**1. Introduction**

Helicenes are polycyclic aromatic hydrocarbons (PAHs) in which aromatic rings are annulated in a helical architecture, giving them unique electronic, photophysical and chiroptical properties [1–4]. Over the past couple of decades, the great advances achieved in this chemistry [2–6] promoted a broad spectrum of material-based applications [7–10], transistors [11,12], and semiconductors [13]. Incorporation of one or more heteroatoms in the helicene scaffolds modulate their physical and optical features, and alter the electronic properties in order to expand their applications [14,15]. With these extra features, the trend in helicene chemistry has begun to move towards heterohelicenes after the domination of carbohelicenes [16–21]. Another approach to promote characteristics of helicenes is to induce multihelicity which means combining two or more helical scaffolds in a single molecule [22,23]. Multiple helicenes show a lot of favorable properties due to their amplified non-planarity, diverse conformations, and maximized intermolecular interactions [24,25]. Various smart core scaffolds were used to induce this multihelicity such as perylene diimide (PDI) that afforded valuable twisted structures for different material-based applications [26–29]. Hence, a lot of efforts were dedicated for designing and synthesizing multiple heterohelicenes [30], in particular, double heterohelicenes [14]. After the first report of double helicene reported by Rajca, many examples of these double heterohelicenes were conducted and exhibited clear superiority over their single counterparts, especially in terms of optical properties (Figure 1a) [31–40]. However, during that frantic pursuit to promote the properties of helicenes, another problem, in particular, synthetic difficulty emerged. With the increase in structural complexity, the synthesis of multiple heterohelicenes becomes more challenging and requires many steps. Although few reports succeeded to introduce effective short-step synthetic protocols for double heterohelicene, most of these successes were concentrated in the double hetero[5]helicene derivatives (Figure 1b) [41–43]. In 2016, Narita, Cao, and Müllen introduced an efficient two-step synthesis of a highly strained OBO-fused double hetero[7]helicene **K** via the nucleophilic aromatic substitution reaction of hexabromobenzene, followed by a sequential

**Citation:** Salem, M.S.H.; Sabri, A.; Khalid, M.I.; Sasai, H.; Takizawa, S. Two-Step Synthesis, Structure, and Optical Features of a Double Hetero[7]helicene. *Molecules* **2022**, *27*, 9068. https://doi.org/10.3390/ molecules27249068

Academic Editor: Mircea Darabantu

Received: 29 November 2022 Accepted: 14 December 2022 Published: 19 December 2022

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step of demethylation and C-H aryl borylation (Figure 1c) [44]. Earlier in the same year, Hatakeyama showed the potential of this synthetic approach to afford their boron-fused double hetero[5]helicene **I** (Figure 1b) [42]. In 2021, Wang and coworkers developed the first examples of B,*N*-embedded double hetero[7]helicenes **L** that showed excellent chiroptical features in the visible range [45]. With only two steps, they succeeded to prepare this double hetero[7]helicene **L** via the nucleophilic aromatic substitution of dibromotetrafluorobenzene with carbazole, followed by a tandem process of substitution with BBr3 and C-H aryl borylation [45].

**Figure 1.** The selected examples of double heterohelicenes in short-step synthesis: (**a**) Double hetero[5–7]helicenes (more than four-step synthesis); (**b**) Double hetero[5]helicenes (two- or threestep synthesis); (**c**) Double hetero[7]helicenes (two-step synthesis).

Notably, these examples (Figure 1c) represent a quantum leap in the short-step synthesis of double hetero[7]helicenes via the tandem process of nucleophilic substitution with BBr3 followed by C-H aryl borylation [44,45]. As part of our effort to explore the

electrochemical domino syntheses, we were interested in designing effective sequential reactions to access double helicene motifs [46,47]. Herein, a facile preparation of a double aza-oxa[7]helicene with a phenylene linker has been established through acid-mediated annulation with the electrochemical sequential reaction (oxidative coupling and dehydrative cyclization). We also studied the structural and optical features via x-ray crystallographic analysis, spectrophotometric analysis, and DFT calculations.
