A Suite of Organoplatinum (II) Triangular Metalloprism: Ag-gregation

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A Suite of Organoplatinum (II) Triangular Metalloprism: Ag-gregationInduced Emission and Coordination Sequence In-duced Emission Tuning Naifang Liu, Tingting Lin, MINGDA WU, He-kuan Luo, Sheng-Li Huang, and T. S. Andy Hor J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b01283 • Publication Date (Web): 31 May 2019 Downloaded from http://pubs.acs.org on May 31, 2019

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Journal of the American Chemical Society

A Suite of Organoplatinum (II) Triangular Metalloprism: Aggregation-Induced Emission and Coordination Sequence Induced Emission Tuning Naifang Liu,‡ Tingting Lin,§ Mingda Wu,§ He-Kuan Luo,§ Sheng-Li Huang,*,† T. S. Andy Hor*,‖ † MOE Key Laboratory of Cluster Science, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing, 100081, China ‡ Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore § Institute of Materials Research and Engineering, A*STAR, 2 Fusionopolis Way, Innovis, Singapore, 138634, Singapore ‖ Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China Supporting Information Placeholder ABSTRACT: A series of triangular metalloprisms with kinetically inert Pt-N bond have been synthesized from the stepwise assembly of a Pt-corner, linear linker 4,4'-bipy (4,4'bipy = 4,4'-bipyridine) and triangular ligand [tpb or tpt, tpb = tris(4-pyridyl)benzene, tpt = tris(4-pyridyl)triazine]. The use of an unsymmetrical [Pt(HL)]-corner (H2L = 2,6diphenylpyridine) leads to novel isostructural products. Phenyl rotation at the metal-corners endows these complexes with good aggregation-induced emission (AIE) function, with varied activities across the isostructural complexes. The coordination sequence of electron-deficient ligand tpt also imparts significant influence on the complex emission. These organoplatinum triangular metalloprisms thus provide a good model to study the influence of building blocks and coordination sequence on the luminescence of supramolecules.

organoplatinum(II) complexes harnessing Pt···Pt and π···π interactions.15 Stang et al. adopted AIEgens ligands, i.e. TPEbased pyridyl unit, to construct emissive Pt(II) metallasupramolecules.6 Compared with an abundance of organic AIE molecules, AIE-active metal complexes, in particular, AIE metallasupramolecules are relatively rare due to the limited AIE generating modes of metal complex and critical combination of AIEgens with metallasupramolecular architectures. We herein report a series of Pt(II) trigonal prismatic assemblies that use [Pt(HL)] as metal-corners which carry phenyl ring whose rotations can be restricted to induce emission in an aggregated state. To the best of our knowledge, no AIE metallosupramolecule derived from AIEgens metalcorner has been reported. With the aim to advance luminescent metallosupramolecule with sophisticated AIE effects, we have designed a set of new Pt(II)-based AIE metallosupramolecules (Figure 1a). Upon molecular aggregation, the trigonal metalloprisms exhibit typical AIE effects with markedly increased quantum yields (-).

Since the concept of AIE was coined by Tang et al. in 2001,1 numerous organic molecules with AIE effects have been designed for OLEDs devices,2 cell imaging,3 photodynamic therapy,4 liquid crystal display,5 etc. Some AIE luminogens (AIEgens) like tetraphenylethene (TPE) were often incorporated into the building blocks of metallosupramolecules,6 metal-organic frameworks7 and covalent-organic frameworks8 to construct luminescent materials based on the restriction of intramolecular rotation. In the burgeoning field of metallosupramolecular chemistry, coordination-driven self-assembly provides a powerful tool to construct metallasupramolecules with different levels of structural complexity.9,10 Luminescent supramolecular architectures have attracted considerable attentions due to their rich excited states, structures with precision control, building block designs and emission tuning especially notable in host-guest adducts.11 These resulted in a range of photoluminescence applications in photo-responsive guest uptake/release,12 photodynamic therapy,13 biological imaging14 etc. Che et al. reported some AIE-active

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agreement with their theoretical distributions. Similarly, samples of the presumed isomers trans-n also showed peaks that match the simulated isotope pattern for the intact 2+ charged cages (2117.28 for trans-1 and 2120.79 for trans-2, respectively. Figure S19-S20).

Figure 1. (a) Synthetic method and sequence-specific control over the construction of triangular metalloprism isomers (tpb for cis-1 and trans-1, tpt for cis-2 and trans-2). (b) The photographs of metallacage cis-2 in CH2Cl2/hexane mixtures with different fractions of hexane on excitation at 365 nm using an UV lamp at 298 K. It is often difficult to selectively synthesize stereoisomeric metallosupramolecules with multi-components due to the kinetic lability of many coordination bonds.16a The Pt-ligand bond is more kinetically inert, and the ligand-exchange labilities can be tuned by careful selection of Pt-corner protecting groups. The cyclometalated (C^N)-group facilitates the formation of Pt-N bond but the de-coordination is still irreversible under ambient conditions, whereas the (C^N)group can also make the Pt-N bonds trans to the carbon donor more labile. Therefore one can design a strategy in selective construction of Pt-isomers by adjusting the sequence of addition of N-donor linkers. Herein, the controlled synthesis of stereoisomeric metalloprism was achieved (Figure 1a).16b The metal-corner [(Pt-L)(dmso)] was treated with a bridging ligand 4,4'-bipy to yield binuclear [(Pt-L)2(4,4'-bipy)] quantitatively. After quantitative treatment with HCSA (HCSA = camphorsulfonic acid) (Figure S11-S14), a tri-pyridyl ligand (tpb or tpt) was added to the mixture to give a trigonal prism salt cis-n·6CSA. The HCSA can protonate one carbon of [(C^N^C)-Pt]-corner thus cleave one Pt-C bond. CSA ions temporarily occupy the released coordination site at Pt-corner, effectively stabilizing the intermediate. Metathesis with Tetrabutylammonium Triflate gives the hexa-OTf- salt in high yield. Reversal of the addition sequence of 4,4'-bipy and tripyridyl ligand resulted in pure trigonal prism salt trans-n. The well-defined signals in 1H NMR spectra evidently indicated the formation of a discrete assembly with highly symmetric structure (Figure S15-S18). Both NMR and ESI-MS analyses are consistent with the proposed molecular structure. The latter analysis of the OTfsalts show the molecular peaks on the parent complexes cis-1 and cis-2 of [(cis-1) − 2OTf]2+ (m/z = 2117.29, Figure 2a) and [(cis-2) − 2OTf]2+ (m/z = 2120.31, Figure 2b), suggesting that the triangular prism structures are intact in solution. The experimental peaks were all isotopically resolved and in good

Figure 2. Experimental (red) and calculated (blue) ESI-MS spectra (2+) of cis-1 (a) and cis-2 (b). Both cis-1 and trans-1 show two absorption bands at ca. 245 nm and 275 nm due to the presence of donors and acceptors with --conjugated systems. Incorporating Pt-pyridyl moieties into the system generated metal-to-ligand charge transfer (MLCT)17 absorption band at over 330 nm (Figure S25-S26). Notably, upon increasing the hexane content in CH2Cl2 solutions, all absorption bands exhibited red-shift with varying extent and accompanied by a dramatic growth at ca. 345 nm, which is attributed to stronger --- stacking interactions of the complexes when in closer proximity state.18 Compared to the absorption spectrum of cis-1, cis-2 shows only one absorption band below 300 nm due to incorporation of --deficient ligand tpt. The MLCT absorption band shows little change upon hexane addition. The emission spectra of cis-1, cis-2 and trans-1 were determined in pure CH2Cl2 and CH2Cl2/hexane mixture (Figure 3a, S27-S28) and trans-2 was non-emissive. The tpbbased cis-1 and trans-1 showed slight enhancement of pure MLCT emission at ca. 480 nm and 520 nm with addition of < 30% hexane. Upon further addition of hexane, AIE appeared and the intensity of this emission was drastically enhanced to an approximately 10-fold accompanied by slight increase of MLCT emission. This is a rare example of a metallasupramolecules that shows two set of emission bands from different luminescence mechanism in a single spectrum. Consequently, the tpb-based cis-1 and trans-1 exhibit weak green emission at the outset due to pure MLCT emission. With the addition of hexane, the two triangular metalloprisms showed emission colour change from orange to yellow because of the dual emission mechanisms taking effect (Figure 3c, S31). Meanwhile, their emission maximum undergoes a ca. 70 nm red shift (Figure 3b, S29) and AIE effects gradually become a predominant emission mechanism. Although cis-2 has a similar structure to cis-1, the incorporation of --deficient ligand tpt resulted in the absence of MLCT emission. Thus, cis-2 is non-emissive when hexane is less than 30%. A luminescence enhancement has been observed at ca. 560 nm and 620 nm for cis-2 upon further


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Journal of the American Chemical Society addition of hexane to over 40% hexane-CH2Cl2 (v/v). The combined emission spectra of cis-2 in different hexane content (Figure S28, S30), clearly indicated a pure AIE effects. In contrast, trans-2 is non-emissive both in solution and in solid state (Figure S32), which is ascribed to the coordination sequence of 4,4'-bipy and tpt, as well as the electron deficient characteristic of tpt.

Figure 3. Emission spectrum and plot of maximum emission intensity and wavelength of trigonal metallaprism cis-1 (a-b). The photographs show the CH2Cl2 solution of cis-1 with increasing hexane content from 0% to 90% under 365 nm UV light (c). To demonstrate the formation of nanoaggregates in CH2Cl2/hexane solution, the morphology of the trigonal metalloprism cis-1 and cis-2 was investigated by transmission electron microscopy (TEM). In 80% hexane-CH2Cl2 mixture, cis-1 self-assemble into cross-linking netlike aggregates with size over 100 nm (Figure 4a), while cis-2 exhibit globular structures upon high hexane content (Figure 4c). Dynamic Light Scatting (DLS) experiments were conducted to measure the nanoaggregate sizes in different hexane content. Under higher hexane concentration, the size of nanoaggregates increases (Figure 4b and 4d). The rotation of pendant phenyl ring of Pt-corner leads to non-emission of herein reported triangular metalloprisms. To understand the dynamic feature of phenyl ring, we constructed cis-2 model containing only cationic skeleton for evaluating the dynamic feature of phenyl ring. Figure 5 illustrates the molecular structure and two optimized conformations of cis-2. Two minimum torsion energies were observed at 53° and -127°, demonstrating the existence of stable conformations. The rotation of phenyl rings are not completely free in dilute solution, but as a pendulum motion within a certain range centered at 53° or -127°, further confirming the availability of [Pt(HL)]-AIEgens.

Figure 4. TEM images prepared from a solution of cis-1 (a) and cis-2 (c) in 80% hexane-CH2Cl2 mixture. Size distributions of cis-1 (b) and cis-2 (d) in CH2Cl2/hexane mixtures were characterized by DLS. The percentages represent the hexane content. These triangular metalloprisms provide the ideal model to investigate the structure-property relationship of isostructural metallosupramolecules. Although the building blocks are the same, the luminescent properties of cis-n and trans-n are completely different in some cases. In tpb-based complexes cis-1 and trans-1, cis-trans structural changes induced little luminescence change. However, the luminescent properties of tpt-based cis-2 and trans-2 are completely different. The cis-2 showed obvious AIE phenomenon, while trans-2 is non-emissive. The combination of emissive [(PtL)2(4,4'-bipy)] arm with tpt generate the luminous cis-2. However, the prior combination of tpt and Pt would produce an emission quenching intermediate product [(Pt-L)3tpt], whose further combination with 4,4'-bipy still yield the nonemissive trans-2. The question here is how does the coordination sequence of tpt impose such significant influence on emissivity of the metallosupramolecular isomers? The geometric positioning (trans or cis) of the Pt-N bond with respect to the carbon donor governs its lability and length, thus the complex emission. DFT calculations were performed to estimate Pt-N bonds length. The trans-to-carbon Pt-N bonds invariably show bonds of extended lengths (~ 2.3Å), whereas, the other three bond distance are less than 2.1 Å (Figure 5, S39). This phenomenon could be further confirmed in the crystal structure analysis. As shown in S116b and S219, the trans-to-carbon Pt-N bonds are significantly longer than their cis counterparts (Figure S40). Both the strong “trans influence” from carbon donor and steric hindrance of pendent phenyl ring are deemed to be responsible for the elongated Pt-N bond. The next question is which factor plays a key role? The bond comparison of tridentate ligand-based system was also carried out (Figure S41). Although there is no steric group in S3, the Pt-N bond trans to carbon is still longer than the cis partner (2.096 vs. 2.020 Å with 0.076 Å difference).20 In S4, the presence of a steric group significantly raises the bond difference from 0.076 Å to 0.189 Å.21 This suggest that the PtN bond elongation is mainly governed by the trans carbon donor with additional contribution from the steric groups.



The kinetically inert character of the Pt-(C,N) corner units fosters the controlled synthesis of a pair of isomers whereas the steric group helps to widen the gap of the Pt-N lengths. The use of electron-deficient building blocks eventually completes the strategy in using this iso-structural system as model to exemplify the structure-property relationship of supramolecules. There are two main mechanisms of aggregation inducedemission in metal complexes: (i) aggregation restricted movement of flexible organic moieties on metal and (ii) aggregation facilitated metallophilic interactions. In the complexes reported here, the planes of coordinating pyridyl groups from both linear and trigonal ligands are perpendicular to the equatorial Pt plane. This precludes close proximity or strong Pt···Pt interactions. The AIE mechanism of cis-1, trans-1 and cis-2 are originated from aggregation induced rigidification of phenyl rings at the Pt-corners.

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Experimental details on the synthesis and characterization of cis-1, cis-2, trans-1 and trans-2, absorption and emission spectra and DFT calculations.

AUTHOR INFORMATION Corresponding Author [email protected] [email protected]

ORCID Sheng-li Huang: 0000-0003-2222-987X T. S. Andy Hor: 0000-0001-7533-1590

Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by “the Fundamental Research Funds for the Central Universities”. We acknowledge IMRE and A*STAR Computational Resource Center for infrastructure support.

REFERENCES

Figure 5. The optimized structure of cis-2 with rotor angle (N79-C80-C96-C99) at 53° (a) and -127° (b). We have assembled a series of triangular metalloprisms with functional [Pt(HL)] corner units using different coordination sequence. The emission spectra of tpb-based cis1 and trans-1 exhibit two characteristic bands traced to MLCT emission and AIE effects. Both these two kinds of emission are enhanced following the particle aggregation. However, the tptbased cis-2 and trans-2 complexes showed different characteristics that are traced to the --deficient building block tpt. Cis-2 is MLCT non-emissive but is AIE-active, while trans2 is non-emissive both in solution and solid state. DFT calculation indicated that the carbon donor and steric effect lead to elongated Pt-N bonds when the second ligand 4,4'-bipy approaches the metal. The effective block of electron-flow to the --deficient [(Pt-L)3(tpt)] results in the non-emissive trans2. Taking advantage the simple concepts of relatively inert PtN bond and trans effect of a carbon donor, we have successfully exploited the luminescence tuning in geometric isomers of metallosupramolecules. The use of dual emissive sources, different addition sequence in the experimental assembly, judicious choice of spacer linker, and very importantly, rotational control using a functional peripheral metal-corner has collectively created a model for further exploration in stereo-differentiating metallosupramolecular systems.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

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A Suite of Organoplatinum (II) Triangular Metalloprism: Ag-gregation

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