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. 2025 Apr 18;3(9):535–540. doi: 10.1021/prechem.5c00032

Synthesis of Chrysene-Based Nanographenes by a Successive APEX Reaction

Yuichi Nakashige †, Hidefumi Nakatsuji †, Kaho Matsushima ‡, Kazuo Murakami †, Hideto Ito ‡,*, Kenichiro Itami ‡,#,§,*
PMCID: PMC12458009  PMID: 41001094

Abstract

Chrysene-based nanographenes (ChrNGs), despite their relatively small structures, have been reported to exhibit low highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gaps and strong long-wavelength fluorescence, making them attractive for various applications. However, the precise synthesis of ChrNGs remains challenging, and their availability is limited compared with other classes of nanographenes. Herein, we report the synthesis of novel ChrNGs by a successive annulative π-extension (APEX) reaction. Using diphenylacetylene and benzonaphthosilole in a Pd/o-chloranil catalytic system, successive APEX afforded ChrNGs of various lengths and degrees of oxidation. Furthermore, exhaustive separation and further π-extension by cyclodehydrogenation afforded ChrNGs with more flat and rigid structures. Photophysical measurements of the obtained ChrNGs showed a variety of absorption and emission properties, including intense multicolor emission.

Keywords: nanographene, chrysene, polycyclic aromatic hydrocarbon, direct arylation, annulative π-extension

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Introduction

Nanographenes and related polycyclic aromatic hydrocarbons (PAHs) have attracted much attention in organic synthesis, physical organic chemistry, and materials science because of their extraordinary electronic, optical, and mechanical properties. − According to various established synthetic methods such as cross-coupling reactions and Scholl reactions, easily available aromatic building blocks have been successfully used to produce larger PAH structures. Thus, derived nanographenes sometimes possess unique structural, photophysical, and radical characteristics from the original PAH cores. For example, perylene-based nanographene, − so-called rylene, possesses a very narrow highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gap (Figure , A). Corannulene-based giant nanographene − exhibits a grossly warped structure based on its bowl-shaped mother corannulene and peripheral seven-membered rings (Figure , B). Pyrene-based nanographenes, such as hexabenzo­[a,c,fg,j,l,op]­tetracene, − possess a large HOMO–LUMO energy gap and a twisted structure induced by four cove-regions and have recently been in the spotlight because of their excellent hole-transporting ability and robustness in organic light-emitting diodes (Figure , C). Chrysene-based nanographenes (ChrNGs) have been reported to exhibit small HOMO–LUMO energy gaps and strong long-wavelength emission, which are expected to lead to applications in light-emitting materials and bioimaging. − ChrNGs are synthesized by a sequential iodination-benzannulation of bi­(naphthylphenyl)­diyne, followed by photochemical cyclodehydroiodination (Figure , D), palladium-catalyzed intramolecular arylations of 7,14-di­(2-chlorophenyl)­zethrene, and the oxidative cyclodehydrogenation of 7,14-diarylzethrenes mediated by iron­(III) chloride and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (Figure , E and F). These are followed by a process involving denitrative alkenylation and intramolecular cyclodehydrogenation (Figure , G), and the palladium-catalyzed annulative π-extension (APEX) reaction of chrysene with diiodobiphenyl (Figure , H). While these methods have enabled precise synthesis of ChrNGs, examples of their synthesis remain limited compared to other nanographenes, and the availability of ChrNGs is thus highly restricted. This could be due to the low availability of chrysene as a starting material and the lack of construction methods for chrysene. Therefore, the development of diverse methods for ChrNG synthesis is essential to advance the research and applications of nanographenes.

1.

1

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Reported perylene-, corannulene-, pyrene-, and chrysene-based nanographenes A–H.

Previously, we developed various APEX reactions for efficient synthesis of nanographenes and polycyclic aromatic compounds. − ,− The APEX reaction involves the π-extension of nonfunctionalized template aromatic compounds and the construction of one or more new aromatic rings in a single step. We previously demonstrated Pd-catalyzed APEX reactions of phenanthrene/pyrene derivatives or diphenylacetylene (2) using dimethyldibenzosilole 1 as a π-extending agent (Figure a). In these reactions, the most olefinic K-region (convex armchair edge) and reactive alkyne moiety are selectively π-extended to various phenanthrene- and pyrene-based nanographenes, but further π-extension and synthesis of ChrNGs have not been achieved in these APEX systems. Herein, we report the synthesis of ChrNGs by a successive APEX reaction using diphenylacetylene (2) and the newly designed benzonaphthosilole 3 (Figure b) as a π-extending agent. We envisioned that the APEX of alkyne 2 with silole 3 would afford diphenylchrysene 4 as an initial product, and then the successive APEX reaction with silole 3 might occur at a newly generated K-region on 4, leading to further π-extended ChrNGs. After examining various reaction conditions and performing further derivatization via cyclodehydrogenation, five ChrNGs isomers composed of a common chrysene core were separated and isolated, and photophysical measurements were performed and compared. Consequently, a series of newly synthesized ChrNGs exhibited diverse and dramatic changes in their photophysical and electronic properties, albeit with slight differences in ring fusion.

2.

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(a) Our previous reports on APEX reactions and (b) this work.

Results and Discussion

Successive APEX Reaction and Synthesis of ChrNGs

Based on a previously reported catalytic system for the APEX reaction of dibenzosilole derivatives with Pd catalysts and o-chloranil, , we examined the synthesis of ChrNGs via successive APEX reactions using diphenylacetylene (2) and benzonaphthosilole 3. In the initial screening using the Pd catalyst, Ag salt, and o-chloranil, the reaction of alkyne 2 with silole 3 gave a variety of oligomeric products, as determined by MALDI-TOF MS analysis. To isolate and identify the distinct oligomer structures, various conditions of Pd salts, Ag salts, catalyst loadings, solvents, reaction temperatures, and reaction times were examined. As a result, the reaction of diphenylacetylene 2 (1.0 equiv) with benzonaphthosilole 3 (2.0 equiv), 20 mol % Pd­(OAc)2, 40 mol % AgSbF6, and o-chloranil (4.0 equiv) in toluene at 40 °C for 2 h resulted in full conversion of the starting materials and formation of ChrNGs with relatively small molecular weights (m/z = 436–1206 in MALDI-TOF MS) (Figure a and S8 in Supporting Information (SI)). While the m/z peak at 692 (C54H44) was a major peak in the MALDI-TOF MS spectrum, repeated size-exclusion chromatography (SEC) resulted in the isolation of the alkyne-APEX product 4 in 29% yield and ChrNG 5 (C55H44) in 1.6% yield. In this reaction, further elongation and cyclodehydrogenation occurred simultaneously, which were observed as mass peaks at m/z = [M + (C21H16) n ]+ and [M – (2H) n ] n . Owing to the complicated reaction profile and similar retention time of each product in the SEC separation, the isolated yield of 5 was lower than that of the crude mixture, and it was difficult to isolate other longer oligomers.

3.

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(a, b) Synthesis of ChrNGs by successive APEX of diphenylacetylene (2) with benzonaphthosilole 3. X-ray crystallographic structures of (c) 4, (d) 6, and (e) 8.

The structure of diphenylchrysene 4 was clearly confirmed by single-crystal X-ray diffraction analysis (Figure c), and the chrysene structure was clearly constructed using the APEX of 2 with 3. Comparing the bond lengths of each aromatic peripheral region, the bond length between C11 and C12 (K-region) was 0.016–0.046 Å shorter than those in other regions. These findings suggest that the C11–C12 bond exhibits the most olefinic character and is the most reactive site in the APEX reaction. Density functional theory (DFT) calculations of intermediate 4 at the B3LPY/6-31G­(d) level of theory revealed that the HOMO and LOMO tend to be delocalized in the K-region (see Figure S14 in SI), which allow K-region selective direct arylation via the coordination of an aryl-palladium species.

Moreover, the oxidative cyclodehydrogenation reaction (Scholl reaction) of ChrNG 5 with DDQ and triflic acid (TfOH) resulted in further π-extended ChrNGs. Depending on the quantity of these reagents, ChrNG 6 (C55H40) and 7 (C55H36) were isolated as the major products. The structure of 6 was confirmed by X-ray diffraction analysis, which revealed that ChrNG 6 has 9 planar aromatic rings (blue color-filled rings) and a twisted terminal naphtho-fused moiety (red color-filled rings) attributed to a fjord-region ([5]­helicene moiety). The structure of 7 was determined using NMR including 1H–1H COSY NMR spectroscopy.

Furthermore, ChrNG 8 was isolated in 1.6% by the APEX reaction under slightly modified conditions in 1,2-dichloroethane (Figure b). In this reaction, the alkyne-APEX product 4, ChrNG 5, other longer oligomers, and oxidized products were produced (see the MALDI-TOF MS spectra in Figure S9). While the potential yield of 8 can be higher than the isolated yield because the mass peak intensity of 8 was the second highest in the MALDI-TOF MS. However, it was difficult to separate 8 from other structurally similar isomers and oligomers by silica gel column chromatography and SEC as in the case of isolation of 5. The formation of ChrNG 8 can be rationalized by the successive APEX reaction followed by simultaneous in situ oxidation of 5 in the fjord-region in the presence of excess amounts of Pd and Ag salts. Previously, we observed a similar in situ oxidation in the fjord-region when chrysene was employed in a Pd-catalyzed K-region-selective APEX. An almost planar nanographene structure (blue color-filled rings in Figure e) was elucidated by X-ray crystallographic analysis of ChrNG 8. Interestingly, unlike the cyclodehydrogenation of 5, the Scholl reaction of 8 with DDQ primarily afforded ChrNG 9 (C55H36), a structural isomer of ChrNG 7.

Photophysical Properties

The UV–vis absorption and fluorescence spectra of ChrNGs 5, 6, 7, 8, and 9 were measured in dichloromethane to elucidate the relationship between their structural differences and photophysical properties (Figure a–c). These solutions exhibited well-resolved absorption bands between 400 and 650 nm with a weak absorption shoulder peak between 410 and 440 nm (ChrNG 5, molar absorptivity (ε) at λ = 420 nm (ε 420) = 1.06 × 10 3 M–1·cm–1), and absorption maxima at 481 nm (ChrNG 8, ε 481 = 1.47 × 10 4 M–1·cm–1), 486 nm (ChrNG 6, ε 486 = 2.48 × 10 4 M–1·cm–1), 553 nm (ChrNG 9, ε 553 = 3.17 × 10 4 M–1·cm–1), and 578 nm (ChrNG 7, ε 578 = 1.53 × 10 4 M–1·cm–1).

4.

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Photophysical properties and electronic structures of ChrNGs 5, 6, 7, 8, and 9. (a) UV–vis absorption spectra of the ChrNGs in CH2Cl2. (b) Emission spectra with excitation wavelengths 380 nm for 5, 430 nm for 6, 520 nm for 7, 430 nm for 8, and 500 nm for 9 and (c) emission colors of the ChrNGs in CH2Cl2 under irradiation of a 365 nm UV light. (d) Pictorial frontier molecular orbitals (isovalue = 0.02) and possible transitions calculated by TD-DFT. Geometry optimizations of model compounds 5′, 6′, 7′, 8′, and 9′, which possess Me groups instead of n Bu groups, were calculated at the B3LYP/6-31G­(d) level, and single-point calculations of the optimized structures were at performed at the B3LYP/6-311++(d,p) level.

Next, time-dependent density functional theory (TD-DFT) calculations were conducted using model compounds (ChrNG 5′, 6′, 7′, 8′, and 9′) in which the butyl groups were replaced with simple methyl groups at the B3LYP/6-311++G­(d,p)//B3LYP/6-31G­(d) level of theory (Figure d). The calculation results indicated that the absorption in the long-wavelength regions of ChrNGs 6, 7, 8, and 9 can be attributed mainly to the allowed HOMO–LUMO transitions with relatively large oscillator strength (f) values (f = 0.363–0.500), whereas the weak shoulder peak observed in ChrNG 5 at 410–440 nm can be attributed to a forbidden-like HOMO–LUMO transition with a small f value (f = 0.0205). Actually, the relatively small ε value was also observed in ChrNG 5. The relatively sharp absorption peaks observed at the shorter wavelength region (e.g., ChrNG 6: 456 nm; ChrNG 7: 539 nm; ChrNG 8: 451 and 426 nm; ChrNG 9: 514 and 480 nm) could be derived from the transitions to vibrational levels due to the rigid frameworks of ChrNGs 6, 7, 8, and 9, which are typically observed in the rigid nanographene structures. , Reflecting the absorption characteristics and wavelengths, the ChrNGs exhibited weak blue emission with an emission maximum at 458 nm (ChrNG 5, quantum yield (Φ 458) = 0.05) and strong emissions with emission maxima at 496 nm (ChrNG 8, blue-green, Φ 496 = 0.48), 511 nm (ChrNG 6, green, Φ 511 = 0.57), 563 nm (ChrNG 9, yellow, Φ 563 = 0.49), and 600 nm (ChrNG 7, orange, Φ 600 = 0.50).

It is reasonable to assume that ChrNGs with a higher degree of oxidation (6–9) exhibit intense emissions based on transitions to vibrational levels, which can be attributed to the rigid nanographene structures of ChrNGs in both the ground and excited states. Compared to isomers with the same molecular weight (6 vs 8 and 7 vs 9), five-membered ring fusion at peri-positions significantly altered the electronic structure and decreased the HOMO and LUMO energy levels. According to the depictions of the HOMO and LUMO orbitals of 6′ and 7′, the presence of the fluorene-substructure seems to elongate the π-conjugation over entire molecules, which can narrow the HOMO–LUMO gap and induce red-shifts in absorption and emission.

Interestingly, compounds 6/6′ and 7/7′ exhibited stark contrast emission colors and HOMO/LUMO levels compared to those of the complete structural isomers 8/8′ and 9/9′. In the Scholl reaction of 5 to 6, the tetrabenzo­[c,g,l,p]­chrysene core of 5 is π-extended to the indeno­[1,2,3-lm]­dinaphtho­[1,2-b:1’,2’,3’,4’-pqr]­perylene core (in 6) with a large LUMO energy decrease (−0.65 eV) compared to the HOMO energy change (+0.12 eV) (Figure ). This was attributed to the emergence of the fluoranthene core (five-membered ring peri-benzo-fusion), which is also a well-known phenomenon in π-extension of related polycyclic aromatic compounds. − In contrast, comparable decreasing LUMO energy levels and increasing HOMO energy levels were observed in the formation of the benzo­[lm]­chryseno­[1,12,11,10-opqrab]­perylene core (in 8). This can also be explained by the typical hexagonal ring fusion, which is often observed in nanographene synthesis. Finally, π-extensions from 6 to 7 and 8 to 9 resulted in comparable HOMO/LUMO energy increasing/decreasing because both π-extensions involved hexagonal ring fusions. As a result, ChrNG 7 possesses a lower LUMO energy level owing to the five-membered peri-benzo-fusion than ChrNG 9, which has all hexagonal ring fusions. These aspects of HOMO/LUMO variation, depending on slight structural changes, are beneficial for the design and implementation of nanographene-based organic electronic devices.

5.

5

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Changes in HOMO/LUMO energy levels in π-extension.

Conclusions

In this study, we synthesized various ChrNGs via successive APEX reactions using diphenylacetylene and benzonaphthosilole in a Pd/o-chloranil catalytic system. The APEX reaction first occurred between diphenylacetylene and benzonaphthosilole to produce diphenylchrysene as the initial intermediate. Successive APEX reactions occurred in the K-region of this intermediate to afford a mixture of ChrNGs of various lengths and degrees of oxidation. Highly fused ChrNGs were also synthesized via cyclodehydrogenation. The optoelectronic properties of the obtained ChrNGs by spectroscopic measurements and DFT calculations revealed their diverse absorption and emission properties in the visible light region and their intense multicolor emission properties based on their rigid structures.

Supplementary Material

pc5c00032_si_001.pdf (3MB, pdf)
pc5c00032_si_002.cif (812.8KB, cif)
pc5c00032_si_003.cif (992.9KB, cif)
pc5c00032_si_004.cif (629.8KB, cif)

Acknowledgments

This study was supported by the JST-ERATO program (JPMJER1302 to K.I.), JSPS KAKENHI (JP1905463 to K.I., JP21H01931 to H.I.), Sumitomo Foundation (2300884), Foundation of Public Interest of Tatematsu (22B025 to H.I.), Kondo Memorial Foundation (2022-03 to H.I.), and NAGAI Foundation of Science & Technology (to H.I.). We thank Dr. Keigo E. Yamada, Dr. Hiroki Shudo, Mr. Daiki Imoto, and Mr. Takato Mori (Nagoya University) for their assistance with the X-ray diffraction analysis. DART-MS measurements were conducted using resources from the Chemical Instrumentation Facility (CIF), Research Center for Materials Science (RCMS), Nagoya University. The computations were performed at the Research Center for Computational Science, Okazaki, Japan (Project Nos. 23-IMS-C061 and 24-IMS-C059).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/prechem.5c00032.

  • Experimental procedures, 1H and 13C NMR spectra, data characterization of all new compounds, optical properties, and computational data (PDF)

  • Crystallographic information for 4 (CIF)

  • Crystallographic information for 6 (CIF)

  • Crystallographic information for 8 (CIF)

The authors declare no competing financial interest.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

pc5c00032_si_001.pdf (3MB, pdf)
pc5c00032_si_002.cif (812.8KB, cif)
pc5c00032_si_003.cif (992.9KB, cif)
pc5c00032_si_004.cif (629.8KB, cif)

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