Controlling Self-Assembly Mechanisms through Rational Molecular

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

Controlling Self-Assembly Mechanisms through Rational Molecular Design in Oligo(p‑phenyleneethynylene)-Containing Alkynylplatinum(II) 2,6-Bis(N‑alkylbenzimidazol-2′-yl)pyridine Amphiphiles Michael Ho-Yeung Chan,† Sammual Yu-Lut Leung,† 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, People’s Republic of China S Supporting Information *

ABSTRACT: The systematic control over association mechanisms of self-assembled materials has been demonstrated through the rational design and synthesis of a series of amphiphilic dinuclear alkynylplatinum(II) bzimpy (bzimpy = 2,6-bis(N-alkylbenzimidazol-2′-yl)pyridine) complexes containing the shape-persistent oligo(p-phenyleneethynylene)s. Multistage morphological transformations from plates to fibers and to spherical nanostructures under different solvent compositions have been demonstrated. The subtle balances between multiple noncovalent interactions including Pt···Pt, hydrophobic, hydrophilic, and π−π stacking interactions are found to have profound impact on the supramolecular assembly of the system, in which a change in the association mechanism from isodesmic to cooperative and back to isodesmic growth has been observed upon increasing hydrophilicity of the complexes.

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unique memory capability3g and water-soluble double complex salts9 via supramolecular assembly associated with the formation of Pt···Pt and π−π stacking interactions. Such selfassembly behaviors and drastic spectroscopic responses to external stimuli have rendered this class of complexes a promising candidate not only for the monitoring of the selfassembly processes, but also for the exploration on new classes of self-assembled materials in supramolecular chemistry. In addition to the construction of well-defined supramolecular architectures, the studies on the association mechanism of the self-assembled architectures have opened up a new area of research for the quantification of the selfassembly processes.10 In particular, there is a surge of interest in the fundamental understanding of both the cooperative and isodesmic growth self-assembly mechanisms in which mathematical models have been developed for the thermodynamic quantification of the aggregates.10a,c However, the investigation on the relationship between the self-assembly mechanisms and the rational molecular design using various organic moieties is rarely explored, not to mention the utilization of the metallosupramolecular π-conjugated amphiphiles involving directional noncovalent metal−metal interactions.3h,4b Thus, it would be interesting to explore the factors that govern the correlation between the molecular structures and the self-

INTRODUCTION Self-assembly of molecules has been one of the most attractive strategies for the constructions of sophisticated supramolecular architectures through multiple noncovalent interactions.1 The construction of self-assembled materials through the utilization of amphiphilic molecules has aroused tremendous interest owing to the subtle balances of intermolecular forces like hydrogen bonding, hydrophobic, hydrophilic, and π−π stacking interactions.2−6 However, the utilization of noncovalent metal− metal interactions in the stabilization of supramolecular systems is relatively under-explored. This has led to the design and utilization of metallosupramolecular π-conjugated amphiphiles for the construction of self-assembled materials. Not only could these amphiphiles display intriguing spectroscopic and luminescent behaviors,3−6 but also they could exhibit directional noncovalent metal−metal interactions,3,5 distinctive from the pure organic amphiphilic counterparts. More importantly, these additional noncovalent metal−metal interactions have served to precisely control the formation of distinct and unique supramolecular architectures.3,5 In view of the intriguing spectroscopic properties7 and the Pt···Pt interactions in platinum(II) polypyridine complexes,7d−i our group has recently reported alkynylplatinum(II) bzimpy (bzimpy = 2,6bis(N-alkylbenzimidazol-2′-yl)pyridine) complexes that show intriguing self-assembly behaviors ranging from metallogel formation8 to discrete morphological transformations associated with drastic color changes3d as well as to the generation of © XXXX American Chemical Society

Received: April 4, 2018

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DOI: 10.1021/jacs.8b03628 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society Scheme 1. Molecular Structures of Alkynylplatinum(II) bzimpy Complexes 1−12

assembly mechanisms. It is envisaged that with the directional noncovalent metal−metal interactions, which provide additional driving forces to the aggregates, self-assembled materials with intriguing aggregation and spectroscopic behaviors could be prepared. Moreover, it is anticipated that through judicious modification of the hydrophobicity of the metallosupramolecular π-conjugated amphiphiles, the systematic control over the aggregation mechanisms with drastic spectroscopic responses due to the directional metal−metal interactions could be achieved. More importantly, these drastic spectroscopic responses imparted from the directional metal−metal interactions could spectroscopically probe the possible changes of the aggregates upon the application of various external stimuli, which could provide further insights into the stability and the quantification of these self-assembled materials, distinctive from the self-assembled materials obtained from purely organic scaffolds. Thus, 2,6-bis(N-alkylbenzimidazol-2′-yl)pyridine (bzimpy) and the shape-persistent oligo(p-phenyleneethynylene) derivatives would be excellent candidates as the pincer ligand and alkynyl ligands respectively for the design of metallosupramolecular π-conjugated amphiphiles since these moieties not only possess extensive π-conjugated surfaces for intermolecular association, but also provide the ease of systematic modification in the hydrophobicity and hydrophilicity of the resulting complexes, which can ultimately lead to the generation of high-ordered hierarchical architectures

resulting from the directional noncovalent intermolecular metal−metal interactions. Herein are described the design and synthesis of a series of dinuclear alkynylplatinum(II) bzimpy complexes with oligo(pphenyleneethynylene) backbone (Scheme 1). It is envisaged that through judicious modifications of the hydrophobicity of the metallosupramolecular π-conjugated amphiphiles, an exploration into the possible systematic control over the aggregation mechanisms with drastic spectroscopic responses imparted from the directional metal−metal interactions could lead to an in-depth understanding of the interplay of the various noncovalent interactions and the factors that govern these interactions. With these understandings, we believe that a control over the self-assembly mechanism through rational molecular design could be achieved. It is envisaged that the present work could provide further insights into the relationships between the self-assembly mechanisms and the rational molecular designs, particularly with the involvement of noncovalent metal−metal interactions in metallosupramolecular π-conjugated amphiphiles, distinctive from previously reported literatures involving mainly the pure organic systems,10−13 ultimately leading to the construction of controlled assemblies.

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RESULTS AND DISCUSSION All the complexes dissolve in DMSO to give yellow solutions at 298 K at a concentration of 10−5 M. 1−12 exhibit similar B


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Figure 1. UV−Vis absorption spectral changes of [{C12bzim-Pt}2-pPE3](PF6)2 (4) in DMSO solutions upon increasing water content from (a) 0 to 20% and (b) 20 to 90% in the concentration regime of 10−5 M.

the concentration regime of 10−5 M to understand the effect of solvents in the aggregation processes. Surprisingly, no precipitation occurs in such a highly polar medium with 90% water for all the complexes at this concentration regime, which prompts the subsequent investigation on the solution-state aggregation behaviors. All the complexes give similar UV−vis absorption spectral changes upon increasing water content in their DMSO solutions as depicted in Figure 1 and Figures S8− S18, Supporting Information. Upon increasing the water content in their DMSO solutions to a critical point (i.e., 20% water in DMSO, v/v), a color change from yellow to orange has been observed, which is accompanied by an abrupt and dramatic growth in the absorption bands at ∼500 nm. These lower-energy absorption bands are assignable to the metal− metal-to-ligand charge transfer (MMLCT) transitions associated with the strengthening of the Pt···Pt and π−π stacking interactions of the complexes in more polar media due to intermolecular association. Interestingly, further addition of water beyond this critical point to 90% water in DMSO results in a gradual decrease in the MMLCT absorption bands at ∼500 nm for all the complexes. However, such decreases are not attributed to the deaggregation of the complexes but instead due to the strengthening of Pt···Pt interactions. These could be realized from the overlaid spectra between their DMSO and 90% water−DMSO (v/v) solutions, in which the absorption tails at ∼530 nm are significantly enhanced at 90% water− DMSO (v/v), indicative of the presence of Pt···Pt interactions in 90% water−DMSO (v/v) solutions. This strengthening of the Pt···Pt interactions is also supported by a red shift of the emission bands upon further addition of water beyond the critical point (Figures S19−22, Supporting Information). Hence, these results suggest that the complex would associate into a closer proximity with enhanced Pt···Pt and π−π stacking interactions upon further addition of water. In sharp contrast to the 1H NMR signals in DMSO-d6 solutions at room temperature, upfield-shifting and broadening of the 1H NMR signals have been observed for [{C12bzim-Pt}2-6TEG-pPE3](PF6)2 (12) at 10% of D2O in DMSO-d6 (Figure 2a). Further increasing the D2O content to up to 30% results in a featureless NMR spectrum, suggesting intermolecular association of the complexes with increasing water content, which is in line with

absorption spectra with the observation of intense high-energy absorption bands at 356−374 nm as well as lower-energy absorption bands at 437−446 nm (Figure S1, Supporting Information). The high-energy absorption bands are assigned as intraligand [π → π*] transitions of the alkynyl ligands and the bzimpy ligands, while the lower-energy bands are assigned as the admixture of intraligand [π → π*] transitions of the bzimpy ligands and metal-to-ligand charge transfer (MLCT) [dπ(Pt) → π*(bzimpy)] transitions with ligand-to-ligand charge transfer (LLCT) [π(alkynyl) → π*(bzimpy)] character.3g,8,9 Their electronic absorption data have been summarized in Table S1, Supporting Information. The absorption tail at ∼520 nm is found to show insignificant changes of the absorption profiles upon heating (Figures S2−S3, Supporting Information), which might be attributed to the insignificant involvement of the Pt···Pt interactions in DMSO. However, the ground state aggregation behaviors of these complexes are believed to be significant as revealed by the temperaturedependent 1H NMR experiments of the complexes in DMSOd6 (Figures S4−S6, Supporting Information) and the dynamic light scattering studies of the DMSO solutions of selected complexes at concentrations of 10−5 M (Tables S2−S3, Supporting Information). Upon an increase in temperature from 298 to 353 K, the 1H NMR signals of the benzimidazolyl protons become downfield-shifted and well-resolved, indicative of the disruption of intermolecular association between the complexes at high temperatures at this relatively low concentration. Moreover, the 1H NMR signals become broad and upfield-shifted with increasing concentration from 10−5 to 10−4 M (Figure S7, Supporting Information), as illustrated by [{C12bzim-Pt}2-6TEG-pPE3](PF6)2 (12), further suggestive of the intermolecular association of the complexes in DMSO solutions. Furthermore, the dynamic light scattering studies of the DMSO solutions of selected complexes also reveal the formation of aggregates with hydrodynamic diameter of about 1000 nm, indicative of intermolecular association of the complexes in DMSO solutions. Spectroscopic Studies on Solvent-Induced Self-Assembly Processes. The self-assembly behaviors of all complexes in DMSO−water mixtures are further investigated by the solvent-dependent UV−vis absorption spectroscopy at C


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the number of triethylene-glycol chains in the backbone from zero (4), to two (9), and to six (12). These intriguing experimental findings further prompt the investigation on their emissive properties in DMSO−water mixtures since luminescence studies represent a relatively more sensitive way to probe the microenvironmental changes based on the changes in the extent of Pt···Pt interactions. Upon photoexcitation, all the complexes are nonemissive in degassed DMSO solutions at 10−5 M concentration regime at 298 K. Interestingly, drastic luminescence enhancements have been observed upon the addition of water into their DMSO solutions leading to red phosphorescence with emission maxima at ∼700 nm (Figure 3). The emergence of this lower-energy emission at ∼700 nm could be attributed to 3 MMLCT emission (Figures S19−22, Supporting Information),3g,8,9 suggestive of the formation of Pt···Pt and π−π stacking interactions during the water-induced aggregation processes, which also coincide with the solvent-dependent UV−vis absorption studies. It is noteworthy that a red shift of the emission band has been observed upon further addition of water beyond the critical point of water content. For instance, a luminescence enhancement has been observed at ∼650 nm for [{C12bzim-Pt}2-pPE3](PF6)2 (4) upon gradual addition of water to reach 20% water−DMSO (v/v) (Figure S21, Supporting Information). Upon further addition of water beyond this critical point, an approximately 10-fold increase in luminescence intensity accompanied by a red shift has been observed. These observations suggest the strengthening of the Pt···Pt interactions upon increasing water content. A comparison between the emission spectra of the complexes at 90% water content of their DMSO solutions (Figure 4) reveals a direct correlation between the extent of Pt···Pt and π−π stacking interactions and the molecular design of the complexes. In sharp contrast to [{C8bzim-Pt}2-pPE3](PF6)2 (2), which emits at 707 nm, a blue-shifted emission band with emission maximum at 668 nm has been observed for [{2EHbzim-Pt}2-pPE3](PF6)2 (3), suggesting a smaller extent of Pt···Pt and π−π stacking interactions (Figure 4a), attributed to the more sterically hindered 2-ethylhexyl chains than the noctyl chains, which would ultimately suppress the intermolecular association of 3. On the other hand, with increasing number of triethylene-glycol chains in the pPE3 backbone, a slight red shift in emission maxima from [{C12bzim-Pt}2pPE3](PF6)2 (4) to [{C12bzim-Pt}2-2TEG-pPE3](PF6)2 (9) and to [{C12bzim-Pt}2-6TEG-pPE3](PF6)2 (12) is observed (Figure 4b), suggesting the insignificant alteration of the extent of Pt···Pt interactions in highly polar media (90% water− DMSO (v/v)) by these well-solvated hydrophilic chains. The slight red shift might be attributed to the electronic effect of the π-donating TEG chains that rendered a more electron-rich Pt

Figure 2. (a) 1H NMR spectral traces of [{C12bzim-Pt}2-6TEGpPE3](PF6)2 (12) upon increasing D2O content in DMSO-d6 from 0 to 30% (bottom to top) at 298 K. (b) 1H NMR spectral traces of [{C12bzim-Pt}2-6TEG-pPE3](PF6)2 (12) with 20% D2O content in DMSO-d6 with increasing temperature from 298 to 353 K ([Pt] = ∼10−5 M).

the experimental findings in the UV−vis and emission spectroscopic studies. The 1H NMR signals for 12 at 20% D2O content in DMSO-d6 are found to be more downfieldshifted and well-resolved upon heating from 298 to 353 K (Figure 2b) due to the dissociation of the complexes at high temperatures. It is worth mentioning that the water content required to reach the maximum growth of the MMLCT absorption bands at ∼500 nm (i.e., the critical point of water content) is dependent on the hydrophobicity of the complexes. Specifically, more hydrophobic complexes would be aggregated to reach the maximum growth of the MMLCT absorption band with a lower water content. For instance, the amount of water required to reach the maximum growth of MMLCT absorption band is found to gradually decrease from 50% ([{C6bzim-Pt}2pPE3](PF6)2 (1)) to 20% ([{C16bzim-Pt}2-pPE3](PF6)2 (5)) upon increasing the alkyl chain length of the complexes from −C6H13 to −C16H33, while the amount of water required is increased from 20% ([{C12bzim-Pt}2-pPE3](PF6)2 (4)) to 30% ([{C12bzim-Pt}2-2TEG-pPE3](PF6)2 (9)) and to 40% ([{C12bzim-Pt}2-6TEG-pPE3](PF6)2 (12)) upon increasing

Figure 3. Photograph showing the DMSO solutions of [{C12bzim-Pt}2-pPE3](PF6)2 (4) with increasing water content from 0 to 90% (left to right; with 10% interval) under UV light. D


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Figure 4. Normalized emission spectra of (a) [{C8bzim-Pt}2-pPE3](PF6)2 (2) and [{2EHbzim-Pt}2-pPE3](PF6)2 (3); (b) [{C12bzim-Pt}2pPE3](PF6)2 (4), [{C12bzim-Pt}2-2TEG-pPE3](PF6)2 (9), and [{C12bzim-Pt}2-6TEG-pPE3](PF6)2 (12); and (c) [{C6bzim-Pt}2-pPE3](PF6)2 (1), [{C12bzim-Pt}2-pPE3](PF6)2 (4), and [{C16bzim-Pt}2-pPE3](PF6)2 (5) in 90% water content in DMSO solutions at 298 K in the concentration regime of 10−5 M.

center, which might result in the destabilization of the dπ(Pt) orbitals. Other than the hydrophilicity, the increasing hydrophobicity of the complexes might contribute to the changes in the extent of Pt···Pt interactions. However, only a slight shift in the emission maxima of the structureless emission bands has been observed from [{C6bzim-Pt}2-pPE3](PF6)2 (1) to [{C12bzim-Pt}2-pPE3](PF6)2 (4) and to [{C16bzim-Pt}2pPE3](PF6)2 (5) upon increasing alkyl chain length from −C6H13 to −C12H25 and to −C16H33, respectively (Figure 4c). Such insignificant changes might suggest that the hydrophobic interactions are not capable of further enhancing the extent of Pt···Pt interactions due to the leveling-off of the hydrophobicity effect. In other words, the system might have already attained maximum extent of Pt···Pt interactions. From the above results, the steric factors have been found to pose the most significant effect on altering the extent of Pt···Pt interactions at such highly polar media (90% water−DMSO (v/v)) when compared to the hydrophilicity and the hydrophobicity of the complexes. Studies on Self-Assembly Mechanisms by Temperature-Dependent UV−vis Spectroscopic Analysis. The hydrophobicity/hydrophilicity of 1−12 is fine-tuned through systematic modification on the hydrophobicity of the bzimpy pincer ligand and the hydrophilicity of the p-phenylene ethynylene backbone. It is envisaged that such systematic modification could provide an in-depth understanding into the principles and mechanistic behaviors governing the supramolecular assemblies, monitored through the drastic spectroscopic responses imparted by the formation of metal−metal interactions. In light of the appearance of MMLCT absorption upon increasing water content in the DMSO solutions of the complexes, variable-temperature UV−vis absorption spectroscopic measurements are conducted in DMSO-water mixtures under similar concentration of 10−5 M for all the complexes (Figures 5 and 6 and Figures S23−S32, Supporting Information). With [{C12bzim-Pt}2-pPE3](PF6)2 (4) as a representative example for illustration purposes, the appearance of the MMLCT absorption band at 20% water−DMSO (v/v) of 4 could act as a spectroscopic probe to quantify and examine the aggregation properties and mechanisms for the self-

Figure 5. UV−Vis absorption spectral traces on cooling a solution of [{C12bzim-Pt}2-pPE3](PF6)2 (4) in 20% water in DMSO (1.1 × 10−5 M) at a cooling rate of 0.5 K min−1. (Inset) Degree of aggregation at 510 nm as a function of temperature with the curve fitted at the elongation (red line) and nucleation (green line) regime based on the nucleation−elongation model.

assembly processes upon changing in temperatures (Figure 5). Upon cooling the 20% water−DMSO (v/v) solution of 4 ([Pt] = ∼10−5 M) from 369 to 290 K at a rate of 0.5 K min−1, a drastic growth of the MMLCT absorption band at ∼500 nm is observed, which is attributed to the formation of intermolecular Pt···Pt and π−π stacking interactions. Interestingly, the plot of the degree of aggregation at 510 nm against temperature of the cooling curve of 4 is observed to be clearly nonsigmoidal (Figure 5, inset), indicative of a cooperative growth mechanism in the supramolecular assemblies. Hence, temperature-dependent nucleation−elongation model has been applied to provide further insights and thermodynamic parameters for the assemblies.10a,b,14 The enthalpy release (ΔH) in the elongation regime and the elongation temperature (Te) are found to be about −76.2 kJ mol−1 and 333 K, respectively. The equilibrium constant for the nucleation step is found to be 2.94 × 10−3, and E


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polymerization is found to be 315 molecules at room temperature. It is believed that 4 would be aggregated upon decreasing temperature to form a nucleus size of about 7 molecules, which then further elongates cooperatively to larger aggregates beyond the elongation temperature. In light of these intriguing findings, the variable-temperature UV−vis absorption studies for all the complexes are carried out under similar concentration of 10−5 M and at the solvent composition that the complexes attained the maximum growth of the MMLCT absorption. Together with the employment of both the temperature-dependent isodesmic and cooperative model in fitting the absorption data, their self-assembly mechanisms and thermodynamic parameters have been obtained and summarized in Tables 1 (isodesmic growth) and 2 (cooperative growth). All the melting curves are obtained with a slow cooling rate of 0.5 K min−1 to ensure that the selfassembly processes are under thermodynamic control.10a Effect of Alkyl Chain Length on Self-Assembly Mechanism. Complexes 1−2 and 4−5 are designed with increasing alkyl chain length on the bzimpy pincer ligand from n-hexyl (1) to n-hexadecyl (5) to further investigate the selfassembly mechanism of the aggregates. In sharp contrast to [{C12bzim-Pt}2-pPE3](PF6)2 (4), cooperativity has not been observed for the self-assembly of [{C6bzim-Pt}2-pPE3](PF6)2 (1) at 50% water−DMSO (v/v). Instead, a sigmoidal cooling curve is obtained from the plot of degree of aggregation against temperature, which is suggestive of an isodesmic self-assembly mechanism,10c with the enthalpy change and melting temperature determined to be −118 kJ mol−1 and 340 K, respectively. In contrast to the cooperative growth mechanism, which undergoes an unfavorable nucleation step, the isodesmic growth of 1 is governed by a single equilibrium constant Ke as shown in Table 1. In addition, the enthalpy changes determined from the van’t Hoff analysis are found to show good agreement with those determined from the isodesmic model. Such characteristic differences between 1 (isodesmic mechanism) and 4 (cooperative mechanism) have led to the further investigation on the effect of alkyl chain length from nhexyl (1) to n-hexadecyl (5) on the change in self-assembly mechanism. More interestingly, upon increasing alkyl chain length from n-hexyl (1) to n-octyl (2), a change in the growth mechanism from isodesmic (1) to cooperative (2) has been observed. The plot of degree of aggregation against temperature of 2 is nonsigmoidal, indicative of a cooperative growth

Figure 6. (a) UV−Vis absorption spectral traces on cooling a solution of [{C12bzim-Pt}2-6TEG-pPE3](PF6)2 (12) in 40% water in DMSO (1.1 × 10−5 M) at a cooling rate of 0.5 K min−1. (Inset) Degree of aggregation at 530 nm as a function of temperature with the curve fitted (red line) based on the temperature-dependent isodesmic model. (b) Van’t Hoff plot of the equilibrium constant Ke at various temperatures with the corresponding thermodynamic parameters.

this relatively low value suggests an unfavorable nucleation process. At the nucleation temperature, a nucleus size of 7 is obtained, while the number-average degree of

Table 1. Thermodynamic Parameters for the Self-Assembly for 1, 5, 6, and 10−12 complex

ΔH /kJ mol−1

1

−113.2 −118.1b −120.4a −127.6b −108.7a −115.0b −128.1a −126.9b −147.0a −142.4b −102.1a −113.1b

5 6 10 11 12

a

Tmb / K

Keb / M−1

ΔSa / J mol−1 K−1

ΔGa / kJ mol−1

DPN

340

7

5.72 × 10

−243

−40.8

10

328

4.41 × 106

−276

−38.2

7

320

1.23 × 106

−249

−34.5

4

320

1.67 × 106

−309

−36.0

5

334

1.40 × 107

−349

−43.0

12

335

4.95 × 106

−214

−38.3

8

Determined from the van’t Hoff plot from the variable-temperature UV−vis absorption studies. bDetermined by fitting the experimental data of the functions of normalized degree of aggregation against temperature according to the temperature-dependent isodesmic model. a

F


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Journal of the American Chemical Society Table 2. Thermodynamic Parameters for the Self-Assembly for 2−4 and 7−9 complex

ϕSAT

ΔHe / kJ mol−1

Te / K

2 3 4 7 8 9

1.239 1.145 1.026 1.031 1.094 0.981

−45.4 −39.4 −76.2 −59.2 −46.5 −94.7

335 353 333 359 351 358

mechanism. Further increase in the alkyl chain length from noctyl (2) to n-dodecyl (4) has not been found to alter the cooperative growth of the aggregates. Surprisingly, upon further increase of the chain length to n-hexadecyl (5), a change in the growth mechanism from cooperative (4) to isodesmic (5) has been observed since the plot of degree of aggregation against temperature for 5 is found to be clearly sigmoidal. From the above intriguing observations, the self-assembly mechanisms are found to be significantly influenced by the overall hydrophobicity of the complexes. The breakpoint of the changes in the mechanisms has been found to be in between −C6H13 and −C8H17 and between −C12H25 and −C16H33. On the basis of the above intriguing results obtained for the pPE3 series (1−2 and 4−5) upon increasing alkyl chain length, further modifications of the complexes are carried out to further verify the changes in the self-assembly mechanism as well as the breakpoint of the change in mechanisms. Hence, the 2TEGpPE3 series (6−11) has been designed with the incorporation of two more hydrophilic TEG chains on the pPE3 backbone to increase the overall hydrophilicity of the complexes when compared to the pPE3 series. A comparison between the cooling curves of 6−11 (Figures S27−S32, Supporting Information) also reveals a change in the aggregation mechanism from isodesmic ([{C6bzim-Pt}2-2TEG-pPE3](PF6)2 (6)) to cooperative ([{C8bzim-Pt}2-2TEG-pPE3](PF6)2 (7) and [{C12bzim-Pt}2-2TEG-pPE3](PF6)2 (9)) and then to isodesmic ([{C16bzim-Pt}2-2TEG-pPE3](PF6)2 (10) and [{C18bzim-Pt}2-2TEG-pPE3](PF6)2 (11)) upon a gradual increase in alkyl chain length from −C6H13 to −C18H37. Moreover, the breakpoint of the change in self-assembly mechanism for the 2TEG-pPE3 series is found to be in good agreement with the pPE3 series (between −C6H13 and −C8H17, and between −C12H25 and −C16H33). From the above observations of the pPE3 series and the 2TEG-pPE3 series, the enhanced cooperativity in a self-assembly system could be attributed to the increase in the noncovalent hydrophobic− hydrophobic interactions introduced by longer alkyl chains.3h,f,15−17 However, such cooperativity would be destroyed upon further introduction of these hydrophobic− hydrophobic interactions by very long alkyl chains, which might disrupt the delicate balance of multiple noncovalent interactions in the self-assembly and the disruption of the cooperativity. This phenomenon could also be observed in the related organic oligo(p-phenyleneethynylene) counterparts incorporated with very long hydrophobic alkyl chains.10f Such results demonstrate that the cooperative growth mechanism could be achieved through a subtle balance between multiple noncovalent interactions such as metal−metal interactions, hydrophobic, hydrophilic, and π−π stacking interactions. In sharp contrast to cooperative growth, the isodesmic mechanism is found to occur when the overall hydrophobicity/hydrophilicity of the complexes become too extreme such as in

Ka 3.18 5.28 2.94 1.38 1.41 9.44

× × × × × ×

10−3 10−4 10−3 10−3 10−3 10−4

7 12 7 9 9 10

90 310 315 760 270 6700

[{C 16 bzim-Pt} 2 -pPE3](PF 6 ) 2 (5) (hydrophobic) and [{C6bzim-Pt}2-2TEG-pPE3](PF6)2 (6) (hydrophilic). Apart from the changes in the aggregation mechanisms, the differences in the release in enthalpy and the number-averaged degree of polymerization at room temperature during aggregation have provided profound significances in the stability of the aggregates. Hence, the enthalpy release during the self-assembly processes within the same self-assembly mechanism (either cooperative or isodesmic) would be compared to gain significant insight into the stability of the aggregates. For the complexes undergoing cooperative growth mechanism, the enthalpy changes of [{C8bzim-Pt}2-pPE3](PF6)2 (2) (−45.4 kJ mol−1) are found to be less negative than that of [{C12bzim-Pt}2-pPE3](PF6)2 (4) (−76.2 kJ mol−1) due to the longer chain length in 4, which would offer greater noncovalent hydrophobic interactions in addition to the Pt···Pt and π−π stacking interactions, resulting in the formation of more stable aggregates.3f,h,15−17 Similar results have been observed for [{C8bzim-Pt}2-2TEG-pPE3](PF6)2 (7) (−59.2 kJ mol−1) and [{C12bzim-Pt}2-2TEG-pPE3](PF6)2 (9) (−94.7 kJ mol−1), which strongly suggests the role of hydrophobic interactions in the stabilization of the aggregates. Furthermore, [{C12bzim-Pt}2-pPE3](PF6)2 (4) with n-dodecyl chains exhibits a higher number-averaged degree of polymerization at room temperature (315) than [{C8bzim-Pt}2-pPE3](PF6)2 (2) (90), attributed to the longer alkyl chains in [{C12bzimPt}2-pPE3](PF6)2 (4), which possesses a larger extent of hydrophobic interactions. Similar and consistent results have been obtained by comparing [{C8bzim-Pt}2-2TEG-pPE3](PF6)2 (7) (760) to [{C12bzim-Pt}2-2TEG-pPE3](PF6)2 (9) (6700), which further confirms the role of hydrophobic interactions in governing the release in enthalpy and the number-averaged degree of polymerization. On the other hand, for the complexes undergoing isodesmic growth mechanism, the increasing alkyl chain length has also been found to enhance the stability of the aggregates. These are reflected by the enthalpy release of [{C6bzim-Pt}2-pPE3](PF6)2 (1) (−118.1 kJ mol−1) and [{C16bzim-Pt}2-pPE3](PF6)2 (5) (−127.6 kJ mol−1) and [{C18bzim-Pt}2-2TEG-pPE3](PF6)2 (11) (−142.4 kJ mol−1) due to the greater noncovalent hydrophobic interactions upon increasing alkyl chain length from −C6H13 to −C16H33 and to −C18H37. More importantly, the self-assembly processes of these complexes are not driven by entropy as revealed from the negative ΔS values, but instead enthalpically driven, leading to spontaneous self-assembly processes resulting in the negative ΔG values. Hence, the presence of multiple noncovalent interactions including Pt···Pt, hydrophobic, hydrophilic, and π−π stacking interactions is of paramount importance to the spontaneous self-assembly behaviors. Effect of Branched Alkyl Chain on Self-Assembly Behaviors. Since the steric bulkiness of the complexes might also be one of the contributing factors affecting the selfG


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Figure 7. TEM images prepared from a solution of [{C16bzim-Pt}2-2TEG-pPE3](PF6)2 (10) (2 × 10−5 M) in (a) pure DMSO, (b) 20% water− DMSO mixture, and (c) 90% water−DMSO mixture. TEM images (b and c) are negatively stained with uranyl acetate.

the complexes with overall hydrophobicity/hydrophilicity at the two extremes, such as [{C16bzim-Pt}2-pPE3](PF6)2 (5) (hydrophobic), [{C6bzim-Pt}2-2TEG-pPE3](PF6)2 (6), and [{C 12 bzim-Pt} 2 -6TEG-pPE3](PF 6 ) 2 (12) (hydrophilic), would result in the imbalance of these multiple noncovalent interactions, which would ultimately lead to the isodesmic growth mechanism. Solvent-Induced Morphological Transformations Probed by Electron Microscopy and Dynamic Light Scattering Studies. In light of the drastic and abrupt changes in the solvent-dependent UV−vis absorption spectra, electron microscopy has been employed to provide further insights on the morphologies formed in different DMSO−water solvent compositions. Complexes 1, 4, and 9−11 have been selected to investigate the morphologies under different solvent compositions, which might contribute to the drastic and abrupt changes in the absorption spectra. Interestingly, increasing water composition in the DMSO solutions of the complexes result in a morphological transformation from plate to fibers and to spherical aggregates at concentration of 10−5 M (Figure 7 and Figures S33−36, Supporting Information). Morphological transformations have also been observed in related alkynylplatinum(II) complexes3f,d,n and are consistent with the different UV−vis absorption behavior at various solvent compositions under cooling. In their DMSO solutions at 10−5 M, the complexes would adopt plate-like nanostructures as shown in the TEM images (Figure 7a and Figures S33a−36a, Supporting Information). It is believed that the relatively hydrophobic complexes with extended π-conjugation would already be aggregated in a polar solvent such as DMSO to form the plate-like aggregates, which coincides with the intermolecular association behavior from the results in 1H NMR spectroscopic studies and the dynamic light scattering studies (Table S2−S3, Supporting Information) in DMSO solutions. Upon increasing water content in DMSO to a critical point where the maximum growth of the MMLCT absorption band has been reached, a morphological transformation from plate to fibrous nanostructures has been observed from the TEM images prepared from these complexes due to the switching-on of the Pt···Pt interactions as revealed by the UV−vis and emission spectroscopy, which could lead to the formation of fibrous nanostructures resulting from the delicate balance between multiple noncovalent interactions such as Pt···Pt, hydrophobic, hydrophilic, and π−π stacking interactions in DMSO−water mixtures (Figure 7b and Figures S33b−36b, Supporting Information). Moreover, further increasing the water composition to 90% would lead to another morphological transformation to spherical aggregates (Figure 7c and Figures S33c−36c, Supporting Information). At this highly polar medium, the hydrophilic interactions introduced by the

assembly mechanism, branched-chain analogues [{2EHbzimPt}2-pPE3](PF6)2 (3) and [{2EHbzim-Pt}2-2TEG-pPE3](PF6)2 (8) are designed through the incorporation of 2ethylhexyl chains on the bzimpy ligands to increase the bulkiness of the complexes. However, no significant changes in the self-assembly mechanism are observed for both [{2EHbzim-Pt}2-pPE3](PF6)2 (3) and [{2EHbzim-Pt}2-2TEGpPE3](PF6)2 (8) when compared to [{C8bzim-Pt}2-pPE3](PF6)2 (2) and [{C8bzim-Pt}2-2TEG-pPE3](PF6)2 (7), respectively. Interestingly, the branched chain analogues [{2EHbzim-Pt}2-pPE3](PF6)2 (3) (−39.4 kJ mol−1) and [{2EHbzim-Pt}2-2TEG-pPE3](PF6)2 (8) (−46.5 kJ mol−1) show less negative enthalpy changes when compared to [{C8bzim-Pt}2-pPE3](PF6)2 (2) (−45.4 kJ mol−1) and [{C8bzim-Pt}2-2TEG-pPE3](PF6)2 (7) (−59.2 kJ mol−1), respectively, possibly due to the introduction of the sterically bulkier alkyl chain, which would suppress the intermolecular association of the complexes. This would result in weaker noncovalent hydrophobic interactions and hence the formation of less thermodynamically stable aggregates. Effect of Hydrophilicity of the Backbone on SelfAssembly Mechanism. In addition to the variations in the alkyl chain lengths in the bzimpy ligand, it is also interesting to investigate the effect of altering the hydrophilicity of the pphenylene ethynylene with the introduction of triethyleneglycol pendants. From [{C12bzim-Pt}2-pPE3](PF6)2 (4) to [{C12bzim-Pt}2-2TEG-pPE3](PF6)2 (9) and to [{C12bzimPt}2-6TEG-pPE3](PF6)2 (12), a change in the self-assembly mechanism from cooperative (4 and 9) to isodesmic (12) has been observed upon a gradual increase in hydrophilicity of the backbone. These increases in the overall hydrophilicity of the complexes are also in line with the observations in the changes of self-assembly mechanism from cooperative ([{C8bzim-Pt}22TEG-pPE3](PF6)2 (7)) to isodesmic ([{C6bzim-Pt}2-2TEGpPE3](PF6)2 (6)) upon reduction of alkyl chain length. Such an increase in the hydrophilicity of the backbone would alter the overall balance between multiple noncovalent interactions including Pt···Pt, hydrophobic, hydrophilic, and π−π stacking interactions, which would lead to the disruption of cooperativity of the self-assembly. With the above-mentioned experimental findings on the effect of alkyl chain lengths, steric bulkiness and the hydrophilicity of the backbone upon modification of the overall hydrophobicity/hydrophilicity of the complexes, it is believed that the cooperativity could only be achieved by suitable design of a certain hydrophobicity/hydrophilicity of the molecules with a delicate balance of different kinds of noncovalent interactions such as metal−metal, hydrophobic, hydrophilic, and π−π stacking interactions, which synergistically assisted the formation of the aggregates cooperatively. On the other hand, H


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hydrophilic−hydrophilic, and π−π stacking interactions. In particular, a change in the aggregation mechanism from isodesmic to cooperative and then to isodesmic is observed upon a gradual increase in alkyl chain length, while the cooperativity of the branched chain analogues is maintained despite the formation of less stable aggregates resulting from the steric hindrance. Moreover, the self-assembly mechanism from cooperative to isodesmic has also been observed upon gradual increase in hydrophilicity of the backbone. It is believed that the cooperativity could only be achieved by suitable design of a certain hydrophobicity/hydrophilicity of the molecules with a delicate balance of different kinds of noncovalent interactions such as metal−metal, hydrophobic, hydrophilic, and π−π stacking interactions, which synergistically assisted the formation of the aggregates cooperatively. On the other hand, the complexes with overall hydrophobicity/hydrophilicity at the two extremes would result in the imbalance of these multiple noncovalent interactions, which would ultimately lead to the isodesmic growth mechanism. More importantly, the fundamental understanding of the role of Pt···Pt interactions and the control of the self-assembly mechanisms of this class of metallosupramolecular π-conjugated amphiphiles have been demonstrated, providing further insights into the molecular engineering and rational molecular design of this class of complexes, resulting in the formation of aggregates with desirable morphologies and self-assembly mechanism.

triethylene-glycol moiety would be minimized as these TEG chains would likely be solvated and thus maximize the Pt···Pt, hydrophobic, and π−π stacking interactions resulting in the formation of spherical aggregates at high water content. Such strengthening of the Pt···Pt interactions could also be revealed from the most red-shifted 3MMLCT emission band upon increasing water content to 90%. With these interplay of multiple noncovalent interactions, three stages of morphological transformation could result, which ultimately could lead to the systematic constructions of distinct supramolecular architectures with solvent compositions. The three-stage of morphological transformation could further be probed by the dynamic light scattering studies in various DMSO−water mixtures. [{C12bzim-Pt}2-pPE3](PF6)2 (4) and [{C12bzim-Pt}2-2TEG-pPE3](PF6)2 (9) have been selected for the investigation on the size of the aggregates in different solvent compositions. Interestingly, the size of the aggregates is found to be reduced from ∼1000 nm to ∼800 nm and then to ∼100 nm upon increasing water content in their DMSO solution to the critical point (maximum growth of MMLCT absorption) and to 90% (Tables S2−S3, Supporting Information), respectively. These observations are consistent with the size of the aggregates observed for [{C12bzim-Pt}2pPE3](PF6)2 (4) and [{C12bzim-Pt}2-2TEG-pPE3](PF6)2 (9) in their electron micrographs prepared from various solvent compositions. The larger-sized aggregates are resulted from the plate-like (∼1000 nm) and fibrous nanostructures (∼800 nm), while the smaller-sized aggregates (∼100 nm) are attributed to the spherical nanostructures. However, the morphologies or sizes of the aggregates in the same solvent system are found to result in insignificant differences upon changing the alkyl chain lengths or increasing the number of hydrophilic TEG chains in the backbone. These observations are understandable and might be attributed to the insignificant alteration on the curvature of the complexes upon a change in the alkyl chain length due to the presence of rigid rod segment in the complexes. On the other hand, the morphological transformations in different solvent compositions could be attributed to the interplay and balance of multiple noncovalent interactions including Pt···Pt, hydrophobic, hydrophilic, and π−π stacking interactions such that various kinds of morphologies could be resulted.

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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.8b03628. Photophysical measurements and instrumentation; temperature-dependent nucleation−elongation model and isodesmic model in curve fitting; synthetic procedures of 1−12; electronic absorption spectra of 1−12 in DMSO solutions at 298 K; variable-temperature UV−vis absorption spectral changes of 1−12 in DMSO solutions at 10−5 M upon heating; 1H NMR spectral traces of 1− 12 in DMSO-d6 at various temperatures in aryl region; 1 H NMR spectral traces of 12 in DMSO-d6 at 298 K in aryl region in various concentration regimes; UV−vis absorption spectral changes of 1−3, 5−12 in DMSO solutions upon increasing water content; emission spectral changes of 2−4 and 9 upon increasing water content in DMSO solutions; UV−vis absorption spectral traces of 1−3, 5−11 upon cooling; TEM images prepared from solutions of 1, 4, 9 and 11; dynamic light scattering data of 4 and 9 (PDF)

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CONCLUSION In conclusion, a series of amphiphilic alkynylplatinum(II) bzimpy complexes has been synthesized in which their selfassembly mechanism can be systematically controlled by the variation of the overall hydrophobicity/hydrophilicity. The bzimpy pincer ligands have been judiciously incorporated into amphiphilic alkynylplatinum(II) complexes because of their viability in the ease of modification of the alkyl chains as well as providing an extensive π-conjugated surface for intermolecular associations. In light of the directional Pt···Pt interactions in the constructions of aggregates, the self-assembly behaviors can be easily probed by their drastic and rich spectroscopic responses, in which their morphological transformations and the systematic control of self-assembly mechanisms have been demonstrated. The present study illustrates the control over the selfassembly mechanism through a systematic approach by the modification of the hydrophobicity/hydrophilicity of the complexes, in which the self-assembly mechanisms have been found to be governed by the subtle balances of noncovalent interactions including Pt···Pt, hydrophobic−hydrophobic,

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

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 the support from The University of Hong Kong and the URC Strategically Oriented Research Theme on Functional Materials for Molecular Electronics. This I


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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 the Hong Kong Special Administrative Region, P. R. China (HKU 17334216). M.H.-Y.C. acknowledges the receipt of a postgraduate studentship and a University Postgraduate Fellowship and S.Y.-L.L. acknowledges the receipt of a University Postdoctoral Fellowship, both of which are administered by The University of Hong Kong. We also thank Frankie Yu-Fee Chan at the Electron Microscope Unit of The University of Hong Kong for the helpful technical assistance.

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Controlling Self-Assembly Mechanisms through Rational Molecular

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