Donor/acceptor-substituted radiaannulene segments of 6,6,12-graphyne

Abstract

A selection of five donor/acceptor-functionalized radiaannulenes was efficiently synthesized via a series of palladium-catalyzed couplings. These donor/acceptor segments of the elusive 6,6,12-graphyne allotrope are strong chromophores and undergo reductions into dianions corrrelating with Hammett substituent constants. Crystal data reveal planar structures of the cyclic cores, but twisting of the aryl substituents with torsional angles independent on the donor/acceptor character.

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Graphynes are allotropes of carbon formed by introducing additional carbon atoms between the sp²-hybridized carbon atoms of graphene and hold vast potential for energy storage, spintronics, chemical sensing, optoelectronics, and catalysis.¹ One specific example is the 6,6,12-graphyne, depicted in Fig. 1. Theoretical studies predict that this graphyne derivative exhibits direction-dependent electronic conduction and high charge carrier mobility, making it and its segments highly attractive for electronic devices.² However, efforts to synthesize 6,6,12-graphyne and its smaller segments remain limited due to the structural complexity, requiring classical synthetic strategies starting from simpler building blocks. The largest synthesized segments to date are “vertical trimers” of acetylenic radiaannulene (RA) macrocycles (Fig. 1) and acyclic “diagonal trimers”.³ RAs can be considered hybrids of dehydrobenzannulene and an expanded radialene,⁴ and they contain both endo- and exo-cyclic double bonds (Fig. 1). Another known^3a structural motif of 6,6,12-graphyne is the bis-dehydroannulene highlighted in dark green, containing a central tetraethynylethene (TEE) unit highlighted in black in Fig. 1.

Fig. 1 Structure of 6,6,12-graphyne with selected segments highlighted. The actual structure corresponding to the segment labelled 1 has Si(i-Pr)₃ groups at the terminal alkynes.

RAs based on two TEE units connected by ortho-phenylene or other conjugated rings have been found to be electron acceptors.^3–5 Thus, the monomer 1 (protected with triisopropylsilyl groups at the terminal alkynes) exhibits reductions in cyclic voltammetry studies.^3a This electron-accepting behavior can be described by the formation of a 14π Hückel-aromatic ring upon accommodating two electrons.

In this work, we describe how the electronic properties of RAs are tuned by further expanding the macrocycle in the “horizontal” direction by four alkynes (purple-colored structure in Fig. 1) connected to a variety of different electron-donating or withdrawing aryl groups. Such compounds were prepared from a new convenient RA building block (2) incorporating four reactive positions (C–Br) for Sonogashira coupling reactions (Scheme 1).

Scheme 1 Synthesis of tetra-bromo core 2 by four-fold palladium-catalyzed Sonogashira reactions from compounds 3 and 4.

First, we subjected 1,2-dibromo-4,5-diethynyl 3 and the known⁶ vinylic dibromide 4 to a cyclization reaction under Sonogashira conditions (Scheme 1), furnishing the RA 2 in a yield of 13%. Here the more reactive vinylic dibromides conveniently reacted rather than the bromobenzene sites.

Next, we subjected RA 2 to four-fold Sonogashira coupling reactions with five different arylacetylenes 6a–e (Scheme 2). These reactions were performed using Pd(PhCN)₂Cl₂ with P(t-Bu)₃·HBF₄ as catalytic system according to the general procedure of Hundertmark et al.⁷ for the coupling of less reactive bromobenzenes. The substituted RAs 6a–e were obtained in yields of 13–48%. Compound 1 was synthesized according to known procedure.^3a All compounds were fully characterized by ¹H NMR and ¹³C NMR spectroscopies as well as by high-resolution mass spectrometry (HRMS).

Scheme 2 Synthesis of substituted radiaannulenes, 6a–e from the tetra-bromo core 2 by four-fold palladium-catalyzed Sonogashira reactions.

Moreover, we managed to grow single crystals suitable for X-ray crystallography of 6a–d (Fig. 2).⁸ All compounds showed high planarity of the central radiaannulene as expected and previously observed.^3a Thermal ellipsoids are omitted for clarity due to the size of the Si(i-Pr)₃ groups (more details can be found in ESI,† Fig. S53–S56). Compounds 6b and 6d showed π–π stacking with a distance of 3.525 and 3.536 Å, respectively. The length of the alkyne linking the central core unit and an aryl substituent group does not change between electron donating or withdrawing aryl groups. The torsion angle of the substituents showed to be paired over the diagonal, and it twists between 1° and 41° with no correlation to the electron-donating or withdrawing character of the aryl groups (see ESI† for values of all angles).

Fig. 2 Molecular structures of compounds (a) 6a, (b) 6b, (c) 6c, and (d) 6d obtained by X-ray diffraction crystallography. Atoms are colored grey (carbon), purple (nitrogen), and red (oxygen); hydrogen atoms and Si(i-Pr)₃ groups were omitted for clarity.

The UV-vis absorption spectroscopic properties of 1, 2, and the five substituted RAs were evaluated in CH₂Cl₂ at 25 °C and are shown in Fig. 3. Absorption maxima and molar absorptivities are listed in Table 1. Compounds 1 and 2 have similar spectra, with a slightly redshifted (ca. 10 nm) longest-wavelength absorption maximum for 2 but a reduced molar absorptivity.

Fig. 3 UV-vis absorption spectra of (a) reference compounds 1, 2, and 6c, and (b) substituted radiaannulenes 6a–e. All spectra are recorded in CH₂Cl₂ at 25 °C.

Table 1 UV-vis absorption maxima (λ_max) and molar absorptivities in brackets (ε) of compounds 1, 2 and 6a–e recorded in CH₂Cl₂ at 25 °C. sh = shoulder

Compound	λ max [nm] (ε [10^3 M^−1 cm^−1])
1	256 (30), 267 (sh, 41), 274 (sh, 49), 283 (82), 316 (30), 331 (32), 347 (sh, 19), 373 (24), 416 (sh, 42), 427 (45), 452 (56)
2	266 (35), 281 (sh, 44), 291 (61), 297 (76), 322 (sh, 34), 336 (34), 355 (27), 383 (28), 397 (sh, 28), 419 (29), 431 (28), 459 (36)
6a	270 (sh 40), 285 (sh, 50), 303 (sh, 65), 326 (92), 368 (93), 445 (sh, 73), 503 (sh, 51)
6b	251 (35), 306 (sh, 52), 347 (81), 377 (sh, 51), 438 (sh, 29), 454 (32), 482 (46)
6c	326 (sh, 92), 340 (96), 372 (sh, 43), 400 (sh, 24), 434 (sh, 26), 449 (29), 477 (42)
6d	273 (50), 336 (sh, 140), 352 (195), 375 (sh, 72), 403 (sh, 38), 436 (sh, 43), 451 (50), 479 (82)
6e	256 (54), 267 (54), 333 (sh, 166), 348 (230), 373 (sh, 66), 402 (sh, 40), 435 (sh, 48), 450 (58), 479 (98)

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All the extended RAs 6a–e have a more intense absorption at around 350 nm compared to those of 1 and 2. This absorption is particularly intense for 6d and 6e incorporating acceptor substituents (CN and Ac, respectively). They all exhibit redshifted longest-wavelength absorptions; around 480 nm for 6b–6e and at 508 nm for 6a, while those of 1 and 2 are at 427 and 459 nm, respectively. A dilution series was performed for all compounds (concentration range 0.5–13 mM), and for each compound selected absorptions were found to follow Lambert–Beer's law (see ESI†), and associations between molecules are therefore assumed to be insignificant. A bathochromic shift of the most intense absorption is observed for 6a when moving to solvents with a higher dielectric constant (cyclohexane, toluene, EtOAc, CH₂Cl₂, and DMF; Fig. 4).

Fig. 4 Normalized absorption spectra of 6a measured in cyclohexane, toluene, EtOAc, CH₂Cl₂, and DMF at 25 °C; normalized for each compound using the most intense absorption.

Subjecting the anilino-substituted RA 6a to trifluoroacetic acid (TFA) was found to blueshift the longest-wavelength absorption to 477 nm (close to that of the other RAs) as well as resulting in an increase of the absorption around 350 nm, resembling the spectral properties of the acceptor-substituted compounds 6d and 6e (Fig. 5). This is explained by the conversion of the electron-donating amino groups into electron-withdrawing ammonium groups as shown in Scheme 3. The spectrum corresponding to the neutral compound was regenerated upon addition of the base triethylamine. Here the increased absorption from 315 nm and below corresponds to the absorption of TFA and Et₃N in CH₂Cl₂ (see ESI,† Fig. S8). Fig. S9 in ESI† shows the spectra of the three RAs with electron-withdrawing substituents (6a(4H⁺), 6d, and 6e) (normalized with respect to longest-wavelength absorption maximum).

Fig. 5 UV-vis absorption spectra of 6a (1.22 μM; solid curve), 6a + TFA (generating 6a(4H⁺); dashed curve). Addition of Et₃N regenerates the spectrum of 6a (slightly smaller absorptions due to dilution). Spectra measured in CH₂Cl₂ at 25 °C.

Scheme 3 Protonation of 6a generates 6a(4H⁺). TFA = trifluoroacetic acid.

The redox properties of the compounds were studied by cyclic voltammetry and differential pulse voltammetry in CH₂Cl₂ + 0.1 M n-Bu₄NPF₆. RA 2 experienced a quasi-reversible reduction at −1.44 V (vs. Fc/Fc⁺), and the electron-withdrawing bromo substituents thus render it a stronger electron acceptor than the previously studied,^3a parent RA 1 with triisopropylsilyl substituents at the alkynyl units (−1.50 V). Compound 2 also underwent a second reduction at −1.65 V. The extended RAs 6a–6e all underwent a first irreversible reduction, followed by a reversible reduction (Fig. 6). The first reduction occurred in all cases at less negative potential than observed for 1 and 2, in particular for acceptor-substituted RAs 6d and 6e. Interestingly, the second reductions were to a larger extent influenced by the electronic character of the substituents. All compounds experienced irreversible oxidations around 1.10 to 1.30 V (see ESI† for full CVs and DPVs).

Fig. 6 Cyclic voltammetry of 6a–e (0.4–0.5 mM) in CH₂Cl₂ with n-Bu₄NPF₆ (0.1 M) as supporting electrolyte. Scan rate: 0.1 V s⁻¹. Ag/Ag⁺ was used as reference electrode, Pt as working electrode, and Pt wire as counter electrode. The potentials listed are the half-wave potentials (E_½) of the reversible reductions derived from the CVs. TIPS = triisopropylsilyl (Si(i-Pr)₃).

When the half-wave potential for the second reductions (by which aromatic dianions are generated) for the five substituted RAs are plotted versus the Hammett substituent σ-values,⁹ a linear correlation is obtained (Fig. 7): E^2nd_1/2/V = 0.11 × σ − 1.64. As anticipated, electron-withdrawing groups (6d and 6e) facilitate reduction, resulting in less negative reduction potentials – while electron-donating groups (6a and 6b) render reduction more difficult. This correlation demonstrates how Hammett substituent constants can conveniently be used to predict redox properties of substituted radiaannulenes.

Fig. 7 Hammett plot showing the linear relationship between the substituent σ-values and the half-wave potentials for the second reduction.

In conclusion, a novel bromo-functionalized radiaannulene was prepared and explored as a useful building block for introducing arylethynyl substituents through palladium-catalyzed Sonogashira reactions. These substituents increased the electron acceptor properties of the compounds, and a linear correlation with Hammett substituent constants was observed for generation of the dianions. The arylethynyl substituents resulted in redshifted longest-wavelength absorptions. Overall spectral behaviors depended on the donor or acceptor characters of the substituents, and this character could be reversibly changed by acid/base treatment for the radiaannulene incorporating 4-(dimethylamino)phenylethynyl substituents. X-Ray crystallography showed that all compounds are planar in the central core, but the peripheral aryl groups were found to be twisted from co-planarity.

Future work will focus on further acetylenic scaffolding of the donor/acceptor radiaannulenes along the vertical direction (after removing the triisopropylsilyl protecting groups) as well as on generation of oligomeric radiaannulenes taking advantage of the bromo-substituted radiaannulene building block. Thereby larger segments of 6,6,12-graphyne can be imagined with properties tuned by the peripheral substituents.

MBN and PLK conceptualized the work (overall supervised by MBN). PLK wrote a first paper draft that was iterated by all authors. PLK did the synthesis work and solvent studies; the synthesis and studies of compound 6c were also performed by MHJ. AS performed the X-ray crystallographic studies.

The Independent Research Fund Denmark, Natural Sciences (grant 3103-00001B) is acknowledged for financially supporting this work. The authors also acknowledge Prof. R. R. Tykwinski (University of Alberta) for assistance with HRMS analytical characterizations and Dr Arianna Lanza (University of Copenhagen) for helping refining X-ray crystal data.

Data availability

The data supporting the findings in this article have been included in the ESI.†

Conflicts of interest

There are no conflicts to declare.

Notes and references

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8. Deposition Numbers 2448792 (6a), 2448802 (6b), 2448784 (6c), 2448788 (6d) contain the supplementary crystallographic data for this paper.
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Footnote

† Electronic supplementary information (ESI) available: Synthetic protocols, UV-Vis and NMR spectroscopic data, electrochemical data, X-ray crystallographic data. CCDC 2448792, 2448802, 2448784 and 2448788. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5cc02685g

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