Fuller-Rylenes: Cross-Dimensional Molecular Carbons

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Dec 2020Fuller-Rylenes: Cross-Dimensional Molecular Carbons Jiajing Feng, Yanan Wu, Qin Yu, Yunpeng Liu, Wei Jiang, Dong Wang and Zhaohui Wang Jiajing Feng Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084 , Yanan Wu Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084 , Qin Yu Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084 , Yunpeng Liu Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084 , Wei Jiang *Corresponding author(s): E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Dong Wang *Corresponding author(s): E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084 and Zhaohui Wang *Corresponding author(s): E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084 University of Chinese Academy of Sciences, Beijing 100049 https://doi.org/10.31635/ccschem.020.202000148 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Fuller-rylenes is a useful model to tailor the properties of cross-dimensional molecular carbons to define their scope for specific applications. Herein we present a straightforward synthetic strategy to hybridize planar rylene dyes and spherical fullerene into esthetic nanostructures containing features from both subunits via one-pot Pd-catalyzed [3 + 2] or [4 + 2] cyclization reactions. Single-crystal X-ray diffraction analysis revealed conclusively the molecular configurations and distinct self-assembly crystal arrangements resulting from the different extension directions of the planar π-systems on the fullerene ball. Our strategy allowed for easy structural and electronic variations; especially, the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) orbital profiles of the two fuller-rylenes, namely, Fuller-PMI and Fuller-PDI (where PMI refers to as perylene monoamide, and PDI, perylene diimide) molecules, determined by combining fullerene with perylene, which featured different edge structures. Download figure Download PowerPoint Introduction Synthetic molecular carbons (SMCs) have been the subject of intense research leading to the birth of innovative materials science.1–3 The most attractive feature of SMCs is the diversity of their topological structures, which include zero-dimensional (0D)-fullerenes, 1D-carbon nanotubes, and 2D-nanographenes with positive or negative curvatures.4–11 Although the focused investigations of various SMCs have changed the perceptions of organic and materials scientists considerably, the atom-precise synthesis of cross-dimensional molecular carbons, such as fullerene/nanographene hybrids, remains a formidable challenge.12–15 Favorably, the discovery of the first molecular allotrope of carbon, the fullerene, provided a fascinating carbon skeleton for endo- and exohedral chemical modifications.16–18 Over the past three decades, a diverse range of reactions have been developed toward functionalized C60 or C70, such as free-radical reactions, a variety of cycloadditions, and recently, palladium (Pd)-catalyzed cyclization and decarboxylative annulation via C–H bond activation.19–23 However, hypothetical fullerene/nanographene hybrids remain a fertile ground, awaiting exploration, likely due to the difficulties associated with their synthesis. We have developed a particular interest in the construction of new forms of molecular carbon by precise bottom-up organic synthesis due to their remarkable electronic and optical functionalities.24–28 Herein, we present a straightforward synthetic strategy to prepare fuller-rylenes, an interesting model to refine the properties and unique applications for cross-dimensional molecular carbons (Figure 1). Specifically, our new fabrication of two molecular carbon derivatives is expected to possess, and potentially enhance, the outstanding physical/chemical properties of spherical fullerenes (e.g., electron mobility, cage functionalization, free-radical or reactive oxygen species (ROS) scavengers, and others), and planar rylene dyes (e.g., UV–vis absorption, edge functionalization, and others).29 Moreover, it would also open the possibility for tuning the electronic structures and frontier orbitals across the integrated hybrids, allowing for the manipulation of supramolecular interactions and their eventual distinct applications as functional materials. The edge topologies of nanographene molecules, such as armchair and zigzag structures, are essential to their chemical and physical properties.30–33 Therefore, the critical issue for the successful construction of cross-dimensional fullerene/nanographene hybrids is establishing a universal method to combine spherical and planar π-molecules with different edges. Here, we chose perylene monoimide (PMI) with a zigzag periphery and perylene diimide (PDI) with armchair bay-region as model compounds. The synthesis of fuller-rylenes was achieved by one-pot Pd-catalyzed [3 + 2] and [4 + 2] cyclization reactions between pristine C60 and the two perylene dyes (PMI and PDI) through peri- and bay-fusion, respectively. Such integrated hybrids, namely Fuller-PMI and Fuller-PDI, should inherit the rich scientific heritage of both fullerene and rylenes. Single-crystal X-ray diffraction analysis revealed the molecular configurations of the fuller-rylenes with five- or six-membered carbon-ring linkages. Also, dense crystal packing arrangements resulting in concave and convex interactions between neighboring C60 and perylene subunits were observed in both fuller-rylenes. Specifically, the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) profiles of Fuller-PMI and Fuller-PDI were determined by combining fullerene with perylene, which featured distinct edge structures. Results and Discussion Synthesis and characterization of the fuller-rylenes Details of the syntheses of the cross-dimensional fuller-rylenes are provided in the Supporting Information Section 2. As shown in Scheme 1, perylene imide is a kind of ideal model due to its perfect edge structures, featuring an intriguing zigzag periphery in PMI and armchair bay-regions in PDI. Inspired by the reported method for arene-fused C60,23 we first integrated C60 into PMI via peri-fusion by [3 + 2] cyclization to form a five-membered carbon-ring linkage. The readily available starting material, PMI, was synthesized according to a previous report.34 Further bromination of PMI in acetic acid with liquid bromine gave peri-substituted perylene monobromide 1 with a relatively high yield of 96%. Then, we mixed electron-deficient perylene monobromide 1 with C60 via one-pot cyclization using Pd(OAc)2 as the catalyst, PCy3·HBF4 as the ligand, and K2CO3 as the base in 1-methylnaphthalene (1-MeNp) at 200 °C, and the target Fuller-PMI was generated in a good yield of 48%. Scheme 1 | Synthetic routes to Fuller-PMI and Fuller-PDI. Download figure Download PowerPoint The next challenge was to explore further cross-dimensional fuller-rylenes syntheses via different edge functionalizations. With bay-substituted perylene monobromide 235 as the precursor in hand, we initially examined the same reaction conditions used for generating Fuller-PMI to construct Fuller-PDI via bay-fusion; however, only a trace amount of the target product could be detected. The results of carefully scanning different palladium catalysts, ligand compounds, and bases are summarized in Supporting Information Table S1. Then, we modified the protocol and discovered eventually that in the presence of 10 mol% Pd(dppf)Cl2 and 20 mol% PPh3, and using 4 equiv of K3PO4 as the base and 1-methylnaphthalene (1-MeNp) as the solvent, Fuller-PDI was generated in moderate yield of 34% at 200 °C. The one-pot domino process evidenced the formation of two new C–C bonds, involving C–H transformation. The two fuller-rylenes were purified easily by standard column chromatography on silica gel and subsequent recrystallization. Fuller-PDI was soluble in common solvents, whereas Fuller-PMI was limited in solubility, even in halogenated or aromatic hydrocarbon solvents. Both fuller-rylenes were characterized by high-resolution mass spectroscopy (HRMS) and nuclear magnetic resonance (NMR) spectroscopy. The measured 1H NMR spectra, shown in Supporting Information Figure S1, coincided well with the aromatic protons in the perylene subunits for both Fuller-PMI and Fuller-PDI, where only one characteristic singlet appeared in the Fuller-PDI. Figure 1 | (a) Representative functionalized C60 derivatives and (b) our strategy toward the generation of cross-dimensional molecular carbons by edge functionalization. Download figure Download PowerPoint Theoretical calculations on the electronic structures of the fuller-rylenes Theoretical calculations were performed to understand the electronic structures of the fuller-rylenes. The energies and shapes of frontier orbitals for model C60, PMI, PDI, and the two fuller-rylenes, calculated by density functional theory (DFT) at the B3LYP/6-31G(d, p) level are shown in Figure 2. Remarkably, Fuller-PMI exhibited distinct "orbital partitioning" properties separated by the five-membered carbon-ring linkage,36,37 with the LUMO entirely localized on the C60 fragment and the HOMO localized on the PMI portion. In contrast, the HOMO of Fuller-PDI was delocalized on the C60 portion and the connecting bonds, whereas the LUMO, although localized mainly on PDI, was also delocalized on the C60 fragment. These results correlated well with the trend of the computed energy levels in which the HOMO and LUMO levels of PDI were substantially lower than those of PMI and close to C60. This unique arrangement resulted in the stronger intramolecular electronic coupling between the PDI and C60 subunits, even in an instance of an sp3 separator. Thus, both PDI and C60 contributed to the frontier molecular orbitals of Fuller-PDI, which determined its corresponding optoelectronic properties and performance. Figure 2 | Energies and shapes of B3LYP/6-31G(d, p) frontier orbitals for model C60, PMI, PDI, Fuller-PMI, and Fuller-PDI. Download figure Download PowerPoint Optical and electrochemical properties of the fuller-rylenes To investigate the optoelectronic properties of the fuller-rylenes constructed by different edge fusions, UV–vis absorption measurements were performed in an ortho-dichlorobenzene (o-DCB) solution with the parent C60, PMI, and PDI for comparison. As portrayed in Figure 3, the two fuller-rylenes displayed entirely different absorption features. Fuller-PMI made via peri-fusion integrated the characteristics of both C60 and PMI with a markedly enhanced light-absorbing capability ranging from 400 to 600 nm. The absorption maximum was apparent at 534 nm, which redshifted 25 nm relative to that of PMI, primarily due to the push–pull effect induced by the perylene dye on the C60 sphere. According to the time-dependent DFT calculations, the maximum absorption peak arose from a HOMO to LUMO+2 transition, which was attributable to a local excitation (LE) by π–π* transitions on the PMI moiety of the complex ( Supporting Information Figure S4). Figure 3 | UV–Vis absorption spectra in o-DCB solution (1 × 10−5 mol L−1) (top) and cyclic voltammograms in an o-DCB solution of C60 (black), PMI (pink), PDI (green), Fuller-PMI (violet), and Fuller-PDI (dark cyan) against the Fc/Fc+ redox couple (Ag/AgCl) electrode as the reference electrode at a scanning rate of 100 mV/s). Download figure Download PowerPoint In contrast, Fuller-PDI displayed broader and more complicated absorption bands ranging from 400 to 650 nm, thus resulting in a significant redshift at the onset of absorption by ∼ 100 nm relative to that of the parent PDI. As revealed by the frontier molecular orbital (FMO) distribution, this broadening, likely, was attributable to an intramolecular electronic coupling between the PDI and the C60 fragments, resulting in evident intramolecular charge-transfer characteristics in the spectral transition. The maximum peak of Fuller-PDI appeared at 534 nm, and although the peak position was the same as that of Fuller-PMI, it was due mainly to the transition from HOMO-4 to LUMO that included both LE and intramolecular charge-transfer excitation (ICTE) processes. Additionally, the absorption peaks ∼ 600 nm originated from two nearly degenerate excitation states involving HOMO-1 to LUMO, and HOMO to LUMO+1 transitions, which combined the LE process between the PDI and C60 fragments and ICTE process from C60 to the PDI unit ( Supporting Information Figure S4 and Table S4). The optical energy gaps of Fuller-PMI and Fuller-PDI, according to their onsets of absorption, were 2.20 and 1.99 eV, respectively ( Supporting Information Table S3). The electrochemical properties of the fuller-rylenes were illuminated by cyclic voltammetry (CV) measurements performed in an o-DCB solution because C60 did not dissolve in dichloromethane (DCM; CH2Cl2). As shown in Figure 3 and Supporting Information Figure S2, both Fuller-PMI and Fuller-PDI showed well-defined, reversible reduction peaks involving five electrons, and Fuller-PMI possessed an additional reversible oxidation peak in the test window derived from the PMI fragment. Supporting Information Table S3 summarizes the electrochemical values of C60, PMI, PDI, Fuller-PMI, and Fuller-PDI. The half-wave potential (E1/2) versus the ferrocene redox couple (Fc/Fc+) for Fuller-PMI: −1.14, −1.50, −1.58, −2.10, −2.28, and 0.87 V, and for Fuller-PDI: −1.05, −1.21, −1.38, −1.65, and −2.22 V. From the results, the first half-wave reduction potential of Fuller-PMI is almost the same as that of C60, and thus, the estimated LUMO energy level was−3.80 eV, demonstrating that Fuller-PMI is rather fullerene-like than PMI-like. Additionally, the first half-wave oxidation potential of Fuller-PMI is closer to that of PMI, and the estimated HOMO energy level was −5.67 eV. On the other hand, the estimated LUMO energy level of Fuller-PDI was −3.88 eV, lower than that of both C60 and PDI, indicating the higher electron affinity of Fuller-PDI, and illustrating further the electronic coupling between the C60 and PDI subunits. The HOMO energy level, calculated according to the LUMO energy level and optical bandgap, was approximately −5.87 eV (Table 1). As revealed by the quantum chemistry calculations, the frontier orbital distributions were consistent with the electrochemical properties of the two fullerene derivatives, and our investigations indicated that the electronic structures and frontier orbital profiles of these fullerene/rylene hybrids could be tuned by combining different π-systems. Table 1 | Optical Properties and Frontier Orbital Energy Levels of C60, PMI, PDI, Fuller-PMI, and Fuller-PDI in Solution Compound λmax(nm)a ɛ(M−1 cm−1)a ELUMO(eV)b EHOMO(eV)b C60 334 74,500 −3.77 −5.96c PMI 509 30,600 −3.34 −5.68 PDI 531 83,200 −3.72 −6.00c Fuller-PMI 534 59,000 −3.80 −5.67 Fuller-PDI 534 24,600 −3.88 −5.87c aMeasured in o-DCB solution (1 × 10−5 mol L−1). bEstimated from the onset of the first reduction or oxidation peak and calculated according to ELUMO = −(4.8 + Eonsetre) eV or EHOMO = −(4.8 + Eonsetox) eV; the Eonset values are versus Fc/Fc+. cCalculated according to EHOMO = (ELUMO − Egopt) eV. Single-crystal structures and packing arrangements of the fuller-rylenes Single crystals of both Fuller-PMI and Fuller-PDI suitable for X-ray analysis were obtained to assess the edge-fusion effect on their structural features. The perylene skeleton stood on or fused onto the 6∶6 ring junctions of the C60 sphere in the Fuller-PMI or Fuller-PDI molecule, respectively, was observed unambiguously in their crystal structures, as shown in Figure 4. Due to the convex and concave interactions between the spheric and planar moieties, the perylene subunits in both of the molecules exhibited a slight bowl-shaped conformation. Additionally, the curvature based on the single-crystal geometry was evaluated by the convenient method of π-orbital axis vector analysis (POAV1), based on the sigma (σ) bond angles,38–42 and the pyramidalization angles, defined as Θσπ − 90°, of each internal sp2 carbon are given in Figures 4a and 4e to describe the local curvature. The results showed that Fuller-PMI had an average local curvature for the six sp2 carbons of the central benzene of 0.8° and highest local curvature of 1.7°, whereas Fuller-PDI had an average local curvature of 0.5° and highest local curvature of 1.1°. As a result, the bowl depth of the perylene core in Fuller-PDI (0.33 Å and 0.57 Å, calculated from the two nitrogen atoms to the mean plane of the central benzene), was shallower than that of Fuller-PMI (0.85 Å), which was comparable with that of corannulene (0.87 Å).43 The larger curvature of the perylene core in Fuller-PMI was possibly due to improved shape recognition from π-extension along the long molecular axis and intense D–A interaction from the pronounced difference in electronic properties between the PMI and C60 constituents. Figure 4 | Single-crystal structures of Fuller-PMI and Fuller-PDI: (a and e) top view and POAV1 pyramidalization angles (Θσπ − 90°); (b and f) side view and bowl depth of perylene core; (c and g) pairs of adjacent molecules with convex and concave overlapping along the π–π stacking direction; (d and h) concave and convex packing arrangements (along the a-axis for Fuller-PMI and c-axis for Fuller-PDI). Thermal ellipsoids indicate a 50% probability. Purple, green, and gray atoms are carbon; blue and orange are N and O, respectively. Hydrogen atoms and long alkyl chains are omitted for clarity. Download figure Download PowerPoint Furthermore, a unique pair with a head-to-tail antiparallel arrangement was formed in Fuller-PMI, with short π–π distances of 3.13–3.36 Å between the concave–convex pair. There were also intense π–π interactions ranging from 3.20 to 3.35 Å between the convex–convex pair. In addition to the aforementioned strong interactions across the sphere and the plane, short contacts of 3.18–3.36 Å between fullerenes within columns were observed, together with close C–H•••π interactions of 2.20–2.89 Å between columns, as shown in Figure 4d. In particular, this induced weak chirality in the perylene skeleton between the pairs, which could be assigned to M- and P-enantiomers (Figures 4c and 4d, M showed in purple and P in gray). Figure 4d depicts the packing mode of Fuller-PMI along the a-axis, in which the M- and P-enantiomers are arranged alternatively. Compared with Fuller-PMI, a repeated pattern of the π-stacking sequence of the M– Penantiomers along the c-axis (Figures 4g and 4h, M shown in green and P in gray) could, also, be observed in Fuller-PDI; however, the perylene bowl between the adjacent columns was bent in the opposite direction, different from the same-directional bowl generated in Fuller-PMI. Besides, there were multiple π–π interactions of 3.19–3.39 Å between the concave–convex pair and 3.22–3.37 Å between the convex–convex pair. Besides, in the absence of fullerene–fullerene interactions, however, a π–π contact of 3.33 Å in the perylene core, C=O•••π contact of 3.04 Å, and C–H•••π contacts of 2.72–2.88 Å were observed between the adjacent arrays. Experimental Methods Experimental Methods are available in the Supporting Information. Conclusion We have presented a straightforward synthetic strategy toward the fabrication of fuller-rylenes, a cross-dimensional molecular carbon system, by edge functionalization. The synthesis and purification of the fuller-rylenes were facile and could be scaled up. The perylene skeletons exhibited a slightly bowl-shaped conformation in both of the fuller-rylenes, in which concave and convex packing arrangements between spheric and planar moieties were observed. Additionally, different molecular configurations and intermolecular arrangements arose from the different extension directions of the planar π-systems on the fullerene ball. Especially, the LUMO and HOMO orbital profiles of Fuller-PMI and Fuller-PDI were determined by combining fullerene and perylene with different edge structures. Our exploration paves the way for the synergetic advantages of combining homologous series of rylene dyes integrated with different types of fullerenes, which might become a new generation of promising candidates for applications in optoelectronics and free radical scavenging activities in medicine. Supporting Information Supporting Information is available. Conflict of Interest The authors declare no competing financial interests. Funding Information This work was supported by the National Key R&D Program of China (2017YFA0204701) and the National Natural Science Foundation of China (NSFC) (nos. 21790361, 21734009, 51673202, and 21901138). References 1. Segawa Y.; Levine D. R.; Itami K.Topologically Unique Molecular Nanocarbons.Acc. Chem. Res.2019, 52, 2760–2767. Google Scholar 2. 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