Helical Synthetic Nanographenes with Atomic Precision

Abstract

Conspectus Understanding and harnessing the properties of nanoscale molecular entities are considered as new frontiers in basic chemistry. In this regard, synthetic nanographene with atomic precision has attracted much attention recently. For instance, taking advantage of the marvelous bonding capability of carbon, flat, curved, ribbon-type, or cone-shaped nanographenes have been prepared in highly controllable and elegant manner, allowing one to explore fascinating molecular architectures with intriguing optical, electrochemical, and magnetic characteristics. This stands in stark contrast to other carbon-rich nanomaterials, such as graphite oxides or carbon quantum dots, which preclude thorough investigations because of complicate structural defects. Undoubtedly, synthetic nanographene contributes strongly to modern aromatic chemistry and represents a vibrant field that may deliver transforming functional materials crucial for optoelectronics, nanotechnologies, and biomedicine. Nonetheless, in many cases, synthesis and characterization of nanographene compounds are highly demanding. Low solubility, high molecular strain, undesired selectivity, as well as incomplete or excessive C–C bond formation are common impediments, that require formidable efforts to control the molecular geometry, to modulate the edge structure, to achieve accurate doping, or to push the upper size boundary. These endeavors are indispensable for establishing structure–property relationships, and lay down foundation for exploring synthetic nanographenes at a high level of sophistications. In this Account, we summarize our contributions to this field by presenting a series of helical synthetic nanographenes, such as hexapole [7]helicene (H7H), nitrogen-doped H7H, hexapole [9]helicene (H9H), superhelicene, and supertwistacene. This kind of giant synthetic nanographene reaches the size domain of carbon quantum dots, albeit has precise atomic structure. It provides a unique platform to study aromatic chemistry and chirality at the nanoscale. We discuss synthetic methods and point out, in particular, the strengths and pitfalls of Scholl oxidation, which are expected to be valuable for making synthetic nanographenes in general. In addition, we illustrate their exciting electrochemical and photophysical performance, which include, but are not limited to, reversible multielectron redox chemistry, record high panchromatic absorption, impressive photothermal behavior, and extremely strong Cotton effect. These unusual characteristics are convincingly traced back to their three-dimensional conjugated architectures, highlighting the critical roles of π-electron delocalization, heteroatom-doping, substitution, and molecular symmetry in determining nanographenes’ properties and functions. Lastly, we put forward our understanding on the challenges and opportunities that lies ahead and hope this Account will inspire ever more ambitious achievements from this attractive area of research.

First-Principles Modeling of Bottom-Up Synthesis of Carbon Quantum Dots

“…Note that the formation of the above-described bushy structures on the edges and the negative effect of these structures on the growth of nanographenes were observed with atomic precision in multiple experimental studies devoted to the growth of nanographenes from various precursor organic molecules (see for review Ref. [55]). For this reason, we propose that the formation of two additional rings in the structure shown in Figure 3g is the upper limit for the growth of the nanographene.…”

π-Extended Helical Multilayer Nanographenes with Layer-Dependent Chiroptical Properties

“…Circularly polarized luminescence (CPL) materials have garnered significant interest due to their wide range of potential applications in fields like biological science, CPL lasers, spintronics, and more. − Within this context, helical nanographenes (NGs) emerge as a captivating class, distinguished by their inherent chirality, distinct dynamic characteristics, and exceptional chiroptical properties. − With the incorporation of [ n ]­helicene units into NGs, a variety of helical NGs with diverse topologies have been reported and considered to be the impetus for the development of novel tailor-made chiroptical materials with high CPL brightness. − Among them, helical NGs extended along the perpendicular direction to the helical axis are garnering increased attention in recent years due to the extended π-electron delocalization, enhanced chirality stability, and upgraded photoluminescence. − …”

Pyrene bridged double[7]helicene embedded with a heptagonal ring

“…Overcoming the shortcomings of pristine [n]helicenes, helicene fused polycyclic aromatic hydrocarbons (PAHs) have shown a significant increase in the overall functional properties. [11][12][13][14][15][16][17][18][19][20][21][22][23][24] Moreover during the last few years multiple [n]helicene-embedded nanographenes with numerous aromatic cores have been developed. 23,[25][26][27][28][29][30] Preserving their core properties, multi[n]helicene showed unique characteristics due to an increased π-conjugation and molecular packing.…”