On-Surface Strain-Driven Synthesis of Nonalternant Non-Benzenoid Aromatic Compounds Containing Four- to Eight-Membered Rings

Abstract

The synthesis of polycyclic aromatic hydrocarbons containing various non-benzenoid rings remains a big challenge facing contemporary organic chemistry despite a considerable effort made over the last decades. Herein, we present a novel route, employing on-surface chemistry, to synthesize nonalternant polycyclic aromatic hydrocarbons containing up to four distinct kinds of non-benzenoid rings. We show that the surface-induced mechanical constraints imposed on strained helical reactants play a decisive role leading to the formation of products, energetically unfavorable in solution, with a peculiar ring current stabilizing the aromatic character of the π-conjugated system. Determination of the chemical and electronic structures of the most frequent product reveals its closed-shell character and low band gap. The present study renders a new route for the synthesis of novel nonalternant polycyclic aromatic hydrocarbons or other hydrocarbons driven by internal stress imposed by the surface not available by traditional approaches of organic chemistry in solution.

Step-Assisted On-Surface Synthesis of Graphene Nanoribbons Embedded with Periodic Divacancies

“…In this work, by introduction of additional steric hindrance at the periodic divacancy sites, the cyclodehydrogenation temperature has been increased to around 720 K. A similar phenomenon with enhanced steric hindrance was observed due to the enhanced π–π interactions between polymer chains . Considering the difference of the strain structures in these processes and the role of strain in the on-surface synthesis, − one can utilize the steric hindrance effect to achieve controllability of the cyclodehydrogenation reactions for multifarious extended π systems. Following the mechanism revealed in this work, and considering the formation of polymer chains that can be well tuned with high selectivity by different annealing procedures (Figure S7) and the template effect of the step edges (Figures S4 and S6), one may achieve well-aligned nonplanar porous GNRs with high density on a highly stepped vicinal surface, such as the Au(877) with {100}-type steps, which could utilize the chemical heterogeneity of the substrate surfaces for designer multifunctional properties.…”

Defect-Induced π-Magnetism into Non-Benzenoid Nanographenes

“…The formation of such NGs is attributed to the oxidative ring closure of the majority of the methyl groups, together with the dissociation of several of them per NG prior to cyclization. Such removal of methyl moieties was previously reported in the synthesis of several NGs [30,37,[42][43][44][45] and GNRs [22,46] and is concomitant to the annealing step at temperatures where the oxidative ring closure is expected to occur on Au(111), therefore being not possible to achieve the synthesis of E.…”

Resolving Atomic‐Scale Defects in Conjugated Polymers On‐Surfaces

“…This intramolecular contrast is the result of the short‐ranged Pauli repulsion being maximized in the areas of higher electron density so that minor variations in electron density assigned to specific bonding order can be resolved in nc‐AFM images as features with different brightness or shape, thus pointing to an alternation in the character of the bond. On the other hand, such features can be further varied due to anisotropic charge distribution in the molecular backbone, [53] distinct interaction with the surface [54] and varying with the tip‐sample distance. Second, Clar's aromatic π‐sextet rule states that a resonance structure of polycyclic aromatic hydrocarbons with the maximal number of nonadjacent π‐sextets represents the most stable form or the major resonance contributor.…”