Exploring a Route to Cyclic Acenes by On‐Surface Synthesis

Impact of Di- and Poly-Radical Characters on the Relative Energy of the Doubly Excited and La States of Linear Acenes and Cyclacenes

Linear and cyclic acenes are polycyclic aromatic hydrocarbons that can be viewed as building blocks of graphene nanoribbons and carbon nanotubes, respectively. While short linear acenes demonstrated remarkable efficiency in several optoelectronic applications, the longer members are unstable and difficult to synthesize as their cyclic counterparts. Recent progress in on-surface synthesis, a powerful tool to prepare highly reactive species, opens promising perspectives and motivates the computational investigations of these potentially functional molecules. Owing to their di- and poly-radical character, low-lying excited states dominated by doubly excited configurations are expected to become more important for longer members of both linear and cyclic molecules. In this work, we investigate the lowest-lying La and the doubly excited (DE) state of linear acenes and cyclacenes, with different computational approaches, to assess the influence of the di-/poly-radical characters (increasing with the molecular dimensions) on their relative order. We show that DFT/MRCI calculations correctly reproduce the crossing of the two states for longer linear acenes, while TDUDFT calculations fail to predict the correct excitation energy trend of the DE state. The study suggests a similarity in the excited electronic state pattern of long linear and cyclic acenes leading ultimately to a lowest lying dark DE state for both.

Sulphur-Embedded Hydrocarbon Belts: Synthesis, Structure and Redox Chemistry of Cyclothianthrenes

Cyclothianthrenes, a series of sulphur-embedded hydrocarbon belts proposed a decade ago, were successfully constructed through a stepwise bottom-up synthesis. The belt [6]cyclothianthrene ([6]CT) is the smallest and most strained member of the family yet reported. Both [6]CT and [8]CT are the first examples of cyclothianthrene characterized by single crystal X-ray diffraction. An unprecedented chiral belt [7]CT and a Möbius-shaped [9]CT were also achieved via modular synthesis. Crystallographic and computational studies show that belts [6]CT-[8]CT have prism-like conformations with well-defined tubular cavities which have potential for guest molecule inclusion. Cyclic voltammograms further revealed that these belts are redox-active. The success of constructing sulphur-embedded hydrocarbon belts, that is, cyclothianthrenes, greatly enriches the chemistry of heteroatom-doped molecular belts and tubes.

Synthesis of a Sidewall Fragment of a (12,0) Carbon Nanotube

Synthesis of a carbon nanobelt (CNB) is a very challenging task in organic chemistry. Herein, we report the successful synthesis of an octabenzo[12]cyclacene based CNB (6), which can be regarded as a sidewall fragment of a (12,0) carbon nanotube. The key intermediate compound, a tetraepoxy nanobelt (5), was first synthesized by Diels-Alder reaction, and subsequent reductive aromatization gave the fully conjugated CNB 6. X-ray crystallographic analysis unambiguously confirmed the belt-shaped structure of 6. ¹ H NMR spectrum and theoretical calculations (ACID, NICS, and 2D/3D ICSS) revealed localized aromaticity and stronger shielding chemical environment in the inner region of the belt. The optical properties (absorption and emission) of 6 were studied and correlated to its electronic structure. Strain analysis indicates that the phenyl substituents at the zigzag edges are crucial to the successful synthesis of 6. This report presents a new strategy towards highly strained CNBs.

Synthetic Strategies of Carbon Nanobelts and Related Belt-Shaped Polycyclic Aromatic Hydrocarbons

The development of carbon nanobelts and related belt-shaped polycyclic aromatic hydrocarbons has gained momentum in recent years. This Minireview focuses on the synthetic strategies used in constructing these aesthetically appealing molecular nanocarbons. Examples of carbon nanobelts and related belt-shaped polycyclic aromatic hydrocarbons reported in recent years as well as some representative synthetic attempts in earlier times are discussed.