Molecular Engineering of Platinum(II) Terpyridine Complexes with

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Molecular Engineering of Platinum(II) Terpyridine Complexes with Tetraphenylethylene-Modified Alkynyl Ligands: Supramolecular Assembly via Pt···Pt and/or π−π Stacking Interactions and the Formation of Various Superstructures Heung-Kiu Cheng, Margaret Ching-Lam Yeung, 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 S Supporting Information *

ABSTRACT: A series of platinum(II) terpyridine complexes with tetraphenylethylene-modified alkynyl ligands has been designed and synthesized. The introduction of the tetraphenylethylene motif has led to aggregation-induced emission (AIE) properties, which upon self-assembly led to the formation of metal−metal-to-ligand charge transfer (MMLCT) behavior stabilized by Pt···Pt and/or π−π interactions. Tuning the steric bulk or hydrophilicity through molecular engineering of the platinum(II) complexes has been found to alter their spectroscopic properties and result in interesting superstructures (including nanorods, nanospheres, nanowires, and nanoleaves) in the self-assembly process. The eye-catching color and emission changes upon varying the solvent compositions may have potential applications in chemosensing materials for the detection of microenvironment changes. Furthermore, the importance of the directional Pt···Pt and/or π−π interactions on the construction of distinctive superstructures has also been examined by UV−vis absorption and emission spectroscopy and transmission electron microscopy. This work represents the interplay of both inter- and intramolecular interactions as well as the energies of the two different chromophoric/luminophoric systems that may open up a new route for the development of platinum(II)−AIE hybrids as functional materials. KEYWORDS: platinum(II) terpyridine complexes, alkynyl ligands, self-assembly, π−π interactions, aggregation-induced emission

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biomolecules,19,20 and so on, resulting in interesting color and luminescence changes for stimuli signaling. It is worth noting that, with further introduction of other noncovalent interactions such as hydrogen bonding or hydrophobic− hydrophobic interactions, the alkynylplatinum(II) complexes are found to yield a series of superstructures with different morphologies upon modulation of solvent compositions.15,16,25−28 The drastic color changes in the visible region and luminescence changes in the near-infrared (NIR) region due to the development of metal−metal-to-ligand charge transfer (MMLCT) state stabilized by Pt···Pt and/or π−π

INTRODUCTION Square-planar d8 platinum(II) polypyridine complexes are wellknown to exhibit intriguing photophysical properties in addition to their strong tendency to form high-ordered oligomers or extended linear chains in the solid state through the assistance of Pt···Pt and/or π−π stacking interactions.1−13 In recent years, Yam and co-workers demonstrated that alkynylplatinum(II) complexes with terpyridine, 2,6-bis(benzimidazol-2′-yl)pyridine, and 2,6-bis(1-alkylpyrazol-3-yl)pyridine ligands are successful classes of transition metal complexes for the design of functional materials.14−24 On the basis of the variation of the extent of Pt···Pt and/or π−π interactions, some of these complexes are found to be responsive to different types of stimuli such as solvents,14−16,25,26 pH,17 temperature,27,28 polyelectrolytes,18 © XXXX American Chemical Society

Received: August 8, 2017 Accepted: September 25, 2017

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DOI: 10.1021/acsami.7b11807 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

formation of nanostructures in the area of supramolecular chemistry. One can also gain insights into the tuning of the luminescence properties of the resultant supramolecular aggregates through the interplay of the energies of the two different chromophores and luminophores in their unaggregated and aggregated states, as well as the various inter- and intramolecular interactions, which may contribute to the development of new functional materials. Herein, we report the design, synthesis, and characterization of a series of alkynylplatinum(II) terpyridine complexes containing a tetraphenylethylene (TPE) unit linked by different lengths of hydrophilic flexible spacers. The spectroscopic changes of all the complexes upon the addition of water together with the morphological transformation of the superstructures have also been investigated and discussed.

interactions upon the addition of stimuli, as well as the capability of formation of distinctive superstructures in the selfassembly process, demonstrated this class of alkynylplatinum(II) complexes as potential candidates for advanced functional materials. On the contrary, traditional fluorophores like anthracene and fluorescein are only emissive in dilute solution but are nonemissive to weakly emissive in the solid state or in concentrated solution due to aggregation-caused quenching (ACQ).29,30 Unlike common fluorophores and similar to the MMLCT excited state characteristics, aggregation-induced emission (AIE) molecules are strongly emissive in the aggregated state. This has been attributed to the restriction of the intramolecular rotation (RIR) process which would slow down the nonradiative pathway and facilitate the radiative process.31,32 The AIE molecules have received much attention since their first report in 2001.33 Besides the superior emission properties, some of the AIE molecules have also been shown to display different kinds of superstructures such as spheres,34−36 rods,34,37 vesicles,37 and fibers.38 Combining both the photophysical and morphological properties, the AIE fluorophores have been applied in a variety of research areas including organic light-emitting diodes,39,40 sensing materials,40−42 biological and imaging probes,43−45 etc. On one hand, the self-assembly process of platinum(II) complexes would give rise to the formation of an MMLCT excited state stabilized by the Pt···Pt and/or π−π interactions, resulting in drastic color and luminescence changes with the formation of superstructures. On the other hand, the principle behind the AIE system involves the recovery of emission via an RIR process upon self-assembly with the formation of aggregates. Although both the alkynylplatinum(II) complexes and the AIE systems exhibit a strong tendency toward supramolecular assembly along with their formation of interesting superstructures, their superstructures are found to be stabilized by different types of noncovalent interactions and their distinctive photophysical properties are attributed to different radiative mechanisms. Specifically, the spectroscopic changes of platinum(II) complexes are mainly associated with aggregation and deaggregation processes, with the turning-on of a low-energy emission due to aggregate formation via noncovalent metal−metal and π−π stacking interactions, the mechanism of which is quite different from that of AIE. Unlike AIE in which the emission turn-on is due to the locking of molecular motion via restricted intramolecular rotation that leads to the slowing down of nonradiative decay processes, the aggregate emission in platinum(II) complexes is originated from exciton coupling arising from Pt···Pt and π−π interactions that leads to a narrower HOMO−LUMO gap for its emission; the emission energy of this can be readily tuned via a change in the extent of the Pt···Pt and π−π stacking interactions. Similar tuning of emission energy is not possible for AIE molecules as their emission is a result of the rigidification brought about by RIR. Interestingly, the research on the supramolecular chemistry of platinum(II)−AIE hybrids is virtually unknown despite few reports on the luminescence behavior of platinum(II) complexes with an RIR-induced AIE property being known.36,41,46,47 In view of such rarity and the different natures of these two functionalities in favoring the formation of superstructures, it is believed that the integration of AIE moieties with the alkynylplatinum(II) terpyridine system would provide insights into the molecular packing and morphological

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RESULTS AND DISCUSSION Synthesis. Alkynylplatinum(II) terpyridine complexes 1a− 5a and 1b−5b (Scheme 1) are synthesized by copper(I)-

Scheme 1. Structures of TPE-Functionalized Alkynylplatinum(II) Terpyridine Complexes

catalyzed dehydrohalogenation reaction of chloroplatinum(II) terpyridine precursors with the corresponding TPE-functionalized alkynyl ligands (L1−L5) in dichloromethane or DMF (Schemes S1 and S2 in Supporting Information). The complexes with terpyridine ligands (1a−5a) are purified by diffusion of diethyl ether vapor into the concentrated acetonitrile solution of the complexes. On the other hand, the complexes with tri-tert-butylterpyridine ligands (1b−5b) are purified by column chromatography on silica gel using a dichloromethane−acetone mixture (4:1, v/v) as eluent, followed by recrystallization via slow diffusion of diethyl ether vapor into the concentrated dichloromethane solution of the complexes. All the complexes have been characterized by 1H NMR and IR spectroscopy, FAB mass spectrometry, and elemental analyses. B


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Figure 1. UV−vis absorption spectra of 1a in acetonitrile ([Pt] = 50 μM) with increasing water content (a) from 0% to 50% and (b) from 50% to 90%. (c) Photo showing the color changes of 1a in acetonitrile with different water contents (from left to right: 30%, 40%, 50%, 55%, 60%, 65%, 70%, 90% water content).

Photophysical Studies. Dissolution of complexes 1a−5a and 1b−5b in acetonitrile leads to UV−vis spectra that exhibit intense absorption bands at around 284−341 nm and less intense absorption bands at ca. 385−470 nm. The photophysical data of all complexes are summarized in Table S1. According to the previous literature reports,14−24 the highenergy bands are tentatively assigned as intraligand (IL) [π → π*] transitions of the terpyridine ligands and alkynyl ligands while the low-energy bands are assigned as a mixture of metalto-ligand charge transfer (MLCT) [dπ(Pt) → π*(terpyridine)] and alkynyl-to-terpyridine ligand-to-ligand charge transfer (LLCT) [π(alkynyl) → π*(terpyridine)] transitions. In addition, complexes 1a−5a show weak absorption tails at about 500 nm, and their concentration-dependent UV−vis absorption spectra (Figures S1−S5) reveal a deviation from Beer’s Law, suggesting that the lower-energy absorption tails are MMLCT transitions, typical of the formation of Pt···Pt and/or π−π interactions. In contrast, the concentrationdependent UV−vis spectra of 1b−5b (Figures S6−S10) are found to follow Beer’s Law, indicative of the absence of MMLCT formation in acetonitrile solution, as expected of the sterically more demanding platinum(II) tri-tert-butylterpyridine system. Furthermore, all the complexes are weakly emissive or nonemissive in acetonitrile solution, probably due to the nonradiative decay introduced by the fast rotations of the phenyl rings on the TPE moiety. Solvent-Induced Self-Assembly Studies. The concentration of complexes 1a−5a and 1b−5b is kept at 50 μM for investigation, and since they are insoluble in water, water is used as the poor solvent for the studies. At low water content (0−50%, Figure 1a), the MLCT/LLCT absorption band of 1a is found to exhibit a negative solvatochromism, arising from an increase in the solvent polarity. Upon further increasing the water percentage (Figure 1b), an emergence of a low-energy absorption tail at about 500−650 nm with the decline of the MLCT/LLCT band is observed. On the basis of the

concentration-dependent UV−vis absorption study of 1a (Figure S1), this absorption tail can be assigned as the MMLCT absorption, suggesting that the presence of a high content of the poor solvent would bring the platinum(II) terpyridine units into close proximity and lead to their selfassembly via the Pt···Pt and/or π−π stacking interactions. Moreover, according to the previous AIE studies, such an absorption tail can also be caused by the Mie scattering effect of nanostructures.31,32 This implies that 1a would be aggregated into superstructures at high water content. In addition, similar observations have also been found in the cases of 2a−5a (Figures S11−S14). These suggest that the introduction of spacers would not prohibit the aggregation. Notably, 2a is found to have the lowest energy of the MMLCT band (580 nm) among all the complexes at 90% water content (Figure S11). This may indicate that 2a has a better packing arrangement assisted by the Pt···Pt and/or π−π stacking interactions. The assembly of the tri-tert-butylterpyridine series in mixed solvent has also been studied, and the results are shown in Figures S15−S19. For the tri-tert-butylterpyridine-containing counterparts 1b−5b, a similar negative solvatochromism has also been observed from 0% to 60% water content (Figures S15a−S19a). Emergence of lower-energy absorption tails is observed in 1b−5b on further increasing the water content (Figures S15b−S19b). In addition to the Mie scattering effect of the nanostructures,31,32 these absorption tails can be attributed to the π−π stacking interactions of the aromatic moieties upon aggregation. The lower-energy absorption of 5b at 90% water content shows a linear relationship with Beer’s Law, further indicating the absence of Pt···Pt interactions in the aggregation process (Figure S19e). More interestingly, the absorbance of the absorption tails at 85% water content is slightly higher than that at 90% water content for 3b−5b (Figures S17c−S19c). This phenomenon can be explained by the size of the superstructures. The dynamic light scattering C


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Figure 2. Emission spectra of (a) 1a and (b) 2a in acetonitrile ([Pt] = 50 μM) with different water contents (the asterisk denoted an instrumental artifact). Excitation wavelengths of 1a and 2a at 350 and 365 nm, respectively.

Figure 3. Emission spectra of (a) 3a, (b) 4a, and (c) 5a in acetonitrile ([Pt] = 50 μM) with different water contents (the asterisk denoted an instrumental artifact). Excitation wavelength at 348 nm.

(DLS) studies reveal that the superstructures of 3b−5b formed at 85% water content have larger hydrodynamic diameters for the superstructures than those at 90% water content (Figures S17d−S19d). As a result the scattering effect of larger superstructures caused the higher absorbance at 85% water content for 3b−5b. Upon photoexcitation at the isosbestic point, 1a remains weakly emissive (λem = 650 nm) at lower water content (0− 70%, Figure 2a). When the water percentage reaches 90% (Figure 2a), an appearance of a red to NIR emission band centered at 786 nm is observed. According to the previous spectroscopic studies14−24 together with the ground state UV− vis absorption studies, this emission band can be attributed to the 3MMLCT emission since the aggregated molecules would facilitate the Pt···Pt and/or π−π stacking interactions. Additionally, the 43-fold emission enhancement at the emission maximum can also be contributed by the RIR process of the TPE moiety in the aggregated state, resulting in more efficient radiative processes. Furthermore, similar emission enhancements have also been observed in 2a−5a at high water content (Figure 2b and Figure 3). These suggest that the introduction of spacers between the platinum(II) terpyridine unit and the TPE moiety would not prohibit the AIE behavior upon selfassembly. However, the spacers would alter the wavelength of the 3MMLCT emission: 2a (793 nm) > 1a (786 nm) > 3a−5a (765 nm) (Figure S20). The higher energy of the 3MMLCT emission bands of 3a−5a would probably be due to the floppy nature of the ethylene glycol units which would disturb the packing arrangement, hence weakening the Pt···Pt and/or π−π stacking interactions. Also, as TPE is well-known to be a propeller-shaped molecule, the molecular packing is likely to be

affected if the TPE moiety is too close to the platinum(II) center as in the case of 1a. In addition, a weak emission band at about 470 nm can be observed in the solutions of 3a−5a at high water content (Figure 3), which is not found in 1a and 2a. This greenish-blue emission can be ascribed to the fluorescence from the TPE moiety according to the emission studies of L1− L5 (Figures S21−S25) and other TPEs in the literature.31,32,40,42,44 Notably, the greenish-blue emission became more dominant while the red to NIR emission became weaker when the length of the spacers is increased (Figure 4). This suggests that the length of the spacers plays an important role

Figure 4. Emission spectra of 3a−5a in acetonitrile ([Pt] = 50 μM) at 90% water content (the asterisk denoted an instrumental artifact). Excitation wavelength at 348 nm. D


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Figure 5. Emission spectra of (a) 3b, (b) 4b, and (c) 5b in acetonitrile ([Pt] = 50 μM) with different water contents. Excitation wavelength at 340 nm.

emission enhancement. Complexes 1b−5b are very weakly emissive in acetonitrile with emission maxima at about 520− 650 nm, which are tentatively assigned as admixtures of 3 MLCT [dπ(Pt) → π*(tpy)] and 3LLCT [π(alkynyl) → π*(tpy)] excited states (Figure 5 and Figures S26−S30). Complexes 1b and 2b are found to only exhibit weak emission enhancements upon addition of water (Figures S26 and S27), which are quite different from that observed in their counterparts 1a and 2a. Interestingly, a more obvious emission enhancement is observed in 3b−5b, indicating that the AIE behavior can be recovered upon lengthening the spacers, as shown in the aggregation studies of 3b−5b. At high water content (>80%), appearance of emission bands centered at about 670 nm is observed for 3b−5b (Figure 5). The close resemblance of the excitation spectra of the two emission maxima at different water contents suggested that these emission bands at ca. 670 nm originated from the excimeric emission of the complexes upon aggregation. These observations can be attributed to the differences in the distance between the bulky tert-butyl groups and the AIE-active TPE motif. If the TPE is too close to the tert-butyl groups just like 1b and 2b, the TPE cannot aggregate properly, resulting in a less effective RIR process and hence the weakening of the AIE effect. Once there is sufficient space between the tert-butyl group and the TPE unit through the lengthening of the ethylene glycol spacers, the RIR effect can become effective. As a result, stronger AIE effects have been observed for 3b−5b. Interestingly, the greenish-blue emission bands (ca. 470 nm) have been almost completely quenched, which is different from the study of 3a−5a. This implied that the quenching process would be more efficient in the series of the tri-tertbutylterpyridine complexes. Electron Microscopic Studies. In view of the significant color and luminescence changes observed upon the addition of water to the complex solutions, which suggest the occurrence of self-assembly, transmission electron microscopy (TEM) has been employed to probe the possible formation of superstructures by these complexes at different water percentage. Interestingly, the introduction or lengthening of the spacer or modification on the terpyridine ligand would lead to different superstructures at different water content. For the complexes without the spacers, 1a and 1b display nanorods and nanospheres at high water content in acetonitrile solution, respectively (Figure 6). The corresponding selected area electron diffraction (SAED) pattern shows that the nanorods formed by 1a at high water content are crystalline, suggesting a highly ordered packing of complex molecules in the super-

in the quenching process, mainly via energy transfer from the TPE motif to the platinum(II) terpyridine unit. The shorter the spacers, the more efficient quenching would result. The corresponding Förster resonance energy transfer efficiency (E) is calculated from the following equation:48,49 E=1−

IDA ID

where IDA and ID are the integrated areas of the donor emission in the presence and absence of acceptor, respectively. In this study, the IDA is the integrated area of the TPE emission of 3a− 5a in a 90% water−acetonitrile solvent mixture ([Pt] = 50 μM) while ID is the integrated emission area of L3−L5 in a 90% water−acetonitrile solvent mixture ([L] = 50 μM). The efficiencies (E) from the TPE unit in its aggregated state (AIE) to the platinum(II) terpyridine moiety are in the order 3a (99.9%) > 4a (99.5%) > 5a (98.6%). The small difference is probably due to the large photoluminescence quantum yields of the AIE of L3−L5 which would overdominate and overwhelm the weak and quenched TPE emission of 3a−5a. In order to gain further insight into the relationship between the spacer distance and the energy transfer efficiency, the equation for determining the energy transfer efficiency in single molecule systems has been employed to provide a qualitative analysis of the effect of spacer distance on the energy transfer efficiency.50−52 The apparent energy transfer efficiency (Eapp) in single molecules is calculated from the following equation: iA Eapp = iA + iD where iD and iA are the integrated areas of the donor emission and acceptor emission of the single molecule, respectively, assuming iD = ID and iA = IA where ID and IA refer to the integrated areas of the respective donor and acceptor in the ensemble.53 In this case, the iD is taken to be the integrated area of the AIE of TPE (λem = 470 nm) of 3a−5a in a 90% water− acetonitrile solvent mixture ([Pt] = 50 μM) while iA is the integrated area of the 3MMLCT emission (λem = 765 nm) of 3a−5a in 90% water−acetonitrile solvent ([Pt] = 50 μM). The apparent efficiencies (Eapp) from the AIE of the TPE unit to the platinum(II) terpyridine moiety are in the order 3a (92.4%) > 4a (76.6%) > 5a (61.7%). This trend further indicates that the complex with the shortest spacer would have the largest apparent energy transfer efficiency. Upon addition of water to the acetonitrile solution of the tritert-butylterpyridine-containing complexes 1b−5b, AIE is also observed at high water content but with different extents of E


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Interestingly, various superstructures have been discovered after the introduction or lengthening of the spacers between the alkynylplatinum(II) terpyridine unit and the TPE moiety. Long wire-like nanostructures are observed in the solution of 2a at high water content (Figure 7a). Once the hydrophilic ethylene glycol is incorporated as a spacer, 3a is found to display a kind of nanoleaf with some short fragments of nanorods (length

Molecular Engineering of Platinum(II) Terpyridine Complexes with

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