A Family of Superhelicenes: Easily Tunable, Chiral Nanographenes by Merging Helicity with Planar π Systems

Abstract

Incorporating helicity into large polycyclic aromatic hydrocarbons (PAHs) constitutes a new field of research at the interface between chemistry and material sciences. Lately, interest in the design of π-extended helicenes has surged. This new class of twisted, chiral nanographenes not only reveals unique characteristics but also finds its way into emerging applications such as spintronics. Insights into their structure-property relationships and on-demand tuning are scarce. To close these knowledge gaps, we designed a straightforward synthetic route towards a full-fledged family of πextended helicenes: superhelicenes. Common are two hexa-perihexabenzocoronenes (HBCs) connected via a central 5-membered ring. By means of structurally altering this 5-membered ring, we realized a versatile library of molecular building blocks. Not only the superhelicene structure, but also their features are tuned with ease. In-depth physico-chemical characterizations served as a proof of concept thereof. The superhelicene enantiomers were separated, their circular dichroism was measured in preliminary studies and concluded with an enantiomeric assignment. Our work was roundedoff by crystal structure analyses. Mixed stacks of M-and P-isomers led to twisted molecular wires. Using such stacks, charge-carrier mobilities were calculated, giving reason to expect outstanding hole transporting properties.

Superhelicenes in the gas phase: experimental and computational evidence of stable radical cation dimers

“…In both cases, no obvious signs of dimerization were found. For A three oxidation states at 0.9 V, 1.2 V, and 1.5 V could be identified, 26 while B showed oxidation at 1.0 V, and 1.3 V. 27 This is in line with the boundaries in which oxidation occurred in earlier ESI studies, [28][29][30] so that it seems perfectly reasonable to assume that the observed ions are the result of electrochemical oxidation inside the ESI source. The key experiment to further characterize the mixed doubly charged dimer ion is a so-called MS 2 experiment, in which the ion of interest is isolated from other ions and subjected to energyresolved collision-induced dissociations (ER-CID).…”

“…22 Following our earlier studies into the gasphase behaviour of hexabenzocoronene-substituted porphyrins, [23][24][25] the present study investigates the formation of radical cation dimers of the recently synthesized so-called superhelicenes. 26,27 These were identified as a fully-fledged family of p-extended helicenes with freely tunable physico-chemical characteristics and the prospect of further functionalization. Individual superhelicenes displayed remarkably high fluorescence quantum yields, while others showed absorption covering almost the entire visible region.…”

“…Potential as hole transporting material was demonstrated by calculations based on the crystal structures, and the separation of enantiomers enables the exploitation of chiroptical properties. 26,27 Fig. 1 displays the structure of the two superhelicenes under investigation.…”

Helically Chiral π‐Expanded Azocines Through Regioselective Beckmann Rearrangement and Their Charged States

“…[7] Moreover, the properties of these chiral structures can be further altered by incorporation of heteroatoms, such as nitrogen or oxygen. [8,9] For instance, substitution of one C(sp 2 )-H unit in an 8-membered ring with a nitrogen results in an azocine unit, which is formally antiaromatic according to Hückel, due to its 8 π-electrons. [10] To escape antiaromaticity, the azocine unit adopts a boat-like geometry.…”

Nanographenes Out of Planarity

“…37 The helical arrangement of NGs leads to inherently chiral PAHs, exhibiting strong Cotton effects and improved solubility compared to their flat analogues, such as those prepared by the groups of Wang, Martín, and Jux, among others. 34 35 38 In addition, their use as chiral dopants in emissive materials has been successfully demonstrated. 39…”