Functionalized Helical Building Blocks for NanoelectronicsKhrystofor 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 Chemistryand 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 andcreated for the cis- and trans-dibrominated perylenediimides.The syntheses are simple and provide these useful materials onthe gram scale. To demonstrate their synthetic versatility, thesebuilding blocks were used to create new dimeric perylenedii-mide helixes. Two of these helical dimers are twistacenes, andone is a helicene. Crucially, each possesses regiochemicallydefined functionality that allows the dimer helix to beelaborated into higher oligomers. It would be very difficultto prepare these helical PDI building blocks regioselectively without the methods described.

This Letter details regioselective, gram-scale syntheses offunctionalized perylene-3,4,9,10-tetracarboxylic diimides

(PDI, Figure 1A) for the construction of helical PDI-basedbuilding blocks for nanoelectronics. Variants of PDI, a readily

available, intensely absorbing, electron-deficient dye mole-cule,1,2 are ubiquitous in organic electronics and optoelec-tronics.3−8 Notably, the power conversion efficiencies oforganic photovoltaics incorporating π-conjugated oligomersand polymers of PDI rival their fullerene-based counter-parts.9−18 Narrowband photodetectors utilizing films of PDI-nanoribbons match the record-setting performances of thoseusing single-crystal perovskites,19 and PDI-nanoribbons func-tion as electron acceptors in highly efficient perovskite solarcells.20

The efficient preparation of PDI-based materials hinges onthe facile preparation and isolation of useful building blocks.Direct bromination of the bays of PDI (positions 1, 6, 7, and 12in Figure 1A,B) provides the swiftest route toward expandingthe π-system for tailoring optical and electronic properties.Unfortunately, this dibromination of PDI at room temperatureaffords a 5:1 mixture of 1,7- to 1,6-dibromoPDI (trans-1 andcis-2, respectively; Figure 1B),21 which cannot be readilyresolved: multiweek fractional crystallizations provide trans-1exclusively.21,22 Alternative routes can also furnish pure trans-1but require additional synthetic steps, protracted fractionalcrystallizations, and/or limited choice of R group.23,24

Consequently, some oligomerizations and polymerizations ofPDI simply forego the separation of trans-1 and cis-2, resultingin regioirregularity.1,2,6,8,10,11 The lack of any method to easilyproduce large quantities of pure trans-1 or cis-2 (or theirsynthetic equivalents) inhibits the preparation and subsequentstudy of novel regiopure PDI-based materials.25−29

Here, we present highly regioselective, scalable syntheses oftrans- and cis-building blocks (Figure 1C,D) for the preparationof regiopure PDI derivatives. These new building blocks, 1-bromo-7-methoxyPDI (3), 1-bromo-6,12-dimethoxyPDI (4),

Received: February 13, 2018Published: March 14, 2018

Figure 1. (A) Parent PDI, whose bay substituents can reside on thesame naphthalene (cis configuration) or different naphthalenes (transconfiguration). (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.

Letter

pubs.acs.org/OrgLettCite This: Org. Lett. 2018, 20, 1991−1994

© 2018 American Chemical Society 1991 DOI: 10.1021/acs.orglett.8b00541Org. Lett. 2018, 20, 1991−1994

and 1,6-dibromo-7-methoxyPDI (5), provide access to awellspring of interesting macromolecular architectures forelectronic and optoelectronic applications. To demonstratethe efficacy of these building blocks, we used 3 and 4 to preparetwistacene PDI dimers (hPDI2),7 such as ribbons 6b−d and7b−d (Figure 2A), whose functional groups enable step-growth

oligomerization and further synthetic transformations. More-over, we used 4 to create bay-functionalized PDI-dimerhelicene 8 (Figure 2B), whose functionality allows it to beelaborated into extended helical π-systems with impressivechiroptical properties.30 Our preparation of these regiodefinedPDI-dimer nanoribbons underscores the synthetic versatility ofthe trans- and cis-building blocks.Compounds 3, 4, and 5 originate from 1-methoxyPDI

(Scheme 1). The methoxy group plays several roles: (1) itactivates select sites in the bay of PDI for bromination; (2) itcan be transformed into an active coupling partner, such as atrifluoromethanesulfonate (triflate), for palladium-catalyzedreactions;31−34 and (3) it can be eliminated during thephotocyclizations that are used to prepare various nanorib-bons.35,36

To synthesize 3, we brominated methoxy-PDI (Scheme 1A).This bromination is operationally trivial because it requires nocatalyst and only a simple aqueous workup. We isolated 3 in80% yield. The π-electron donating ability of the methoxysubstituent directs bromination to the 7-position, resulting inthe near-exclusive formation of the trans-product.37 Specifically,the selectivity for this bromination is 98:2 trans-to-cis [see theSupporting Information (SI) for HPLC analysis, Figure S1B].We modulated the ratio of monobromination versus

dibromination (the ratio of 3 versus 5 in Scheme 1A) byaltering the number of equivalents of bromine and the durationof the reaction (see SI for details). For the synthesis of 5, weachieved a 96:4 ratio of 5 to its trans-isomer (see SI for HPLCanalysis, Figure S2).38 The regioselectivity in the bromination

of 3 to yield 5 underscores the superior directing effect of themethoxy group via π-electron donation relative to that of thebromine substituent.Scheme 1B details the synthesis of 4, also a cis-building block.

We accessed 4 from 3 by first substituting the bromine for amethoxy substituent to yield the dimethoxyPDI 9. Subsequentbromination afforded 4. An alternative synthesis of 4 iscontained in the SI. In summary, the reactions that produce 3,4, and 5 are highly regioselective and scalable, thereby enablingeasy isolation of all three building blocks on a multigram scale.We tested the effectiveness of 3, 4, and 5 as trans- and cis-

building blocks by synthesizing the trans- and cis-bis(triflate)hPDI2 isomers 6d and 7d (Figure 2A). For the trans-hPDI2series (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 reactorwith iodine as an oxidant provided the fused trans-dimethoxy-hPDI2 (6b) in 89% yield.40 Through two high-yieldingfunctional group manipulations (78% over the two steps), weformed 6d. The regiochemical purity of 6b (and consequently6d) is ≥97% relative to 7b by HPLC analysis (Figure S3B).41

For the cis-hPDI2 series (Scheme 2B), we utilized a Stillecoupling between the cis-building block 4 and the stannane 11(see SI for its synthesis) to furnish 12. The subsequentcyclizations, which include both an oxidative photocyclizationand a photocyclization that eliminates MeOH,35,36 produced 7bwith regiochemical purity ≥96% (see Figure S3C).42

Subsequent functional group manipulations provided hPDI27d in high overall yield. In summary, by using regiodefinedbuilding blocks 3 and 4, we dictated the synthesis of regiopurehPDI2.Creating 6b−d and 7b−d or their equivalents would be

difficult without the use of the trans- and cis-building blocks, 3

Figure 2. (A) trans- and cis-Building blocks enable the synthesis ofregiodefined hPDI2 twistacene ribbons 6 and 7. The bay substituentscan reside on opposite sides (trans configuration) or on the same side(cis configuration) of hPDI2. (B) cis-Building block 4 allowspreparation of functionalized PDI-helicene 8.

Scheme 1. Building Block Synthesesa

aFor all syntheses, R = CH(C5H11)2. (A) Synthesis of the trans-building block, 1-bromo-7-methoxyPDI (3), and cis-building block,1,6-dibromo-7-methoxyPDI (5). (B) Synthesis of the cis-buildingblock, 1-bromo-6,12-dimethoxyPDI (4).

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and 4/5. For example, the direct dibromination of the parenthPDI2 (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 canbe used to extend them in a step-growth fashion to createlonger ribbons. For instance, the combination of cis-buildingblock 5 with 11 would provide cis-dimethoxy-hPDI3 (thetrimeric PDI).Nanoribbons 6 and 7 can be combined with other aryl

substrates to offer a variety of new materials. However, theirutility hinges on the efficacy of the triflates (6d and 7d) intransition-metal-catalyzed cross-coupling reactions.31−34 Wefound that these PDI-triflates undergo palladium-catalyzedcross-coupling reactions in high yield. For instance, 6d wassubjected to Suzuki cross-coupling43 with thiophene-2-boronicacid pinacol ester to give the double-coupled product in 80%yield. Similarly, a Stille coupling of 2-(tributylstannyl)thiophenewith 6d provided the same product in comparable yield (seeScheme S2A,B in the SI for details).Scheme 3 underscores the effectiveness of 3 as a trans-

building block. Demethylation of 3 unmasks 13 in 86% isolated

yield. Triflation of 13 produces 14, which readily undergoesSuzuki cross-coupling to the known 1,7-diphenylPDI 15.44 Thebuilding block 14 will allow longer oligomers of the hPDIseries to be created. These hPDI oligomers will also carryuseful and well-defined functionality. For example, thecombination of 14 with 11 would provide trans-dimethoxy-hPDI3.Scheme 4 highlights the utility of 4 as a cis-building block.

The photocyclization of 16 eliminates methanol to give theracemic dimethoxyPDI helicene 8. In subsequent unpublished

work, we have found that the iodine is not needed for thecyclizations that involve the elimination of methanol. We areusing 8 to further extend the length of this helix. It would bedifficult to produce this regiochemically defined functionalityon the termini of the helix without using the chemistrydescribed here. Because these helicenes are functionalized, theycan be easily incorporated into supramolecular helices andconjugated, helical macromolecules that would be useful in anumber of material applications. The chiral, optical, andchiroptical properties of the functionalized helicenes are thefocus of ongoing and future research.This Letter describes operationally simple and scalable

synthetic methods to create regiochemically well-defined PDIbuilding blocks. Simple sequences of halogenation andnucleophilic aromatic substitution provide molecular replace-ments for 1,7- and 1,6-dibromoPDI with near-perfectregioselectivity. From these building blocks, we created twotypes of functionalized helixes: PDI-twistacene and PDI-helicene. These new helical building blocks are also easilysynthesized and provide a path to creating larger PDI-twistacene oligomers, PDI-helicene oligomers, and PDI-macro-cycles45 with atomic precision.

■ ASSOCIATED CONTENT*S Supporting Information

The Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.or-glett.8b00541.

Synthetic procedures, additional figures/schemes, andcharacterization data (PDF)

■ AUTHOR INFORMATIONCorresponding Authors

*E-mail: fwn2@columbia.edu.*E-mail: cn37@columbia.edu.ORCID

Colin Nuckolls: 0000-0002-0384-5493Notes

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTSPrimary support of this research was provided by the Office ofNaval Research (award no. N00014-16-1-2921 and N00014-17-1-2205). C.N. thanks Sheldon and Dorothea Buckler for theirgenerous support. K.K. thanks Columbia University’s RabiScholars Program for support of this research through agenerous stipend. The Columbia University Shared MaterialsCharacterization Laboratory (SMCL) was used extensively forthis research. We are grateful to Columbia University forsupport of this facility.

Scheme 2. Functionalized PDI-twistacenesa

a(A) Synthesis of 6b−d. (B) Synthesis of 7b−d.

Scheme 3. Synthetic Utility of the trans-Building Block 3

Scheme 4. Synthesis of functionalized PDI-helicene 8

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■ REFERENCES(1) Wurthner, F.; Saha-Moller, C. R.; Fimmel, B.; Ogi, S.;Leowanawat, P.; Schmidt, D. Chem. Rev. 2016, 116, 962.(2) Huang, C.; Barlow, S.; Marder, S. R. J. Org. Chem. 2011, 76, 2386.(3) Anthony, J. E.; Facchetti, A.; Heeney, M.; Marder, S. R.; Zhan, X.Adv. Mater. 2010, 22, 3876.(4) Liu, Z.; Zhang, G.; Cai, Z.; Chen, X.; Luo, H.; Li, Y.; Wang, J.;Zhang, D. Adv. Mater. 2014, 26, 6965.(5) Jiang, W.; Li, Y.; Wang, Z. Acc. Chem. Res. 2014, 47, 3135.(6) Zhan, X.; Facchetti, A.; Barlow, S.; Marks, T. J.; Ratner, M. A.;Wasielewski, M. R.; Marder, S. R. Adv. Mater. 2011, 23, 268.(7) Zhong, Y.; Kumar, B.; Oh, S.; Trinh, M. T.; Wu, Y.; Elbert, K.; Li,P.; Zhu, X.; Xiao, S.; Ng, F.; Steigerwald, M. L.; Nuckolls, C. J. Am.Chem. Soc. 2014, 136, 8122.(8) Feng, J.; Jiang, W.; Wang, Z. Chem. - Asian J. 2018, 13, 20.(9) Zhong, Y.; Trinh, M. T.; Chen, R.; Purdum, G. E.; Khlyabich, P.P.; Sezen, M.; Oh, S.; Zhu, H.; Fowler, B.; Zhang, B.; Wang, W.; Nam,C. Y.; Sfeir, M. Y.; Black, C. T.; Steigerwald, M. L.; Loo, Y. L.; Ng, F.;Zhu, X. Y.; Nuckolls, C. Nat. Commun. 2015, 6, 8242.(10) Guo, Y.; Li, Y.; Awartani, O.; Han, H.; Zhao, J.; Ade, H.; Yan,H.; Zhao, D. Adv. Mater. 2017, 29, 1700309.(11) Guo, Y.; Li, Y.; Awartani, O.; Zhao, J.; Han, H.; Ade, H.; Zhao,D.; Yan, H. Adv. Mater. 2016, 28, 8483.(12) Gao, G.; Liang, N.; Geng, H.; Jiang, W.; Fu, H.; Feng, J.; Hou, J.;Feng, X.; Wang, Z. J. Am. Chem. Soc. 2017, 139, 15914.(13) Zhang, J.; Li, Y.; Huang, J.; Hu, H.; Zhang, G.; Ma, T.; Chow, P.C. Y.; Ade, H.; Pan, D.; Yan, H. J. Am. Chem. Soc. 2017, 139, 16092.(14) Wu, Q.; Zhao, D.; Schneider, A. M.; Chen, W.; Yu, L. J. Am.Chem. Soc. 2016, 138, 7248.(15) Meng, D.; Sun, D.; Zhong, C.; Liu, T.; Fan, B.; Huo, L.; Li, Y.;Jiang, W.; Choi, H.; Kim, T.; Kim, J. Y.; Sun, Y.; Wang, Z.; Heeger, A.J. J. Am. Chem. Soc. 2016, 138, 375.(16) Sisto, T. J.; Zhong, Y.; Zhang, B.; Trinh, M. T.; Miyata, K.;Zhong, X.; Zhu, X. Y.; Steigerwald, M. L.; Ng, F.; Nuckolls, C. J. Am.Chem. Soc. 2017, 139, 5648.(17) Zhan, C.; Yao, J. Chem. Mater. 2016, 28, 1948.(18) Meng, D.; Fu, H.; Xiao, C.; Meng, X.; Winands, T.; Ma, W.;Wei, W.; Fan, B.; Huo, L.; Doltsinis, N. L.; Li, Y.; Sun, Y.; Wang, Z. J.Am. Chem. Soc. 2016, 138, 10184.(19) Zhong, Y.; Sisto, T. J.; Zhang, B.; Miyata, K.; Zhu, X. Y.;Steigerwald, M. L.; Ng, F.; Nuckolls, C. J. Am. Chem. Soc. 2017, 139,5644.(20) Castro, E.; Sisto, T. J.; Romero, E. L.; Liu, F.; Peurifoy, S. R.;Wang, J.; Zhu, X.; Nuckolls, C.; Echegoyen, L. Angew. Chem., Int. Ed.2017, 56, 14648.(21) Rajasingh, P.; Cohen, R.; Shirman, E.; Shimon, L. J. W.;Rybtchinski, B. J. Org. Chem. 2007, 72, 5973.(22) Wurthner, F.; Stepanenko, V.; Chen, Z.; Saha-Moller, C. R.;Kocher, N.; Stalke, D. J. Org. Chem. 2004, 69, 7933.(23) Sengupta, S.; Dubey, R. K.; Hoek, R. W. M.; Van Eeden, S. P. P.;Gunbas, D. D.; Grozema, F. C.; Sudholter, E. J. R.; Jager, W. F. J. Org.Chem. 2014, 79, 6655.(24) Ma, J.; Yin, L.; Zou, G.; Zhang, Q. Eur. J. Org. Chem. 2015,2015, 3296.(25) It has been shown that the different substitution pattern on thePDI (1,7- versus 1,6-) can lead to different charge transport propertiesof some bay disubstituted PDI derivatives. See refs 26−29.(26) Dubey, R. K.; Niemi, M.; Kaunisto, K.; Efimov, A.; Tkachenko,N. V.; Lemmetyinen, H. Chem. - Eur. J. 2013, 19, 6791.(27) Dubey, R. K.; Efimov, A.; Lemmetyinen, H. Chem. Mater. 2011,23, 778.(28) Ahrens, M. J.; Tauber, M. J.; Wasielewski, M. R. J. Org. Chem.2006, 71, 2107.(29) Goretzki, G.; Davies, E. S.; Argent, S. P.; Alsindi, W. Z.; Blake,A. J.; Warren, J. E.; Mcmaster, J.; Champness, N. R. J. Org. Chem.2008, 73, 8808.(30) Schuster, N. J.; Paley, D. W.; Jockusch, S.; Ng, F.; Steigerwald,M. L.; Nuckolls, C. Angew. Chem., Int. Ed. 2016, 55, 13519.(31) Scott, W. J.; Stille, J. K. J. Am. Chem. Soc. 1986, 108, 3033.

(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 relaysynthesis to a PDI derivative with an established structure (see SIScheme 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 ref16 (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 self-coupling of trans-3 and cis-3 creates the trans-hPDI2 6b.(42) The minor regioisomer is trans-6b, stemming from thecombination 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.8b00541Org. Lett. 2018, 20, 1991−1994

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