Formation of 1D Infinite Chains Directed by Metal ... - ACS Publications

Article Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Formation of 1D Infinite Chains Directed by Metal−Metal and/or π−π Stacking Interactions of Water-Soluble Platinum(II) 2,6-Bis(benzimidazol-2′-yl)pyridine Double Complex Salts Victor Chun-Hei Wong,† Charlotte Po,† Sammual Yu-Lut Leung,† Alan Kwun-Wa Chan,† Siyuan Yang,‡ Bairen Zhu,‡ Xiaodong Cui,‡ and Vivian Wing-Wah Yam*,† †

Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee (Hong Kong)), and Department of Chemistry, The University of Hong Kong Pokfulam Road, Hong Kong, P. R. China ‡ Department of Physics, The University of Hong Kong Pokfulam Road, Hong Kong, P. R. China S Supporting Information *

ABSTRACT: A new class of water-soluble double complex salts (DCSs), [Pt{bzimpy(TEG)2}Cl][Pt{bzimpy(PrSO3)2}Cl] and its alkylplatinum(II) bzimpy derivatives (bzimpy = 2,6-bis(benzimidazol-2′-yl)pyridine, has been demonstrated to exhibit strong aggregation in water through Pt···Pt and π−π stacking interactions to give a variety of distinctive nanostructures based on the formation of one-dimensional (1D) infinite chains. The self-association process can be systemically controlled by varying the solvent composition and temperature and has been studied by 1H NMR, 2D NOESY NMR, mass spectrometry, electron and confocal fluorescence microscopy, UV−vis absorption, and emission spectroscopy.

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INTRODUCTION Square-planar platinum(II) polypyridine complexes of d8 electronic configuration have aroused enormous attention, not only as a result of their intriguing spectroscopic and luminescence properties but also their tendency to form metal−metal and/or π−π stacking interactions.1−4 Early studies on metal− metal interactions in the platinum(II) polypyridine system were mainly confined to the solid-state polymorphism of various platinum(II) complexes arising from the differences in metal− metal and/or π−π separations.1−4 The enhanced solubility and room-temperature phosphorescent properties of alkynylplatinum(II) terpyridine3f,g and alkynylplatinum(II) bzimpy (bzimpy = 2,6-bis(benzimidazol-2′-yl)pyridine) systems have accelerated the study and understanding of metal−metal interactions in the solution state, with drastic color changes observed upon polyelectrolyte-induced3a−d and solvent-induced3e−h aggregation, demonstrating the involvement of Pt···Pt interactions in solutions. With the rich and interesting spectroscopic and luminescence properties, platinum(II) polypyridine complexes have also been employed as versatile spectroscopic probes for biomolecules.3b,k−m Furthermore, the aggregation process has been demonstrated to display the potential for the construction of supramolecular architectures by exploiting these metal−metal interactions.3,4 Platinum(II) polypyridine complexes are also capable of forming metallogels, well-defined supramolecular structures and liquid crystal phases through the self-assembly processes.3,4 Double complex salts (DCSs) represent one of the platinum(II) systems renowned for forming infinite linear metal © XXXX American Chemical Society

atom chains by columnar stacks of alternating square planar cations and anions.1a,5−11 The first DCS reported is the Magnus’ salt,5 [Pt(NH3)4][PtCl4], in 1828. However, it was only after more than 100 years later that in-depth investigations revealed its correct structure through X-ray structure determination.6 It was found that the Magnus’ salt consisted of two distinct forms, in which the Pt···Pt separation of the pink form is >5 Å, and the one with surprising green color shows linear chains of platinum(II) metal centers with Pt···Pt contacts of 3.23−3.25 Å.6 The insoluble nature of the Magnus’ salt has hindered further detailed studies. It was only until the remarkable effort in fabricating soluble Magnus’ salt derivatives by Caseri and colleagues that detailed investigations became possible.7 The color of the derivatives is found to be closely dependent on the distances between the metal centers.7e Re-investigations of the Magnus’ salt showed that the materials display interesting band properties for potential conductive application in the field of molecular electronics.8 Mann and co-workers reported a series of DCSs with the anion [Pt(CN)4]2−, in which most of them were found to exhibit interesting vapochromic and vapoluminescent behaviors for the development of selective chemosensors.9 The area of platinum(II) polypyridine DCSs has been relatively less explored and most of the DCSs involve either the anion, [Pt(CN)4]2−,1f,9d or nonplatinum(II) polypyridine ions,10 probably because of the poor solubility problem. Recently, DCSs of Received: September 13, 2017

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DOI: 10.1021/jacs.7b09770 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society Scheme 1. Structures of Complexes 1−3 and Their Respective Precursor Complexes, [Pt{bzimpy(TEG)2}Cl]Cl, [Pt{bzimpy(TEG)2}(CC−C6H5)]PF6, [Pt{bzimpy(PrSO3)2}Cl]PPN and [Pt{bzimpy(PrSO3)2}(CC−C6H5)]PPN

Figure S1). Water-soluble platinum(II) double complex salt complex 1 was eventually isolated as a purple precipitate by the salt metathesis reaction between its corresponding cationic precursor complex, [Pt{bzimpy(TEG)2}Cl]Cl, and its anionic precursor complex in PPN salt, [Pt{bzimpy(PrSO3)2}Cl]PPN, in equal amounts in dichloromethane solution. Similarly, complexes 2 and 3 were successfully synthesized with their corresponding precursor complexes, respectively, under the same ratio in dichloromethane solution. All the complexes have been characterized by 1H NMR spectroscopy, IR spectroscopy, high-resolution ESI mass spectrometry, and satisfactory elemental analyses. Dissolution of complexes 1−3 in pure DMSO at room temperature under dilute condition at 10−5 M gives a clear yellow solution that shows very intense intraligand (IL) [π → π*(bzimpy)] absorptions at ca. 315−371 nm and less intense absorption tails at ca. 452 nm in the UV−vis absorption spectra, as shown in Figure 1 and Figure S2. The low-energy absorption tails are assigned as metal-to-ligand charge transfer (MLCT) [dπ(Pt) → π*(bzimpy)] transitions, with some ligand-to-ligand charge transfer (LLCT) [π(CC) → π*(bzimpy)] character in the cases of complexes 2−3. The lack of additional lower-energy absorptions suggests that all the double complex salts exist as discrete charged species with no significant self-aggregation in pure DMSO solution under dilute conditions. In contrast, aqueous solution of complex 1 shows a clear red solution, while those of complexes 2−3 give orange solutions under the same concentration. Upon the increase of water content in DMSO solution, the solution color of complex 1 is found to change from yellow (100% DMSO) to orange (30% H2O) and finally to red (beyond 50% H2O), accompanied by a growth of a new absorption band at ca. 566 nm with clear isosbestic points in the UV−vis absorption spectral traces, as illustrated in Figure 1. The newly appeared absorption band is typical of metal−metal-to-ligand chargetransfer (MMLCT) [dσ*(Pt2) → π*(bzimpy)] transitions, arising from the presence of Pt···Pt and/or π−π stacking interactions assisted by the hydrophobic−hydrophobic interactions between the platinum(II) bzimpy moieties upon an increase in solvent polarity. Electrostatic interactions between the oppositely charged ions also facilitate the ion-pair formation that brings the platinum(II) bzimpy moieties into close proximity. A plot of the normalized degree of aggregation against the volume fraction of

[Pt(bzq)(CNR)2][Pt(bzq)(CN)2] (bzq = 7,8-benzoquinolinate), where both ions contain polypyridine moieties, were reported.11 However, these DCSs were unstable and showed thermally irreversible ligand rearrangements that resulted in the eventual formation of the corresponding netural complexes, [Pt(bzq)(CN)(CNR)].11 Thus, an understanding of DSCs of platinum(II) polypyridines has been limited. In addition to the metal− metal and electrostatic interactions between oppositely charged complexes, the DCSs containing polypyridine moieties in both the cations and anions are expected to possess π−π stacking interactions during the molecular association processes. Thanks to the ease of functionalization of the bzimpy pincer ligands, herein, we report a new strategy in designing DCSs in which both complex ions contain the platinum(II) pincer complex ions. This new class of DCSs, [Pt{bzimpy(TEG)2}Cl][Pt{bzimpy(PrSO3)2}Cl] (1) and its alkylplatinum(II) bzimpy derivatives, [Pt{bzimpy(TEG)2}(CC−C6H5)][Pt{bzimpy(PrSO3)2}Cl] (2) and [Pt{bzimpy(TEG)2}(CC−C6H5)][Pt{bzimpy(PrSO3)2}(CC−C6H5)] (3) (Scheme 1), contains hydrophilic TEG chains in its cations and negatively charged hydrophilic sulfonate groups in its anions. While conventional methods for preparation of DCSs are done by salt metathesis in aqueous medium,1a,e,5,7,9−11 the DSCs reported in this article are prepared in dichloromethane. To the best of our knowledge, these complexes are the first report of highly water-soluble platinum(II) DCSs. The water-soluble DSCs are found to show interesting spectroscopic and luminescence changes upon a variation of the solvent composition; such changes are found to be associated with formation of well-defined morphologies.

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RESULTS AND DISCUSSION

Attempts to synthesize platinum(II) DCS complex 1 in aqueous medium by metathesis reaction between its corresponding watersoluble cationic precursor complex, [Pt{bzimpy(TEG)2}Cl]Cl, and anionic precursor complex, [Pt{bzimpy(PrSO3)2}Cl]K, did not produce the DCS as a precipitate, indicating its high water solubility. A titration experiment was conducted by mixing [Pt{bzimpy(TEG)2}Cl]Cl and [Pt{bzimpy(PrSO3)2}Cl]K in aqueous solution, and a binding stoichiometry of 1:1 was clearly obtained from the Jobs’ plot (Supporting Information (SI), B


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observation of an emerging low-energy structureless emission band at ca. 672−697 nm with photoexcitation at 395 nm upon an increase in water content in the DMSO solutions (Figure 2a

Figure 2. (a) Corrected emission spectra of complex 1 (2.24 × 10−5 M) upon increasing the water content in DMSO. (b) Normalized emission intensity of complexes 1−3 in 90% water in DMSO (v/v).

Figure 1. (a) UV−vis absorption spectra of complex 1 (2.24 × 10−5 M) upon increasing the water content in DMSO. The inset shows a plot of normalized degree of aggregation (monitored at the absorbance at 566 nm) as a function of DMSO volume fraction with curve fitting to the equilibrium model. (b) UV−vis absorption spectra of complex 1 in water (9.89 × 10−5 M) upon increasing temperature from 10 to 80 °C. (c) Solutions of complex 1 in water at room temperature and upon heating.

and Figure S4), and the photophysical data are tabulated in Table S1. Such intense emission, assigned as 3MMLCT emission, in high water content (beyond 30% H2O) could be ascribed to the presence of Pt···Pt and π−π stacking interactions with the self-assembly induced by the reduced solvation of the platinum(II) bzimpy moieties under increased solvent polarity, further supporting the formation of aggregates synergistically induced by metal−metal, π−π stacking, hydrophobic−hydrophobic, and electrostatic interactions. Further support comes from the 1H NMR studies at various DMSO-water compositions (10−4 M) (Figure 3a, b). Complex 1 is found to undergo the aggregation process in aqueous media. In pure DMSO, well-resolved proton signals with chemical shifts and splitting patterns consistent with the chemical formulation are observed. Increasing the water composition has led to a gradual upfield shift and broadening of the proton signals corresponding to the bzimpy moieties and has finally ended up with very broad and poorly resolved proton signals at water content beyond 40%. Similar upfield shifts have also been found in other related systems upon aggregation.3h,13 This suggests the existence of substantial π−π stacking interactions in the platinum(II) bzimpy moieties at high water composition. Temperature-dependent 1H NMR studies of complex 1 in pure D2O also reveal aggregate formation and the occurrence of a deaggregation process with increasing temperature (Figure 3c). Proton signals in the aromatic region become better resolved and downfield shifted with increasing temperature. It is worth noting that the proton signals are not completely well-resolved even at 80 °C, indicating that there is still a substantial extent of Pt···Pt and/or π−π stacking interactions at such a high temperature, which is consistent with the findings obtained in the temperature-dependent UV−vis study. Although there exists no significant aggregation in dilute DMSO solution (10−5 M), high concentrations of the complexes

DMSO as the good solvent results in a clear sigmoidal curve, indicating an isodesmic growth mechanism for the aggregation process (Figure 1a, inset).12 The aggregation process is believed to involve a non-nucleated pathway with the thermodynamic parameters ΔG° and m determined to be −41.5 ± 1.1 kJmol−1 and 39.3 ± 2.2 kJmol−1 respectively (SI), suggesting that the aggregation process is spontaneous. The corresponding plot also indicates that a critical water content of around 40% is required to trigger the formation of aggregates, as revealed by the growth of the MMLCT band, demonstrating the formation of metal−metal interactions. Similarly, upon increase of water content in DMSO solutions for complex 2 (Figure S2a), an additional absorption band appears at ca. 566 nm, which is assigned as MMLCT band. The growth of a new absorption band for complex 3 appears to be less obvious (Figure S2b). Interestingly, for both complexes 1 and 2, the low-energy absorption bands at ca. 566 nm are found to be highly sensitive to temperature. The absorption bands are found to show a drop in intensity and a blue shift in the absorption maxima with a concomitant growth of a high-energy absorption band at ca. 380 nm with an isosbestic point at ca. 403 nm upon increasing the temperature (Figure 1b and Figure S3). It is believed that the DCSs would undergo a deaggregation process, attributed to a weakening of the intermolecular Pt···Pt and/or π−π stacking interactions and hence leading to a drop and blue shift of the MMLCT absorption bands. Surprisingly, the low-energy absorption band is not fully suppressed, which reveals the incomplete deaggregation even at such high temperatures. For all the complexes, the growth of the MMLCT transition band in the UV−vis absorption spectra is concomitant with the C


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Figure 4. (a) UV−vis absorption spectra of complex 1 in DMSO in the concentration range of 1.48 × 10−3 to 8.32 × 10−6 M. The apparent absorbance values have been obtained by correcting to a 1 cm path length equivalence. (b) UV−vis absorption spectra of complex 1 in DMSO (1.04 × 10−3 M) upon increasing temperature from 20 to 80 °C. Figure 3. (a) 1H NMR spectral traces at different compositions of D2O in DMSO-d6 for complex 1 (6.18 × 10−4 M, 400 MHz, 298 K). (b) Solutions of complex 1 in D2O-DMSO-d6 mixture (volume percentage of D2O in DMSO-d6 from left to right: 0, 10, 20, 30, 40, 50%). (c) 1H NMR spectra of complex 1 in D2O (5.44 × 10−4 M) at 25, 40, 55, 70, 80 °C, respectively.

in DMSO solutions (10−3 M) give rise to aggregation. Upon an increase of concentration of complex 1 from the range of 8.32 × 10−6 to 1.48 × 10−3 M, a growth of new low-energy absorption bands is observed when the concentration reaches beyond 5 × 10−4 M. By monitoring the absorbance at 550 nm, a significant deviation from the Beer’s law is found (Figure 4a), which is commonly observed in related platinum(II) polypyridine systems.3h,4d,14 This suggests the occurrence of groundstate aggregation between the oppositely charged ions, probably due to the formation of the ion-pair complexes, in high-concentration DMSO solution, and the low-energy absorption band is assigned to be originated from the MMLCT transition due to the formation of Pt···Pt and π−π stacking interactions. Furthermore, the low-energy absorption band is sensitive to temperature. Upon the increase in temperature, there is a drop in the low-energy absorption band revealing the deaggregation process at high temperature (Figure 4b). The temperature-dependent 1 H NMR studies have provided further evidence for the aggregation process in high-concentration DMSO solution (10−3 M). The aromatic proton signals from the bzimpy moieties are clearly downfield-shifted at high temperature (67 °C) when compared to that recorded at room temperature (25 °C) (Figure 5). Such downfield shifts have also been observed in other related systems during deaggregation processes.3h,13a,b On the contrary, under dilute condition (10−5 M), there is negligible change in the chemical shifts of the aromatic proton signals, further confirming the occurrence of aggregation in concentrated DMSO solution. To verify the identity of the aggregate species of the DCSs in aqueous solution, electron microscopies have been employed to

Figure 5. 1H NMR spectra of complex 1 in DMSO-d6 (1.01 × 10−3 M) at 25 and 67 °C, respectively.

rationalize the spectral changes observed in the UV−vis absorption, emission, and NMR experiments. TEM images observed from DMSO solutions (10−4 M) of complex 1 indicate the absence of distinct and well-defined nanosize aggregates, which is in good agreement with the lack of aggregation observed in the UV−vis spectra. TEM images of complex 1 prepared from pure water under similar concentration give nanofibers with uniform diameters of ca. 60 nm and lengths ranging from ca. 5 to 20 μm (Figure 6a). Similar nanofiber architectures have also been evidenced by both scanning electron microscopy (SEM) and atomic force microscopy (AFM) images on the sample prepared on silica wafers (Figure 6b, c). The height of the nanowire structure is found to be ca. 20 nm with a width of ca. 100 nm (Figure 6c inset), indicating a flattened structure on silica wafer in the SEM and AFM studies, possibly caused by the interaction of the soft materials with the surface.15 In contrast, complex 2 instead of forming straight nanofibers forms more flexible nanofibers of smaller diameter in aqueous solution (Figure 6d), and complex 3 only forms much shorter nanorods. The magnified transmission electron microscopy (TEM) image shows that the nanarod is formed by a bundle of small nanofibers of diameter of ca. 4 nm (Figure 6f). It can be seen that the replacement of chloro ligands with phenyl alkynyl ligands poses negative effects on the formation of straight and D


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Figure 6. (a) TEM, (b) SEM, and (c) AFM images of the superstructures prepared from complex 1 (6 × 10−4 M) in water. Inset of (c): Height profile of the nanostructures at the selected cross-section (indicated as red line). TEM images prepared from (d) complex 2 (5 × 10−4 M) and (e) 3 (5 × 10−4 M) in water. (f) Magnified TEM image prepared from 3 (5 × 10−4 M) in water.

as illustrated in Figure S5b, while [Pt{bzimpy(PrSO3)2}Cl]K shows a vesicle architecture with a diameter of ca. 200 nm which has been reported by our group previously (Figure S5a).3h [Pt{bzimpy(TEG)2}Cl]Cl and [Pt{bzimpy(PrSO3)2}Cl]K are both chloroplatinum(II) bzimpy complexes with different substituents on the pincer ligands. In contrast to the hydrophobic nature of the platinum(II) bzimpy moieties, the anionic sulfonate groups in [Pt{bzimpy(PrSO3)2}Cl]K are strongly hydrophilic in nature. It is believed that, in water, the sulfonate head would point outward, while the platinum(II) bzimpy moieties would be forced to pack closely through Pt···Pt and/or π−π stacking interactions to avoid unfavorable contact with solvent molecules, resulting in a multibilayer vesicle structure.3h Similarly, the TEG groups in [Pt{bzimpy(TEG)2}Cl]Cl are hydrophilic but with less affinity for water when compared with the anionic sulfonate heads on [Pt{bzimpy(PrSO3)2}Cl]K. It is likely that, in water, the TEG groups would play a similar role as the sulfonate head groups, leading to formation of nanosheet aggregates. The morphology of complex 1 in aqueous solution is distinctly different from its precursor complexes, and the nanofibers formed are believed to arise from the charge-by-charge assembly via the alternate stacking of cationic and anionic complexes. The alternate stacking of oppositely charged ions has been supported by 2D 1H−1H NOESY NMR experiments (Figure 8). The protons on the TEG side chains of the cation and those of the alkyl chains of the sulfonate head groups of the anion display NOE signals with the phenyl rings of the bzimpy moieties of their counterpart complex ions, respectively, in the 2D 1H−1H NOESY NMR spectrum of a concentrated solution of complex 1 in concentrated DMSO (10−3 M), with a distance of 3.43−3.80 Å estimated from the integrals of the cross-peaks (Table S2).23 This implies that the cation and the anion adopt an alternate stacking in a twisted head-to-tail manner, which is in good agreement with the crystal packing of a related alkynylplatinum(II) bzimpy complex reported.16 On the contrary, at 67 °C, the disappearance of the NOE signals evidence the deaggregation process in the high-temperature regime. Further

long nanofibers, which may be rationalized by the fact that the phenyl alkynyl ligand is comparatively more sterically demanding and thus avoids the formation of large aggregates in this DCS system. The existence of nanofibers in aqueous solution of complex 1 is further confirmed by confocal fluorescence microscopy. Unlike TEM, SEM, and AFM, in which the studies are in the dried state, confocal fluorescence microcopy allows the direct examination of the nanoaggregates formed in solution. Luminescent nanofibers of 10−20 μm long are observed from the solution of complex 1 (Figure 7) and are similar in size but

Figure 7. Confocal fluorescence microscopy image of complex 1 prepared from aqueous solution (5 × 10−4 M).

slightly larger than those observed in the dried state of the sample determined by TEM, SEM, and AFM, probably due to the shrinkage of the structures by the loss of solvent during the drying process. Interestingly, compared to nanofibers formed by complex 1, its cationic precursor complex, [Pt{bzimpy(TEG)2}Cl]Cl, and anionic precursor complex, [Pt{bzimpy(PrSO3)2}Cl]K, demonstrate completely different nanomorphologies as illustrated by TEM images prepared from aqueous solutions of same concentration. [Pt{bzimpy(TEG)2}Cl]Cl exists as nanosheet structure, E


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Figure 8. Partial 1H−1H NOESY spectrum of complex 1 (1.02 × 10−3 M, 500 MHz, 298 K) for the Pt(bzimpy) moieties and their corresponding side chains in DMSO-d6.

support for the charge-by-charge assembly comes from the mass spectrometric studies. A m/z signal corresponding to the binding of cation and anion with the dissociation of a chloro ligand, [[Pt{bzimpy(TEG)2}Cl]+[Pt{bzimpy(PrSO3)2}Cl]−Cl]+, is observed in high-resolution positive MALDI-MS spectrum of complex 1 (Figure 9). Furthermore, m/z signals corresponding

Figure 9. Positive high-resolution MALDI mass spectrum of complex 1 and the expanded ion cluster of [1−Cl]− and its corresponding simulated isotope pattern.

to sandwiched ions, [[Pt{bzimpy(PrSO3)2}Cl]+[Pt{bzimpy(TEG)2}Cl]+[Pt{bzimpy(PrSO3)2}Cl]]− and the corresponding sandwiched ions of its derivatives, are observed for all three DCSs in their corresponding high-resolution negative ESI-MS spectra conducted in aqueous media (Figure S6). Synchrotron XRD measurements have been performed on the bulk sample of complex 1 and d spacings of 14.82, 8.32, 7.38, 5.47, 4.00, 3.34 Å are obtained, as shown in Figure 10a. A characteristic XRD pattern in a ratio of ca. √7:2:√3:1 corresponding to 14.82, 8.32, 7.38, 5.47 Å is observed, indicating the formation of a hexagonal columnar (Colh) structure with a lattice parameter of a = 14.82 Å. In addition, the peak at 2θ = 26.10° (d spacing = 3.34 Å) represents the presence of periodical π−π stacking and Pt···Pt interactions in the solid state of the charge-by-charge assembly. This indicates that the nanofibers are assembled via Colh packing that consists of alternate stacks of oppositely charged complex ions with the involvement of π−π stacking and Pt···Pt interactions, which is schematically depicted in Figure 10b. From the electron micrographs, fluorescence images, and the spectroscopic changes observed, a proposed mechanism for the

Figure 10. (a) Wide-angle XRD patterns obtained by the synchrotron radiation source on the bulk sample of complex 1. Numerical values indicate d spacing in Å. (b) Schematic representation of the selfassembled complex 1 to form the nanofiber with hexagonal columnar structure.

aggregation process of complex 1 is suggested and is depicted in Figure 11. In dilute pure DMSO, complex 1 would be wellsolvated without significant ground-state aggregation. However, upon an increase in concentration beyond 5 × 10−4 M, oppositely charged complex ions would be brought together by Pt···Pt and π−π stacking interactions to form ion-pairs in the crowded environment even though the ion-pairs are still being well-solvated by the DMSO solvent molecules with no further formation of aggregates of the DCS observed. However, upon the increase in water content in DMSO, the Pt(bzimpy) moieties F


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Figure 11. (a) Proposed self-assembly model for complex 1 showing the assembly induced by solvent and concentration modulation. (b) Proposed self-assembly model for complex 1 showing the assembly induced by temperature modulation in water.

1 nA is obtained upon a bias voltage of +8 and −8 V. The I−V profile is nonlinear, suggesting that the nanofibers behave as semiconductors (Figure S7b). The electrical conductivity is believed to be associated with the oppositely charged complex ions aligned in 1D anisotropic manner by the electrostatic, metal−metal, and π−π stacking interactions. Similar observations have been found for nanostructures formed by Pt(II) and Rh(I) complexes previously reported.1a,4g,8,17−20 The above results suggest that the nanofibers prepared by complex 1 could serve as a potential material for 1D charge transport in the application of molecular electronics.

would start to come into close proximities to avoid unfavorable contacts with the aqueous media. At the same time, the hydrophilic TEG chains in the cations and the sulfonate head groups in the anions, which have high affinity toward water, would point outward and interact with the water molecules, allowing the construction of stable and extended aggregates. Eventually, the continued alternate stacking of the oppositely charged complex ions would lead to the construction of onedimensional (1D) infinite arrays of Pt(II) moieties, assisted by the Pt···Pt, π−π stacking, hydrophobic−hydrophobic as well as electrostatic interactions. These 1D infinite chains of alternating positively and negatively charged ions would further assemble together into hexagonal columnar structures through interchain hydrophobic−hydrophobic and electrostatic interactions, resulting in the formation of nanofibers, as shown in the TEM images, and giving rise to the poorly resolved NMR signals and the MMLCT absorption and emission bands. Upon increasing the temperature in water, the infinite chains are found to deaggregate and result in dispersion of oppositely charged ions, which is also supported by the better resolved NMR signals and suppression of the MMLCT absorption band at elevated temperatures. The charge-transporting properties along the direction of nanofibers prepared by complex 1 have been preliminarily explored using field-effect transistor (FET) configuration.4g,17 An aqueous solution (10−3 M) of complex 1 was drop-cast onto a silicon wafer with prefabricated gold electrodes. Instead of obtaining a single nanofiber across the two electrodes, a number of nanofibers or a mesh of nanofibers are obtained that bridge between electrodes with a channel length of approximately 20 μm (Figure S7a). The IDS vs VDS output shows that a current of

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CONCLUSION To conclude, a new water-soluble DCS system consisting of chloroplatinum(II) and/or alkynylplatinum(II) bzimpy complexes of opposite charges has been reported, which demonstrates the capability to form highly ordered nanofibers and nanorods that can be systematically controlled by the variation of solvent composition and temperature. Such self-association process is governed by the formation of Pt···Pt and π−π stacking interactions as well as electrostatic interactions between the oppositely charged complex ions. The interesting morphological transformation from randomly dispersed complex ions to ordered nanofibers and nanorods in different solvent media, which is accompanied by spectroscopic changes, has been demonstrated by 1D and 2D NMR, UV−vis absorption, and emission studies as well as the electron and confocal fluorescence microscopy. The stacking topology of the different charged ions has been characterized by mass spectrometry. The present study illustrates that the employment of hydrophilic side chains G


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(0.04 g, 0.39 mmol), and a catalytic amount of CuI. The reaction mixture was heated under reflux under a N2 atmosphere overnight. After cooling to room temperature, diethyl ether was added to precipitate out the orange solid from the reaction mixture. The crude product obtained by filtration was further purified by slow diffusion of diethyl ether to dichloromethane solution of the complex to give an orange solid. Subsequent salt metathesis to the PF6− salt was done by dissolving the solid in MeOH with the addition of NH4PF6. The orange solid precipitated out was filtered and washed with MeOH. Yield: 0.11 g, 0.11 mmol, 87%. 1H NMR (400 MHz, DMSO-d6, 298 K, δ/ppm): δ 3.04 (s, 6H, −OCH3), 3.13−3.15 (m, 4H, −CH2O−), 3.17−3.21 (m, 8H, −CH2O−), 3.42−3.46 (m, 4H, −CH2O−), 3.84− 3.86 (m, 4H, −CH2O−), 4.89 (t, 4H, J = 3.9 Hz, −CH2N−), 7.36 (t, 1H, J = 8.0 Hz, phenylacetylene), 7.44−7.51 (m, 4H phenylacetylene), 7,53−7.61 (m, 4H, benzimidazolyl), 7.75 (d, 2H, J = 8.0 Hz, benzimidazolyl), 8.31 (t, 1H, J = 8.0 Hz, pyridyl), 8.41 (d, 2H, J = 8.0 Hz, benzimidazolyl), 8.59 (d, 2H, J = 8.0 Hz, pyridyl). Positive FAB-MS: m/z: 899 [M − Cl]−. IR (KBr): ν = 2120 cm−1 ν(CC). Elemental analysis calcd for C41H46F6N5O6PPt: C, 47.13; H, 4.44; N, 6.70; found: C, 47.06; H, 4.44; N, 6.62. [Pt{bzimpy(TEG)2}Cl][Pt{bzimpy(PrSO3)2}Cl] (1). Complex 1 was prepared by mixing equimolar amount of [Pt{bzimpy(TEG)2}Cl]Cl and [Pt{bzimpy(PrSO3)2}Cl]PPN. To a red solution of [Pt{bzimpy(PrSO3)2}Cl]PPN (0.15 g, 0.11 mmol) in dichloromethane (25 mL) was added dropwise an orange solution of [Pt{bzimpy(TEG)2}Cl]Cl (0.100 g, 0.11 mmol) in dichloromethane (25 mL). The mixture was stirred at room temperature for 1 h. The DCS was precipitated as a purple solid and was collected by filtration. The complex was purified by washing with dichloromethane solution. Yield: 0.15 g, 0.09 mmol, 84%. 1H NMR (400 MHz, DMSO-d6, 298 K, δ/ppm): δ 2.07−2.09 (m, 4H, −CH2−), 2.66−2.68 (m, 4H, −CH2SO3), 3.07 (s, 6H, −OCH3), 3.20−3.39 (m, 12H, −CH2O−), 3.78−3.80 (m, 4H, −CH2O−), 4.56 (t, 4H, J = 4.0 Hz, −CH2N−), 4.73 (t, 4H, J = 3.9 Hz, −CH2N−), 7.15 (t, 2H, J = 8.0 Hz, benzimidazolyl), 7.27 (t, 2H, J = 8.0 Hz, benzimidazolyl), 7.31 (t, 2H, J = 8.0 Hz, benzimidazolyl), 7.39 (t, 2H, J = 8.0 Hz, benzimidazolyl), 7.59 (m, 4H, benzimidazolyl), 7.59 (d, 2H, J = 8.0 Hz, benzimidazolyl), 7.86 (d, 2H, J = 8.0 Hz, benzimidazolyl), 8.39 (t, 1H, J = 8.0 Hz, pyridyl), 8.47 (t, 1H, J = 8.0 Hz, pyridyl), 8.47 (d, 2H, J = 8.0 Hz, pyridyl), 8.58 ppm (d, 2H, J = 8.0 Hz, pyridyl). Positive HR-ESI-MS: m/z found (calcd for C33H41ClN5O6Pt+ [Pt{bzimpy(TEG)2}Cl]+) 834.2339 (834.2388). Negative HR-ESI-MS: m/z found (calcd for C25H23ClN5O6PtS2− [Pt{bzimpy(PrSO3)2}Cl]−) 783.0421 (783.0433). [Pt{bzimpy(TEG)2}(CC−C6H5)][Pt{bzimpy(PrSO3)2}Cl] (2). Complex 2 was prepared according to the procedure described for complex 1, except [Pt{bzimpy(TEG)2}(CC−C6H5)]PF6 (0.05 g, 0.05 mmol) was used in place of [Pt{bzimpy(TEG)2}Cl]Cl to give complex 2 as an orange solid. Yield: 0.07 g, 0.04 mmol, 79%. 1H NMR (400 MHz, DMSO-d6, 298 K, δ/ppm): δ 2.10−2.15 (m, 4H, −CH2−), 2.64−2.66 (m, 4H, −CH2SO3), 3.06 (s, 6H, −OCH3), 3.16−3.18 (m, 4H, −CH2O−), 3.20−3.24 (m, 8H, −CH2O−), 3.89 (t, 4H, J = 4.0 Hz, −CH2O−), 4.80 (t, 4H, J = 4.0 Hz, −CH2N−), 4.99 (t, 4H, J = 3.9 Hz, −CH2N−), 7.33 (t, 1H, J = 8.0 Hz, phenylacetylene), 7.41− 7.46 (m, 4H, benzimidazoly), 7.43−7.49 (m, 4H, phenylacetylene), 7.53−7.58 (m, 4H, benzimidazolyl), 7.81 (m, 2H, benzimidazolyl), 7.81 (m, 2H, benzimidazolyl), 8.08 (d, 2H, J = 8.0 Hz, benzimidazolyl), 8.26 (t, 1H, J = 8.0 Hz, pyridyl), 8.47 (d, 2H, J = 8.0 Hz, benzimidazolyl), 8.47 (t, 1H, J = 8.0 Hz, pyridyl), 8.66 (d, 2H, J = 8.0 Hz, pyridyl), 8.69 ppm (d, 2H, J = 8.0 Hz, pyridyl). IR (KBr): ν = 2120 cm−1 ν(CC). Positive HR-ESI-MS: m/z found (calcd for C41H46N5O6Pt+ [Pt{bzimpy(TEG)2}(CC−C6H5)]+) 899.3037 (899.3094). Negative HR-ESI-MS: m/z found (calcd for C25H23ClN5O6PtS2− [Pt{bzimpy(PrSO3)2}Cl]−) 783.0409 (783.0433). [Pt{bzimpy(TEG)2}(CC−C6H5)][Pt{bzimpy(PrSO3)2}(CC−C6H5)] (3). Complex 3 was prepared according to the procedure described for complex 1, except [Pt{bzimpy(TEG)2}(CC−C6H5)]PF6 (0.05 g, 0.05 mmol) and [Pt{bzimpy(PrSO3)2}(CC−C6H5)]PPN (0.07 g, 0.05 mmol) were used in place of [Pt{bzimpy(TEG)2}Cl]Cl and [Pt{bzimpy(PrSO3)2}Cl]PPN, respectively, to give 3 as an orange solid. Yield: 0.08 g, 0.05 mmol, 83%. 1H NMR (400 MHz, DMSO-d6,

with both cations and anions containing platinum(II) bzimpy moieties would open up a new strategy to construct watersoluble DCSs that can assemble into interesting and highly ordered nanostructures. In addition to extending the studies on DCSs to aqueous medium, the present system has also demonstrated the use of noncovalent intermolecular forces to synergistically direct the molecules to assemble in a 1D anisotropic manner, serving as candidates for the development of a new class of functional supramolecular soft materials for potential applications in 1D charge transport.

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EXPERIMENTAL SECTION

Materials and Reagents. Potassium tetrachloroplatinate(II) (K2[PtCl4]) (Chem. Pur.), 1,3-propane sultone (Alfa Aesar), phenylacetylene (Sigma-Aldrich Co. Ltd.), and bis(triphenylphosphine)iminium chloride ([PPN]Cl) were purchased from the corresponding chemical company. 2,6-Bis(benzimidazol-2′-yl)pyridine (bzimpy),21 potassium salt of 2,6-bis(1-(3-propylsufonate)benzimidazol-2′-yl)pyridine [bzimpy(PrSO3)2]K2),3h [Pt{bzimpy(PrSO3)2}Cl]K,3h [Pt{bzimpy(PrSO3)2}Cl]PPN,3h [Pt{bzimpy(PrSO3)2}(CC−C6H5)]PPN,3i 2,6-bis(1-(TEG)benzimidadozl-2′-yl)pyridine [bzimpy(TEG)2],22 and [Pt{bzimpy(TEG)2}Cl]Cl22 were prepared according to literature procedures. All other reagents, unless specified, were of analytical grade and were used as received without further purification. Physical Measurements and Instrumentation. 1H NMR spectra were recorded on a Bruker AVANCE 400 or 500 (400 and 500 MHz) Fourier-transform NMR spectrometer. FAB mass spectra were recorded on a Thermo Scientific DFS high-resolution magnetic sector mass spectrometer. High-resolution ESI mass spectra were collected on a Bruker maXis II high-resolution LC-QTOF mass spectrometer. High-resolution MALDI-TOF mass spectrometry was performed on a Bruker ultrafleXtreme mass spectrometer. IR spectra were obtained as KBr disk on a Bio-Rad FTS-7 Fourier-transform infrared spectrophotometer (4000−400 cm−1). Elemental analyses of the complexes were performed on a Flash EA 1112 elemental analyzer at the Institute of Chemistry, Chinese Academy of Sciences. The UV−vis spectra were recorded using a Varian Cary 50 UV−vis spectrophotometer with the monitoring of temperature using the Varian Cary single-cell Peltier thermostat. Emission spectra were obtained from a Spex Fluorolog-3 model FL3-211 fluorescence spectrofluorometer equipped with an R2658P PMT detector. TEM experiments were performed on a Philips CM100 transmission electron microscope with an accelerating voltage of 200 kV. SEM experiments were performed on a Hitachi S4800 FEG scanning electron microscope operating at 4.0−6.0 kV. The samples for TEM and SEM were prepared by drop casting dilute solutions onto a carbon-coated copper grid and silicon wafer, respectively, which was then allowed to undergo slow evaporation in air for 15 min to remove the excess solvent. Topographical images and phase images of AFM were obtained from an Asylem MFP3D atomic force microscope with an ARC2 SPM Controller under constant temperature and atmospheric pressure. Samples for AFM measurements were prepared by drop casting dilute solutions onto a silicon wafer. Confocal fluorescence microscopy imaging experiments were performed on a Carl Zeiss LSM 780 using a diode-pumped solid-state laser with excitation wavelengths of 561 nm. The solution samples were dropped onto microscopic slides, covered with coverslips, and then sealed from the atmosphere to prevent evaporation of the solvent. The charge-transporting properties along the direction of nanofibers prepared by complex 1 have been preliminarily explored using field-effect transistor (FET) configuration. An aqueous solution (10−3 M) of complex 1 was drop-cast onto a silicon wafer with prefabricated gold electrodes. I−V curves were measured by using a Desert Cryogenics probe station. The IDS vs VDS output characteristic were obtained without back-gate voltage where D and S denote drain and source, respectively. Synthesis. [Pt{bzimpy(TEG)2}(CC−C6H5)]PF6. To a solution of [Pt{bzimpy(TEG)2}Cl]Cl (0.10 g, 0.12 mmol) in degassed dichloromethane (40 mL) were added triethylamine (1 mL), HCC−C6H5 H


Article

Journal of the American Chemical Society 298 K, δ/ppm): δ 2.08−2.13 (m, 4H, −CH2−), 2.60−2.63 (m, 4H, −CH2SO3), 3.04 (s, 6H, −OCH3), 3.13−3.14 (m, 4H, −CH2O−), 3.17−3.20 (m, 8H, −CH2O−), 3.33−3.35 (m, 4H, −CH2O−), 3.82 (t, 4H, J = 3.9 Hz, −CH2O−), 4.75 (t, 4H, J = 4.0 Hz, −CH2N−), 4.84 (t, 4H, J = 3.9 Hz, −CH2N−), 7.31−7.35 (m, 4H, phenylacetylene), 7.39−7.45 (m, 4H, benzimidazoly), 7.44−7.50 (m, 6H, phenylacetylene), 7.48−7.54 (m, 4H, benzimidazolyl), 7.68 (m, 2H, benzimidazolyl), 7.69 (m, 2H, benzimidazolyl), 8.03 (t, 1H, J = 8.0 Hz, pyridyl), 8.18 (d, 2H, J = 8.0 Hz, benzimidazolyl), 8.22 (t, 1H, J = 8.0 Hz, pyridyl), 8.34 (d, 2H, J = 8.0 Hz, benzimidazolyl), 8.51 (d, 2H, J = 8.0 Hz, pyridyl), 8.64 ppm (d, 2H, J = 8.0 Hz, pyridyl). IR (KBr): ν = 2120 cm−1 ν(CC). Positive HR-ESI-MS: m/z found (calcd for C41H46N5O6Pt+ [Pt{bzimpy(TEG)2}(CC−C6H5)]+) 899.3087 (899.3094). Negative HR-ESI-MS: m/z found (calcd for C33H28N5O6PtS2− [Pt{bzimpy(PrSO3)2}(CC−C6H5)]−) 849.1130 (849.1137).

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the Li Ka Shing Faculty of Medicine of The University of Hong Kong is also thanked for providing assistance for the confocal fluorescence microscopy. We also thank Mr. Li Dian for his technical assistance in locating the nanofibers on the silicon wafer with prefabricated gold electrodes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b09770. UV−vis absorption data of solution of mixtures of [Pt{bzimpy(TEG)2}Cl]Cl and [Pt{bzimpy(PrSO3)2}Cl] K in different mole ratios and corresponding Job’s plot; UV−vis absorption and emission data for 2 and 3; Table of photophysical data of 1−3; TEM data of [Pt{bzimpy(TEG)2}Cl]Cl and [Pt{bzimpy(PrSO3)2}Cl]K; mass spectrometry data of 1−3; SEM data of a FET device of 1 and the IDS vs VDS output characteristics of the device; curve-fitting with the nucleation-elongation model; determination of distances between protons by 1H−1H NOESY spectroscopy (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Xiaodong Cui: 0000-0002-2013-8336 Vivian Wing-Wah Yam: 0000-0001-8349-4429 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V.W.-W.Y. acknowledges UGC funding administered by The University of Hong Kong for supporting the Electrospray Ionization Quadrupole Time-of-Flight Mass Spectrometry Facilities under the Support for Interdisciplinary Research in Chemical Science and the support from the University Research Committee (URC) Strategic Research Theme on New Materials of The University of Hong Kong. This work has been supported by the University Grants Committee Areas of Excellence (AoE) Scheme (AoE/P-03/08) and a General Research Fund (GRF) grant from the Research Grants Council of Hong Kong Special Administrative Region, P. R. China (HKU 17334216). V.C.-H.W. acknowledges the receipt of a Hong Kong Ph.D. Fellowship administered by the Research Grants Council of Hong Kong Special Administrative Region, People’s Republic of China. We also thank Mr. Frankie Yu-Fee Chan of the Electron Microscope Unit of The University of Hong Kong for his helpful technical assistance. The Beijing Synchrotron Radiation Facility (BSRF) is also thanked for providing beamline time (beamline 4B9A) in the synchrotron radiation X-ray diffraction facilities. We also thank Dr. Liaoyuan Yao for X-ray diffraction data collection. The Faculty Core Facility of I


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Formation of 1D Infinite Chains Directed by Metal ... - ACS Publications

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