Letter pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Functionalized Helical Building Blocks for Nanoelectronics Khrystofor Khokhlov,‡ Nathaniel J. Schuster,‡ Fay Ng,*,‡ and Colin Nuckolls*,†,‡ †
Institute of Advanced Materials and Nanotechnology, The State Key Laboratory of Refractories and Metallurgy, School of Chemistry and Chemical Engineering, Wuhan University of Science and Technology, Wuhan 430081, China ‡ Department of Chemistry, Columbia University, New York, New York 10027, United States S Supporting Information *
ABSTRACT: Molecular building blocks are designed and created for the cis- and trans-dibrominated perylenediimides. The syntheses are simple and provide these useful materials on the gram scale. To demonstrate their synthetic versatility, these building blocks were used to create new dimeric perylenediimide helixes. Two of these helical dimers are twistacenes, and one is a helicene. Crucially, each possesses regiochemically defined functionality that allows the dimer helix to be elaborated into higher oligomers. It would be very difficult to prepare these helical PDI building blocks regioselectively without the methods described.
T
available, intensely absorbing, electron-deficient dye molecule,1,2 are ubiquitous in organic electronics and optoelectronics.3−8 Notably, the power conversion efficiencies of organic photovoltaics incorporating π-conjugated oligomers and polymers of PDI rival their fullerene-based counterparts.9−18 Narrowband photodetectors utilizing films of PDInanoribbons match the record-setting performances of those using single-crystal perovskites,19 and PDI-nanoribbons function as electron acceptors in highly efficient perovskite solar cells.20 The efficient preparation of PDI-based materials hinges on the facile preparation and isolation of useful building blocks. Direct bromination of the bays of PDI (positions 1, 6, 7, and 12 in Figure 1A,B) provides the swiftest route toward expanding the π-system for tailoring optical and electronic properties. Unfortunately, this dibromination of PDI at room temperature affords a 5:1 mixture of 1,7- to 1,6-dibromoPDI (trans-1 and cis-2, respectively; Figure 1B),21 which cannot be readily resolved: multiweek fractional crystallizations provide trans-1 exclusively.21,22 Alternative routes can also furnish pure trans-1 but require additional synthetic steps, protracted fractional crystallizations, and/or limited choice of R group.23,24 Consequently, some oligomerizations and polymerizations of PDI simply forego the separation of trans-1 and cis-2, resulting in regioirregularity.1,2,6,8,10,11 The lack of any method to easily produce large quantities of pure trans-1 or cis-2 (or their synthetic equivalents) inhibits the preparation and subsequent study of novel regiopure PDI-based materials.25−29 Here, we present highly regioselective, scalable syntheses of trans- and cis-building blocks (Figure 1C,D) for the preparation of regiopure PDI derivatives. These new building blocks, 1bromo-7-methoxyPDI (3), 1-bromo-6,12-dimethoxyPDI (4),
his Letter details regioselective, gram-scale syntheses of functionalized perylene-3,4,9,10-tetracarboxylic diimides (PDI, Figure 1A) for the construction of helical PDI-based building blocks for nanoelectronics. Variants of PDI, a readily
Figure 1. (A) Parent PDI, whose bay substituents can reside on the same naphthalene (cis configuration) or different naphthalenes (trans configuration). (B) 1,7- and 1,6-DibromoPDI (trans-1 and cis-2, respectively). (C) Compound 3 is a trans-building block. (D) Compounds 4 and 5 are cis-building blocks. In this work, R = CH(C5H11)2. © XXXX American Chemical Society
Received: February 13, 2018
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DOI: 10.1021/acs.orglett.8b00541 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 1. Building Block Synthesesa
and 1,6-dibromo-7-methoxyPDI (5), provide access to a wellspring of interesting macromolecular architectures for electronic and optoelectronic applications. To demonstrate the efficacy of these building blocks, we used 3 and 4 to prepare twistacene PDI dimers (hPDI2),7 such as ribbons 6b−d and 7b−d (Figure 2A), whose functional groups enable step-growth
a
For all syntheses, R = CH(C5H11)2. (A) Synthesis of the transbuilding block, 1-bromo-7-methoxyPDI (3), and cis-building block, 1,6-dibromo-7-methoxyPDI (5). (B) Synthesis of the cis-building block, 1-bromo-6,12-dimethoxyPDI (4).
Figure 2. (A) trans- and cis-Building blocks enable the synthesis of regiodefined hPDI2 twistacene ribbons 6 and 7. The bay substituents can reside on opposite sides (trans configuration) or on the same side (cis configuration) of hPDI2. (B) cis-Building block 4 allows preparation of functionalized PDI-helicene 8.
of 3 to yield 5 underscores the superior directing effect of the methoxy group via π-electron donation relative to that of the bromine substituent. Scheme 1B details the synthesis of 4, also a cis-building block. We accessed 4 from 3 by first substituting the bromine for a methoxy substituent to yield the dimethoxyPDI 9. Subsequent bromination afforded 4. An alternative synthesis of 4 is contained in the SI. In summary, the reactions that produce 3, 4, and 5 are highly regioselective and scalable, thereby enabling easy isolation of all three building blocks on a multigram scale. We tested the effectiveness of 3, 4, and 5 as trans- and cisbuilding blocks by synthesizing the trans- and cis-bis(triflate) hPDI2 isomers 6d and 7d (Figure 2A). For the trans-hPDI2 series (Scheme 2A), 3 reacted cleanly with trans-1,2-bis(tributystannyl)ethylene via Stille coupling to give 10 in 93% yield.39 Mallory photocyclization of 10 in an LED flow reactor with iodine as an oxidant provided the fused trans-dimethoxyhPDI2 (6b) in 89% yield.40 Through two high-yielding functional group manipulations (78% over the two steps), we formed 6d. The regiochemical purity of 6b (and consequently 6d) is ≥97% relative to 7b by HPLC analysis (Figure S3B).41 For the cis-hPDI2 series (Scheme 2B), we utilized a Stille coupling between the cis-building block 4 and the stannane 11 (see SI for its synthesis) to furnish 12. The subsequent cyclizations, which include both an oxidative photocyclization and a photocyclization that eliminates MeOH,35,36 produced 7b with regiochemical purity ≥96% (see Figure S3C). 42 Subsequent functional group manipulations provided hPDI2 7d in high overall yield. In summary, by using regiodefined building blocks 3 and 4, we dictated the synthesis of regiopure hPDI2. Creating 6b−d and 7b−d or their equivalents would be difficult without the use of the trans- and cis-building blocks, 3
oligomerization and further synthetic transformations. Moreover, we used 4 to create bay-functionalized PDI-dimer helicene 8 (Figure 2B), whose functionality allows it to be elaborated into extended helical π-systems with impressive chiroptical properties.30 Our preparation of these regiodefined PDI-dimer nanoribbons underscores the synthetic versatility of the trans- and cis-building blocks. Compounds 3, 4, and 5 originate from 1-methoxyPDI (Scheme 1). The methoxy group plays several roles: (1) it activates select sites in the bay of PDI for bromination; (2) it can be transformed into an active coupling partner, such as a trifluoromethanesulfonate (triflate), for palladium-catalyzed reactions;31−34 and (3) it can be eliminated during the photocyclizations that are used to prepare various nanoribbons.35,36 To synthesize 3, we brominated methoxy-PDI (Scheme 1A). This bromination is operationally trivial because it requires no catalyst and only a simple aqueous workup. We isolated 3 in 80% yield. The π-electron donating ability of the methoxy substituent directs bromination to the 7-position, resulting in the near-exclusive formation of the trans-product.37 Specifically, the selectivity for this bromination is 98:2 trans-to-cis [see the Supporting Information (SI) for HPLC analysis, Figure S1B]. We modulated the ratio of monobromination versus dibromination (the ratio of 3 versus 5 in Scheme 1A) by altering the number of equivalents of bromine and the duration of the reaction (see SI for details). For the synthesis of 5, we achieved a 96:4 ratio of 5 to its trans-isomer (see SI for HPLC analysis, Figure S2).38 The regioselectivity in the bromination B
DOI: 10.1021/acs.orglett.8b00541 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 2. Functionalized PDI-twistacenesa
a
Scheme 4. Synthesis of functionalized PDI-helicene 8
work, we have found that the iodine is not needed for the cyclizations that involve the elimination of methanol. We are using 8 to further extend the length of this helix. It would be difficult to produce this regiochemically defined functionality on the termini of the helix without using the chemistry described here. Because these helicenes are functionalized, they can be easily incorporated into supramolecular helices and conjugated, helical macromolecules that would be useful in a number of material applications. The chiral, optical, and chiroptical properties of the functionalized helicenes are the focus of ongoing and future research. This Letter describes operationally simple and scalable synthetic methods to create regiochemically well-defined PDI building blocks. Simple sequences of halogenation and nucleophilic aromatic substitution provide molecular replacements for 1,7- and 1,6-dibromoPDI with near-perfect regioselectivity. From these building blocks, we created two types of functionalized helixes: PDI-twistacene and PDIhelicene. These new helical building blocks are also easily synthesized and provide a path to creating larger PDItwistacene oligomers, PDI-helicene oligomers, and PDI-macrocycles45 with atomic precision.
(A) Synthesis of 6b−d. (B) Synthesis of 7b−d.
and 4/5. For example, the direct dibromination of the parent hPDI2 (6/7 X = H, Figure 2A) gives a 50:50 mixture of hPDI2 (6/7 X = Br, Figure 2A), which is difficult to separate.7 Here, we prepared 6d with atomic precision on a multigram scale. The functionality on the termini of the twistacene ribbons can be used to extend them in a step-growth fashion to create longer ribbons. For instance, the combination of cis-building block 5 with 11 would provide cis-dimethoxy-hPDI3 (the trimeric PDI). Nanoribbons 6 and 7 can be combined with other aryl substrates to offer a variety of new materials. However, their utility hinges on the efficacy of the triflates (6d and 7d) in transition-metal-catalyzed cross-coupling reactions.31−34 We found that these PDI-triflates undergo palladium-catalyzed cross-coupling reactions in high yield. For instance, 6d was subjected to Suzuki cross-coupling43 with thiophene-2-boronic acid pinacol ester to give the double-coupled product in 80% yield. Similarly, a Stille coupling of 2-(tributylstannyl)thiophene with 6d provided the same product in comparable yield (see Scheme S2A,B in the SI for details). Scheme 3 underscores the effectiveness of 3 as a transbuilding block. Demethylation of 3 unmasks 13 in 86% isolated
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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/acs.orglett.8b00541. Synthetic procedures, additional figures/schemes, and characterization data (PDF)
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Scheme 3. Synthetic Utility of the trans-Building Block 3
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Colin Nuckolls: 0000-0002-0384-5493 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Primary support of this research was provided by the Office of Naval Research (award no. N00014-16-1-2921 and N00014-171-2205). C.N. thanks Sheldon and Dorothea Buckler for their generous support. K.K. thanks Columbia University’s Rabi Scholars Program for support of this research through a generous stipend. The Columbia University Shared Materials Characterization Laboratory (SMCL) was used extensively for this research. We are grateful to Columbia University for support of this facility.
yield. Triflation of 13 produces 14, which readily undergoes Suzuki cross-coupling to the known 1,7-diphenylPDI 15.44 The building block 14 will allow longer oligomers of the hPDI series to be created. These hPDI oligomers will also carry useful and well-defined functionality. For example, the combination of 14 with 11 would provide trans-dimethoxyhPDI3. Scheme 4 highlights the utility of 4 as a cis-building block. The photocyclization of 16 eliminates methanol to give the racemic dimethoxyPDI helicene 8. In subsequent unpublished C
DOI: 10.1021/acs.orglett.8b00541 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
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(32) Jutand, A.; Mosleh, A. Organometallics 1995, 14, 1810. (33) Farina, V.; Krishnan, B.; Marshall, D. R.; Roth, G. P. J. Org. Chem. 1993, 58, 5434. (34) Lutz, C.; Bleicher, K. H. Tetrahedron Lett. 2002, 43, 2211. (35) Mallory, F. B.; Rudolph, M. J.; Oh, S. M. J. Org. Chem. 1989, 54, 4619. (36) Jørgensen, K. B. Molecules 2010, 15, 4334. (37) Zhang, X.; Zhan, C.; Zhang, X.; Yao, J. Tetrahedron 2013, 69, 8155. (38) The structure determination of 5 was supported by relay synthesis to a PDI derivative with an established structure (see SI Scheme for details). (39) Milstein, J.; Stille, J. K. J. Am. Chem. Soc. 1978, 100, 3636. (40) For specifics on the LED flow reactor conditions, see SI of ref 16 (Sisto et al.). (41) The minor regioisomer cis-7b arises from the combination of 2% cis-3 with 98% trans-3 during the Stille coupling reaction; the selfcoupling of trans-3 and cis-3 creates the trans-hPDI2 6b. (42) The minor regioisomer is trans-6b, stemming from the combination of 2% cis-isomer of 11 (originating from 3). (43) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457. (44) Ball, M.; Zhong, Y.; Fowler, B.; Zhang, B.; Li, P.; Etkin, G.; Paley, D. W.; Decatur, J.; Dalsania, A. K.; Li, H.; Xiao, S.; Ng, F.; Steigerwald, M. L.; Nuckolls, C. J. Am. Chem. Soc. 2016, 138, 12861. (45) Ball, M.; Fowler, B.; Li, P.; Joyce, L. A.; Li, F.; Liu, T.; Paley, D.; Zhong, Y.; Li, H.; Xiao, S.; Ng, F.; Steigerwald, M. L.; Nuckolls, C. J. Am. Chem. Soc. 2015, 137, 9982.
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DOI: 10.1021/acs.orglett.8b00541 Org. Lett. XXXX, XXX, XXX−XXX
