Design and Synthesis of Heteroatom-Containing Polycyclic Aromatic Hydrocarbons with B–N and B–O Bonds

Abstract

We developed a new synthetic strategy for iterative BO-containing acene derivatives. This approach is both simple and efficient, enabling two-ring expansion in only two steps: ipso-iodination directed by a trimethylsilyl group, followed by Suzuki cross-coupling and subsequent condensation for BO annulation. Repeating this cycle allowed the sequential introduction of BO units in a zigzag pattern along the acene edges. Through this strategy, we successfully synthesized anti-BO-acene derivatives. This methodology expands the structural diversity of acenes and provides fundamental insight into how BO unit incorporation modulates their electronic structures and photophysical properties. Overall, this study establishes a practical molecular design platform that overcomes previous synthetic limitations and highlights the potential of anti-BO-acene derivatives as promising candidates for next-generation organic electronic materials. Chapter 2: Synthesis of Triphenyl-fused Azaphenalenes Derivatives with Multiple Heteroatoms A triangular PAH, 1H-phenalene, first reported by Mayer in 1922, is composed of three fused rings sharing edges (Figure 10a).55 This molecule contains one sp³-hybridized carbon, which limits the extent of π-conjugation and results in non-aromatic behavior. In principle, while fully conjugated structures of 1H-phenalene can be generated as cationic, radical, or anionic intermediates, the inherent instability and ionic nature of these species make such approaches practically challenging.56 A more effective strategy involves the incorporation of heteroatoms such as nitrogen atom, allowing the formation of fully conjugated, neutral systems that maintain both electronic stability and planarity. From this analysis, the nitrogen-containing analogue azaphenalene was first reported by Windgassen in 1959 (Figure 10b).57 Figure 10. Chemical structures of (a) 1H-phenalene, (b) azaphenalene, (c) and (d) more nitrogen- containing azaphenalene derivatives. The lone pair electrons on the central nitrogen enables azaphenalene to achieve a 14 π-electron system, thereby restoring aromaticity and maintaining both planarity and continuous conjugation. Despite these advantages, studies on azaphenalene have remained limited due to synthetic challenges.58 Nevertheless, phenalene and its derivatives continue to serve as valuable model systems for investigating electronic structures.59,60 Recently, nitrogen-substituted azaphenalene derivatives have been developed (Figure 10c and d),61,62 attracting increasing attention for their tunable electronic properties and potential applications. In particular, tricycloquinazoline (Figure 10d), which is triphenyl- fused azaphenalene derivative, can be efficiently synthesized with high yields via a one-pot condensation reaction starting from 2-amino-1-benzonitrile as the precursor,61 providing a versatile platform for further functional studies (Figure 11a). Figure 11. (a) Synthetic method of tricycloquinazoline, (b) The proposed synthetic method of heteroatom-containing azaphenalene derivatives. In this study, tricycloquinazoline was selected as a model compound. Incorporation of nitrogen atoms into the phenalene framework enhances the overall structural stability, while its straightforward one-step synthesis further highlights its synthetic accessibility. This simple and efficient synthetic method through condensation, combined with the improved structural stability, inspired us to further explore the potential of this molecule. Based on this molecular platform, we aimed to tune the energy gap at the atomic level by replacing the central N–C–N unit with N–B–N or N–B–O units. BN substitution provides isoelectronic stabilization, whereas BO substitution introduces a larger electronegativity difference and a distinct dipole moment, thereby will be expected to impart unique optical and electronic properties.20–24 The transformation of boronic acids (–B(OH)2) into dichloroboranes (–BCl2) using SiCl4 has been previously reported.63,64 Building upon this precedent, we designed a cyclization reaction via electrophilic borylation, taking advantage of the electrophilic nature of electron-deficient boron centers. This approach, reminiscent of the efficient and straightforward synthesis of tricycloquinazoline, enables the facile incorporation of boron atoms into the molecular framework. Through this process, we envisioned that triphenyl-fused azaphenalene derivatives incorporating BN or BO units could be obtained. Herein, we present a straightforward one-pot strategy for synthesizing azaphenalene derivatives using boronic acid as the boron source, allowing controlled incorporation of BN or BO units into the model compound (Figure 11b). The resulting compounds were systematically characterized by spectroscopic and electrochemical techniques, revealing clear differences in their electronic structures and optical properties depending on the type of substitution. Notably, BO-azaphenalene exhibited promising performance in perovskite solar cells (PSCs), highlighting its potential for broader application in organic electronic devices such as OLEDs and OFETs.