), undecanethiol (UDT), N,N,N-trimetylo-11-mercaptoundecyloamonium chloride (TMA), and citrate ions. The reactions with DDA- and UDT-coated AuNPs were carried out in toluene, the reactions using TMA- and citrate-coated AuNPs were conducted at toluene-water interface. It was found that for all C 60 derivatives the ligand exchange took place for each type of the AuNPs employed. We observed that the rate of the reaction decreased whilst solubility in organic solvents increased significantly with the length of the substituent R in the C 60 derivative (Scheme 1). For the ligands substituted with Me (4a) and OMe (4b) the exchange reactions proceeded rapidly. For ligand 4d substituted with the O[n-C 16 H 34 ] chain these reactions were very slow and needed 3-4 days to be completed. Also, the solubility of 4d in organic solvents was so high that it prevented removal of the unreacted ligand from the post reaction mixture. The exchange reactions with 4c proceeded at moderate speed, and were complete after 12–24 h. Because of both the moderate reaction rate and average solubility in toluene, allowing effective control over the ligand exchange process, compound 4c, O-Butyl 4-(azahomo(C 60 -I h )fullerene)benzothioate, was selected for further investigations. In the following, it is referred to as the C 60 -BCT-OBu ligand. UV-Vis absorbance study of C 60 -BCT-OBu. UV-Vis absorbance spectroscopy study of the toluene solution of the C 60 BCT-OBu ligand was performed to characterize its optoelectronic properties (Fig. 1). The recorded spectrum resembles that of pristine C 60 . This is an expected result given small size of the linker and weak conjugation of the fullerene double bonds. To determine the optical energy band gap, E g , of this ligand, we employed the standard Tauc’s plot based on the UV-Vis absorbance data. To construct the Tauc’s plot, the absorption coefficient, α, was expressed as a function of the photon energy, hν, through the relation for indirect allowed transitions, (αhν)1/2 = C(hν – E g ), where C is a constant. The resulting Tauc’s plot is depicted as an inset in Fig. 1. A linear extrapolation of the above relation yielded the optical energy band gap E g = 2.76 eV. It is noted that this value is equal to that of pristine C 60 .16

Figure. 1. UV-Vis absorbance spectrum of the toluene solution of C 60 -BTC-OBu. Inset: Tauc’s plot derived from the UV-Vis spectrum, yielding the energy band gap E g = 2.76 eV.

Characterization of the bonding between the C 60 -BCT-OBu ligand and AuNP

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Ligand exchange with DDA-coated AuNPs. We first investigated product of the ligand exchange reaction on the surface of the AuNPs coated with DDA. The DDA-coated AuNPs had average diameter of ~8 nm, and were synthesized according to the literature procedure.38 To carry out the reaction, toluene solution of C 60 -BCT-OBu was added in one portion into vigorously stirred toluene dispersion of the AuNPs. Upon addition of the ligand, the AuNPs solution immediately changed color from purple to blue, and dark precipitate started to appear. The reaction ran for 10 h, and then the precipitate was collected, thoroughly purified with toluene from residual free C 60 -BCT-OBu ligand, and dried. To establish the nature of the bonding between the C 60 -BCT-OBu ligand and the AuNP core, high-resolution X-ray Photoelectron Spectroscopy (XPS) experiments were conducted. The original S 2p XPS spectrum recorded for the precipitate deposited onto a silicon substrate is shown together with deconvoluted peaks in Fig. 2C. Three chemical states of sulfur were identified. Two of these states are represented by two spin-orbit split peaks (S 2p 3/2 and S 2p 1/2 ) separated by 1.18 eV. The first state – signals at binding energy (BE) 161.10 and 162.28 eV, respectively – is assigned to the sulfur bound S-Au to the AuNPs42 (Fig. 2A). The second one (signals at BE 163.90 and 165.08 eV) is attributed to the thione group (C=S) in the C 60 -BCT-OBu molecule. The presence of the latter signals in the XPS spectrum indicates that also pristine (unreacted) C 60 -BCT-OBu ligands were present in the precipitate. Most likely, they were stuck to the bounded ligands or/and attached to the surface of the AuNPs through dispersion forces as shown in Fig. 2B. The third state represented by signals at BE 168.22 and 169.40 eV (see Fig. S2) is attributed to sulfonate groups belonging most likely to sulfur S(V) surface species in the form Au-SO 3 -C-.16 These compounds are formed due to light and air exposure during sample processing.42 Based on the areas of the S-Au, C=S, and sulfonate signals, it is found that the amount unbound ligands in the precipitate was about 16%. The C 1s XPS spectrum displayed also carbon signals at BE of 284.6 and 285.7 eV, characteristic of C-C and C-S bonds, respectively43 (see Fig. S3). To confirm the presence of the fullerene spheres in the ligand exchange product, Surface Enhanced Raman Spectroscopy (SERS) studies44 were also performed. The recorded SERS spectrum (Fig. 2D) displayed clearly two peaks at 762 and 1458 cm-1 that are characteristic of the C 60 spheres. To find the amount of the ligand bound to the AuNP core, the precipitate was characterized with TGA analysis. It was calculated based on the percentage weight loss occurring between 30.0 and 985.91 °C, which corresponds to the degradation of C 60 -BCT-OBu (Fig. S12). The weight loss measured over this temperature range was 13.45% (Fig. S13). For the mean diameter of the gold core of 8 nm this mass loss gives 523 ligand molecules per AuNP. According to the XPS studies discussed above, ~16% of the ligand molecules present in the precipitate are unbound. Thus, the actual number of the C 60 -BCT-OBu ligand bound to single AuNP is estimated as N lig = 440. Note that the maximal number of the C 60 -BCT-OBu molecules that can attach to the surface of the core, calculated based on the closepacking of the C 60 spheres, is ~800. The obtained value of N lig is roughly a half of this number. Ligand exchange with TMA-coated AuNPs. To further investigate the interaction between C 60 -BCT-OBu and the surface of the AuNP, we conducted the ligand exchange reaction with AuNPs coated with a thiol-terminated ligand, in conditions that


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prevented aggregation of the AuNPs. For this purpose, we employed TMA-coated AuNPs39 (~8 nm in diameter) deposited on a hydrophilic solid substrate (silicon or quartz slide). After the deposition of the AuNPs, the slide was immersed in the toluene solution of C 60 -BCT-OBu. The ligand exchange reaction ran for 1 h, then the slide was thoroughly rinsed with toluene and dried. SEM studies confirmed that the AuNPs remained immobilized on the surface during the whole process. To examine the character of bonding between C 60 -BCT-OBu and the AuNP core, we compared the XPR spectra obtained for the TMA-coated AuNPs before and after the ligand exchange. High-resolution Au 4f XPS spectra recorded for the TMA-coated AuNPs before and after the ligand exchange are shown in Fig. 3A and Fig. 3B, respectively. In each case, by spectral deconvolution, we identified two spin-orbit split peaks (Au 4f 7/2 and Au 4f 5/2 ) separated by 3.67 eV. Positions of the peaks corresponding to bulk gold, Au(0), were found at BE 83.64 and 87.31 eV before, and at BE 83.60 and 87.27 eV after the ligand exchange. The peaks related to the oxidized gold state, Au(I), were located at BE 84.62 and 88.29 eV before, and at BE 84.40 and 88.07 eV after the ligand exchange. The relative contribution of the Au(I) species of gold in the surface area of the TMA-coated AuNPs was estimated before and after ligand exchange to be 21% and 42%, respectively (see Table S1 for more detail). From the comparison of the XPS spectra, it follows that – within the precision of the instrument (± 0.1 eV) – the reaction did not alter the position of the oxidized state, which corresponds to the Au-S bonding.45 The most consistent interpretation of the substantial growth of the Au(I) contribution to the Au 4f signal is a change in the molecular environment of the AuNP core and/or an increase of the number of attached ligands per NP. Taken together, these two facts (unaltered position of the oxidized gold Au(I) state accompanied by the change in molecular decoration of the AuNP surface after the reaction) provide confirmation that the C 60 -BCTOBu ligands form the Au-S bonding with the surface of the AuNPs. The UV-Vis spectroscopy, performed for the transparent quartz slides, revealed the presence of C 60 spheres (Fig. 3C). Properties of the C 60 -BCT-OBu-coated AuNPs. First the morphology of the precipitate formed in the ligand exchange reaction on the DDA-coated AuNPs was investigated using Scanning Electron Microscopy (SEM). SEM images (Fig. 4A) showed that it is composed of big chunks of aggregated NPs. Sonication of this material in toluene for 6 h allowed to break up these big aggregates into smaller pieces that were then analyzed using Scanning Transmission Electron Microscopy (STEM). The STEM micrographs (Fig. 4B) revealed that the aggregated AuNPs formed densely packed structures. Remarkably, the AuNPs did not coalesce, but remained separated from one another. We found that the precipitate was absolutely insoluble in organic solvents, such as toluene, carbon disulfide, chloroform, chlorobenzene, 1,2-dichlorobenzene, THF, dodecane, or hexadecane. In addition, we studied precipitate created in the ligand exchange reaction on the UDT-coated AuNPs38 (~8 nm in diameter). We followed the experimental procedures employed for the reaction with DDA-coated AuNPs, and obtained precipitate that was also completely insoluble in all the organic solvents mentioned. The fact that the aggregates of the C 60 BCT-OBu@AuNPs did not dissolve in toluene – that is, in a solvent in which the C 60 -BCT-OBu ligand is completely soluble – is noteworthy. It indicates that the aggregates are stabilized by forces stronger than the π-π dispersion attraction between the C 60 spheres.46 Otherwise, toluene would dissolve the aggregates. Most likely, the aggregated AuNPs were joined by

a layer of ligands forming an alternating head-tail structure that is illustrated schematically in Fig. 4C. Such arrangement of the ligands between two neighboring AuNPs provides additional linkage through dispersive forces anchoring C 60 sphere to the gold surface.

Figure 2. The C 60 -BTC-OBu ligand forming Au-S bonding with the AuNP gold surface (A), and the same ligand in its pristine form attached to the surface of AuNP through dispersive forces (B). C) High-resolution S 2p XPS spectrum recorded for the DDA-coated AuNPs after the ligand exchange reaction. The spectrum displays signals of Au-S bonding (S 2p 3/2 = 161.1 eV) and thione group (S 2p 3/2 = 163.9 eV). D) SERS spectra of the precipitate (black line) and the C 60 -BTC-OBu ligand (green line) exhibiting two signals (indicated by the arrows) characteristic of the C 60 sphere.



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of the unbound ligands would stain the solution in red revealing the presence of charged fullerene derivative.

Figure 4. SEM (A) and STEM (B) micrographs of the precipitate obtained in the ligand exchange reaction. Scheme (C) shows the proposed arrangements of the C 60 -BCT-OBu ligands on the surfaces of two adjacent AuNPs.

Figure 3. High-resolution Au 4f XPS spectrum obtained for the immobilized TMA-coated AuNPs before (A) and after (B) the ligand exchange reaction with C 60 -BCT-OBu. In both cases the spectrum exhibits two pairs of the spin-orbit split signals (Au 4f 7/2 and Au 4f 5/2 ) corresponding to bulk gold, Au(0), and oxidized gold state, Au(I). C) Comparison of UV-Vis spectra recorded for the TMA-coated AuNPs deposited on a quartz plate before (dashed line) and after (solid line) the ligand exchange. The broad shoulderpeak in the UV range in the latter spectrum is due to the presence of C 60 .

Electrical charging of the precipitate by LiNaph in THF. To check the ability of the C 60 -BCT-OBu@AuNPs to accumulate electric charge we performed a series of experiments in which the precipitate was treated with a THF solution of lithium naphtalenide (LiNaph). First, pre-prepared THF solution of LiNaph was added dropwise into a THF suspension of the precipitate. The experiment was carried out under argon atmosphere with complete exclusion of air and water. Upon introduction of LiNaph the color of the suspension turned immediately from grey to purple-red, and, with the addition of LiNaph, gradually changed to brownish red. Vanishing of the greenish color characteristic of LiNaph was caused by the oxidation of naphtalenide anion radicals by the C 60 -BCT-OBu@AuNPs. Finally, the color turned greenish indicating that the ability of the NPs to absorb electrons had been exhausted and LiNaph was in excess. The resulting suspension tended to agglomerate, which was manifested by formation of a gradient of purple color when left undisturbed overnight. After extended period of time (few days to a week) all material settled on the bottom creating purple sediment, leaving the THF supernatant clear and colorless. The lack of color of the supernatant showed that there was no release of the unbound ligand form the C 60 -BCT-OBu@AuNP precipitate during the process. Otherwise, even a small portion

Although the above experiment proved that the precipitate absorbs electrons from LiNaph, it was not suitable to establish how many electrons can be absorbed by a single AuNP. The reason for this was that to reach the saturation LiNaph had to be introduced in a molar amount exceeding that of AuNPs by three orders of magnitude. Because both the C 60 -BCT-OBu@AuNPs and Naph can absorb electrons, knowledge of the equilibrium distribution of electrons among these two species would be necessary for a reliable determination of the mean number of electrons accumulated on a single AuNP. To determine the number of electrons that the C 60 -BCTOBu@AuNPs is capable of absorbing, we carried out other experiments in which THF suspension of the precipitate was mixed with predetermined small amount of THF solution of LiNaph, and brought in contact with metallic Li. In this experimental setup Naph molecules acted as a charge carriers transferring repeatedly electrons from the piece of lithium metal to the AuNPs, as explained schematically in Fig. 5A. The mixture was then agitated in ultrasonic bath for 60 minutes. To ensure that this amount of time was enough to reach the electron saturation, we conducted experiments with three different Naph to AuNPs molar ratios, varying from about 16 to 146 (the amount of the AuNPs was the same). For each amount of Naph the resulted solutions were slightly turbid indicating that the AuNPs exhibited a tendency to aggregate. The DLS measurements revealed that in all experiments the sizes of the aggregates were distributed in the range of 140-200 nm. The recorded UV-Vis spectra displayed maximum absorption located at about 534 nm. The Zeta-potential of the solutions was found to be about 15 mV. For some DLS scans it was possible to identify a clear signal of individual NPs, and estimate the mean hydrodynamic diameter as ~10.6 nm. Such representative DLS profile is shown in Fig. S11. The mean diameter of the AuNP core was 8 nm, and the length of the C 60 -BCT-OBu molecule is ~1.3 nm, of which 0.7 nm is the C 60 sphere. That is, distance from the terminating


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sulfur atom to the middle of the C 60 sphere is about 0.95 nm. Thus, the effective (volume-averaged) diameter of the C 60 BCT-OBu@AuNP is expected to be slightly greater than (8 + 2 × 0.95) = 9.9 nm. The measured value (10.6 nm) agrees quite well with this estimate. To calculate the mean number of electrons absorbed by the NPs, the following procedure was employed: Some amount of the solution was collected and quenched with water, which lead to rapid precipitation of the AuNPs. Next, pH of the supernatant was measured and used to calculate the total amount of hydroxyl anions, 𝑛𝑛OH , in the sample. Because the analyzed supernatants were basic with pH > 9.5, one could assume that electrons gathered on the C 60 -BCT-OBu@AuNPs and naphtalenide radicals were the only source of OH − anions, produced in reaction H2 O + 𝑒𝑒 − → OH − + 12H2 . Thus, the amount of electrons absorbed on the AuNPs was calculated by subtracting the amount of OH − anions generated by naphtalenide radiNaph Naph cals, 𝑛𝑛OH , from 𝑛𝑛OH . The quantity 𝑛𝑛OH is proportional to the Naph number of moles of Naph, 𝑛𝑛Naph , through the relation 𝑛𝑛OH = 𝛼𝛼𝑛𝑛Naph , where α is the mean number of OH − generated by a single Naph molecule. To determine α we performed a series of experiments in which THF solutions of pure LiNaph were quenched with water. Based on the pH measurements, we found α = 1.59 ± 0.09. Note that this result is in line with the reported ability of Naph to absorb one or two electrons in reaction with Li metal in THF.47 We also found that THF had, compared with Naph, a negligible small contribution to the generation of OH − anions in the quenching experiments (~1.4 10-6 moles of OH − per mL of THF, see Fig. S4). Results for three values of the Naph:AuNP molar ratios employed are shown in Table 1. The mean number of electrons per AuNP (electron:AuNP) was calculated as the ratio (𝑛𝑛OH − 𝛼𝛼𝑛𝑛Naph )/𝑛𝑛NP , where 𝑛𝑛NP is the number of AuNPs moles in the sample analyzed. As seen, in each case pH similar values of pH and electron:AuNP ratios were obtained. This result indicates that the electron saturation was achieved independently of the Naph amount used, and the value of ~4,500 can be thus taken as a rough estimate of the maximal number of electrons which a single AuNP is capable to absorb in the LiNaph-mediated charging process. To highlight the effect of fullerene functionalization on the charging process, we determined the amount of electrons that can be absorbed by pristine C 60 and UDT-coated AuNPs. UDT was used because it possesses relatively short alkyl chain and forms the same S-Au bond as C 60 -BCT-OBu, which makes it a good reference ligand. We employed the same procedure and setup as for the C 60 -BCT-OBu@AuNPs. It was found that single C 60 molecule can absorb ~2 electrons (2.15 ± 0.10). We also established that single UTD-coated AuNP absorbed on average less than one electron (0.31 ± 0.60). These results show that the amount of electrons accumulated by the C 60 -BCT-OBu@AuNP (~4,500) cannot be due neither to the gold core nor the C 60 spheres alone. Thiol-coated AuNPs were shown48 to get charged upon contact with electrode due to the self-capacitance effects. In such conditions electrons are transferred to equalize the potential of the electrode and that of the core of the AuNPs. Here, we employ completely different experimental conditions. Electrodes are not used, and the electron transfer can occur only upon contact of the AuNP with the naphtalenide anion. Our results show that the UDT ligands form an isolating layer that effectively prevents transfer of electrons from LiNaph to the core.

Only the C 60 -BCT-OBu ligands facilitate charging of the AuNP. Table 1. Values of pH and the corresponding numbers of electrons absorbed by a single AuNP (electron:AuNP) obtained in experiments with different Naph:AuNP ratios. Naph:AuNP

pH

electron:AuNP (× 103)

16.62

9.62 ± 0.04

4.73 ± 0.48

73.10

9.58 ± 0.09

4.49 ± 0.97

146.19

9.56 ± 0.11

4.35 ± 1.14

Surfactant-aided dissolution of the precipitate in toluene. We employed cationic surfactant CTAB to dissolve the precipitate obtained in the ligand exchange reaction with the DDAcoated AuNPs. Because CTAB is completely insoluble in toluene, in our experiments it was introduced in a form of turbid toluene suspension to the precipitate immersed in a small amount of toluene. Upon addition of the surfactant, after ~10 min. of sonication, the system became clear, and showed no traces of turbidity. The UV-Vis spectrum of the obtained solution displayed maximum absorption peak located at 526 nm, indicating the absence of aggregated NPs. (see Fig. S4). DLS studies confirmed that the solution contained only individual NPs. Their mean hydrodynamic diameter was determined to be D h = 13.28 ± 0.20 nm (Fig. S6). Because the mean hydrodynamic diameter of the C 60 -BCT-OBu@AuNPs was found to be ~10.6 nm, it follows that the C 60 -BCT-OBu@AuNPs are surrounded by an extra shell of the thickness of 1.3 nm, which is commensurable with the length of a free flexible hexadecyl chain49 at room temperature (~1.6 nm). The obtained value of D h can be thus attributed to the presence CTAB adsorbed on the C 60 -BCT-OBu ligands. Of course, if the surfactant molecules formed a dense spherical layer the hydrodynamic diameter would be markedly greater than 13.28 nm. This is however not the case here because CTAB are attached to the C 60 spheres and cannot form any ordered layer around the AuNP. The outer layer of alkyl chains renders the NPs solvophilic with respect to toluene, facilitating full dissolution of the precipitate. In addition to CTAB, we employed two other cationic surfactants: BDAC and DDAC. In each case we also observed complete dissolution of the precipitate into individual AuNPs, as it was confirmed by DLS and UV-Vis absorbance studies (Figs. S7 - S10). Note that in terms of the dissolution rate, DDAC proved to be the most effective among the three cationic surfactant used. In our attempts to dissolve the precipitate, we applied also anionic surfactant SDS and nonionic surfactant C 12 E 10 , and followed the experimental procedures employed for the cationic surfactants. Remarkably, it was found that neither SDS nor C 12 E 10 was able to dissolve the C 60 -BCT-OBu@AuNP aggregates in the slightest degree. DISCUSSION Mechanism of LiNaph-mediated charging of the C 60 BCT-OBu@AuNPs. Fig. 5A explains the proposed mechanism of electron transport from the naphthalide anion radical to the C 60 -BCT-OBu@AuNP. Because the electron affinity (LUMO) of Naph, EA N = -0.2 eV, is substantially higher than that of C 60 , EA F = 2.6 eV, electrons from the ionized Naph are transferred to the fullerene sphere. EA F is also much higher than



the work function of metallic gold, Φ Au ≈ 5.2 eV. The C 60 sphere is coupled with the π-conjugated linker that is anchored to the Au core by sulfur atom of the thione group. Such bonding is known to donate lone pair onto gold surface atoms and conduct charge,23,31 facilitating further transfer of the electrons from the C 60 to the core, as shown schematically in Fig. 5A. The electron transfer causes charging of the Au core, which is accompanied by rising the Fermi level by an extra electrostatic potential E el (Fig. 5C). This two-step electron transport from the naphthalide anions to the core continues until the energy of electrons on the Au core reaches the LUMO level of the C 60 sphere. Then, the Au core and the fullerene spheres form a an equipotential heterostructure shown in Fig. 5B. Electrons transferred from the naphthalide anions are distributed within this structure in such a way that their energy on the metallic core and on the C 60 spheres is the same. The charging process ceases when the energy of electrons on the heterostrucure attains LUMO level of naphthalene. This condition combined with the equipotential condition yields the following set of two equations: core , ΦAu − 𝐸𝐸𝐸𝐸N = ∆𝐸𝐸el

𝐸𝐸𝐸𝐸F − 𝐸𝐸𝐸𝐸N = core ∆𝐸𝐸el

F ∆𝐸𝐸el ,

(1) (2)

F ∆𝐸𝐸el

and denote change of the potential on the where core and the fullerene sphere, respectively, due to the absorpcore tion of electrons. To estimate ∆𝐸𝐸el we make a simplifying assumption and employ the image-charge approximation for the interaction of electron inside the Au core and the Li+ cation. In this approach, they form a pair, and the distance of the electron from the surface of the core is equal to the radius, r Li = 0.9 Å, of the Li+ cation adhering to the surface. Within this approximation the charge-charge interaction energy, E cc , of the pair is calculated as 𝐸𝐸cc = −

𝑘𝑘𝑒𝑒 2 𝜀𝜀r 𝑑𝑑

,

(3)

where d = 2r Li = 1.8 Å, ε r = 7.6 is dielectric permittivity of THF, k = 8.988 × 109 Nm2C-2 is the electrostatic constant, and e is the elementary charge. It is further assumed that the electronLi+ pairs form electric dipoles having the dipole moment p = ed (see Fig. 5B). The energy of electrostatic interaction between such two dipoles, E dd , is estimated as 𝐸𝐸dd ≈

𝑘𝑘𝑝𝑝2

3 𝜀𝜀r 𝑟𝑟dd

=

𝑘𝑘𝑒𝑒 2 𝑑𝑑 2 3 𝜀𝜀r 𝑟𝑟dd

,

(4)

where r dd is mean distance between the dipoles on the surface. If one assumes that there are N e of the electron-Li+ pairs arranged hexagonally on the surface of the core, the distance r dd can be calculated from the relation: 2 𝑟𝑟dd =

2π

√3𝑁𝑁e

2 𝐷𝐷core .

(5)

The charge-charge and dipole-dipole electrostatic energies contribute to the total raising of the energy of the electrons on the core that is given by the following formula: core ∆𝐸𝐸el = 𝑁𝑁e 𝐸𝐸cc + 3𝑁𝑁e 𝐸𝐸dd (6) For the core of the diameter D core = 8 nm Eq. (1) with Eqs. (3) – (6) gives the estimation of the absorbed electrons as N e ≈ 3,300. The maximum number of Li+ cations that fits on the surface of the core is of the order of 103. The calculated N e corresponds thus to ~33% of the available surface. The TGA/XPS studies showed that the number of ligands bound to the surface is 440. Since the area of the Au-S bond is ~0.2 nm2, they occupy ~44% of the surface of the core. If follows that the calculated number of Li+ cations can easily fit in the remaining space.

Page 8 of 13

AuNPs possess ability to accumulate electric charge resulting from their self-capacitance. To estimate the electrical capacitance of the AuNPs used, we approximate them by metal spheres of immersed in THF. The capacitance is calculated as C = ε r D core /2k = 3.38 aF. In the Naph-mediated charging, the voltage ∆V = (Φ Au - EA F )/e = 2.6 V determines roughly the limiting capacitive potential generated by the electrons gathered on the core. It follows that the capacitive effects would contribute at most to ∆VC ≈ 50 electrons. This rules out the possibility that the charge accumulated on the C 60 -BCT-OBu@AuNP could be due to the capacitive contributions. F To calculate ∆𝐸𝐸el first note that because C 60 is not a conducting sphere, the charge-image model is rather inadequate to model interaction between the absorbed electrons and the lithium cations. Instead, one has to assume that electrons are localized on the surface of the sphere. Due to a high degree of delocalization of electrons, one can assume that they are more or less uniformly distributed over the C 60 sphere. Consequently, the Li+ cations attached to the surface can also be treated as a uniformly positively charged cloud, approximated as an outer sphere. It is explained in Fig. 5B. Within this approach, the exF , of the electrons on the C 60 surface tra electrostatic energy, ∆𝐸𝐸el is calculated as: F ∆𝐸𝐸el =

𝑘𝑘𝑒𝑒 2 𝑁𝑁F2 (𝑅𝑅F+Li −𝑅𝑅F ) 2𝜀𝜀𝑟𝑟 𝑅𝑅F+Li 𝑅𝑅F

,

(7)

where N F is the number of electrons absorbed by the fullerene sphere. Here R F denotes the radius of the sphere. (R F = 0.35 nm), and R F+Li stands for the outer positively charged sphere formed by Li+ (cf. Fig. 5B). One can assume that the distance between the inner and outer spheres is roughly of the order of the size of the Li+ radius, that is, ~0.1 nm. Thus, R F+Li ≈ R F + 0.1 nm = 0.45 nm. From Eqs. (2) and (7) one gets 𝑁𝑁F = 6.6. Remarkably, this result agrees well with the reported50 ability of pristine C 60 to form in THF salts containing up to six Li+ ions. In view of our experimental results showing that C 60 absorbs on average 2.15 electrons, the obtained value of N F should be treated rather as a maximal capacity of the fullerene sphere for electrons. To estimate the amount of all electrons absorbed by the shell we employ the experimental value N F = 2.15, and calculate it as the product N lig × N F = 946. The total number of electrons absorbed by the C 60 -BCTOBu@AuNPs is a sum N e + N lig × N F = 4,246. It agrees well with the mean number of electrons absorbed by a single C 60 BCT-OBu@AuNPs obtained in experiments (~4,500). At this point, a comment about the unbound and oxidized ligands present in the precipitate is made. Within our method we could not establish if they contribute to the absorption of electrons. However, according to the XPS data (Fig. 2C), the unbound ligands account for only 16% of the total amount of C 60 -BCT-OBu contained in the precipitate. It means that they can at most contribute to ~5% of the total number of electrons accepted by C 60 BCT-OBu@AuNP. Also, we could not establish whether the sulfonate moieties contribute to the electron absorption. The sulfonate group can accept a maximum of six electrons (S(V) to S(-I)). The XPS studies (Fig. S3) revealed that ~30% of the ligands are in the oxidized form. Thus, assuming that sulfur atoms are reduced by Li, one can estimate that the sulfonate groups could contribute to at most ~18% of all electrons absorbed by the C 60 -BCT-OBu@AuNP. Finally, note that the presented model in which the Li+ cations adhere tightly to the core and the fullerene spheres is supported


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by experimental data. The surprisingly small value of Z-potential measured (about -15 mV) indicates that Li+ effectively screen the negative charge accumulated on the C 60 -BCTOBu@AuNPs and explains their tendency to from aggregates. Dissolution of the aggregates of C 60 -BCT-OBu@AuNPs by cationic surfactants. In the following, we seek to explain the surfactant-mediated dissolution process in terms of the model developed in the previous section. We present arguments that the attachment of the chains to the ligand shell of the AuNP is facilitated by ionic bonding between cationic surfactant head group and negatively charged fullerene sphere. First, consider the possibility that the surfactant molecule adsorbs electrostatically on the Au core with the amino heads (-N+(CH 3 ) 3 ) tethered to the gold surface.51 If this mechanism was operative, the surfactant molecules would bind to isolated (free) spots on the core, and their flexible alkyl chains would be entangled with neighboring C 60 -BCT-OBu molecules. Thus, in the case of CTAB, the hexadecyl chains of the free length of ~1.6 nm would hardly protrude above the layer of the C 60 -BCTOBu ligands, in contradiction with the results of the DLS studies. The same concerns DDAC molecule possessing alkyl chains of the free length of ~1 nm. Furthermore, in the case of BDAC, due to the presence of benzyl moiety, the binding would involve arrangement of the surfactant molecule in the position almost parallel to the surface of the core. Consequently, the alkyl chain would not stick out beyond the C 60 -BCT-OBu layer at all. In view of the above facts, the electrostatic binding between the Au surface and the amino heads of the cationic surfactants can be excluded as the mechanism responsible for dissolving of the aggregates. Next, consider the mechanism in which the surfactant molecule is anchored to the C 60 sphere through dipole–induced dipole attraction. Energy of this type of interactions is ~10-2 eV, which is of the order of thermal energy. For this reason the scenario in which undissociated surfactant molecules anchor to neutral fullerene spheres can be ruled out. This conclusion is supported by the experimental observation that nonionic surfactant C 12 E 10 , possessing highly polar polyethylene oxide groups, was not capable of dissolving the aggregates.

Figure 5: A) LiNaph-mediated charging process. B) Equipotential heterostructure composed of the AuNP core and the C 60 spheres. The dashed lines represent conducting molecular linkage. Energy diagram for the electron transfer from LiNaph to the core (C), and to the equipotential heterostructure (D).

It follows that the attachment of the alkyl chain to C 60 is possible only through the charge–charge electrostatic interaction. The proposed mechanism of the charging is explained in Fig. 6A. Electron from the metal core is transferred to the C 60 sphere via the π-conjugated hydrocarbon linkage. The cationic surfactant head binds then to the negatively charged fullerene sphere, and the halide anion concurrently binds to the positive charge (a hole) created at the surface of the Au core as a result of the electron transfer. Because CTAB, BDAC, and DDAC molecules do not dissociate in toluene due to its low dielectric permittivity, creation of ions requires separation of the halogen anion from the cationic head group of the surfactant. Thus, the above process can occur if the dissociation energy of surfactant, surf , is higher than the sum of the ionic binding energies of the 𝐸𝐸cc cationic head – quaternary ammonium (QA) cation, with fullerQA−F ene anion, 𝐸𝐸cc , and the halide anion, X − , with positively X−Au , that is charged Au core, 𝐸𝐸cc QA−F

surf X−Au > 𝐸𝐸cc + 𝐸𝐸cc . (8) 𝐸𝐸cc The electrostatic charge-charge energies can be crudely estimated by Eq. (3) with ε r = 2.38 (dielectric constant of toluene). The radius of Br − and Cl− is, respectively, r Br = 0.2 and r Cl = 0.18 nm. The radius of QA cation can be approximated by that of the NH 4 + cation, which is r A ≈ 0.15 nm. For the inter-ionic distances d = r Br + r A = 0.35 nm and d = r Cl + r A = 0.33 nm Eq. CTAB BDAC DDAC ≈ -1.7 eV, and 𝐸𝐸cc = 𝐸𝐸cc ≈ -1.8 eV, re(3) gives 𝐸𝐸cc spectively. To provide a rough estimate of the binding energies,



we employ the image-charge approximation. For d = 2r Br , and Br−Au ≈ -1.5 d = 2r Cl Eq. (3) gives the ionic binding energies 𝐸𝐸cc Cl−Au eV, and 𝐸𝐸cc ≈ -1.7 eV, respectively. To estimate the binding energy between the QA cation and C 60 we assume that the absorbed electron is localized at the surface and, consequently, d QA−F ≈ -4.0 eV. The above figures ≈ r A . Eq. (3) gives then 𝐸𝐸cc show that condition (8) is satisfied for all cationic surfactants used. The second condition necessary for the charge separation/ion binding process to occur concerns the electronic structure of the C 60 -BCT-OBu@AuNP (Fig. 6B). It is possible when energy of electron before the process, -Φ Au , is higher than the sum of energy of electron transferred at the fullerene sphere, -EA F + QA−F 𝐸𝐸cc , and the electrostatic energy of the hole created in the Au X−Au , core, 𝐸𝐸cc QA−F

𝐸𝐸𝐸𝐸F − ΦAu > 𝐸𝐸cc

X−Au + 𝐸𝐸cc .

(9)

Figure 6: Explanation of the charge separation and binding of the cationic surfactant (CTAB) to the AuNP (A), and the corresponding energy diagram for this process (B).

Page 10 of 13

chains. One of the synthesized ligands, O-Butyl 4-(azahomo(C 60 -I h )fullerene)benzothioate (C 60 -BCT-OBu), has been chosen to investigate in detail the nature of the interactions between the ligand and the gold core of the AuNPs. High-resolution XPS spectroscopy studies have revealed that the new C 60 derivatives possess the ability to bind chemically to the surface of the AuNP core through the Au-S covalent bonds with Au atoms. That is, we have demonstrated that thioketone and thionoesters can be successfully employed as new anchoring moieties. The UV-Vis absorbance spectroscopy studies have shown the value of the optical band gap equal to 2.76 eV, which is identical to that of pristine C 60 . The synthesized C 60 -BCT-OBu@AuNPs exhibit strong propensity to aggregate and form hardly soluble precipitate. They can be however easily dissolved into individual AuNPs in toluene solution of cationic surfactants. The C 60 -BCTOBu@AuNPs precipitate have been found to exhibit an extraordinary capability to accumulate negative charge. To demonstrate that it can perform as an efficient “electron-sponge” material, we have conducted experiments in which C 60 -BCTOBu@AuNP aggregates suspended in THF are charged in a LiNaph-mediated process. It has been determined that a single AuNP in the aggregate can accept on average as many as ~4,500 electrons. To explain the properties of the AuNPs we have developed a model in which C 60 -BCT-OBu@AuNP is represented by an equipotential heterostructure composed of C 60 spheres connected with the AuNP core by conducting molecular linkers. This model explains the exceptional ability of the AuNPs to absorb electrons, and correctly predicts the observed capacitance of the AuNPs.

Interestingly, condition (9) can be also derived in terms of the Fermi levels, as explained in Fig. 6B. Before the charge separation the Fermi level (𝐸𝐸Fermi ) of electron on the core is -Φ Au . ∗ ) in the heterostrucAfter the separation, the Fermi level (𝐸𝐸Fermi ture comprising the core and the attached C 60 -BCT-OBu ligands is calculated – by analogy to an intrinsic semiconductor – as an average of the Fermi level of electron in the C 60 sphere (QA−F EA F + 𝐸𝐸cc ) and that of the hole generated at the core (-Φ Au QA−F X−Au ). The requirement that -Φ Au > (-EA F + 𝐸𝐸cc )/2 + (+ 𝐸𝐸cc X−Au Φ Au + 𝐸𝐸cc )/2 reproduces the condition (9). For the system under investigation, the above condition is satisfied for CTAB, DDAC, and BDAC, which supports the proposed charge separation mechanism. Note that the charge separation/binding mechanism could be also operative for anionic surfactant SDS. In this case however, the Na+ cation would attach to the C 60 sphere and the anionic sulfate head to the gold core. Consequently, the alkyl chain would not stick out beyond the C 60 -BCT-OBu layer. This explains why SDS was not able to dissolve the precipitate.

ASSOCIATED CONTENT

SUMMARY AND CONCLUSIONS To sum up, we have developed first synthetic protocol to obtain fully conjugated fullerene C 60 derivatives that are capable of binding covalently to the gold surface. Using this protocol, we have synthesized four thionoester- and thioketone-substituted azahomo-[60]fullerenes. We have found that both the solubility of the obtained C 60 derivatives in organic solvents and the speed of ligand-exchange reactions on AuNP depends on and can be controlled by the length of the substituent alkyl

Notes

Supporting Information. The supporting Information is available free of charge via the Internet at http://pubs.acs.org. Detailed synthetic protocols for all compounds; 1H and 13C NMR spectra; TGA and XPS spectra of material, UV-Vis, and DLS characterization of dispersed AuNPs.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources This work was financed by the National Science Centre (NCN) OPUS grant No. 2016/23/B/ST5/02747.

The authors declare no competing financial interest.

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Gold Nanoparticles Functionalized with Fully Conjugated Fullerene

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