Rotational Motion of Ligand-Cysteine on CdSe Magic-Sized Clusters

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Rotational Motion of Ligand-Cysteine on CdSe Magic-Sized Clusters Takuya Kurihara, Akihiro Matano, Yasuto Noda, and Kiyonori Takegoshi J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019

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The Journal of Physical Chemistry

Rotational Motion of Ligand-Cysteine on CdSe Magic-Sized Clusters Takuya Kurihara,† Akihiro Matano,† Yasuto Noda,*,† and Kiyonori Takegoshi† †Division

of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa-

Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan

ABSTRACT

Structure and dynamics of organic ligands capping on the surface of clusters significantly influence the property and function of the clusters. Recently, we revealed that, in cysteine-capped (CdSe)34 magic-sized cluster (CdSe-Cys), ligand-cysteine has two capping structures, namely, monodentate ligand-cysteine with sulfur–cadmium bond and bidentate one with sulfur– and nitrogen–cadmium bonds. In this work, we examine the motion of ligand-cysteine capping on (CdSe)34 by performing solid-state 2H nuclear magnetic resonance (NMR) spectroscopy in CdSe-Cys with deuterated methanediyl group (–CD2–).

2H

quadrupoler Carr–Purcell–Meiboom–Gill (QCPMG) and

quadrupolar-echo spectra of CdSe-Cys were measured. Temperature dependence of line width and spin-lattice relaxation time of spikelets in the QCPMG spectra suggested that monodentate ligandcysteine undergoes molecular motion. The quadrupolar-echo spectra were a sum of a motionally-

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narrowed peak at the center of the spectra and a typical 2H Pake pattern, which are assigned to monodentate and bidentate ligand-cysteine, respectively. The line shape analysis for the 2H quadrupolar-echo spectra obtained at 0–40°C revealed that the CH2 group in monodentate ligandcysteine undergoes pseudoisotropic rotation by the combination of two intramolecular rotations around the Cd–S and S–C bonds with the activation energy of ~19 kJ/mol. Further, no evidence was found for exchange of the two capping structures.


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INTRODUCTION Nanoparticles (NPs) have attracted interest because of their unique optical and electrical properties. In NPs composed of an inorganic core and an organic layer, the organic layer plays an important role for stability and solubility of NPs. Further, it was shown that the organic ligands can also affect various physical/chemical properties and functions of NPs. Improvement in the quantum yield of photoluminescence (PL) of semiconductor NPs by ligand modification1–4 is of interest for applications to light-emitting devices, cellular imaging, and so on. The ligands can not only cap the core, but also link one NP to another NP or substrate. Such NP–NP and NP– substrate complexes are potentially applicable to nanoelectronic devices.5–7 In addition to these effects due to a “static” capping structure, dynamics of the ligand molecules plays an important role in electronic structure and dynamics,8,9 formation of self-assembled monolayers,10–12 catalytic activity,13,14 surface enhanced effects,15 nanomedical application,16 and so on. It has further been reported that fluctuations in the surface and interface structure of NPs affect the optical phenomena, such as photoannealing and photobrightening.17 Therefore, investigation of the dynamic as well as static structure at the surface and interface is essential for fundamental study and various applications of NPs. Clusters, ultrasmall NPs with up to several hundreds of core atoms, show dramatic size effects. In particular, clusters composed of a specific, magic number of core atoms can be highly stable. Kasuya et al. reported ligand-capped CdSe magic-sized clusters (MSCs) (CdSe)13 and (CdSe)34.18 One of the attractive properties of CdSe MSCs is PL depending on the structure of ligands. So far, various studies for controlling and improving the PL property using the ligand exchange and the postsynthetic ligand addition methods have been reported.19–25 However, characterization of the surface and interface structure of CdSe MSCs, which would give insights


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into the optical property, still remains challenging. In our recent nuclear magnetic resonance (NMR) studies, we showed the existence of amine–cadmium bond in cysteine-capped (CdSe)34 MSCs (CdSe-Cys) and determined the ratio of amine–cadmium bond formation (~43%) as well as the bond length (~0.24 nm).26 Further studies by infrared (IR), X-ray photoelectron (XPS), and multinuclear solid-state NMR spectroscopies revealed two capping structures of ligandcysteine; one forms sulfur– and nitrogen–cadmium bidentate coordination, whereas the other forms only the sulfur–cadmium monodentate coordination (Figure 1).27 It was also unraveled that the carboxylate group of ligand-cysteine has ionic interaction with counter ion Na+ in the solid-state sample. In this study, motional effects of ligand-cysteine was suggested, such as, by narrower 1H magic-angle spinning NMR lines and faster spin-lattice relaxation times of CdSeCys compared to those in crystalline cysteine. Herein, we report more detailed studies on molecular motion of ligand-cysteine by solid-state 2H NMR spectroscopy of CdSe-Cys with using partially deuterated ligand-cysteine.

Figure 1. Two capping structures of ligand-cysteine on (CdSe)34 MSC revealed in our previous work.27


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The dynamics of ligands have been examined by various NMR experiments.11,13,14,28–30 Among them, solid-state 2H NMR spectroscopy is a powerful method to characterize segment motion of ligands in solid samples. The shape of resonance lines in 2H quadrupolar echo spectra sensitively reflects molecular motion with a motional rate k ranging from 103 to 107 s−1, and thus analysis of the temperature dependence of the line shape gives information on local dynamic structure.31–34 In 2H quadrupolar Carr–Purcell–Meiboom–Gill (QCPMG) spectra having a wide spikelet pattern, both the overall spectral shape and the spike width and height is sensitive to motion with k = 102–108 s−1.35,36 In this work, we synthesized CdSe-Cys with the deuterated methanediyl group (CdSe-[CD2]Cys) and studied solid-state 2H QCPMG and quadrupolar-echo NMR of CdSe-(CD2)Cys. The 2H QCPMG spectra suggested the presence of molecular motion of monodentate ligand-cysteine. The 2H quadrupolar-echo spectra showed a motionallynarrowed peak and a typical Pake pattern, which were assigned to monodentate and bidentate ligand-cysteine, respectively. The line shape analysis for the 2H quadrupolar-echo spectra obtained at 0–40°C revealed that the CD2 group in mobile monodentate ligand-cysteine undergoes pseudoisotropic rotation by the combination of two intramolecular rotations around the Cd–S and S–C bonds. The activation energy of the pseudoisotropic rotation is ca. 19 kJ/mol, which small value indicates that the dynamics of monodentate ligand-cysteine is almost unrestricted by the electrostatic interaction with the counter ions.


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EXPERIMENTAL SECTION Synthesis and Solidification of CdSe-(CD2)Cys Cadmium (shot, 99%), selenium (powder, 99%), sodium sulfite (Na2SO3) (97%), sodium hydrate (NaOH) (93%), 60–61% nitric acid (HNO3), and acetone (99.5%) were purchased from Wako Pure Chemicals. 3,3-d2-cysteine (2H 98%) and L-cysteine (97%) were obtained from Cambridge Isotope Laboratories and Aldrich, respectively. All chemical reagents were used as received. With 3,3-d2-cysteine, CdSe-(CD2)Cys was synthesized in aqueous solution and solidified by the method that we reported previously.26 Firstly, selenium aq. was synthesized by stirring 50 mg of selenium, ~240 mg of Na2SO3, and 12.5 ml of distilled H2O at 950 rpm in a brown glass vial at 105°C on a hotplate stirrer overnight. 1 M L-cysteine aq. was prepared from 197.4 mg of 3,3d2-cysteine, 18.6 mg of L-cysteine, and 1.78 ml of distilled H2O. Then, cadmium-cysteine complex aq. was obtained by mixing cadmium hydroxide prepared from 50 mg of cadmium, 1.78 ml of 1 M L-cysteine aq., and 8 ml of 1 M NaOH aq. CdSe-(CD2)Cys solution was obtained by adding 4.45 ml of selenium aq. to the cadmium-cysteine complex aq. and was kept at room temperature for 24 h. Successful preparation of CdSe-(CD2)Cys was confirmed by UV-vis measurement (see Figure S1 in Supporting Information). Then, CdSe-(CD2)Cys was precipitated by adding ~7.5 ml of acetone into the synthesis solution, and was collected by centrifugation at 10,000 rpm for 5 min. Finally, CdSe-(CD2)Cys powder was obtained after vacuum drying.


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Solid-State 2H NMR Spectroscopy 2H

QCPMG and quadrupolar-echo NMR experiments for CdSe-(CD2)Cys were performed in a

magnetic field of 7.05 T (2H 46.1 MHz) with an OPENCORE spectrometer and a 2H static 5 mm probe. The 2H QCPMG spectra were measured with a 90° pulse length of 2.5 μs and echo intervals of 100 μs. The quadrupolar-echo spectra were obtained with a 90° pulse length of 2.5 μs and a 90° pulse interval of 30 μs. Sample temperature was controlled by cooled N2 gas flow and calibrated by using the 2H chemical shift values of deuterated methanol (CD3OD).37 2H

quadrupolar-echo spectra in the presence of molecular motion were simulated with

EXPRESS38 using a tetrahedral four-site jump model (Figure 2).39 In this model, molecular rotation is pseudoisotropic, and is characterized by the site-to-site jump rate krot and the Euler angles (α, β, γ) = (0°, 0°, 0°), (0°, β’, 0°), (0°, β’, 120°), and (0°, β’, 240°) (109.47° ≤ β’ ≤ 180°) which define the transformation of the principal axis frame of the electric field gradient tensor into an intermediate jump frame. The anisotropy of the pseudoisotropic molecular rotation is expressed by the anisotropy parameter A as

𝐴=

1 (2 + 3(3cos2 𝛽′ ― 1)). 8

When β’ = 109.47°, the rotation is isotropic (A = 0), whereas β’ = 180° corresponds to the largest anisotropy (A = 1).


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Figure 2. Tetrahedral four-site jump model used for the simulation of the 2H quadrupolar-echo spectra under isotropic and pseudoisotropic rotation. Euler angles (α, β, γ) transforming the principal axis frame of the electric field gradient tensor into an intermediate jump frame are shown.

RESULTS AND DISCUSSION 2H

QCPMG NMR of CdSe-(CD2)Cys

In the 2H QCPMG spectrum measured at −50°C (Figure 3a), spikelets spaced by an interval of 10 kHz were observed. In the spectrum obtained at 40°C (Figure 3b), broader spikelets, in addition to the narrow spikelets similar to those in Figure 3a, were also observed in the region ranging from −10 to 10 kHz. The spikelet in 2H QCPMG spectra is known to broaden at the motional rate k of the order of 103–105 s−1.35 Thus, the broadening of the spikelets in the central region of the spectrum obtained at 40°C indicates molecular motion of the CD2 group of ligandcysteine, while the narrow spikelets appearing in the spectra at 40°C suggests co-existence of such ligand-cysteine that remains rigid. Importantly, our previous work revealed that, in CdSeCys, ligand-cysteine has the two capping structures, monodentate ligand-cysteine having the


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sulfur–cadmium bond and bidentate one having the sulfur– and nitrogen–cadmium bonds (Figure 1).27 It is envisaged that the CH2 group in monodentate ligand-cysteine is more mobile as compared to that in bidentate one. Hence, we ascribed the broad and narrow spikelets observed at 40°C to mobile monodentate and rigid bidentate ligand-cysteine, respectively.

Figure 3. 2H QCPMG spectra of CdSe-(CD2)Cys measured at −50°C (a) and 40°C (b, c). Recycling delay (RD) periods of the QCPMG sequence were set to 8 s (a), 1 s (b), and 0.01 s (c).

To examine the difference in the mobility of the CD2 group corresponding to the narrow and broad spikelets, we measured another QCPMG spectrum at 40°C under the same conditions except for the much shorter recycling delay (RD) of 0.01 s (Figure 3c) than that in Figure 3b (RD = 1 s). In Figure 3c, the broad spikelets became dominant; the D magnetizations of the CD2 group with higher mobility, and thereby those with shorter spin-lattice relaxation time T1, are emphasized in the spectrum. To further examine the molecular motion of the CD2 group in


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ligand-cysteine in detail, we measured the 2H quadrupolar-echo spectra by setting RD to 0.01 s, so that the signal of mobile ligand-cysteine can be mainly observed.

2H

quadrupolar-echo NMR of CdSe-(CD2)Cys

Figure 4a shows the 2H quadrupolar-echo spectra of CdSe-(CD2)Cys measured at 0–40°C.


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Figure 4. (a) Experimental 2H quadrupolar-echo spectra of CdSe-(CD2)Cys obtained at 0–40°C with RD = 0.01 s. (b) Simulated 2H quadrupolar-echo spectra for pseudoisotropic rotation (blue solid line), isotropic rotation with the site-to-site jump rate krot = 1 × 107 s−1 (gray), no molecular motion with krot = 0 s−1 (green), and sum of the three components (red). krot and the anisotropy parameter A used for the spectral simulation of pseudoisotropic rotation are shown at the left side of each simulated spectrum. For the spectral simulation of the pseudoisotropic and isotropic


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rotations, CQ = 173 kHz and η = 0.03 obtained by the spectral fitting (see Figure S2) were used. The static spectrum (green) without motion was calculated with CQ = 170.8, 168.8, 168.0, 167.1, and 165.6 kHz at 0, 10, 20, 30, and 40°C, respectively, and η = 0.03. Asterisks indicate artifacts arising from electrical transient effects.

The resonance line at the spectral center became narrower with increasing temperature. Thus, it can be assigned to the mobile CD2 group. In addition, the broad typical Pake pattern having edges at around ±62 kHz was found. We assign the narrow peak at the spectral center and the broad line shape to monodentate and bidentate ligand-cysteine, respectively. It is well-known that the line shape of 2H NMR spectra of isotropically rotating molecules changes from the Pake pattern to a narrow peak with increasing the motional rate.40 The spectral feature at the center position thus suggests that the CD2 group in monodentate ligand-cysteine undergoes isotropic rotation, and we considered that the “isotropic” motion is realized by the combination of two intramolecular rotations around the Cd–S and S–C bonds. We shall refer this rotation to as pseudoisotropic rotation. In this work, we first attempted to simulate the 2H quadrupolar-echo spectra under isotropic rotation by using the tetrahedral four-site jump model with the Euler angle β’ = 109.47° (Figure 2). The simulated line shape of the central peak did not depend on the site-to-site jump rate krot and did not account for the broader line width of the central peak in the experimental spectrum at 40°C (Figure S3). Accordingly, we took anisotropy into account by employing the distorted tetrahedral jump model with β’ > 109.47°, varying the site-to-site jump rate krot and the parameter A defining the deviation from β’ = 109.47° as adjustable parameters. Figure 4b shows


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the 2H quadrupolar-echo spectra simulated with the distorted tetrahedral model (blue solid line), the non-distorted tetrahedral model with krot = 107 s−1 (gray), and no molecular motion (green). The red line is the sum of the three simulated line shapes, which well accounted for the experimental spectra. We here conclude that the CD2 group in monodentate ligand-cysteine undergoes the pseudoisotropic rotation (the blue component) and that in bidentate ligandcysteine has less mobility (the green component). For the gray component, we ascribed it to free mobile cysteine contained in the sample as impurity, which was also observed as the sharp signal components in the 1H MAS NMR results.27 It is noted that quantitative determination of the ratio of the three cysteine species based on the 2H quadrupolar-echo intensity is difficult because the intensity depends also on the rate and mode of the motion.39 For bidentate ligand-cysteine, the best-fit CQ values to the experimental line shapes observed at 0, 10, 20, 30, and 40°C were 170.8, 168.8, 168.0, 167.1, and 165.6 kHz, respectively. The decrease of the CQ value with increasing temperature is presumably assigned to motional averaging of CQ by vibrational motion of the CD2 group in bidentate ligand-cysteine.41 For the pseudoisotropic rotation of monodentate ligand-cysteine, the larger anisotropy parameter A at lower temperature indicates that the pseudoisotropic rotation becomes more anisotropic at lower krot. If the activation energies of the Cd–S and S–C axis rotations have different values, the temperature dependence of each rotational rate is also different, leading to the change of the anisotropy for the pseudoisotropic rotation depending on tempetarure. Lastly, we would like to point out that the temperature dependence of the experimental line shape can be explained only by the anisotropy of the pseudoisotropic rotation and the vibrational motion. This indicates that, in solid-state CdSe-Cys, exchange of the capping structure of ligand-cysteine between the monodentate and bidentate forms does not occur in timescale of 10−7–10−3 s at 0–40°C.


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Figure 5 shows an Arrhenius plot for the krot values. The temperature dependence of krot was fitted to the Arrhenius equation, and the activation energy Ea was obtained to be 19.2 ±0.5 kJ/mol. This value is comparable to Ea = 2–20 kJ/mol of the methyl rotations.42–44 Presumably, the low activation energy for almost isotropic rotation of the sizable CD2 moiety is allowed by high surface curvature due to small diameter of (CdSe)34 (< 2 nm)45 and/or surface geometry of (CdSe)34 where enough space for monodentate ligand-cysteine to rotate exists around the cadmium site. Our previous study revealed that the carboxylate group of ligand-cysteine forms ionic bond with the counter ion Na+ in the solid-state CdSe-Cys.27 The small Ea value of the pseudoisotropic rotation indicates that the electrostatic interaction between the carboxylate group and the counter ion does not prevent the rotation of ligand-cysteine.

Figure 5. Temperature dependence of the site-to-site jump rate krot. The solid line represents the least-squared fit by the Arrhenius law. From the slope, the activation energy of the pseudoisotropic rotation was obtained to be 19.2 ±0.5 kJ/mol.


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CONCLUSION We synthesized CdSe-(CD2)Cys and examined the molecular dynamics of ligand-cysteine on the (CdSe)34 core by solid-state 2H NMR. The 2H QCPMG spectra suggested the presence of facile molecular motion of monodentate ligand-cysteine. In the 2H quadrupolar-echo spectra of CdSe-(CD2)Cys, the motionally-narrowing peak and the broad Pake pattern were assigned to mobile monodentate and rigid bidentate ligand-cysteine, respectively. The line shape analysis for the 2H quadrupolar-echo spectra measured at 0–40°C revealed that the CD2 group in monodentate ligand-cysteine undergoes the pseudoisotropic rotation by the combination of the two intramolecular rotations around the Cd–S and S–C bonds. This result suggests that (CdSe)34 has surface structure where ligand-cysteine can have the mobility. Recently, it has been reported that the PL quantum yield of noble metal clusters can be enhanced by restricting intermolecular rotation and/or vibration of the ligands.30,46–51 As for semiconductor clusters, although the optical property has been attracting attention, to our best knowledge, the effects of ligand mobility on the PL quantum yield has not been reported yet. Verification of the effects is left as future work.


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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Experimental condition for UV-vis; UV-vis spectrum of CdSe-(CD2)Cys; Experimental 2H quadrupolar-echo spectrum of CdSe(CD2)Cys measured at −75°C; Experimental 2H quadrupolarecho spectrum of CdSe(CD2)Cys with simulated line shapes (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by JSPS Grant-in-Aid for Scientific Research on Innovation Areas “Mixed anion” (Grant Number 16H06440) and Grant-in-Aid for JSPS Fellows (Grant Number 18J11973).


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Rotational Motion of Ligand-Cysteine on CdSe Magic-Sized Clusters

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