Alkyne-stabilized ruthenium nanoparticles: manipulation of intraparticle charge delocalization by nanoparticle charge States.
Alkyne-stabilized ruthenium nanoparticles: manipulation of intraparticle charge delocalization by nanoparticle charge States.
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DOI：
10.1002/anie.201004967
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发表时间：
2010-12
影响因子：
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通讯作者：
Xiongwu Kang;N. Zuckerman;J. Konopelski;Shaowei Chen
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文献类型：
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作者：
Xiongwu Kang;N. Zuckerman;J. Konopelski;Shaowei Chen
单层保护的过渡金属纳米粒子是一类独特的功能纳米材料，其性质不仅可以通过金属核和有机保护配体的化学性质来控制，而且可以通过金属-配体界面键合相互作用来控制。后者主要是由最近的进展，在纳米粒子钝化金属-碳共价键，其中粒子内电荷离域可能会发生作为一个结果的强烈的金属-碳界面键合相互作用，在鲜明的对比，纳米粒子功能化的巯基衍生物。例如，当二茂铁部分通过钌-卡宾p键结合到钌纳米颗粒表面上时，在混合价的二茂铁基金属中心之间发生有效的价间转移，如电化学和近红外（NIR）光谱测量和密度泛函计算所示。此外，当荧光团通过相同的共轭键连接到纳米颗粒表面上时，出现了与具有共轭间隔基的二聚衍生物一致的新的发射特性。在最近的一项研究中，还观察到有效的粒子内电荷离域与钌纳米粒子钝化的炔基片段。这一结果归因于钌与配体中碳原子的自旋杂化形成的独特的界面成键作用（Ru C）。在这些研究中，纳米颗粒金属核充当导电介质，以促进共价结合到纳米颗粒表面上的功能部分之间的电荷转移。因此，可以预期的是，粒子内共轭的程度可以很容易地控制的纳米粒子的电荷状态，这是本研究的主要动机。在实验上，利用Ru C纳米粒子的分子电容特性，通过简单的化学还原或氧化来改变纳米粒子的电荷状态。然后通过FTIR光谱、X射线光电子能谱（XPS）和光致发光测量仔细研究了纳米颗粒电荷状态对颗粒光学和电子性质的影响，并与所制备的纳米颗粒进行了比较。用于制备由1-辛炔基片段（Ru-OC）钝化的钌纳米颗粒的合成程序先前已经详细描述。TEM测量表明，纳米粒子表现出（2.55 ± 0.15）nm的平均核直径。然后通过化学氧化还原反应改变纳米颗粒的电荷状态。具体地，为了使纳米颗粒带负电荷，在典型的反应中，将5 mg的Ru-OC纳米颗粒溶解在二氯甲烷（ImL）中;然后加入新鲜制备的NaBH 4水溶液（ImL，5 mgmL）。将混合物搅拌30分钟，然后除去水。所得纳米颗粒表现出负净电荷，并表示为Ru-OCRed。通过将纳米颗粒溶液与饱和Ce（SO 4）2水溶液混合30 min，以类似方式制备带正电的纳米颗粒。所得纳米颗粒表示为Ru-OCOx。用低介电有机保护层钝化的过渡金属纳米颗粒长期以来一直被认为是纳米级分子电容器。事实上，基于同心结构模型，纳米颗粒电容（CMPC）可以通过CMPC= 4pe 0 e（r+d）rd d来估计，其中e0是真空介电常数，e是有机保护层的有效介电常数，r是金属芯的半径，并且d是有机保护配体的长度。对于辛炔钝化的钌（Ru-OC）纳米颗粒，r= 1.275 nm，d= 0.848 nm（由Hyperchem估计），并且e= 2.6。因此，纳米颗粒的电容可以估计为约0.92 aF。为了量化还原或氧化后的纳米颗粒电荷状态的变化，我们电化学测量了纳米颗粒的开路电位。结果表明，所制备的Ru-OC纳米粒子的开路电位为+0.140 V（vs. Ag/AgCl）。在通过NaBH 4还原后，其降低至+0.024 V，而在通过硫酸铈氧化后，其增加至+0.250 V。该结果表明，还原的纳米颗粒（Ru-OCRd）表现出每个纳米颗粒0.67个电子的平均充电，而氧化的纳米颗粒（Ru-OCOx）通过每个纳米颗粒0.63个电子的平均放电形成。有趣的是，尽管纳米粒子的电荷状态发生了这些微妙的变化，但观察到纳米粒子的光电特性受到了相当剧烈的影响。图1描绘了在还原或氧化之前和之后的纳米颗粒的FTIR光谱。对于所制备的Ru-OC纳米颗粒，C C伸缩带出现在1965 cm-1处（插图）。与辛炔单体相比，对于辛炔单体，C C伸缩[*] X。W.康，加-地B。Zuckerman，Prof. J. P. Konopelski，Prof. S. W. Chen加州大学化学与生物化学系1156 High Street，圣克鲁斯，CA 95064（USA）传真：（+1）831 -459-2935电子邮件：shaowei@ucsc.edu主页：http://chemistry.ucsc.edu/~ schen
Monolayer-protected transition metal nanoparticles are a unique family of functional nanomaterials in which the properties of the materials can be readily manipulated not only by the chemical nature of the metal cores and the organic protecting ligands, but also the metal–ligand interfacial bonding interactions. The latter is largely motivated by recent progress in nanoparticle passivation by metal–carbon covalent bonds, where intraparticle charge delocalization may occur as a result of the strong metal–carbon interfacial bonding interactions, in sharp contrast to nanoparticles that are functionalized by mercapto derivatives. For instance, when ferrocene moieties are bound onto a ruthenium nanoparticle surface by ruthenium–carbene p bonds, effective intervalence transfer occurs between the ferrocenyl metal centers at mixed valence, as manifested in electrochemical and near-infrared (NIR) spectroscopic measurements and density functional calculations. Furthermore, when fluorophores are attached onto the nanoparticle surface by the same conjugated linkage, novel emission characteristics emerge that are consistent with those of dimeric derivatives with a conjugated spacer. In a more recent study, effective intraparticle charge delocalization was also observed with ruthenium nanoparticles passivated by alkynyl fragments. This result was ascribed to the unique interfacial bonding interactions (Ru C ) formed by ruthenium and sphybridized carbon atoms of the ligands. In these studies, the nanoparticle metal cores serve as the conducting media to facilitate charge transfer between the functional moieties covalently bound onto the nanoparticle surface. Therefore it is anticipated that the extent of intraparticle conjugation may be readily controlled by the nanoparticle charge state, which is the primary motivation of the present study. Experimentally, by exploiting the molecular capacitor characters of Ru C nanoparticles, the charge states of the nanoparticles were varied by simple chemical reduction or oxidation. The impacts of the nanoparticle charge states on the particle optical and electronic properties were then carefully examined by FTIR spectroscopy, X-ray photoelectron spectroscopy (XPS), and photoluminescence measurements, and compared to those of the as-prepared nanoparticles. The synthetic procedure for the preparation of ruthenium nanoparticles passivated by 1-octynyl fragments (Ru-OC) has been detailed previously. TEM measurements showed that the nanoparticles exhibited an average core diameter of (2.55 0.15) nm. The nanoparticle charge states were then varied by chemical redox reactions. Specifically, to render the nanoparticles negatively charged, in a typical reaction, 5 mg of Ru-OC nanoparticles were dissolved in dichloromethane (1 mL); a freshly prepared water solution of NaBH4 (1 mL, 5 mgmL ) was then added. The mixture was stirred for 30 min and then water was removed. The resulting nanoparticles exhibited negative net charges and were denoted as Ru-OCRed. Positively charged nanoparticles were prepared in a similar fashion by mixing the nanoparticle solution with an aqueous solution of saturated Ce(SO4)2 for 30 min. The resulting nanoparticles were denoted as Ru-OCOx. Transition metal nanoparticles passivated with a lowdielectric organic protecting layer have long been known to act as nanoscale molecular capacitors. In fact, based on a concentric structural model, the nanoparticle capacitance (CMPC) can be estimated by CMPC= 4pe0e(r+d) r d, where e0 is the vacuum permittivity, e is the effective dielectric constant of the organic protecting layer, r is the radius of the metal core, and d is the length of the organic protecting ligand. For the octyne-passivated ruthenium (Ru-OC) nanoparticles, r= 1.275 nm, d= 0.848 nm (estimated by Hyperchem), and e= 2.6. Thus, the nanoparticle capacitance can be estimated to be about 0.92 aF. To quantify the change of the nanoparticle charge state after reduction or oxidation, we measured the open circuit potentials of the nanoparticles electrochemically. It was found that the as-prepared Ru-OC nanoparticles exhibited an open circuit potential of + 0.140 V (versus Ag/AgCl). After reduction by NaBH4, it decreased to + 0.024 V, whereas after oxidation by ceric sulfate, it increased to + 0.250 V. This result indicated that the reduced nanoparticles (Ru-OCRed) exhibited an average charging of 0.67 electrons per nanoparticle, whereas the oxidized nanoparticles (Ru-OCOx) were formed by an average discharging of 0.63 electrons per nanoparticle. Interestingly, despite these subtle changes of nanoparticle charge states, rather drastic impacts were observed on the nanoparticle optoelectronic properties. Figure 1 depicts the FTIR spectra of the nanoparticles before and after reduction or oxidation. For the as-prepared Ru-OC nanoparticles, the C C stretching band appeared at 1965 cm 1 (inset). In comparison to octyne monomers, for which the C C stretch[*] X. W. Kang, N. B. Zuckerman, Prof. J. P. Konopelski, Prof. S. W. Chen Department of Chemistry and Biochemistry, University of California 1156 High Street, Santa Cruz, CA 95064 (USA) Fax: (+1)831-459-2935 E-mail: shaowei@ucsc.edu Homepage: http://chemistry.ucsc.edu/~ schen