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. 2025 Sep 1;5(9):4281–4287. doi: 10.1021/jacsau.5c00653

Benzo-Extended [n]Phenacenes: e‑Flow Synthesis and Length-Dependent Properties

Qiang Wang †, Wei-Zhen Wang †, Ruiying Zhang †, Zi’ang Zhai †, Xinyu Chen †, Minggang Li †, Yi Jin †, Lingyun Zhu †, Yuanming Li †,‡,*, Ke-Yin Ye †,*
PMCID: PMC12458015  PMID: 41001632

Abstract

This study presents an efficient method for synthesizing twisted benzo-extended [n]­phenacenes ([n]­BPs) featuring an electrochemical flow (e-flow) Scholl reaction of the corresponding [n]­BP precursors from a one-pot three-component Suzuki–Miyaura coupling reaction. The e-flow Scholl reaction offers advantages such as reduced oxidant usage and overoxidation byproducts, and easy scale-up through extended electrolysis time toward these intricate polycyclic aromatic hydrocarbons. In addition, the increase in molecular length decreases the optical bandgap of [n]­BPs and thus tunes their photophysical properties. This work provides a green and sustainable synthetic strategy for diverse [n]­BPs and enables facile bandgap modulation through π-conjugation extension, offering potential for organic semiconductor applications in optoelectronic devices.

Keywords: Continuous flow, electrosynthesis, Scholl reaction, phenacene, GNRs

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Introduction

The Scholl reaction is among the most adopted reactions directly forging carbon–carbon bonds between adjacent aromatics without additional linkers or noble metal catalysts. However, the requisite uses of excess chemical oxidants in this transformation, e.g., FeCl3 or 2,3-dichloro-5,6-dicyano­benzo­quinone (DDQ), bring forth not only atom-economic and environmental concerns but also unproductive overoxidation, distracting it away from scaled preparation (Scheme a). The electrochemical Scholl reaction has emerged as an eco-friendly alternative due to its mild conditions, high efficiency, and minimized side reactions. In particular, we recently demonstrated that integrating flow electrolysis into the Scholl reaction offers additional advantages such as short reaction times, facile parameter adjustment, reduced electrolyte usage, and easy scale-up through extended electrolysis time. Nevertheless, the synthetic potential of this electrochemical flow (e-flow) Scholl reaction in preparing intricate functional polycyclic aromatic hydrocarbons (PAHs), especially for scaled preparation, remains underdeveloped.

1. a) Typical Reaction Conditions of Scholl Reactions. b) Representative Synthetic Methods of [n]­Phenacenes. c) Current Work.

1

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Ladder-type conjugated molecules with precisely tunable band gaps have emerged as next-generation semiconductors due to their exceptional photoelectric properties. Among them, [n]­phenacenes stand out as the thinnest graphene nanoribbons (GNRs) composed of fused benzene rings in a repeating armchair-shaped pattern and have attracted increasing attention because of the wide application of [n]­phenacenes in semiconductors, including field-effect transistors (FETs), organic light-emitting diodes (OLEDs) and organic photovoltaics. Particularly, the field-effect mobility (μ) of [n]­phenacene-based FETs increases with π-conjugation extension, highlighting the demand for efficient synthetic strategies for extended [n]­phenacenes. The general synthetic strategies for well-defined [n]­phenacenes involve the sequential formation of a linear precursor followed by a ring-closing step (Figure S1). For instance, established methods for [n]­phenacenes synthesis include Wittig reaction, or Perkin condensation, followed by Mallory reaction (Scheme b). Alternatively, Suzuki-Miyaura coupling followed by PtII-catalyzed aromatization or C–H bond activation offers another synthetic route. However, the synthesis and application of precisely defined, longer [n]­phenacenes are still limited because of their low solubility and challenging large-scale synthesis.

In this study, we investigated the length-dependent photophysical properties of twisted benzo-extended [n]­phenacenes ([n]­BPs) through a one-pot three-component Suzuki-Miyaura coupling, termed as the ‘shotgun’ approach, followed by an e-flow Scholl reaction (Scheme c). Three [n]­BPs ([6]­BP, [13]­BP, and [20]­BP) with both armchair and cove edge structures were obtained (n indicates the number of longitudinally fused benzene rings along their backbone). This simplified synthetic method enriches the diversity of phenacene derivatives even in hundred-milligram-scale preparation. Furthermore, comprehensive theoretical and experimental investigations demonstrate that the defined edge structure and length significantly influence the electronic structures and photophysical properties of these intriguing molecules.

Results and Discussion

To construct the anticipated [n]­BPs, 2-bromo biphenyl 1 was introduced as the end block, while 2,7-dibromo-1,8-diarylphenanthrene 2 and naphthalene-2,6-diboronic acid pinacol ester 3 were used as the key link block and the functional block, respectively (Scheme a). Notably, 2 was readily synthesized in two steps on a gram scale (Scheme S1). tert-Butyl substituents were installed to enhance the solubility of the intermediates and the corresponding [n]­BPs. Subjecting 1, 2, and 3 to the Suzuki-Miyaura coupling reaction resulted in the [n]­BP precursors in 37% (4a), 20% (4b), and 8% (4c), respectively.

2. a) Synthetic Routes b) MALDL-TOF MS of [20]­BP .

2

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a Reaction conditions: (I) 1 (2.4 equiv), 2 (1.0 equiv), 3 (2.4 equiv), Pd­(PPh3)4 (0.5 equiv), K2CO3 (25 equiv), 1,4-dioxane:H2O = 5:1; 90 °C, 60 h, isolated yields. (II) Cond. a: FeCl3, r.t., 2 h; Cond. b: DDQ, CF3SO3H, CH2Cl2; Cond. c: in an undivided cell, carbon felt electrodes, n Bu4NBF4 (0.1 M), DCM: TFA = 9:1, DDQ (20 mol %), r.t., I = 8 mA, 1 h; Cond. d: in an e-flow reactor, a carbon anode, a Ni cathode, n Bu4NBF4 (0.067 M), DCM: TFA= 9:1, DDQ (10 mol %), r.t., E cell = 1.6 V, t R = 2.5 min for [6]­BP and E cell = 1.5 V, t R = 5 min for [13]­BP.

We anticipated that the e-flow Scholl reaction should serve as a green and sustainable alternative to its chemical variant for the downstream ring closure step. To optimize the reaction conditions, 4a was used as a model compound. Indeed, [6]­BP was obtained in 60% yield under the e-flow Scholl conditions (E cell = 1.8 V, v = 0.3 mL min–1, t R = 0.83 min) with 10 mol % of DDQ using a carbon graphite anode and a Ni cathode in dichloromethane/trifluoroacetic acid mixed solvent (Table , entry 1). Cyclic voltammetry studies suggested DDQ served as a redox mediator in the electrochemical oxidation of 4a. The Mulliken population analysis at B3LYP/6–31G­(d) level suggested that the naphthalene moiety of 4a bears the most negative charge and should be the most susceptible site toward initial oxidation (see Supporting Information). In the absence of DDQ, the direct electrochemical oxidation also led to the desired [6]­BP, albeit in much lower yield (entry 2). The presence of trifluoroacetic acid is pivotal to the anticipated reactivity because it not only readily enhances the oxidation potential of DDQ but also acts as a proton donor for the hydrogen evolution reaction at the cathode. One notable feature of this e-flow Scholl reaction is that only a small amount of electrolyte is sufficient to maintain good reaction yield (entry 3). Extensive reaction optimization, including the cell voltage, flow rate, and dose of DDQ, did not afford higher reaction yields (entries 4–6). Notably, we found that concurrently reducing the cell voltage to 1.6 V and flow rate to 0.1 mL min–1 afforded the anticipated [6]­BP in 71% yield (entry 7). In addition, we also conducted constant-current electrolysis. While 4 mA afforded only a slightly lower yield than our standard conditions (entry 8), the yield of electrolysis at 5 mA was dramatically reduced (entry 9). Meanwhile, to maintain the constant current, the cell potential continued to increase owing to the concomitant increase in cell resistance, which ultimately led to unproductive side reactions.

1. Optimization of Reaction Conditions .

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Entry Deviation from standard conditions Yield (%)
1 None 60
2 Without DDQ 30
3 n Bu4NBF4 (0.1 M) 55
4 DDQ (20 mol %) 55
5 E cell = 1.75 V 47
6 v = 0.2 mL min–1 51
7 E cell = 1.60 V, v = 0.1 mL min–1 71
8 I = 4 mA, v = 0.1 mL min–1 68
9 I = 5 mA, v = 0.1 mL min–1 51
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a

Yields determined by 1H NMR analysis using 1,3,5-trimethoxybenzene as the internal standard.

b

Isolated yield.

As a comparison, the chemical Scholl cyclization of 4a with excess FeCl3 only gave [6]­BP in 11% yield (Cond. a). Switching to another typical Scholl reaction condition, i.e., DDQ (2.4 equiv)/CF3SO3H, was less effective (5%, Cond. b). In both reactions, considerable amounts of unidentified overoxidation byproducts were observed by thin-layer chromatography and nuclear magnetic resonance spectroscopy because of excess oxidants and harsh conditions. In addition, we also performed a batch electrochemical Scholl reaction (I = 8 mA) with a catalytic amount of DDQ in an undivided cell, but only obtained [6]­BP in 20% yield (Cond. c). The clean reaction and an improved yield render our e-flow Scholl reaction a powerful tool suited for scale-up preparation of PAHs, a task that is typically challenging for conventional Scholl reactions. Notably, the flow electrolysis of 4a afforded 230 mg [6]­BP in 70% yield simply by extending the electrolysis time without changing the reaction conditions and setups. The long-term operational stability of this e-flow setup was further evaluated by continuously running 10 standard reactions in 25 h (see Supporting Information). Moreover, the fact that only 11 mg of DDQ in this e-flow reaction readily generated 230 mg of [6]­BP represents a significant improvement over its chemical variant, which needs superstoichiometric oxidants, leading to severe environmental concerns and tedious product purifications.

Similarly, the e-flow Scholl reaction of 4b (E cell = 1.5 V, v = 0.05 mL min–1, and t R = 5 min) afforded the π-extended [13]­BP in 40% yield, much higher than reactions with FeCl3 (15%), DDQ (8%), or batch electrolysis. Although without well-resolved 1H and 13C NMR spectra because of its poor solubility, the formation of [20]­BP with stoichiometric DDQ was proved by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) (Scheme b). Besides twisted benzo-extended [n]­phenacenes, we found that this e-flow Scholl approach could also be readily transferable for the facile preparation of diverse poly­(hetero)­aromatic hydrocarbons (see Supporting Information).

Single crystals of [6]­BP suitable for X-ray diffraction measurements were obtained by slow vapor diffusion in CH2Cl2 and MeOH. The X-ray diffraction analyses indicate that the intramolecular distortion increases the solubility of this nonplanar molecule. After forming a 6-membered ring, the benzenoid rings adopt an alternating “up-down” conformation with the presence of bulky tert-butyl groups on the fjord edges of [6]­BP (Figure a). Due to structural distortion and steric hindrance, there is no π-π stacking interaction in the encapsulation structure of [6]­BP (Figure b). Instead, the dominant intermolecular forces are C–H···π interactions, with the closest distance between neighboring molecules being 2.72 Å. These interactions enable the molecules to connect tightly, forming a network-like structure.

1.

1

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(a) X-ray crystal structures of [6]­BP (thermal ellipsoids at 50% probability; solvent molecules omitted for clarity). (b) Packing structure of [6]­BP (hydrogen atoms omitted for clarity).

Properties and DFT Calculations

To determine the electronic properties of [6]­BP and [13]­BP, cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were carried out (Figure ). Both [6]­BP (E ox1 = – 1.25 V and E red1 = – 1.42 V) and [13]­BP (E ox2 = – 1.28 V and E red2 = – 1.48 V) display quasi-reversible redox processes. To further gain a deeper understanding of the length-dependent photophysical properties of [6]­BP and [13]­BP, UV–vis absorption and fluorescence emission measurements were conducted to provide insights into the length-dependent photophysical properties of [6]­BP and [13]­BP (Figure ). In comparison with [6]­BP (λabs = 367 nm), the laterally extended homologue [13]­BP exhibited a red-shifted absorption profile (λabs = 392 nm). The maximum fluorescence emissions of 438 and 455 nm were observed for [6]­BP and [13]­BP, respectively. Transient photoluminescence decay curves reveal an average fluorescence lifetime of 6.9 ns for [6]­BP (ΦF = 6%) and 11.4 ns for [13]­BP (ΦF = 15%), respectively.

2.

2

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CV (full line) and DPV (dotted line) of [6]­BP and [13]­BP in CH2Cl2 solution (1 × 10–4 mol/L) containing 0.1 M n Bu4NPF6 at room temperature at a scan rate of 0.1 V/s.

3.

3

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UV–vis absorption (full line) and fluorescence spectra (dotted line) of [6]­BP and [13]­BP (1.0 × 10–5 M in CH2Cl2).

To further elucidate their photophysical properties, we calculated the energy levels of [6]­BP and [13]­BP using time-dependent density functional theory (TD-DFT) at the B3LYP/6–31G­(d) level of theory (Figure , Figures S8 and S9). Visualization of the results was performed with Multiwfn and VMD software. The calculations reveal a weak band centered at 394 nm for S0→S2 excitations of [13]­BP in which the major contribution to these excitations (>85%) arises from the HOMO→LUMO transition with oscillator strength f = 1.03. A strong absorption band centered at 364 nm for S0→S6 excitations of [13]­BP (Figure a). Additionally, the calculations also confirm the delocalized nature of HOMO–1, HOMO, LUMO, and LUMO+1 orbitals in [13]­BP (Figure b).

4.

4

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Energy diagrams and frontier molecular orbitals of [13]­BP calculated at the B3LYP/6–31G­(d) level of theory.

It is evident from the structural optimization that the HOMO and LUMO orbitals of [n]­BPs are evenly delocalized across the entire molecular skeleton (Figure and Table S10). As the value of n increases continuously, the energy band gap of [n]­BP shows a gradually decreasing trend. Calculations reveal a band gap of 4.02 eV for [6]­phenacene. After longitudinal benzo-π-extension, [6]­BP exhibits a calculated HOMO energy of −5.16 eV and a LUMO energy of −1.39 eV, resulting in a band gap of 3.86 eV. Similarly, [13]­BP displays a HOMO energy of −5.11 eV, a LUMO energy of −1.60 eV, and a band gap of 3.57 eV upon extending its π-conjugated system lengthwise. It is worth noting that [n]­BP compared with [n]­phenacene, which has a lower LUMO and a higher HOMO. Hence, the effective regulation of the band gap of phenacene derivatives can be achieved by longitudinal/transverse π-extension. Additionally, tert-butyl groups were incorporated to enhance solution processability, enabling fabrication via spin-coating or inkjet printing in many application fields such as photodetectors and infrared absorption.

5.

5

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HOMO (bottom), LUMO (top), and Gap (middle) energies of the pristine [n]­phenacenes (blue line) and [n]­BPs (green line) calculated at the B3LYP/6–31G­(d) level of theory.

To elucidate the electronic structures and molecular aromaticity of [13]­BP, we further conducted DFT calculations on its nucleus-independent chemical shifts (NICS) and anisotropy of the induced current densities (ACID) at the B3LYP/6–31g­(d) level of theory based on optimized structures (Figure ). The NICS (1) values (−11.2 to – 6.9 for [13]­BP) indicate complete aromaticity in these six-membered rings, with benzo-extended rings displaying higher NICS(1) values, suggesting their significant contribution to overall aromaticity. Thus, NICS plots demonstrate that the twisted conformation decreases the aromaticity of [13]­BP compared with [13]­phenacene. These findings are further supported by ACID calculations, which reveal a predominantly global clockwise ring current flow along the periphery of the spine (Figure S13).

6.

6

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Calculated NICS (1) values of (a) [13]­phenacene and (b) [13]­BP (the blue ring is the max value of [13]­BP).

Conclusion

In this study, we designed a modular synthetic strategy for various benzo-extended and twisted [n]­BPs via a green and sustainable e-flow Scholl reaction. Remarkably, a considerable quantity of [6]­BP (230 mg) was readily synthesized simply by extending the electrolysis time without changing the reaction conditions and setups, distinguishing this approach from the conventional Scholl reaction, which is typically nontrivial for scaled preparation. Through π-extension, we could readily adjust the band gaps of [n]­BPs, laying the foundation for their further applications. We anticipate that this new approach should provide ready access to many structurally diverse phenacenes and thus facilitate their prospective applications in materials sciences such as organic semiconductors.

Supplementary Material

au5c00653_si_001.pdf (3.4MB, pdf)

Acknowledgments

This work was supported by the State Key Laboratory of Elemento-Organic Chemistry, Nankai University (No. 202309), the Joint Funds of the National Natural Science Foundation of China (No. U24A20567), the National Natural Science Foundation of China (No. 22421002), and the Natural Science Foundation of Fujian Province, China (No. 2024J01236). We thank Prof. Weiguo Huang (Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences) for the single crystal measurement, and Prof. Chaolumen (Inner Mongolia University) for constructive criticism of the manuscript. We thank Taosong Wang for the TOC.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.5c00653.

  • Detailed materials for syntheses and characterization of [6]­BP, [13]­BP, including NMR, mass spectra, crystal data, and DFT calculations (PDF)

§.

These authors contributed equally to this work.

The authors declare no competing financial interest.

References

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