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A Helicene Nanoribbon with Greatly Amplified Chirality Nathaniel J. Schuster,† Raúl Hernández Sánchez,† Daria Bukharina,‡ Nicholas A. Kotov,‡ Nina Berova,† Fay Ng,*,† Michael L. Steigerwald,*,† and Colin Nuckolls*,† †

Department of Chemistry and Columbia Nano Initiative, Columbia University, New York, New York 10027, United States Department of Chemical Engineering and Biointerfaces Institute, University of Michigan, Ann Arbor, Michigan 48109, United States

‡

S Supporting Information *

ABSTRACT: We report the synthesis and characterization of a chiral, shape-persistent, perylene-diimide-based nanoribbon. Specifically, the fusion of three perylenediimide monomers with intervening naphthalene subunits resulted in a helical superstructure with two [6]helicene subcomponents. This π-helix-of-helicenes exhibits very intense electronic circular dichroism, including one of the largest Cotton effects ever observed in the visible range. It also displays more than an order of magnitude increase in circular dichroism for select wavelengths relative to its smaller homologue. These impressive chiroptical properties underscore the potential of this new nanoribbon architecture in the context of chiral electronic materials.

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ere we present a perylene-3,4,9,10-tetracarboxylic-diimide (PDI) π-conjugated helix whose significant electronic circular dichroism (ECD) stems from the integration of two rigid helicene subcomponents within its compact chiral framework. The synthesis and arrangement of increasingly sophisticated chiral materials has invigorated interest in chiroptical switches,1 circularly polarized luminescence,2 nonlinear optics,3 and spin filters.4 Helicenes, helices of angularly fused aromatic subunits surrounding a nonintersecting stereogenic axis, are among the most studied chiral materials.5 The helicene framework can be adapted to promote the assembly of chiral supramolecular aggregates,6 as well as to enable chiroptical switching in response to light, oxidation or reduction, and pH changes.7 Still, enhancing the chiroptical properties of helicenes remains a significant challenge. For instance, the axial elongation of carbohelicenes elicits little change in the magnitude of molar ECD (Δε) beyond [6]helicene.8 Moreover, modifications to the helicene scaffold (e.g., lateral extension,9 the incorporation of heteroatoms,10 or the fusion of multiple helicenes11) rarely increase (but often diminish) Δε relative to the analogous carbohelicene. In contrast, macromolecular helices assembled via the conjugated tethering of multiple helicenes often display large ECD.12 As such, we suspected that a nanoribbon adopting a similar helixof-helicenes structure would exhibit impressive ECD, as well as other properties befitting chiral electronic materials.13 This Communication details the development of a new chiral and shape-persistent nanoribbon architecture, the π-helix-ofPDI-helicenes. Specifically, the fusion of two naphthalene subunits with three PDI monomers results in naphthyl-linked PDI-trimer helix (NP3H, Figure 1). We prepared NP3H as a © XXXX American Chemical Society

Figure 1. Fusion of three PDI monomers with intervening naphthalene subunits gives NP3H, a chiral nanoribbon and homologue of the PDI-helicene NPDH.

mixture of its left- (M) and right-handed (P) helices via iterative cross-couplings and oxidative visible-light cyclizations. Single-crystal X-ray diffraction (SCXRD) revealed extensive intramolecular overlap of the π-surface, which locks the nanoribbon into a helical superstructure. NP3H adopts many of the useful properties of its subcomponents: from PDI, the capacity to accept multiple electrons, the intense absorbance of visible light, and strong fluorescence;14 and from [6]helicene, axial chirality. Unifying these subcomponents as a π-helix-ofhelicenes confers extraordinary chiroptical advantages: the largest Δε of optically pure NP3H triples that of [6]helicene. Received: March 31, 2018

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DOI: 10.1021/jacs.8b03535 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society

Scheme 1. Two Synthetic Routes, Each Consisting of Iterative Cross-Couplings and Oxidative Visible-Light Cyclizations, Yield NP3Ha,b

a

Key: (i/iv) 1,6-Br2PDI/1-BrPDI, Pd(dppf)Cl2, K3PO4, H2O, THF, rt; visible hν, air, EtOAc, rt, 54%/81% overall. (ii/v) 1-BrPDI/1,6-Br2PDI, Pd(dppf)Cl2, K2CO3, H2O, THF, 56 °C, 55%/90%. (iii/vi) Visible hν, I2, PhH, 70 °C, 73%/92% of 6 and 5% of NP3H after 24 h. bSee the Supporting Information for additional information regarding these syntheses.

Steric hindrance may account for the slow conversion of 6 into NP3H. The initial oxidative photocyclization of 5 locks one of the terminal PDI subunits in place, burying the remaining cyclization site in the core of the nascent helix (Figure S1). It follows that 6 can cyclize upon photoexcitation to give the transient dihydro-precursor of NP3H, but oxidants cannot readily access the site to remove the hydrogen atoms. In contrast, the cyclization sites of 3 are located at the relatively uncongested ends of the molecule, thus facilitating hydrogen abstraction. SCXRD verified the π-helix-of-helicenes structure of NP3H (Figure 2). The molecule spans 1.1 nm from the core of one PDI terminus to the core of the other PDI terminus (Figure S2). The helix is remarkably compact: the dihedral angles defined by the two terminal rings of the [6]helicene subcomponents of NP3H measure 40−45°, whereas this same angle for [6]helicene alone is 58.5°.19 This compression results in extensive π-to-π overlap between the PDI subunits (Figure 2a), precluding inversion to give the meso isomer. Indeed, resolution of the enantiomers by preparative chiral high-performance liquid chromatography indicated the absence of other stereoisomers (Figure S3). The shortest carbon-tocarbon distances between adjacent PDI subunits measure only 2.96(3) Å and 3.01(2) Å (Figure 2b), well within twice the van der Waals radius of the carbon atom (3.4 Å). The crystal structure also shows that extensive intermolecular π-to-π interactions mediate the formation of supramolecular columns of NP3H in the solid state (Figure S4). These columns consist of alternating M- and P-NP3H (Figure 2c), whose slip-stacking maximizes contact between the naphthyl and PDI surfaces of adjacent molecules. The considerable ECD of NP3H distinguishes this new πhelix from NPDH, as well as from helicenes in general. Optically pure NP3H exhibits many prominent Cotton effects across the UV−visible range (Figure 3a). The intensities of

NP3H also exhibits more than an order of magnitude enhancement in Δε for select wavelengths in the UV−visible range relative to its smaller homologue, naphthyl-linked PDIdimer helicene (NPDH, Figure 1).15 Two discoveries inspired our synthetic routes toward NP3H. First, the stability of N-methyliminodiacetic acid (MIDA) boronates under anhydrous and certain wet conditions permits the selective Suzuki−Miyaura reaction of vinyl and aryl boronic acids and pinacol boronates.16 Subsequent hydrolysis of the MIDA ligand with mild aqueous base unmasks the boronic acid for further coupling. As such, we anticipated bifunctional naphthalene 1 (Scheme 1) would enable the sequential couplings with PDI subunits necessary to obtain NP3H. Second, acenes fuse preferentially at the peri-position during intramolecular oxidative photocyclizations,17 particularly with PDI and its precursors.15,18 Therefore, we suspected such cyclizations of three PDI monomers with intervening naphthalene subunits would select against undesired isomers of NP3H. Our most efficient synthesis of NP3H entails the expansion of the π-surface of the inner PDI, followed by the installation of the outer PDI subunits (Scheme 1, Inside-Out route). The Suzuki−Miyaura cross-couplings of dibrominated PDI and 1 proceeded chemoselectively at room temperature. Their product cyclized oxidatively upon exposure to air and visible light, yielding 2. Subsequent Suzuki−Miyaura cross-couplings with brominated PDI provided 3. Finally, irradiation of a warm solution of 3 and iodine with visible light provided racemic NP3H in 73% yield within 1 day. From an alternative synthesis, we discovered that the location of the final oxidative photocyclization can hinder the preparation of NP3H. The installation of the π-extended PDI termini (4) on the inner PDI provided 5 (Scheme 1, OutsideIn route), an isomer of 3. Two days of irradiation failed to fully convert 5 into NP3H; instead, the reaction largely stalled at 6. B

DOI: 10.1021/jacs.8b03535 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

Figure 2. Structure of NP3H from SCXRD. Free solvent, the CH(C5H11)2 chains, and hydrogen atoms have been hidden to provide an unobstructed view of the π-surface. Thermal ellipsoids are set at 20% probability. (a) The PDI surfaces have been uniquely colored to clarify the extent of intramolecular overlap. (b) Only ∼3.0 Å separate the closest pairs of carbon atoms on adjacent PDI subunits. (c) The racemic mixture of NP3H assembles into heterochiral columns in the solid state.

several of these effects exceed the plateau in ECD (∼260 M−1 cm−1) encountered upon lengthening the carbohelicenes.8 The largest Cotton effects of NP3H manifest as a bisignate pair centered at ∼390 nm, with enormous |Δε| of 620 M−1 cm−1 at 377 nm and 820 M−1 cm−1 at 407 nm. The latter is one of the largest Δε in the visible range reported for any molecule.20 A corresponding bisignate feature centered at ∼380 nm appears in the ECD spectrum of NPDH, but with significantly less intense maxima: 41 M−1 cm−1 at 355 nm and 56 M−1 cm−1 at 401 nm. What relationships exist between the UV−visible absorbance spectra of NP3H and NPDH (Figure 3b) and the relative strengths of their ECD? NP3H absorbs unpolarized light most intensely at the 0-0 transition (λmax = 513 nm) of the vibronic progression characteristic of many PDI-based compounds,14b,21 including helical foldamers.22 This vibronic progression of NP3H is practically isoenergetic with the analogous progression in NPDH, whose 0-0 transition crests at 514 nm. Moreover, the relative peak intensity of these 0-0 transitions (1.0 × 105 M−1 cm−1 for NP3H vs 0.7 × 105 M−1 cm−1 for NPDH) closely scales with the number of PDI subunits in each molecule (i.e., 3:2). Accordingly, the ECD response of NP3H and NPDH at ∼520 nm may reasonably be anticipated to reflect a sum of the contributions from the individual PDI chromophores; indeed, a 1.3-fold increase in the local maximum |Δε| is observed from NPDH (170 M−1 cm−1 at 523 nm) to NP3H (220 M−1 cm−1 at 517 nm). Moreover, the g-factors (|Δε|/ε, Figure 3c) of both molecules coalesce to 2.5 × 10−3 at ∼520 nm; thus, the π-helix-of-PDI-helicenes structure does not improve sensitivity toward circularly polarized light in this wavelength region. The absorbance peaks of NP3H from 270−410 nm also have analogues in the UV−visible spectrum of NPDH (Figure 3b);

Figure 3. (a) ECD of NP3H and NPDH in THF (10 μM, 1 cm path length). (b) UV−visible absorbance and fluorescence spectra of NP3H (6.3 μM, λex = 409 nm) and NPDH (9.7 μM, λex = 393 nm) in THF (1 cm path length). Fluorescence intensities have been normalized to the largest absorptions of the corresponding species. (c) g-Factors of NP3H and NPDH in THF.

however, their relative intensities utterly fail to account for the largest enhancements in ECD in this wavelength regime observed for NP3H. Specifically, relative to the Cotton effects of NPDH at 355 and 401 nm, the Cotton effects of NP3H at 377 and 407 nm show a 15-fold enhancement in Δε. Obviously, 15-fold enhancements are not observed between peaks in the UV−visible spectra. A comparison of the g-factors (Figure 3c) from 270−410 nm reveals significant amplification in ECD for NP3H relative to NPDH. For instance, NP3H has |Δε|/ε of 7.9 × 10−3 at 377 nm and 8.9 × 10−3 at 407 nm, a 7.2fold and 5.9-fold increase over those transitions of NPDH at 355 and 401 nm. As such, the π-helix-of-PDI-helicenes architecture properly augments ECD in this wavelength region relative to its helicene homologue: it increases both |Δε| and gfactor. C

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Journal of the American Chemical Society The fluorescence quantum yield (Φ) of NP3H in THF under ambient conditions is 44%, whereas that of NPDH is 27%. Rigidity that enforces planarity or near-planarity in polyaromatics generally shortens the Stokes shift and promotes fluorescence.23 Accordingly, the sandwiching of the near-planar PDI core of NP3H by the terminal PDI subunits (Figure 2a) may account for the enhanced Φ relative to that of NPDH. The Stokes shifts also suggest a disparity in their relative rigidities in the excited state (Figure 3b): NP3H exhibits a shift of 10 nm (539 to 549 nm), whereas that of NPDH is 27 nm (514 to 541 nm). The Φ of both PDI-helices well exceeds those of the carbohelicenes. For example, the Φ of [6]helicene in 1,4dioxane at room temperature is 4%.24 Electrochemical reduction via cyclic voltammetry affirms NP3H accepts multiple electrons from −1.1 to −2.0 V vs Fc/ Fc+ (Figure S5; see the Supporting Information for the experimental details). The redox cycle is chemically reversible. Although only four reduction events can readily be distinguished in the voltammogram, spectroelectrochemical measurements (Figures S6 and S7) indicate the addition of six electrons to NP3H (one electron per imide group) in this potential range. In conclusion, we have prepared the shape-persistent π-helixof-helicenes NP3H via iterative palladium-catalyzed crosscouplings and intramolecular oxidative photocyclizations. Visible light induces these cyclizations, a departure from the potentially destructive UV irradiation often used to prepare carbohelicenes and their polyhelicene variants.5,9f,11g,25 NP3H exhibits the extraordinary optical properties of PDI: it absorbs UV and visible light intensely and fluoresces brilliantly. Incorporating PDI into this chiral superstructure confers enormous ECD: the largest Δε of NP3H triples that of [6]helicene. NP3H also exhibits more than an order of magnitude enhancement in Δε for select wavelengths in the UV−visible range relative to NPDH, its smaller homologue. These impressive properties underscore the potential of the πhelix-of-PDI-helicenes in the context of chiral electronic materials. For instance, the ordered aggregation of optically pure NP3H in the solid state may result in significant nonlinear optical phenomena,26 as well as pronounced chiral-induced spin selectivity.4 The synthesis of larger members of this nanoribbon series (e.g., NP4H, NP5H, etc.) will enable the clarification of the relationship between molecular structure and chiroptical response, thereby facilitating further amplification of ECD.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.N. thanks Sheldon and Dorothea Buckler for their generous support. Primary support for this project was provided by the U.S. Department of Energy (DOE), under award no. DESC0014563. Partial support was provided by the Office of Naval Research under award no. N00014-16-1-2921. Instruments in the Nakanishi and Owen laboratories at Columbia University were used for this research. We thank both laboratories for their generosity. N.J.S. recognizes the NSF GRFP for its support (DGE 16-44869). R.H.S. acknowledges the support from the Columbia Nano Initiative Postdoctoral Fellowship. SCXRD was performed at the Shared Materials Characterization Laboratory (SMCL) at Columbia University. Use of the SMCL was made possible by funding from Columbia University.

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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/jacs.8b03535. Data for NP3H (CIF) Synthetic procedures, additional figures/schemes, and characterization data (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] *[email protected] ORCID

Raúl Hernández Sánchez: 0000-0001-6013-2708 Nicholas A. Kotov: 0000-0002-6864-5804 Colin Nuckolls: 0000-0002-0384-5493 D

DOI: 10.1021/jacs.8b03535 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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DOI: 10.1021/jacs.8b03535 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX