Letter pubs.acs.org/macroletters
Soluble PtII-Containing Polymers Based on a 2,6-Bis(1H‑1,2,3-triazol4-yl)-4-ethynylpyridine Ligand Benjamin Schulze,†,‡ Andreas Winter,‡,§,∥ Christian Friebe,‡,§,∥ Eckhard Birckner,⊥ and Ulrich S. Schubert*,‡,§,∥ ‡
Laboratory of Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, Humboldtstr. 10, 07743 Jena, Germany § Jena Center for Soft Matter (JCSM), Philosophenweg 7, 07743 Jena, Germany ∥ Center for Energy and Environmental Chemistry Jena (CEEC Jena), Philosophenweg 7a, 07743 Jena, Germany ⊥ Institute of Physical Chemistry (IPC), Friedrich Schiller University Jena, Lessingstr. 10, 07743 Jena, Germany S Supporting Information *
ABSTRACT: A soluble PtII-acetylide polymer 6 was prepared in a supramolecular polymerization of a PtII-chloro precursor complex 5, which represents a selfcomplementary AB-type monomer. This new type of polymer combines the structural features of the common polyplatinynes and cationic PtII-acetylide complexes. Though the photophysical and electrochemical properties of the material still need to be advanced, a versatile and straightforward method for the preparation of soluble, cationic PtII-acetylide polymers with phosphorescent behavior is offered.
L
inear PtII-containing polymers, in particular, the rigid-rodlike PtII-acetylide polymers (I), represent an important class of materials within the broad field of metallopolymer science due to their rich photophysical properties (Chart 1).1−3 Making use of these, their utilization in various types of optoelectronic applications has been reported (e.g., lightemitting diodes, photovoltaics, or field-effect transistors). Since the heavy PtII ion enables an efficient intersystem crossing,
relatively intense emission originating from triplet excited states is typically observed. This feature is also found in the squareplanar complexes with a tridentate and an acetylide ligand in the coordination sphere of the PtII ion (II);4,5 typically, derivatives of 2,2′:6′,2″-terpyridine (tpy) are used in this respect, though some structural analogs, such as 2,6-bis(1H1,2,3-triazol-4-yl)pyridines (tripy or btp), have recently also attracted attention (Chart 1).6−11 Such complexes have, for instance, been utilized to form stable, charged metallogels with rich luminescence properties.12,13 Moreover, PtII-acetylide complexes have successfully been established as photosensitzers in the field of water reduction.14,15 In order to combine both types of materials, that is, the PtII-acetylide polymers and the PtII-tpy-acetylide complexes, the groups of Ziessel and Che followed a step-controlled protocol to obtain oligomeric PtII complexes (III with n < 4; Chart 1).16−19 However, the considerably high synthetic effort (resulting from the stepwise assembly strategy) and the expected decrease in solubility with increasing chain length (due to the absence of any solubilizing substituents) make this approach ineligible when aiming for higher oligomers or even polymers. According to the models for (metallo-)supramolecular polymerizations, polymers, such as III, can be obtained from
Chart 1. Schematic Representation of Materials Containing PtII-Acetylide Units
Received: December 7, 2016 Accepted: February 1, 2017
© XXXX American Chemical Society
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DOI: 10.1021/acsmacrolett.6b00936 ACS Macro Lett. 2017, 6, 181−184
Letter
ACS Macro Letters Scheme 1. Schematic Representation of the Synthesis of the PtII Metallopolymer 6a
Reaction conditions: (a) (i) Pd(PPh3)4, CuI, LiCl, NEt3, THF; (ii) 2 equiv 2-methylbut-3-yn-2-ol, 0 °C to room temperature, 48 h; (iii) 1.5 equiv TiPS-acetylene, 50 °C, 12 h. (b) NaH, dry toluene, 1 h reflux. (c) EtHex-N3, CuSO4, NaAscorbate, CH2Cl2/EtOH/H2O, 16 h, 40 °C. (d) Pt(COD)Cl2, AgPF6, CH2Cl2, 12 h, rt. (e) (i) 1.2 equiv Bu4NF, CuI (10 mol %), CH2Cl2/NEt3, 48 h, rt; (ii) 1.2 equiv Bu4NF, THF, 48 h, rt.
a
AB-type monomers.20−22 Considering an isodesmic supramolecular polymerization, both a high monomer concentration and a high equilibrium constant are generally required when aiming for high degrees of polymerization (DPs). The advantage of utilizing a self-complementary AB-type monomer is the already intrinsically given ideal stoichiometry of functional groups as the third precondition. Following this general concept, the synthesis of a PtIIacetylide polymer similar to III via a supramolecular polymerization approach was targeted. For this purpose, the cationic PtII complex 5 was chosen as designated AB-type monomer to undergo a polycondensation reaction by formally eliminating a silyl chloride (Scheme 1). Though tpy derivatives are commonly used as ligands for PtII ions, we opted for a tripytype one that bears solubilizing groups but keeps the structure simple and rigid (as in III). The synthesis of the PtII metallopolymer 6 was accomplished following a linear bottom-up procedure, as outlined in Scheme 1. In detail, 2,4,6-tribromopyridine (1) was converted to the protected tris-ethynyl derivative 2 making use of a stepwise Sonogashira cross-coupling procedure. The subtle differences in reactivity of the bromo-substituents of 1 (i.e., increased reactivity for the positions 2 and 6 vs 4)23 enabled the synthesis of 2 with 60% yield. The subsequent selective deprotection with NaH in toluene afforded the 2,6-diethynyl species 3, which was utilized further in a Cu-catalyzed azide− alkyne cycloaddition (CuAAC) reaction (68% over both steps). The obtained bis(triazolyl)pyridine ligand 4 was coordinated to a PtII center with 80% yield. The resulting PtII monomer 5 was thoroughly characterized by NMR spectroscopy and mass spectrometry (see the SI). As shown recently by De Cola and co-workers,9 such compounds are hardly emissive in aerated solutions and weakly emissive under deaerated conditions at room temperature but may feature intense solid-state emission arising from dimers formed due to metallophilic Pt−Pt interactions. The photophysical properties of 5 in solution and in the solid state are in line with those discussed in literature (Figure 1): When fabricated into a thin solid film by spin-coating, 5 becomes highly emissive (λem = 605 nm). Likewise, the bulk material features a bright orange solid-state emission. The solid-state absorption spectrum of 5 is more
Figure 1. UV−vis absorption and emission spectra of 5 in solution (10−6 M in CH2Cl2) and in the solid state. The picture shows the bright solid-state emission of 5 arising from Pt−Pt interactions.
complicated and strongly red-shifted compared to the one measured in solution, which is basically attributed to stacking of the planar molecules involving metallophilic interactions.24 In order to establish a reliable protocol for the synthesis of 6 by polymerization of the self-complementary AB-type monomer 5, two important issues had to be addressed: Efficient deprotection of the TiPS-protected alkyne and abstraction of the chloro-ligand from the PtII center. Moreover, precipitation in course of the polycondensation reaction as well as homocoupling of the free ethynyl moieties had to be avoided. Typically, PtII-acetylide compounds, small molecules as well as polymers, are prepared from terminal alkynes via Hagihara’s method using catalytic amounts of CuI and an amine base.3,22 Alternatively, silyl-protected alkynes might be also used for the metalation of the PtII center (in situ deprotection with KF).25 The acetylide transfer to PtII with preformed AgI-acetylides represents one further promising strategy in this respect though it has not yet been applied in the synthesis of metallopolymers.26 Commonly, fluoride salts, such as Bu4NF or AgF, are employed for the cleavage of the TiPS-alkyne protecting group.27 On the basis of all these methods, a screening was performed to elaborate the most appropriate reaction conditions for the polymerization of 5 (for details, see the SI). In short, Bu4NF was found to be necessary to achieve a sufficient reaction; from the tested conditions, the system CuI/ 182
DOI: 10.1021/acsmacrolett.6b00936 ACS Macro Lett. 2017, 6, 181−184
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ACS Macro Letters
ligand; accordingly, the low-energy absorption band with moderate intensity (λabs = 396 nm) arises from metal-to-ligand charge-transfer (MLCT) [dπ(Pt) → π*(ethynyl-tripy)] transitions (Figure 3a). Moreover, 6b is emissive at room
NEt3/Bu4NF yielded 6a as a defined, low-molar-mass material after 48 h (Scheme 1, Table 1). The DP was increased Table 1. Polymer Characterization by SECa 5 6a 6b
Mn (g mol−1)
Đ
DP
550 3400 7100
1.03 1.78 2.56
1 6 13
a
SEC conditions: DMAc containing 0.08 wt % NH4PF6 as eluent, linear polystyrene calibration.
significantly when an excess of Bu4NF was used: 6b, with an estimated DP of 13, was obtained after 96 h (Table 1). On the contrary, when using AgF/Bu4NF as reagent, partial precipitation was observed and an only ill-defined material could be isolated. It was further checked if an analogous TMS-protected monomer 7 can be utilized instead (for details on the synthesis of 7, see the SI). The more labile TMS group was introduced to ease the deprotection step and, thereby, facilitate the entire polymerization. However, when using DMF/NEt3/CH3OH and catalytic CuI, precipitation was observed upon heating, presumably due to the formation of an insoluble high-molarmass polymer. The polymers 6 were investigated by NMR spectroscopy and showed the characteristic broadened signals in the 1H NMR spectra (see the SI). Since signals arising from the end groups could not be identified clearly, a calculation of the molar mass from these spectra was obsolete. Due to the high kinetic stability of 6, size-exclusion chromatography (SEC) could be utilized to estimate the molar mass (distribution) on the basis of a linear polystyrene calibration.22 The 2D-SEC data obtained in N,N-dimethylacetamide (DMAc) containing 0.08 wt % NH4PF6 are summarized in Table 1. Moreover, 3D-SEC traces were recorded using a photodiode array (PDA) detector in order to correlate the 2D-SEC data with the UV−vis absorption behavior. This analytical method confirmed the presence of the cationic PtII-acetylide entities within the polymer chain (Figure 2 and SI). The photophysical and electrochemical properties of 6b were also studied. With reference to spectroscopic work on structurally related small-molecule complexes,11 the intense high-energy absorption bands (λabs < 320 nm) were assigned to intraligand (IL) [π → π*] transitions of the ethynyl-tripy
Figure 3. (a) UV−vis absorption, emission (λexc = 395 nm) and excitation spectra (λem = 570 nm) of 6b (emission spectra were recorded under N2-purged conditions); for all spectra: 10−6 M in CH3CN, room temperature. (b) UV−vis absorption and emission (λexc = 385 nm) spectra of 6b in the solid state (thin drop-casted film on glass slide).
temperature with a lifetime (τ) and quantum yield (ΦPL) of 0.8 μs and 2.0% in N2-purged solution, respectively. Thin films of 6b were prepared by drop-casting onto glass slides. The absorption and emission spectra of the films were, aside from a small bathochromic shift of the emission signal, very similar to those obtained from dilute solutions (Figure 3b). Thus, the polymer chains did presumably not show the pronounced stacking behavior in the solid state as seen for the monomer 5 or Rowan’s supramolecular polymers.28 This suggests that the PtII sites along the polymer chain were not arranged in a coplanar fashion. However, limited stacking at the more accessible chain ends cannot be fully excluded. The electrochemical behavior of 6b was studied by cyclic voltammetry (CV) measurements in CH3CN containing 0.1 M Bu4NPF6 as supporting electrolyte (see the SI). For 6b, an irreversible anodic wave at about +1.55 V versus Fc+/Fc was found corresponding to the oxidation of the metal center to the +III state.11 The monomer 5 displays a reversible reduction at −1.35 V, whereas the reversibility of this process is expressed only poorly for 6b (E = −0.92 V). UV−vis spectroelec-
Figure 2. 3D-SEC plot of 6b (eluent: DMAc containing 0.08 wt % NH4PF6, linear polystyrene calibration). 183
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(8) Byrne, J. P.; Kitchen, J. A.; Kotova, O.; Leigh, V.; Bell, A. P.; Boland, J. J.; Albrecht, M.; Gunnlaugsson, T. Dalton Trans. 2014, 43, 196−209. (9) Allampally, N. K.; Daniliuc, C.-G.; Strassert, C. A.; De Cola, L. Inorg. Chem. 2015, 54, 1588−1596. (10) Schulze, B.; Schubert, U. S. Chem. Soc. Rev. 2014, 43, 2522− 2571. (11) Li, Y.; Zhao, L.; Tam, A. Y.-Y.; Wong, K. M.-C.; Wu, L.; Yam, V. W.-W. Chem. - Eur. J. 2013, 19, 14496−14505. (12) Tam, A. Y.-Y.; Wong, K. M.-C.; Yam, V. W.-W. J. Am. Chem. Soc. 2009, 131, 6253−6260. (13) Camerel, F.; Ziessel, R.; Donnio, B.; Bourgogne, C.; Guillon, D.; Schmutz, M.; Iacovita, C.; Bucher, J.-P. Angew. Chem., Int. Ed. 2007, 46, 2659−2662. (14) Du, P.; Schneider, J.; Jarosz, P.; Eisenberg, R. J. Am. Chem. Soc. 2006, 128, 7726−7727. (15) Archer, S.; Weinstein, J. A. Coord. Chem. Rev. 2012, 256, 2530− 2561. (16) Ziessel, R.; Diring, S. Tetrahedron Lett. 2006, 47, 4687−4692. (17) Ziessel, R.; Diring, S.; Kadjane, P.; Charbonnière, L.; Retailleau, P.; Philouze, C. Chem. - Asian J. 2007, 2, 975−982. (18) Muro, M. L.; Diring, S.; Wang, X.; Ziessel, R.; Castellano, F. N. Inorg. Chem. 2008, 47, 6796−6803. (19) Kui, S. C. F.; Law, Y. C.; Tong, G. S. M.; Lu, W.; Yuen, M.-Y.; Che, C.-M. Chem. Sci. 2011, 2, 221−228. (20) De Greef, T. F. A.; Smulders, M. M. J.; Wolffs, M.; Schenning, A. P. H. J.; Sijbesma, R. P.; Meijer, E. W. Chem. Rev. 2009, 109, 5687− 5754. (21) Winter, A.; Hager, M. D.; Schubert, U. S. In Polymer Science: A Comprehensive Review; Matyjaszewski, K., Möller, M., Eds.; Elsevier B.V.: Amsterdam, 2012; Vol. 5.13, pp 269−310. (22) Winter, A.; Schubert, U. S. Chem. Soc. Rev. 2016, 45, 5311− 5357. (23) Schulze, B.; Friebe, C.; Hoeppener, S.; Pavlov, G. M.; Winter, A.; Hager, M. D.; Schubert, U. S. Macromol. Rapid Commun. 2012, 33, 597−602. (24) Williams, J. A. G. Top. Curr. Chem. 2007, 281, 205−268. (25) Fan, Y.; Zhu, Y.-M.; Dai, F.-R.; Zhang, L.-Y.; Chen, Z.-N. Dalton Trans. 2007, 3885−3892. (26) Ji, Z.; Azenkeng, A.; Hoffmann, M.; Sun, W. Dalton Trans. 2009, 7725−7733. (27) Kim, S.; Kim, B.; In, J. Synthesis 2009, 2009, 1963−1968. (28) Lee, S. W.; Kumpfer, J. R.; Lin, P. A.; Li, G.; Gao, X. P. A.; Rowan, S. J.; Sankaran, R. M. Macromolecules 2012, 45, 8201−8210.
trochemical measurements were carried out to further investigate the oxidation and reduction processes of 6b (see the SI). Within the time scale of the measurements both processes revealed irreversible character. To conclude, a PtII-tripy-acetylide polymer (6) has been prepared in a straightforward fashion using a preformed protected PtII complex as monomer (5). Thereby, two common limitations, namely low solubility and high synthetic effort, could be bypassed. Though the photophysical and electrochemical properties of 6 still need to be improved, the ease of processing and the film-forming ability as well as the observed solid-state emission indicate the high potential of such polymers for integration into optoelectronic devices, similar to the already established polyplatinyne-type materials.
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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/acsmacrolett.6b00936. Experimental details and characterization data for all compounds (PDF).
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AUTHOR INFORMATION
Corresponding Author
*Fax: +49 3641 948202. E-mail:
[email protected]. Website: www.schubert-group.de. ORCID
Ulrich S. Schubert: 0000-0003-4978-4670 Present Address †
NOVALED GmbH, Tatzberg 49, 01307 Dresden, Germany.
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS B.S. and C.F. are grateful to the Fonds der Chemischen Industrie for a Ph.D. scholarship. Financial support by the Deutsche Forschungsgemeinschaft (DFG, Grant No. SCHU1229-16/1) is kindly acknowledged. The authors also thank Nicole Fritz (ESI MS), Grit Festag (SEC), and Gabriele Sentis (NMR) for the help with the respective measurements.
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REFERENCES
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