Nanographenes as electron-deficient cores of donor-acceptor systems.

Conjugation of nanographenes (NGs) with electro-active molecules can establish donor-acceptor π-systems in which the former generally serve as the electron-donating moieties due to their electronic-rich nature. In contrast, here we report a series of reversed donor-acceptor structures are obtained by C-N coupling of electron-deficient perchlorinated NGs with electron-rich anilines. Selective amination at the vertexes of the NGs is unambiguously shown through X-ray crystallography. By varying the donating ability of the anilino groups, the optical and assembly properties of donor-acceptor NGs can be finely modulated. The electron-deficient concave core of the resulting conjugates can host electron-rich guest molecules by intermolecular donor-acceptor interactions and gives rise to charge-transfer supramolecular architectures.

Introduction

As molecularly defined cutouts of the papan class="Chemical">n class="Chemical">graphene lattice, npan>anographenes (NGs) have been developed to deduce intrinsic structure-property correlations[1-3]. Among various functional characteristics of NGs, their extended sp2-conjugated carbon skeleton offers intriguing optical absorption, photoluminescence and charge transport properties[4-7] and thus promising potential in electronics[8, 9] and optoelectronics[10]. The electron-rich nature of NGs qualifies them as typical p-type semiconductors in organic field-effect transistors[11-14] and as electronic donors in organic photovoltaics[14-16]. Allowing conjugation of donor to acceptor moieties has proven as a powerful concept to modulate the optical absorption and charge transport[17-22]. In such donor-acceptor (D-A) conjugates[23-25], NGs have typically served as donors. Recently, a few studies have switched the electronic characteristics of NGs from electron-rich to electron-poor by multiple introduction of strongly electron-withdrawing groups, such as bisimide[26-29], chloro[30], and fluoro[31, 32] substituents. Such electron-deficient NGs thus provide alternative pathway to D-A conjugates. However, this concept remains challenging due to lack of suitable synthetic protocols.

Here we report an amipan class="Chemical">nation occurring at the vertexes of perchlorinpan>ated NGs by palladium-catalyzed Buchwald–Hartwig coupling[33, 34]. Using this strategy, various electron-rich aniline derivatives are coupled to the periphery of electron-deficient chlorinated NGs, providing a series of nano-sized D-A architectures. The concave molecular structures of these D-A NGs are clearly demonstrated by single-crystal X-ray diffraction, which show a selective amination at the vertexes. The donating strength of attached anilino groups can finely tune the optical absorption and intermolecular interactions. Different from previously reported concave polycyclic aromatic hydrocarbons, the concave electron-deficient skeletons of the D-A NGs can assemble with donor-type guest molecules such as tetrathiafulvalene by intermolecular charge transfer (CT).

Results

Amination of NGs

Arm-chair edges of graphenic structures comprise two types of papan class="Chemical">n class="Chemical">carbon sites[35-37], onclass="Chemical">pan>e at the bay and another at the vertex (Fig. 1). The steric hinpan>drance at the bay position is obviously larger than that at the vertex and will become evenpan> more severe with bulky groups. In case of perchlorinpan>ated NGs, the strong congestion of chloro substituents at the bay has been shown to distort the molecular geometry from planar to curved[30]. We reasoned that the chlorines at the different peripheral sites of NGs possessed different chemical reactivity and could allow selective modification (Fig. 1).

Fig. 1

Arm-chair edge of graphenic structures. Arm-chair edge contains two kinds of carbon sites, one at the bay (cyan point) and another at the vertex (magenta point)

Arm-chair edge of graphenic structures. Arm-chair edge copan class="Chemical">ntains two kinds of n class="Chemical">carbon sites, one at the bay (cyan poinpan>t) anclass="Chemical">pan>d another at the vertex (magenpan>ta poinpan>t)

Takingpan> perchlorinated hexa-peri-hexabenzocoronene (C42Cl18, 1, here the smallest representative NG) with two types of chloro groups as an initial example, we screened the palladium-catalyzed Buchwald–Hartwig C–N cross-coupling between 1 and aniline (Fig. 2 and Supplementary Table 1). Under optimized reaction conditions aniline can, indeed, dominantly couple to the vertexes of 1, yielding the hexakis-anilino chlorinated hexa-peri-hexabenzocoronene (2a) (Fig. 2 and Supplementary Fig. 1). The bulky bidentate phosphine ligand was necessary to produce 2a (Supplementary Table 1).

Fig. 2

Synthesis and crystal structure of 2 and optical properties. a optimized synthetic route for 2. b the doubly concave structure of 2 represented by 2a. The thermal ellipsoids are set at 50% probability level. c UV-Vis absorption spectra of 2 measured in dichloromethane (the concentration of 2 is 10−5 mol·L−1). Pd2(dba)3 = tris(dibenzylideneacetone)dipalladium. BINAP = 2,2′-bis(diphenylphosphino)-1,1′-binaphthalene

Synthesis and crystal structure of 2 and optical properties. a optimized synthetic route for 2. b the doubly concave structure of 2 represented by 2a. The thermal ellipsoids are set at 50% probability level. c UV-Vis absorption spectra of 2 measured in dichloromethane (the concenpan>tration of 2 is 10−5 mol·L−1). Pd2(dba)3 = tris(dibenzylideneacetone)dipalladium. BINAP = 2,2′-bis(diphenylphosphino)-1,1′-binaphthalene

In additiopan class="Chemical">n to NMR spectroscopy (Supplementary Figs. 9 and 10), the structure of 2a was assessed by single crystal X-ray diffraction (Fig. 2b). As depicted inpan> Fig. 2b, the most distinguishable feature of 2a is six substituting anilino groups at the vertexes of NG. The unobstructed remaining chlorines in the bays adopt an up and down conformation, which keeps the structure of 2a doubly concave (Fig. 2). We further demonstrated that the amination was tolerant to different aniline derivatives, because 2b-2e were synthesized and characterized (Fig. 2 and Supplementary Figs. 4, 7 and 11–18).

Optical properties of 2

The UV-Vis spectrum of 2a displays three bands that peak at 452, 482, apan class="Chemical">nd 560 nm (Fig. 2c). The variation of concentration does not affect the absorption profile of 2a, suggesting the absenpan>ce of the π–π stacking in solution (Supplementary Fig. 36). The absorption of 2a showed a bathochromic shift (45 nm) compared with its parent compound 1[30], attributed to the intramolecular CT from anilino groups to the inner NG. An expected solvatochromism with increasing solvent polarity was observed for 2a, but the redshift is relatively small (Supplementary Fig. 35).

Comparingpan> the optical absorption, a gradual bathochromic shift of absorption peaks was revealed (Fig. 2c and Supplemenpan>tary Table 2) from 2a to 2e without concenpan>tration depenpan>denpan>ce (Supplemenpan>tary Figs. 36–40), inpan> which the electron-donating power of peripheral anilino groups increased. Consequently, the optical HOMO-LUMO gap of 2a to 2e decreased from 2.18 to 1.84 eV (Supplementary Table 2). On the other hand, the photoluminescence of compounds 2a, 2b, and 2c shows an emission with λmax located at ~560 nm with an absolute photoluminescence quantum yield (PLQY) of 3.5–5.4% (Supplementary Table 2), whereas the enhanced intramolecular CT in 2d and 2e quenched their photoluminescence dramatically (Supplementary Fig. 41 and Supplementary Table 2). These optical properties of 2 confirmed their D–A characteristics.

Theoretical calculations of 2

Theoretical calculations helped to describe the ipan class="Chemical">ntramolecular CT in 2. LUMOs were generally distributed over the NG moiety whereas the HOMOs were located at the peripheral anilino units (Supplementary Fig. 52). The electron density differences between the first excitation and the ground state showed a decrease at the anilino units and an increase at the inner NG core (Supplementary Fig. 53) in 2. Differently, the weaker donating ability of anilino groups in 2a leads to poor intramolecular CT, therefore the electron density decrease at the anilino groups of 2a is obviously smaller than that in 2e (Supplementary Fig. 53), which comprises the electron-richest functional groups. Theoretical and experimental results thus jointly validate the D-A nature of 2.

Packing structure of 2

The crystal structures of 2a-2e (Supplementary Figs. 31–33 apan class="Chemical">nd Supplementary Methods) revealed their assembly in the solid state. All inner cores of 2a-2e have a C3v symmetrical double concave structure, if ignorin class="Chemical">ng the conformation of peripheral phenpan>yl groups (Fig. 2b and Supplemenpan>tary Figs. 31–33), which impedes the π–π inpan>teractions betweenclass="Chemical">pan> the n class="Chemical">NG cores in the solid (Supplementary Fig. 42). Indeed, the solid-state UV-Vis spectra of 2a–2d display small red shift by 30–35 nm (Supplementary Figs. 43–46 and Supplementary Methods), in comparison with those in solution, which confirms the absence of π–π interactions in the solid state for 2a-2d.

In copan class="Chemical">ntrast, in the crystal of 2e, the peripheral p-N-N-dimethyl anilino groups assemble with the skeleton of NG core in a face-to-face fashion with a π–π interaction distance of 3.30 Å (Fig. 3), forming intermolecular D–A supramolecular networks. The different assembly structure of 2e originates from the stronger donor ability of p-N-N-dimethyl anilino groups, as compared with those in 2a-2d. Remarkably, an obvious red shift by 125 nm for 2e appeared in the solid-state optical absorption (Fig. 3), and the absorption of 2e in the solid extends over the entire range of visible light. The case of 2e suggests that the intermolecular CT creates an extensive absorption of the D–A molecules, which should be useful for the design of conjugated molecules for light harvesting[38].

Fig. 3

Absorption and assembly in the solid state of 2e. The UV-Vis absorption of 2e in solid state (red) measured in a diffuse-reflectance mode was compared with that in dichloromethane (blue, 10−5 mol·L−1). The peripheral groups which do not allow D–A interactions with the inner NG were omitted for clarity

Absorption apan class="Chemical">nd assembly in the solid state of 2e. The UV-Vis absorption of 2e in solid state (red) measured in a diffuse-reflectance mode was compared with that in n class="Chemical">dichloromethane (blue, 10−5 mol·L−1). The peripheral groups which do not allow D–A interactions with the inner n class="Chemical">NG were omitted for clarity

Amination for larger NGs

Encouraged by the successful amipan class="Chemical">nation of 1, we proceeded to larger homologs of 1, C60Cl22 (3) and C78Cl26 (5), which contain six chlorines at the vertexes as well. Under similar reaction conditions, two series of hexakis-aminated chlorinated products 4a–4c (C60Cl16(NHC6H4R)6, R = −H (4a), −nBu (4b), and −OCH3 (4c)) and 6a–6b (C78Cl20(NHC6H4R′)6, R′ = −nBu (6a) and –OCH3 (6b)) were obtained (Fig. 4 and Supplementary Figs. 2, 5, 6, 8 and 19–30). The unambiguous structure of 4 was disclosed by single-crystal X-ray diffraction (Supplementary Methods). The Fig. 4b showed that six anilino groups are indeed connected to the vertexes of 3 while the chlorines at the bay again keep intact. The UV-Vis spectra of 4 and 6 display bathochromic shifts by 39 and 37 nm (Fig. 4c, Supplementary Figs. 47–49 and Supplementary Table 3), respectively, compared with 3 and 5, also due to the intramolecular CT. By theoretical calculations, 4 exhibits a typical distribution of electron density differences as a feature of D-A conjugates (Supplementary Fig. 55).

Fig. 4

Synthesis and crystal of 4 and optical absorption. a optimized synthetic route for 4. b the crystal structure of 4 represented by 4a. The thermal ellipsoids were set at 30% probability level. c UV-Vis absorption spectra of 3, 4a, 5, and 6a. Notably, the molar absorption coefficients of 3, 4, and 6 were listed in Supplementary Figs. 47–49, while that of 5 can not be measured due to its extremely low solubility

Supramolecular assembly between D–A NGs and tetrathiafulvalene

D-Apan> NGs own an electron-deficient concave core, which can act as supramolecular host for electron-rich donor molecules such as tetrathiafulvalene (TTF). Evaporation of the carbon disulfide solution of 2a and TTF led to the formation of crystalline black needles, different from the orange prisms of 2a. X-ray diffraction analysis revealed that a supramolecular complex 2a ⊃ TTF had formed with TTF molecules located at the center of concave NG in a face-to-face manner (Fig. 5a, c). The interfacial distance between TTF and NG core is 3.30 Å implying strong π–π interactions. Further stacking between TTF and 2a builds up a mixed D–A–D–A supramolecular column (Fig. 5c). The columnar supramolecular architecture of 2a ⊃ TTF holds promise for long-range orientations of CT dipoles, making it a potential organic ferroelectric[39, 40]. Compound 4 as acceptor can also host electron-donating TTF (Fig. 5 and Supplementary Fig. 34). One TTF molecule is located above the core of 4, forming sandwich-type complexes (2·4 ⊃ TTF) with close interfacial π–π interactions (3.30 Å) between TTF and inner NG.

Fig. 5

Supramolecular assembly between D-A NGs and TTF. a, b the structures of 2a ⊃ TTF and 4a ⊃ TTF. c one dimensional mixed-stacking of 2a and TTF in the crystal. Peripheral phenyl groups were omitted for clarity. d sandwich-type complex of 2·4a ⊃ TTF. e, f UV-Vis-NIR spectra of 2a, 2a ⊃ TTF, 4a and 2·4a ⊃ TTF in the solid state measured in a diffuse-reflectance mode. CT bands at 650 nm and 780 nm were clearly observed in the spectra of supramolecular complex

Supramolecular assembly between papan class="Chemical">n class="Chemical">D-A class="Chemical">pan> class="Chemical">NGs and TTF. a, b the structures of 2a ⊃ TTF and 4a ⊃ TTF. c one dimensional mixed-stacking of 2a and TTF in the crystal. Peripheral phenyl groups were omitted for clarity. d sandwich-type complex of 2·4a ⊃ TTF. e, f UV-Vis-NIR spectra of 2a, 2a ⊃ TTF, 4a and 2·4a ⊃ TTF in the solid state measured in a diffuse-reflectance mode. CT bands at 650 nm and 780 nm were clearly observed in the spectra of supramolecular complex

The structures of both supramolecular complexes (2a ⊃ TTFpan> and 2·4 ⊃ TTF) suggest the presence of intermolecular CT between TTF and electronic deficient core as proven by CT bands in the absorption spectra of 2a ⊃ TTF and 2·4 ⊃ TTF (Fig. 5). In comparison with 2a ⊃ TTF, a bathochromic shift (130 nm) appeared in the CT band of 2·4 ⊃ TTF (Fig. 5f and Supplementary Fig. 50), attributed to the extended conjugated skeleton of 4. Remarkably, the absorption of 2·4 ⊃ TTF crystals can extend into the NIR region up to 1000 nm. The unambiguous structure and clear CT band of the supramolecular complexes between D–A NGs and donating guests (Fig. 5) characterized these D–A NG conjugates as promising electronic-accepting host for the construction of CT supramolecular architectures.

Discussion

Our aminatiopan class="Chemical">n protocol enabled functionalization at the vertexes of NGs and constructed reversed D-A conjugates. The donating ability of aniline groups affords an efficient approach to modulate the properties of D-A NGs, e.g., absorption, optical gap, luminescence, and assembly. Since the edge halogenation of graphene nanoribbons and graphenic materials was achieved, our functionalization concept can additionally incorporate D-A structures and CT dipoles into these graphenic structures. The resulting D–A NGs undergo assembly with electron-rich molecules by intermolecular D-A interactions, creating CT supramolecular architectures. This opens an avenue toward supramolecular functional materials based on NG acceptors.

Methods

Synthesis of 2

First, the reaction copan class="Chemical">nditions, including amounpan>t of aniline, phosphine ligands and bases, for the palladium-catalyzed Buchwald–Hartwig C-N coupling between 1 and aniline were screened (Supplementary Table 1). The optimized conditions required a large excess of aniline to favor the multiple C–N coupling of 1 with aniline and suppress the homocoupling between 1[41, 42], supported by the control experiments (Supplementary Table 1). The decreased amount of aniline in control experiments resulted in a lower yield of 2a, whereas the experiments with 15 and 30 fold excess of aniline did not affect the yield (Supplementary Table 1).

Under the optimized reactiopan class="Chemical">n conditions, a 25 ml reaction tube was charged with 1 (100 mg, 0.087 mmol), Cs2CO3 (113 mg, 0.348 mmol), Pd2(dba)3, (24 mg, 0.026 mmol) and BINAP (32 mg, 0.052 mmol) under argon, and then an aniline (194 mg, 2.09 mmol, equal to 4-fold according to the molar of chlorine at the vertexes) solution in toluene (5 ml) was added into the tube. The reactants were stirred and refluxed at 105 °C for 36 h. After the reaction, the products were extracted with dichloromethane (DCM) (30 ml) and washed with water (20 ml). The collected organic phase was dried by anhydrous MgSO4. After filtration and evaporation of the solvent, the crude products were separated over a silica column using DCM/petroleum ether (60–90 °C) (1.5:1) as eluent. The red component was collected, yielding 40 mg of 2a. The yield was 31% (Note that the yield of 2a determined by NMR spectroscopy is ~50%, Supplementary Table 1). Due to strong absorption of 2a on silica, the subsequent eluent contained 2a as well (Supplementary Fig. 1). If these components (spot 3 shown in Supplementary Fig. 1) were collected and separated again, the isolated yield can be improved to 38%.

Besides 2a as the major product of C–N couplipapan class="Chemical">n class="Chemical">ng, the byproducts inpan> the reaction were attributed to the oligomers originpan>ating from the competitive palladium-catalyzed C–C homocoupling between 1[41, 42] and a few pentakis-anilino products. The byproducts with more than 6 anilino groups were not observed. Considering that six C-N bonds are formed in one reaction, the achieved yield is over 80% for each C–N bond formation.

2b, 2c, 2d, and 2e were sypan class="Chemical">nthesized by the coupling of 1 to differenpan>t class="Chemical">pan> class="Chemical">aniline derivatives employing similar reaction and separation conditions. The isolated yields of 2b, 2c, 2d, and 2e were 32, 26, 26, and 21%, respectively.

Synthesis of 4

A 25 ml reaction tube was charged with 3 (50 mg, 0.033 mmol), papan class="Chemical">n class="Chemical">Cs2CO3 (43 mg, 0.133 mmol), class="Chemical">pan> class="Chemical">Pd2(dba)3 (9 mg, 0.01 mmol) and BINAP (12 mg, 0.02 mmol) under argon. An aniline (74 mg, 0.792 mmol, equal to 4-fold according to the molar of chlorine at the vertexes of 3) solution in toluene (5 ml) was added into the tube. After stirring at 105 °C for 36 h, 30 ml DCM was added. The organic phase was washed with water (20 ml) and dried over anhydrous MgSO4. After removing the solvent, the crude products were separated by a silica column using DCM/petroleum ether (60–90 °C) (1.5:1) as eluent. The collected component was further purified by high performance liquid chromatography (HPLC) using JAIGEL-2H column (Japan Analytical Industry Ltd., chloroform as eluent), yielding 10 mg of 4a. The yield was 17%.

4b, and 4c were also sypan class="Chemical">nthesized using differenpan>t class="Chemical">pan> class="Chemical">aniline derivatives under similar reaction and separation conditions as those of 4a. Notably, purification by HPLC using JAIGEL-2H column was necessary for 4b and 4c as well. The isolated yields of 4b and 4e were 13 and 20%, respectively, under the optimized conditions.

Synthesis of 6

5 (50 mg, 0.027 mmol), Cs2CO3pan> (36 mg, 0.133 mmol), Pd2(dba)3 (8 mg, 0.0099 mmol), and BINAP (10 mg, 0.0198 mmol) were introduced into a 25 ml reaction tube under argon (Supplementary Fig. 2). Then toluene (5.0 ml) and 4-n-butyl-aniline (96 mg, 0.792 mmol, equal to a 4-fold according to the molar of chlorine at the vertexes) were added. The reactants were stirred for 36 h at 105 °C. The products were extracted with DCM (30 ml), washed with water (20 ml) and dried over anhydrous MgSO4. After removing the solvent, the crude products were separated over a silica column using DCM/petroleum ether (60–90 °C) (1:3) as the eluent. Then, the obtained component was further purified by HPLC using JAIGEL-2H column (chloroform as the eluent), yielding 13 mg of 6a. The yield was 19%. Similarly, 6b was synthesized and isolated, with a yield of 8%.

Crystallography

Usin class="Chemical">ngpan> Olex2[43], all the initial structures were solved with the SHELX-XT structure solution program by the direct method and refinpan>ed with the XL refinpan>emenpan>t package by Least Squares minpan>imization. The crystals of 4a and 4c easily effloresced, thus the structures of 4a and 4c were obtainpan>ed from the crystal structures of their respective supramolecular complexes. For compounpan>d 6, sin class="Chemical">ngle crystals could be obtained which were too tiny to be measured by X-ray diffraction.

Theoretical calculations

All the calculations were performed with the Gaussiapan class="Chemical">n 09 software package[44]. The solvent effects of DCM used inpan> experimenpan>t have beenclass="Chemical">pan> considered using the CPCM continuum solvation model[45]. First, in order to ensure the suitable hybrid functional, we compared the theoretical HOMO–LUMO gap calculated by B3LYP, CAM-B3LYP, M062X, and HSEH1PBE[46] with the experimental optical HOMO–LUMO gap (Supplementary Table 4). The optimized structures by different functionals were also compared with the experimental crystallographic data, taking 2a as an example (Supplementary Fig. 51, Supplementary Table 5). Accordingly, the HSEH1PBE functional is more appropriate for the theoretical calculations (Supplementary Figs. 52–55). Then we calculated the electron density differences between the first excitation state (corresponding to the UV-Vis absorption with maximum wavelength and mainly arising from HOMO to LUMO transition) and the ground state for compounds 2–4 by HSEH1PBE.

Data availability

Supplementary crystallographic data for this manuscript have been deposited at the Cambridge Crystallographic Data Centre under deposition numbers CCDC 1580856–1580862. These data can be obtained free of charge from [http://www.ccdc.cam.ac.uk/data_request/cif]. The authors declare that all the data supporting the finpan>dings of this study are available within the article (and Supplementary Information File), or available from the corresponding author on reasonable request.