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Synthesis of Luminescent Platinum(II) 2,6-Bis(Ndodecylbenzimidazol-2′-yl)pyridine Foldamers and Their Supramolecular Assembly and Metallogel Formation Michael Ho-Yeung Chan, Maggie Ng, Sammual Yu-Lut Leung, Wai Han Lam, 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, Pokfulam, Hong Kong, People’s Republic of China S Supporting Information *

ABSTRACT: Dinuclear alkynylplatinum(II) metallofoldamers with an oligomeric m-phenyleneethynylene backbone have been designed with the incorporation of a sterically undemanding, π-conjugated, and hydrophobic 2,6-bis(N-dodecylbenzimidazol-2′-yl)pyridine pincer ligand. The complex with the optimal chain length has been found to exhibit gelation behavior via stabilization by noncovalent Pt···Pt and π−π stacking interactions in the hierarchical architecture constructed from the single-turn helix. The chain lengths of the complexes have been found to be a critical determinant for their gelation behavior, conformations, and morphologies. Such a gelation process has been found to undergo a cooperative assembly mechanism according to the nucleation−elongation model. Their self-assembly via the Pt···Pt and π−π stacking interactions has been studied by 1H NMR, 2D ROESY NMR (ROESY = rotatingframe Overhauser spectroscopy), electronic absorption, and emission spectroscopy, and density functional theory calculations have provided further insights into the folded state geometry of this class of metallofoldamers.

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diverse applications.8 In contrast to biologically inspired foldamers, m-phenyleneethynylene (mPE) oligomers are representative of abiological foldamers, in which the folding process can be readily mediated by solvents or temperatures.9 Recently, Yam and co-workers demonstrated a series of metalcontaining foldamers with platinum(II) terpyridine moieties that are capable of folding back onto each other to form a single-turn helix.10 In addition, this single-turn helix, which is tightly stabilized by intramolecular Pt···Pt and π−π stacking interactions, has served as an optimum building block for transverse aggregation to form helices of helices in a supramolecular assembly.10c Despite a vast variety of LMWGs that have emerged from the self-assembly of discrete molecules,2−5,7 it has long been known that organogels derived from foldamers are relatively less explored. The development of foldamer organogels has a great potential impact since foldamers have flexible conformations and the tubular cavity of the well-defined helix may be effective for molecular recognition and molecular switching by controlling the environment. In view of the unique metal− metal interactions commonly found in platinum(II) polypyridine complexes6d−g,i−l and the recent demonstration of singleturn helical platinum(II) complex formation,10 it is envisioned

INTRODUCTION Supramolecular chemistry has been an important area of research ranging from biology to materials science because of the sophisticated construction of molecular architectures through different kinds of noncovalent interactions.1 The rapid development of low molecular weight gelators (LMWGs) with the intermolecular forces of hydrogen-bonding, hydrophobic−hydrophobic, and π−π interactions has received enormous attention.2 In particular, the recent exploration of the self-assembly of discrete metal complexes for the formation of LMWGs has also opened up a new area, with the possible involvement of additional intermolecular forces and metal− ligand coordination and demonstration of unprecedented molecular architectures through multiple noncovalent interactions.3−5 More importantly, apart from metal−ligand coordination, the possible involvement of metal···metal interactions with intriguing spectroscopic and luminescent properties6 has made them distinctive from the pure organic analogues.7 Therefore, metal-based supramolecular materials have represented a sound basis for the rational design of novel LMWGs. To date, the research on the construction of peptide-based helical structures has attracted enormous attention in the study of biologically inspired foldamers because their oligomers are able to mimic the secondary structures in proteins and nucleic acids, behaving as bioactive building blocks for biomaterials in © 2017 American Chemical Society

Received: April 11, 2017 Published: June 16, 2017 8639

DOI: 10.1021/jacs.7b03635 J. Am. Chem. Soc. 2017, 139, 8639−8645

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Journal of the American Chemical Society that intermolecular metal−metal interactions are capable of directing the helical platinum(II) complexes into a high degree of hierarchical structure. This would ultimately lead to the preparation of interesting classes of soft materials. Moreover, it is anticipated that suitable modification of the steric bulk and hydrophobicity of the pincer ligands at the two termini might lead to foldamer organogels with luminescence properties, as a lower steric bulk and a larger extent of π-conjugation in the pincer ligands could facilitate the formation of inter- and intramolecular Pt···Pt and π−π stacking interactions. This would lead to the preparation of new classes of luminescent foldamer organogels with interesting spectroscopic properties and self-assembly behaviors, distinctive from the previously reported metallofoldamers and pure organic analogues of foldamer organogels. In this context, the 2,6-bis(N-alkylbenzimidazol-2′-yl)pyridine (bzimpy) ligand has been selected as the pincer ligand of choice. Herein are described the synthesis and supramolecular assembly studies of a series of dinuclear platinum(II) bzimpy complexes containing mPE repeating units (Scheme 1), in

Figure 1. Photographs showing (a) the gel prepared from 2 under ambient light and (b) the gel prepared from 2 under UV light.

Scheme 1. Structure of the Alkynylplatinum(II) bzimpy Complexes 1−3

which the length of the mPE backbone has been varied to determine the optimal length for gel formation. In addition, the strong luminescence behavior of the gelators is found to be attributed to the hexagonal packing in the hierarchical assembly and the extent of the Pt···Pt and π−π interactions. The formation of the intra- and intermolecular Pt···Pt and π−π stacking interactions in the metal-containing organogel has also been studied by 2D ROESY NMR, electronic absorption, and emission spectroscopy.

Figure 2. Partial 1H−1H ROESY spectrum of 2 in the aliphatic alkyl chain region and the aryl region in CD3CN at 298 K.

of 2.65−2.84 Å, estimated from the integrals of the cross-peaks (Table S1, Supporting Information). This indicates that the two terminal platinum(II) bzimpy moieties of 2 and 3 would come close to each other to adopt a helical conformation, presumably with the formation of intramolecular π−π stacking interactions. On the contrary, 1 of a shorter backbone does not exhibit these NOE signals and is believed to adopt an extended chain structure, with the two platinum(II) bzimpy moieties on the same molecule being far away from each other. The difference in their conformations is further revealed by comparing their hydrodynamic radii in DOSY NMR (DOSY = diffusionordered spectroscopy) experiments (Figure S3, Table S2, Supporting Information), in which 2 and 3 have smaller hydrodynamic radii of 10.7 and 11.3 Å, respectively, than 1 does (24.3 Å). These are presumably associated with the coiling of 2 and 3 into folded conformations in CD3CN to give the smaller hydrodynamic radii. The single-turn helices for 2 and 3 would further intermolecularly self-assemble themselves into helical stacks with the formation of both inter- and intramolecular π−π interactions, as supported by the broad aromatic signals in the 1D 1H NMR spectra and strong NOE interactions in the 2D 1H−1H ROESY NMR spectra, respectively. In contrast, 1 would essentially adopt an extended chain given the short length of the backbone that would not allow the two terminal platinum(II) bzimpy moieties within the

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RESULTS AND DISCUSSION All the complexes show different solubilities and gel formation abilities in CH3CN at a concentration of 10−3 M. The solution of 1 remains cloudy, while 2 and 3 dissolve very well in CH3CN. Upon aging, 2 would form a clear and stable orange gel that shows orange luminescence under UV irradiation (Figure 1). As indicated in the 1H NMR spectra in CD3CN at 298 K, all of them exhibit broad upfield-shifted NMR signals in the aromatic region (Figure S1, Supporting Information). Upon an increase in temperature, the 1H NMR signals would become downfield-shifted back to their normal region and well-resolved. This suggests that all the complexes show intermolecular assembly in CD3CN at room temperature under such relatively high concentrations with the involvement of π−π stacking interactions. Interestingly, strong nuclear Overhause effect (NOE) signals between the alkyl chains and the mPE units can also be observed in 2 and 3 in the 2D 1H−1H ROESY NMR (ROESY = rotating-frame Overhauser spectroscopy) spectra (Figure 2; Figure S2, Supporting Information) with a distance 8640

DOI: 10.1021/jacs.7b03635 J. Am. Chem. Soc. 2017, 139, 8639−8645

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Figure 3. (a) Schematic drawing of the extended chain ribbons of 1 and its self-assembly behaviors. (b) Schematic drawing of the single-turn helix of 2 and the formation of the hierarchical architecture stabilized by the Pt···Pt and π−π stacking interactions.

also been observed in some of the related alkynylplatinum(II) bzimpy complexes.7e,11 In contrast to their similar UV−vis absorption spectra, the emission spectra are very different from each other (Figure S8, Supporting Information). 1 of a shorter backbone shows a triplet intraligand (3IL) emission band as well as a low-energy structureless emission band (690 nm). However, 2 and 3 predominantly exhibit structureless emission bands at 695 and 688 nm, respectively, while 3 particularly shows an incomplete switching-off of the monomeric 3IL emission band. In addition, these structureless emission bands in the lower energy regions for 1−3 are found to be concentration-dependent (Figures S9−S11, Supporting Information). Upon increasing the concentration (10−6 to 10−4 M), the emission bands become red-shifted, strongly supportive of their intermolecular assembly nature. Thus, these structureless emission bands are assigned as triplet metal−metal-toligand charge transfer (3MMLCT) phosphorescence, originating from the Pt···Pt and π−π interactions in the self-assembly. However, the aggregation behaviors of 1−3 are different according to their conformation and extent of Pt···Pt and π−π interactions. The unfolded state of 1 would achieve the intermolecular Pt···Pt and π−π interactions solely in the form of extended chain ribbons (Figure 3a). On the contrary, 2 and 3 with a sufficient length of mPE units are capable of folding back onto themselves to form single-turn helices with the formation of intramolecular Pt···Pt and π−π interactions. Furthermore, these helically disposed chains of 2 and 3 would undergo intermolecular association into a hierarchical assembly, driven by the intermolecular Pt···Pt and π−π interactions as depicted schematically in Figure 3b. It is also interesting to note that there is a direct correlation of the extent of Pt···Pt and π−π interactions with the stability of the gel formed for the folded state of the complexes. This can be inferred from the observation that 2, with the largest extent of Pt···Pt and π−π interactions shows the lowest 3MMLCT emission energy (695 nm) and the complete switching-off of its monomeric 3IL emission in CH3CN. This can be attributed to the optimum

same molecule to come close enough to each other. Unlike helical stacks of 2 and 3, the unfolded state of 1 would associate intermolecularly in the form of extended chain ribbons. Such molecular association might account for its poor solubility, because of its extended chain ribbons, which would maximize the extent of π−π interactions in both the bzimpy ligand and the mPE backbones between adjacent molecules in polar solvents. Therefore, the difference in their conformations might account for their different solubilities and gel formation abilities in CH3CN. The yellow to orange solutions of 1−3 in CH3CN are further examined by UV−vis absorption and emission spectroscopy. 1−3 would exhibit similar UV−vis absorption spectra (Table S3, Supporting Information) with intense intraligand [π → π*] transitions of the alkynyl ligands and the bzimpy ligands at 285−358 nm, together with lower energy absorption bands at 445−470 nm, which are assigned as metal-to-ligand charge transfer (MLCT) [dπ(Pt) → π*(bzimpy)] transitions mixed with ligand-to-ligand charge transfer (LLCT) [π(alkynyl) → π*(bzimpy)] character (Figure S4, Supporting Information). The absorption tail at 520 nm in the UV−vis absorption spectra of 1 in the concentration range of 10−6 to 10−4 M does not obey Beer’s law (Figure S5, Supporting Information), which is indicative of ground-state intermolecular association of 1 by Pt···Pt interactions, and hence, the absorption tail is assigned as a metal−metal-to-ligand charge transfer (MMLCT) [dσ*(Pt··· Pt) → π*(bzimpy)] transition. In sharp contrast to 1, 2 and 3 behave differently in the concentration range of 10−6 to 10−4 M in the absorption tail at 520 nm, which might be due to their folded-state conformation. The absorption tails at 520 nm are found to obey Beer’s law (Figures S6 and S7, Supporting Information); however, the ground-state complex aggregation is believed to be significant, as supported by the upfield shift and the broadening of the 1H NMR signals. It is believed that the association constant for the aggregation of the individual folded states is too small to be observable in the MMLCT absorption tails in the UV−vis absorption spectra, which have 8641

DOI: 10.1021/jacs.7b03635 J. Am. Chem. Soc. 2017, 139, 8639−8645

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Journal of the American Chemical Society length of 2 to form a single-turn helix, which acts as an optimum array with a tighter intramolecular Pt···Pt interaction to give such a hierarchical assembly that leads to gel formation with a 3MMLCT luminescence property. The stability of the helical array can be further revealed by comparing the UV−vis absorption spectral traces of 2 and 3 upon increasing the CH2Cl2 content. With reference to previous studies on the Pt(II)-containing foldamers with Pt(II)−tBu3trpy units,10 the structural conformational transition between the random-coil state and the helical strand could be controlled by the CH 2 Cl 2 −CH 3 CN solvent composition. However, it is worthwhile to note that 2 with the two Pt(II)−bzimpy units shows a negligible UV−vis absorption spectral change (Figure S12a, Supporting Information) and incomplete switching-on of the monomeric 3IL emission (Figure S12b) upon increasing the CH2Cl2 content in CH3CN. Particularly, there is no significant increase in the high-energy absorption band at 285 nm, indicative of the lack of a configuration transformation of the mPE units from the cisoid to the transoid form. Compared with the spectroscopic studies of 2, those of 3 exhibit an increase in the high-energy absorption band (Figure S13a, Supporting Information) and the switching-on of the monomeric 3IL emission, comparable to the 3MMLCT emission band (Figure S13b) at high CH2Cl2 content. This may be attributed to the fact that 3 does not possess an optimum helical array for the hierarchical assembly, and this might also account for the lack of gel formation. On the contrary, 2 with the optimum length of the mPE backbone exhibits both an extraordinary stability of the helical conformation and gel formation, which are believed to originate from the more extended π-conjugation of the bzimpy ligands and the less bulky but more hydrophobic hydrocarbon chains. All these structural characteristic of 2 have favored the coming together of the two end groups into close proximity for intraand intermolecular Pt···Pt and π−π interactions to occur and promote the gel formation in CH3CN, distinctive from the previous studies on platinum(II) terpyridine systems. Density functional theory (DFT) calculations have been performed to provide insights into the molecular structure of the folded conformation of 2 (see the Computational Details in the Supporting Information). The Cartesian coordinates of the selected structures are shown in Tables S4 and S5 (Supporting Information). The optimized geometry of the folded structure of the model complex of 2, in which the dodecyl groups of the bzimpy ligand were replaced by methyl groups, is shown in Figure 4. The terminal [Pt(bzimpy)] units are in a head-to-tail orientation allowing the construction of a single helical turn, and the respective coordination planes are nearly parallel to each other. The Pt···Pt distance and interplanar distances of the two [Pt(bzimpy)] coordination planes are 3.428 and 3.463 Å, respectively, indicating the presence of Pt···Pt and π−π stacking interactions in the folded structure of the complex. The change in the free energy of the folded structure relative to the unfolded structure (all-transoid conformation, Figure S14, Supporting Information) is computed to be −10.4 kcal mol−1,12 indicating that the folded structure is thermodynamically more favorable than the unfolded structure. It is worth noting that when comparing the change in free energy in the present system (−10.4 kcal mol−1) and our previously reported tri-tert-butylterpyridine-based metallofoldamers (ca. −2 kcal mol−1),10a,b the folded state of the present system shows enhanced thermodynamic stability, suggestive of the extraordinary stability of the helical conformation resulting from the

Figure 4. Optimized geometry of the folded structure of the model complex of 2. All hydrogen atoms are omitted for clarity.

judicious utilization of the bzimpy pincer ligand of lower steric bulk, more extended π-conjugation and higher hydrophobicity,13 which could facilitate the formation of Pt···Pt and π−π stacking interactions. These computational results are in line with the experimental findings from the electronic absorption and emission spectroscopic studies. The self-assembly mechanism for the gel formation has been further investigated by the variable-temperature UV−vis absorption studies. Upon decreasing the temperature of a concentrated solution of 2 in CH3CN (5.75 × 10−3 M) from 343 to 293 K, there is a growth of the absorption tail at λ > 600 nm associated with the formation of the orange gel. Interestingly, the plot of fraction of aggregated species (αagg) against temperature at 630 nm of the cooling curve of 2 is found to be clearly nonsigmodal (Figure 5), indicative of a

Figure 5. (a) Electronic absorption spectra of the acetonitrile gel of 2 with decreasing temperature from 343 to 293 K. (b) Plots of degree of aggregation monitored at 630 nm against temperature for the heating and cooling of the acetonitrile gel of 2 in the temperature range from 343 to 293 K. The solid line shows the fitted curve in the elongation (black) and the nucleation (blue) regimes.

cooperative supramolecular association mechanism. Hence, the nucleation−elongation model has been employed to further investigate the gel formation process.14 The thermodynamic parameters for the self-assembly process of 2 have been obtained (Table S6, Supporting Information), in which the enthalpy released during the self-assembly for elongation (ΔHe) and the elongation temperature (Te) are found to be about −117.1 kJ mol−1 and 318 K, respectively. In addition, the 8642

DOI: 10.1021/jacs.7b03635 J. Am. Chem. Soc. 2017, 139, 8639−8645

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as shown in the temperature-dependent UV−vis absorption studies of the gel. In sharp contrast to 2, loosely formed nanofibers have been observed in the TEM images prepared from the CH3CN solution of 3 (Figure 6c). This could be attributed to the unstable helical turns of 3 as revealed from the increase in the high-energy absorption band and the switchingon of the monomeric 3IL emission upon increasing the CH2Cl2 content in CH3CN. These observations are also in line with the dynamic light scattering studies on their acetonitrile solutions (Table S7, Supporting Information), in which larger aggregates are only observed in 2 due to its high-order hierarchical architectures. Hence, different kinds of morphologies could be correlated with the aggregation of the extended chain structure and folded-state conformation of the complexes. In addition, from the powder X-ray diffraction analysis of the gel sample prepared from 2, the helical pitch of the stacks of the folded state could be determined from the d-spacing at 4.0 Å, which is attributed to the contribution of intra- and/or intermolecular Pt···Pt and π−π stacking interactions (Figure 8).7o More importantly, the d-spacings at 27.7, 15.3, and 7.62 Å

equilibrium constant for the nucleation step (Ka) is found to be 1.14 × 10−4, and its low value is suggestive of an unfavorable nucleation process. At the elongation temperature, the nucleus size ⟨Nn(Te)⟩ of 21 is obtained, while the number-averaged degree of polymerization is found to be 1500 molecules at room temperature. Hence, it is believed that around 21 molecules of the individual folded state of 2 would stack onto each other to form a nucleus and then further cooperatively elongate to larger aggregates of columnar stacks upon decreasing the temperature beyond the elongation temperature as indicated from the growth of the MMLCT absorption band. Moreover, hysteresis has been observed from the plot of the degree of aggregation monitored at 630 nm against temperature in the heating and the cooling processes of the gel, suggesting that the disruption and the formation of the intermolecular Pt··· Pt and π−π stacking interactions in the gelation process would occur at different temperatures (Figure 5b). In short, the selfassembly processes of 2 would ultimately lead to the construction of high-order hierarchical architectures in a cooperative and synergistic manner assisted by the directional intermolecular Pt···Pt and π−π interactions which lead to the formation of the gel. The different aggregation behaviors of 1−3 have further inspired us to look at their morphologies via electronic microscopy. From the electronic micrographs of the complexes (Figure 6), all the complexes are found to adopt different kinds

Figure 8. Powder X-ray diffraction pattern on the gel sample of 2. Numerical values indicate d-spacings (Å). Figure 6. TEM images prepared from (a) 1, (b) 2, and (c) 3 in acetonitrile solutions ([Pt] = 2 × 10−4 M).

with a ratio of 1:1/ 3 : 1/ 13 are suggestive of the hexagonal packing array of 2 in the solid state, which is also observed in related organic oligo(m-phenyleneethynylene) counterparts.15 They reveal the presence of hierarchy of the folded state in the gel, which is in line with the findings in the UV−vis absorption, emission, and the electronic microscopic studies. Hence, the Pt···Pt and π−π stacking interactions in the single-turn helical platinum(II) complexes would serve as directional noncovalent interactions in building up the hierarchy in the metallogel.

of morphologies in CH3CN solutions. Nanowire morphologies have been observed in the transmission electron microscopy (TEM) images of 1, which could be derived from the aggregation of the extended chain ribbon structure of 1 as revealed from its rather poor solubility and its 3IL emission origin. Such a conformation could allow the ribbons to stack together intermolecularly to result in the formation of the nanowire. Only 2 is formed to give a continuous fibrous network (Figures 6b and 7) resulting from a high-order hierarchical architecture due to the large extent of the Pt···Pt and π−π stacking interactions that results in the gel formation,

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CONCLUSION In conclusion, a series of m-phenyleneethynylene-containing dinuclear alkynylplatinum(II) bzimpy metallofoldamers have been designed. The sterically undemanding, π-conjugated, and hydrophobic bzimpy pincer ligand has been incorporated into the metallofoldamers to facilitate the formation of inter- and intramolecular Pt···Pt and π−π stacking interactions within the reciprocal association of multiple helices, which is distinctive from the alkynylplatinum(II) terpyridine metallofoldamers. This molecular design, together with the optimal chain length and conformation, has allowed the formation of high-order hierarchical architectures that ultimately lead to gelation directed by the intra- and intermolecular Pt···Pt and π−π stacking interactions of the helical platinum(II) complexes. In sharp contrast to the organic organogel counterparts, the luminescent and spectroscopic behaviors can be readily modulated by the metal···metal interactions, such that func-

Figure 7. (a) TEM and (b) SEM images prepared from the acetonitrile gel of 2 ([Pt] = 2 × 10−3 M). 8643

DOI: 10.1021/jacs.7b03635 J. Am. Chem. Soc. 2017, 139, 8639−8645

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tional materials with rich spectroscopic behavior can be designed.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b03635. Photophysical measurements and instrumentation, computational details, nucleation−elongation model in curve fitting, synthetic details of 1−3, variable-temperature 1H NMR experiments for 1−3, partial 1H−1H ROESY spectrum of 3, diffusion-ordered NMR, electronic absorption, concentration-dependent absorption, emission, and concentration-dependent emission spectra of 1−3, optimized geometry of the unfolded structure, diffusion coefficients and hydrodynamic radii obtained from diffusion-ordered NMR spectroscopic studies, Cartesian coordinates of the optimized geometries of the selected structures, thermodynamic parameters for the self-assembly of 2, and dynamic light scattering data of 1−3 (PDF)

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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 URC Strategic Research Theme on New Materials. This work has been supported by the University Grants Committee Areas of Excellence (AoE) Scheme (AoE/P-03/08), a General Research Fund (GRF) grant from the Research Grants Council of the Hong Kong Special Administrative Region, People’s Republic of China (HKU 17334216), and the National Basic Research Program of China (973 Program; 2013CB834701). 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, all of which are administered by The University of Hong Kong. We thank the Computer Center of The University of Hong Kong for providing computational resources. We also thank Mr. Frankie Yu-Fee Chan at the Electron Microscope Unit of The University of Hong Kong for his helpful technical assistance.

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