Kinetic Study on the Self-Assembly of Au(I)âThiolate Lamellar Sheets

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A: Kinetics, Dynamics, Photochemistry, and Excited States

Kinetic Study on the Self-Assembly of Au(I)-Thiolate Lamellar Sheets: Pre-Assembled Precursor vs Molecular Precursor Chuying Dai, Yajiao Hao, Yang Yu, Minjie Li, and Sean Xiao-An Zhang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b02103 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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The Journal of Physical Chemistry

Kinetic Study on the Self-Assembly of Au(I)Thiolate Lamellar Sheets: Pre-Assembled Precursor vs Molecular Precursor Chuying Dai, Yajiao Hao, Yang Yu, Minjie Li,* Sean Xiao-An Zhang AUTHOR ADDRESS State Key Lab of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P.R. China *Corresponding author, email: [email protected] fax: +86-431-85153812 phone: +86-431-85153811

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT:

Molecular

self-assembly has

played

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an

important

role

in

nanofabrication. Due to the weak driving forces of non-covalent bonds, developing molecular nano-assemblies that have both robust preparation conditions and stable structure is a challenge. In our previous work, we have developed a reversible selfassembly system of Au(I)-thiolate coordination polymer (ATCP) to form colloidal lamellar sheets, and demonstrated the high tailorbility and stability of their structures, as well as their promising applications in gold nanocluster/nanoparticle fabrication and UV light shielding. Here, we first reported our progress in exploring a robust and green assembly protocol toward ATCP colloidal lamellar sheets in water by allowing the molecular precursors of HAuCl4 and thiol ligand to form ATCP-pre-assembled intermediates. In this way, colloidal ATCP lamellar sheets can be prepared in a wide synthetic concentration ([Au]0 ≥ 2 × 10-4 M) and a broad assembly temperature (80 100 oC) with similar high yields (> 80%). The assembly kinetics at different conditions are also studied in detail to help understanding the robust assembly process. The robust and green synthetic protocols will pave a way for their real applications.

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1. INTRODUCTION Molecular self-assembly has been an indispensable technique in nanofabrication nowadays. Due to the weak driving forces of non-covalent bonds, the assembled structures are usually susceptible to both the preparation conditions and the assembly processes. Variation of common preparation parameters, such as concentration,1-3 temperature,4,5 and solvents,6-8 may lead to distinct molecular packing modes, and manipulation of assembly processes is also effective to control the structures of the assemblies.9-11 The susceptibility of the molecular assemblies is an advantage when applied in catalysis,12,13 sensing,14,15 and detection.16,17 However, it is also a disadvantage when applied as robust functional materials due to the instable performance upon the change of their environments. Developing molecular nanoassemblies that have both robust preparation conditions and stable structure is still a challenge. Among various types of molecular level self-assembled materials, coordination polymer (CP) is an outstanding one because of its high structural tailorability,18,19 thermal stability,20,21 and order parameter.22 Au(I)-thiolate coordination polymer (ATCP), as a special type of CP, is widely studied because of its broad applications as antirheumatic drugs,23,24 gold precursors,25-27 luminescent materials,28 as well as building blocks to produce various hierarchical self-assembled structures.29 The specialty of ATCP over other types of CPs is that linear Au-S bonds exclusively compose the backbone of the polymer chain. The further assemblies of ATCP are 3



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driven by both inter-chain aurophilic interactions and inter-ligand interactions.30 This makes ATCP assemblies differ from the hot-studied metal-organic framework (MOF) materials that apply only coordination bond as the main driving force.31-33 Therefore, ATCP assemblies usually have more complicated assembly processes and structures. Take ATCP lamellar structure for example, our previous work has revealed that the CP chains first assembled into string-shaped intermediates by inter-chain interactions, and then the string-shaped intermediates coiled, aggregated and crystallized to form lamellar sheets.34 In contrast to MOF materials, ATCP lamellar sheets can show reversible assembly-disassembly behavior by tuning the inter-ligand interactions, because the inter-ligand interactions and inter-chain aurophilic interactions are synergistically interacted in the assemblies.35,36 In the meanwhile, the lamellae still show much stronger thermal stability than H-bond supramolecular polymers, because there are multiple assembly driving forces including Au-S bond, aurophilic interactions and inter-ligand interactions (Figure S1).5 What’s more, we’ve demonstrated the possibility of continuous engineering of structure of the lamellar material by doubleligand co-assembly.36 Therefore, ATCP is very promising to develop into a robust selfassembled nanomaterial to integrate new functions if their synthetic conditions are easy to handle and suitable for industrialization. In this work we report our exciting results on robust synthetic conditions by using the pre-assembled string-shaped intermediates as precursors instead of the conventional molecular precursors. The effects of preparation conditions (concentration and 4


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temperature) on the conversion rate and yield are systematically studied. It reveals that precursors of string-shaped intermediates have several advantages over molecular species: First, two processes of aggregation and crystallization during the string-tolamellar-sheet transformation can be separated, showing a concentration-irreverent conversion rate and yield in a wide concentration range, and the lowest assembly concentration can be 1/100 to 1/1000 of that of conventional H-bond polymer,1-3,37 not reported in other materials as far as we know. Second, although there are some differences between the string-shaped precursors that were prepared at significant different temperatures, their conversion rates and yields into lamellar sheets are all acceptable. Third, the assembly can be performed in a wide range of temperature from 80 °C to 100 °C in water at time-scale of several minutes to hours, and it is much shorter than some self-assembly systems that take days to finish the assembly.38,39 We propose ATCP lamellar sheets are very promising to perform as a robust self-assembled skeleton to align and integrate other functionalities on them.

2. EXPERIMENTAL DETAILS 2.1.1

Materials. All chemicals were commercially available and used without

further purification. Hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O, AR grade) was purchased from Shenyang Jinke Reaction Company, 3-mercaptoropionic acid (MPA ≥ 99%) was purchased from Alfa Aesar Company, sodium hydroxide (NaOH, ≥ 96%) was purchased from Beijing chemical industry reagent company. Sodium 35



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mercaptopropionate (MPA-Na) was obtained by 1: 1 molar ratio neutralization of MPA with NaOH. Water was purified by Milli-Q system. 2.2 Preparation. All glass flasks and burettes were cleaned thoroughly with aqua regia before use. Preparation of Au(I)-MPA String-Shaped Intermediates. The triplicate mixtures of 4 mL MPA-Na (0.05 M) and 1.5 mL NaOH (0.1 M) solution were added into 50 mL HAuCl4 solution (1 × 10-3 M) at room temperature (precursor-RT), 80 °C (precursor80), and 100 °C (precursor-100) respectively. They were then cooled down to room temperature with ice water immediately. Preparation of Au(I)-MPA Lamellar Sheets with String-Shaped Intermediates. Studies on the effects of concentration: 0.5 mL, 1 mL, 2 mL, 4 mL, 6 mL, 8 mL and 10 mL of the above solutions containing string-shaped intermediates were added into purified water to maintain the same volume of 10 mL in Erlenmeyer flasks respectively. The [Au]0 of these solution were 5 × 10-5 M, 1 × 10-4 M, 2 × 10-4 M, 4 × 10-4 M, 6 × 10-4 M, 8 × 10-4 M, and 1 × 10-3 M. The flasks were put on a hot plate set at 300 °C to quickly make the solution boiled, and time was recorded thereon. The mixtures were allowed to react for minutes to hours depending on [Au]0. Study on the effects of temperature: 10 mL of the above solutions containing stringshaped intermediates was added in Erlenmeyer flasks. The [Au]0 of these solutions were 1 × 10-3 M. The flasks were put into oil baths set at 60 °C, 70 °C, 80 °C, and 90 °C, and

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time was recorded thereon. The mixtures were allowed to react for minutes to hours depending on temperature. Preparation of Au(I)-MPA Lamellar Sheets with Molecular Precursors. Study on the effects of concentration: 10 mL water solutions containing 5 × 10-5 M, 1 × 10-4 M, 2 × 10-4 M, 4 × 10-4 M, 6 × 10-4 M, 8 × 10-4 M, and 1 × 10-3 M HAuCl4 were heated to boil on a hotplate set at 300 °C. The mixtures of MPA-Na (0.05 M) and NaOH (0.1 M) with the molar ratio of HAuCl4: MPA-Na: NaOH = 1: 4: 3 were added into the boiling solution, and time was recorded thereon. The mixtures were allowed to react for minutes to hours depending on [Au]0. Study on the effects of temperature: 10 mL HAuCl4 solution (1 × 10-3 M) was added in Erlenmeyer flask. The flasks were put into oil baths set at 60 °C, 70 °C, 80 °C, 90 °C. After five minutes, the mixtures of 0.8 mL MPA-Na (0.05 M) and 0.3 mL NaOH (0.1 M) were added into the HAuCl4 solutions, and time was recorded thereon. The mixtures were allowed to react for minutes to hours depending on temperature. 2.3 Characterizations. UV-Vis spectra were measured using a Shimadzu UV-Vis 2550 spectrophotometer (wavelength resolution: 0.1 nm) with 1 cm light path cuvettes. The scan step was set as 1 nm. The concentration of measurements: 0.2 ml sample solution was diluted with 1.8 ml purified water. Transmission Electron Microscopy (TEM) was performed on JEM-2100F electron microscope at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were operated on a Thermo ESCALAB 250 spectrometer with twin-anode Al Kα (1486.6 eV) X-ray source at 15 7



kV and 17 mA. FT-IR was investigated by a VERTEX 80/80v spectrometer, using KBr as the reference. PXRD data were obtained with an Empyrean diffractometer with Cu Kα radiation (λ = 1.5418 Å). The tube current and voltage were 40 mA and 40 kV, respectively; the scan range (2θ) was from 5.0° to 40.0°; and the scan step was 0.1° min−1. DSC experiment was investigated by a NETZSCH DSC 204 instrument in a N2 atmosphere at a scanning rate of 10 °C min-1 to 400 °C for one cycle. TGA was measured on a NETZSCH STA 449C thermogravimetric analyzer with a heating rate of 10 K min -1 from 30 °C to 800 °C in an air atmosphere. AFM images were measured on a NanoscopeIIIa scanning probe microscope from Digital Instruments in the tapping mode under ambient conditions, and the samples were adsorbed on hydroxy modified silicon slides for AFM characterization.

3. RESULTS AND DISCUSSION 3.1 Application of UV-Vis Spectra to Correlate Abs with Conversion/Yield. ATCP is a coordination polymer formed by linear coordination between Au(I) and thiol ligands. And the synthesis of ATCP involves the reduction of Au(III) ions with thiol ligands first to generate Au(I) and dithiol ether, then the coordination of Au(I) with the remaining thiol ligands to form linear coordination polymer. By reaction of Au(III) with 3-mercaptoropionic acid (MPA) ligands at proper pH values (4.77 – 6.78) at boiling temperature, we have prepared colloidal Au(I)-MPA lamellar sheets in water (Figure S2). It is found that string-shaped intermediates form immediately upon adding the 8


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ligands to the boiling Au(III) solution by inter-chain aurophilic interactions, and then the string-shaped intermediates aggregate and crystallize to form lamellar sheets.34 The Au(I)-MPA lamellar sheets have very characteristic, strong and narrow ligand to metal charge transfer (LMCT) and metal centered charge transfer (MCCT) absorption bands at 394 nm and 350 nm which have distinct difference in spectral shape and position with the string-shaped intermediates and the peak positions will change when the crystal structure changes.34,36 So, the UV-Vis spectrum can be used as a fingerprint of the lamellar structure. By applying UV-Vis spectroscopy to monitor the assembly process, the kinetics involved in the string-sheet transition, as well as the yields of the sheets can be revealed. Although further aggregation of smaller sheets to larger ones can happen, it only results in stronger scattering of the light, but does not change the absorption positions. We need to establish the quantitative relationship between UV-Vis spectra and the concentrations of Au ([Au]) in the lamellar sheets in the first place, and then the conversion rate and yield of the sheets can be easily monitored with UV-Vis spectroscopic method. First, we studied whether Lambert-Beer law is applicable for ATCP lamellar sheets. A series of ATCP lamellar sheets with different average sizes (104 nm to 527 nm) with the same crystal structure (sample 1 to sample 5) were prepared (Figure 1a, Figure S3-S8), they were all separated from their mother solution and re-dispersed into water at different concentrations. We can see that the relationship between the absorbance and concentration of [Au] all obey the Lambert-Beer law when 9



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[Au] is in the concentration range of 10-5 M (Figure 1b-f). However, we found that the molar extinction coefficients (ε) decrease with the sizes of the samples. This is because the absorption of the sheets comes from LMCT and MCCT, which means the numbers of electron transition centers are as many as the numbers of Au or S atoms in the sheets, that’s why we use the concentration of Au atoms ([Au]) rather than the concentration of the sheets ([sheet]) to calculate ε with Lambert-Beer law. When the size and thickness of the sheets goes larger and thicker, the number of the sheets will decrease, leading to more inhomogeneous distribution of the absorption centers in the dispersion solution and less efficient absorption of light since the absorption centers only exist within the sheets. Therefore, the larger size of the sheets, the smaller the extinction coefficients (Figure S9). Ren et al have explored formulas and experimental setup to separate the scattering and absorption of metal nanoparticles accurately with a dualchannel optical fiber-based UV-Vis spectrometer,40 which is very important for the evaluation of certain properties of nanoparticles. Here, due to the narrow absorption band of the assemblies, the light scattering appeared as an absorption tail in long wavelength. When the size is larger, the scattering is stronger, so the absorbance at nonabsorption position can be used as an indicator or their relative sizes. Here, we defined a light scattering coefficient (S) (the absorbance at 500 nm / absorbance at 394 nm) as a standard to judge the light scattering ability for the five different sized samples (0.0095, 0.019, 0.027, 0.031 and 0.037, respectively). Their molar extinction coefficients can be calibrated with the light scattering coefficient to obtain a unified 10


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calibrated molar absorption coefficient (εc) with an emprical Equation 1. In this way, no matter how large the product is, their unified calibrated molar absorption coefficient is the same for all the samples (Table 1). The average value of εc is 25285.2 L/(mol × cm).

𝜀𝑐 = 𝜀 × 4 × ∛𝑆

(1)

After we establish the relationship among molar absorption coefficients (εc), molar extinction coefficients (ε) and light scattering coefficients (S), we can calculate the [Au] in the lamellar sheets and calculate the conversion (C) by reading the absorbance and scattering coefficient for any sample at any stage with Equation 2, in which b is the light pass of the cuvette and [Au]0 is the total concentration of Au in the solution. In this way, we can use UV-Vis spectra to study the assembly dynamics of Au(I)-MPA sheets thereafter. One thing that needs to be mentioned is that the absorbance will sometimes decrease near the end of the reaction, which is caused by partially precipitation or disassembly/decomposition of the sheets, and this part of data will not be used in the analyses. 𝐶 = 𝐴𝑏𝑠/(𝜀𝑐 × 𝑏 × [𝐴𝑢]0 )

(2)

11



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Figure 1. UV-Vis spectra change of the lamellar sheets with [Au] by gradual dilution for Sample 1 (a); The linear fitting of absorption at 394 nm with [Au] in Au(I)-MPA lamellar sheets by gradual dilution of Sample 1 (b), Sample 2 (c), Sample 3 (d), Sample 4 (e) and Sample 5 (f). Table 1. Calculation of Au(I)-MPA Lamellar Sheets with Different Sizes Sample No. 1

[Au] (mol/L) 7.42 × 10-

b (cm) 1

Abs 1.997

ε (L/(mol·cm)) 27394.4

S 0.0095

εc (L/(mol·cm)) 23219.2

1

1.934

25288.6

0.019

27054.1

1

1.678

21268.1

0.027

25651.4

1

1.667

20087.3

0.031

25130.0

1

1.458

19028.6

0.037

25371.5

ε̅c (L/(mol·cm))

5

2

7.68 × 105

3

7.81 × 10-

25285.2

5

4

8.15 × 105

5

7.45 × 105

3.2. Research on the Assembly Kinetics of Three Pre-Assembled StringShaped Intermediates.

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3.2.1 Three String-Shaped Intermediates as Precursors of the Sheets. Three different string-shaped intermediates were prepared by reaction of Au(III) with MPA at three different temperatures (room temperature (RT), 80 °C and 100 °C) and the reaction systems were cooled to RT immediately after mixing the reactants when the preparation temperatures are 80 °C and 100 °C. Because the reduction and coordination reaction to form ATCP chains are very fast and efficient, the CP chains can further aggregate to form string-shaped intermediate at room temperature at near 100% yield, but they cannot further grow to form lamellar sheets at RT. The three string-shaped intermediates are denoted as precursor-RT, precursor-80 and precursor-100, and they were fully characterized with UV-Vis spectra, XPS, IR (Figure S10) and TEM (Figure 2a). XPS and IR show that the compositions of these string-shaped intermediates are the same. TEM images reveal that the length of strings is longer and thus the aggregation is more serious when the synthetic temperature is higher. Then, three precursors were heated to boiling temperature for further growth, and it is found that lamellar sheets with similar sizes and shapes were produced after about 10 minutes with all these three precursors (Figure 2b).

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Figure 2. TEM images of precursor-RT (a1), precursor-80 (a2), precursor-100 (a3) and Au-MPA lamellar sheets assembled by precursor-RT (b1), precursor-80 (b2) and precursor-100 (b3). Scale bar = 200 nm.

We then studied how concentration of the precursors and temperature affect the growth curve with UV-Vis spectra in the following section. 3.2.2 Effects of [Au] 0 on the Conversion Rate and Yield. The concentration of [Au]0 in the solution of string-shaped intermediates is tuned to 5 × 10-5 M, 1 × 10-4 M, 2 × 104

M, 4 × 10-4 M, 6 × 10-4 M, and 8 × 10-4 M respectively by diluting the three precursors

with pure water. Then these precursors are heated to boiling temperature and the growth of the lamellar sheets is monitored with UV-Vis spectroscopy (Figure S11a-c). It’s very interesting to find that both the yield and conversion rate are nearly the same when [Au]0 ≥ 2 × 10-4 M for all the three precursors with an exception caused by an experimental misconduct (Figure 3a-e). It indicates that the string to sheet transition is not equilibrium. The assembly kinetic is close to a first-order reaction in this concentration range. This is because the strings have already aggregated to different 14


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degrees at this concentration range and these aggregates are ready to form the lamellar nuclei. When the concentration goes down to 5 × 10-5 M, both the yield and conversion rate go down. The results indicate that it is very hard for single strings to form nuclei without further aggregation with other strings when [Au]0 ≤ 1 × 10-4 M. Au(I)-MPA lamellar sheets were also prepared with Au(III) and MPA as reactants directly at different concentrations. If the assembly begins with these ions, lamellar sheets cannot form when the concentration of [Au]0 was lowered to 5 × 10-5 M (Figure S11d). The lowest concentration to form lamellar sheets is about 1 × 10-4 M. If the strings are already formed and then diluted as we described before, lamellar sheets can still form at [Au]0 = 5 × 10-5 M. The result is in consistent with the above conclusions that the pre-assembled string aggregates can transform to nuclei directly without further aggregated with other strings, but if a single string does not aggregate with other strings, it is hard for it to form nuclei, and it will grow on the nuclei. Different from applying string-shaped intermediates as precursor, the conversion rate is highly dependent on the [Au]0 when using Au(III) and MPA as precursor. When the concentration of the molecular precursor is more concentrated, the assembly finishes faster. It means that the aggregation of the strings is a rate-determining step in all concentration range. The reason for the difference between using the pre-assembled string-shaped intermediates as precursor and using the molecular species as precursor in assembly is proposed as follows: the aggregation of strings intermediates can happen at room temperature very effectively, and their further crystallization to form lamellar sheets 15



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requires higher temperature (discussed later). So when the concentration of [Au]0 ≥ 2 × 10-4 M, the strings have already aggregated at room temperature to some degree, and their conversion rates into lamellar sheet do no rely on the concentration thereafter. Only when the concentration of [Au]0 is less than 1 × 10-4 M, it shows concentrationdependent conversion rate (Figure 3f). If the reaction was started at boiling temperature with the molecules as precursor, aggregation is still a prerequisite for crystallization, but the crystallization rate is faster than the aggregation rate, so aggregation is the ratelimiting step and it shows concentration-dependent conversion rates. So from this study, we know that we can separate the aggregation and crystallization by using the pre-assembled string-shaped intermediates as precursors, and the lamellar sheets can be obtained with high yield of > 80% in a large concentration range from [Au]0 = 2 × 10-4 M to [Au]0 =1 × 10-3 M with similar conversion rates. And so far, we do not see significant differences among the three string-shaped intermediates as precursors.

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Figure 3. The Conversion-Time curves at different [Au]0 with precursor-RT (a), precursor-80 (b), precursor-100 (c) and molecules (d) (the green dotted lines of the first three graphs are the time when temperature reaches 100 °C). The Conversion-[Au]0 curves with different precursors (e); schematic illustration of assembly of Au(I)-MPA lamellar sheets affected by [Au]0 concentration (f).

The phenomenon that the pre-assembled string-shaped intermediates can further grow into lamellar sheets at very low concentrations is possibly because their lower structural transformation energy barrier and lower solubility in solution. It is a significant difference from other supramolecular systems, such as host-guest polymers and H-bond polymers. In host-guest system, the building blocks tend to form intramolecular species at a low concentration and it turned to intermolecular species at a high concentration.41,42 For H-bond polymers, the degree of polymerization is distinctly dependent on the concentration of the reactant. The viscosity increases slowly before the critical concentration, above which point the viscosity starts to rise much more remarkably.1,43 The concentration-dependent assembled behavior is related to the weak-bonding strength of the non-covalent bonds in nature, and it results in the problem that some assemblies change their structures or even cannot exist after separated from the mother solution,44-46 which greatly limits their real application. Here, the stringshaped intermediates can be used as a concentration-insensitive precursor, which profits from their high stability during diluting and heating. And the strong stability is resulted from i) the much higher strength of Au-S bonds (40–145 kJ mol-1) in the 17



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polymer backbone than H-bond or host-guest interactions, and ii) the co-existence of the aurophilic interactions and inter-ligand H bond interactions to enhance the stability.47,48 Even though Au-S bonds have high bonding strength, reversible breakage and reformation of these bonds are possible, which is a common phenomenon in the drifting of self-assembled monolayer of thoilates on gold substrates,49,50 ligand exchange process on gold nanoparticle,25,26 and our previously reported string-tolamellar-sheet transformation process,34 therefore, both the high bonding strength and the dynamic bonding process enable us to have more control of their assembly. 3.2.3 Effects of Temperature on the Conversion Rate and Yield After we studied the effect of concentration on the assembly kinetics, we further study how temperature affects the assembly. The assembly was controlled at five temperatures of 60 °C, 70 °C, 80 °C, 90 °C and 100 °C. [Au]0 was controlled at 1 × 10-3 M. From the above study, it can be concluded the pre-assembled string-shaped assemblies have already aggregated to different degrees at such a high concentration, and these string-aggregates can further fuse and crystallize to form nuclei themselves. It can be seen that the assembly can proceed at all these temperatures, and the conversion rate goes faster when the temperature is higher. The conversion-time curves show the type of precursor has greatly affected the conversion rates when temperature is ≤ 90 °C (Figure 4). The conversion rate for precursor-RT is significantly faster than that of precursor-80 and precursor-100 when temperature is lower than 100 °C. While the precursor-80 and precursor-100 show similar conversion rates in this temperature range. However, when 18


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the temperature is increased to 100 °C, precursor-RT and precursor-80 do not show difference in both conversion rate and yield.

Figure 4. The Conversion-Time curves at different temperatures with precursor-RT (a), precursoe-80 (b), precursor-100 (c) and molecules (d) at [Au]0 = 1 × 10-3 M.

The above study indicates that temperature has different impacts on the assembly rates for the three precursors. Because it has shown the concentration-irreverent conversion rate at 100 °C when [Au]0 is higher than 2 × 10-4 M, which is the feature of first-order reaction, we studied ln(c)-time curves at different temperatures for the three precursors one by one (Figure 5a, Figure S12).51,52 In these experiments, time is recorded once the precursors are put in the oil bath, and it needs time (2 min to 4 min) to reach the set temperatures, so the beginning of the assembly is not used for the data 19



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analyses; also, the absorbance sometimes decreases near the end of the reaction, which is caused by partially precipitation or disassembly/decomposition of the sheets, and this part of data is not used in the analyses either. Even after removing the beginning part and the ending part, it can be seen that ln(c)-time curve is not strictly linear in the whole temperature range from 60 °C to 100 °C, therefore, the whole assembly is not a typical first-order reaction. However, compared with precursor-RT and precursor-80, it shows a broad linear range in the ln(c)-time curve for precursor-100 in the middle part of the assembly (80% of the assembly has finished during this part). It means the middle part of the assembly can follow first order of reaction for precursor-100 because its aggregation is more severe than the other two, and the concentration-irreverent conversion rate is understandable. By plotting lnk-1/T curve (Figure 5b), the activation energy barrier for the assembly is calculated to be around 110.5 kJ/mol. As for precursor-RT and precursor-80 (Figure S13), the assembly of precursor-80 is similar to precursor-100, but the assembly of precursor-RT deviates obviously from first-order reaction, and it is hard to understand why it still shows concentration-irreverent conversion rates, we will continue to study what other factors have affected the assembly. But from the fact that the precursor-RT has a faster conversion rate than precursor-100, we know its activation energy barrier is lower than that of precursor100. From the fact that precursor-80 has a similar conversion rate to precursor-100, we know its activation energy barrier is similar to that of precursor-100 too. Even though we have not figured out all the factors that affect the assembly, from the above 20


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phenomenon we can conclude that the activation barrier is related to the aggregation size of the precursor. Because the fusion and crystallization of string-aggregates to form lamellar sheets need to break and re-form chemical bonds, it needs to break more bonds simultaneously for larger aggregates, so the activation barriers for precursor-80 and precursor-100 are larger than precursor-RT (Figure 5c). Therefore, besides the broad synthetic concentrations, the assembly can be achieved in a broad temperature range too with acceptable reaction times. But in some H-bond system, when the temperature is higher than 50 °C, the driving force becomes weak.5 The temperature above 80 °C can make some host-guest polymers gradually dissociate.4

Figure 5. The ln(c)-Time curves at different temperatures with precursor-100 (a) (the blue dotted lines are the linear regions); the lnk-1/T curve of precursor-100 (b); schematic illustration of free energy of Au(I)-MPA lamellar sheets with different stringshaped intermediates (c). Finally, we studied the effect of temperature on the quality of the assemblies. Because the self-assembled materials are very hard to be perfectly crystallized and they show only the inter-layer diffraction peaks without the information on intra-layer order, 21



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therefore, XRD cannot be used to assess the quality of the assemblies (Figure S14). However, the UV-Vis spectra of ATCP lamellar sheets are very sensitive to the bonding environments in the assemblies, we use the full-width at half-maxima (FWHM) of the absorption bands as a standard to evaluate the quality of the lamellar sheets. If the assemblies are perfectly crystallized, it will have the narrowest FWHM. When the defects become more, the FWHM of the absorption will become wider. From Figure 6, it can be seen that the FWHMs all have a main tendency to decrease with heating time, indicating a gradually improved crystallinity. However, when the assembly finishes, the FWHMs are all in a very narrow range of 19-26 nm in the temperature range from 60 °C to 100 °C, and the FWHMs fluctuate from 19- 21 nm at 80 °C to 100 °C. So the assembly can have very close quality at the temperature range from 80 °C to 100 °C.

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Figure 6. The FWHM-Time curves for precursor-RT (a), preecursor-80 (b), precursor100 (c) and molecules (d) at different temperatures and the change of FWHM of the final product with assembly temperatures (e).

4. CONCLUSIONS By using pre-assembled string-shaped intermediates as precursors instead of the molecular precursors, the assembly has less dependence on preparation conditions. It can endure a broad assembled concentration ([Au]0 = 2 × 10-4 M to [Au]0 = 1 × 10-3 M) and temperature range (80 °C - 100 °C) without significant differences in yield and quality of the assemblies for all the three string-shaped intermediates. The broad concentration-irrelevant conversion and yield in this protocol is owing to the two processes of aggregation and crystallization have been separated by preparing the string-shaped intermediates and allow them to aggregate at room temperature first. Once the strings have aggregated, they can fuse and crystallize to form nuclei themselves without further aggregate. The minor change in the synthetic procedures leads to robust synthetic conditions, which makes the scale-up and even industrialization of ATCP materials possible. What’s more, this work lays a foundation on control of the size and thickness of the assemblies by controlling the aggregation states of the string-shaped intermediates, which will be reported in our further work. The robust synthetic protocols as disclosed in this paper, as well as their high thermal and colloidal stability, vast surface functional groups and their reversible assembly 23



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behaviors as disclosed in our previous work make the materials very promising to integrate multi-functions on them to apply as catalyst carriers, solution processible functional particles, absorbents and so on.

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ASSOCIATED CONTENT Supporting Information. DSC and TGA data of Au(I)-MPA nanosheet, scheme of the assembly pathway of Au(I)-MPA, morphology and UV-Vis spectra of Sample 1 to Sample 5, scheme of the reasons for the molar extinction coefficients decrease with the sizes of Au(I)-MPA nanosheets, UV-Vis and XPS data of pre-assembled precursors, UV-Vis spectra of Au(I)-MPA nanosheets, ln(c)-Time curves and lnk-1/T curves of precursor-RT and Precursor-80, XRD data of Au(I)-MPA nanosheets (Word) AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Fax: 86-431-85153812 Tel: 86-431-85153811 Present Addresses State Key Lab of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P.R. China Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.


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Kinetic Study on the Self-Assembly of Au(I)âThiolate Lamellar Sheets

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