Helical Bilayer Nanographenes: Impact of the Helicene Length on the Structural, Electrochemical, Photophysical, and Chiroptical Properties

Abstract

Helical bilayer nanographenes (HBNGs) are chiral πextended aromatic compounds consisting of two π−π stacked hexabenzocoronenes (HBCs) joined by a helicene, thus resembling van der Waals layered 2D materials. Herein, we compare [9]HBNG, [10]HBNG, and [11]HBNG helical bilayers endowed with [9], [10], and [11]helicenes embedded in their structure, respectively. Interestingly, the helicene length defines the overlapping degree between the two HBCs (number of benzene rings involved in π−π interactions between the two layers), being 26, 14, and 10 benzene rings, respectively, according to the X-ray analysis. Unexpectedly, the electrochemical study shows that the lesser π-extended system [9]HBNG shows the strongest electron donor character, in part by interlayer exchange resonance, and more red-shifted values of emission. Furthermore, [9]HBNG also shows exceptional chiroptical properties with the biggest values of g abs and g lum (3.6 × 10 −2 ) when compared to [10]HBNG and [11]HBNG owing to the fine alignment in the configuration of [9]HBNG between its electric and magnetic dipole transition moments. Furthermore, spectroelectrochemical studies as well as the fluorescence spectroscopy support the aforementioned experimental findings, thus confirming the strong impact of the helicene length on the properties of this new family of bilayer nanographenes.

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

“…Density functional theory (DFT) calculations at the B3LYP-D3­(BJ)/6-31G­(d) level were performed to gain a deeper insight into the electronic structures of NGs 1 – 3 . It is found that both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are localized on the bridged half side of 1 – 3 rather than the entire conjugated backbone, similar to other reported bilayer and trilayer NGs. , Elongating the molecular backbone from 1 to 3 causes a gradual increase in the HOMO energy level and a decrease in the LUMO energy level, leading to narrower energy gaps for 2 and 3 , which closely matched the experiment results. Based on the results of time-dependent (TD)-DFT calculations, the maximum absorption peaks (λ max ) of 1 (calculated value of 524 nm), 2 (calculated value of 559 nm), and 3 (calculated value of 579 nm) can be assigned to the HOMO → LUMO transition, which concurs with the peak values obtained from experimental measurements (Figure c and Tables S4–S6).…”

“…Alternatively, electron diffraction (ED) that has been proven superior to dealing with the tiny crystals , was applied and successfully resolved the single-crystal structure of NG 3 , revealing its clear tetralayer structure with four HBC subunits linked by three [7]­helicene units with the interlayer distances of 3.56, 3.67, and 4.20 nm, respectively (Figure e, CCDC no. 2289385), comparable with the reported bilayer and trilayer NGs. , In addition, NG 1 crystallized in the P 1̅ space group with obvious intermolecular π–π stacking between P and M isomers (π–π distance: 3.70 Å) (Figure b). For NGs 2 and 3 , the homochiral isomers are arranged along the a -axis and c -axis, respectively, without π–π interaction due to the blocked stacking between π faces caused by the surrounding tert -butyl groups (Figure d,f).…”

“…The enantiomers of 1 – 3 present strong circular dichroism (CD) and circularly polarized luminescence (CPL) activity with large dissymmetric factors ( g ). Notably, the chiral NGs 1 – 3 demonstrate unprecedentedly high CPL brightness up to 168 M –1 cm –1 , much higher than the reported chiral NGs with typical CPL brightness values underneath 90 M –1 cm –1 , ,− rendering them promising candidates for CPL emitters. Our demonstrated strategy not only offers the modular synthesis of helical multilayer NGs with remarkable CPL brightness but also holds the potential to be extended for the synthesis of other π-extended helical NGs or even helical graphene nanoribbons (GNRs), which could offer valuable insights into the intricate interplay between the structure-determined π extension and the properties of helical multilayer NGs.…”

“…Recent works suggest that helical NGs bearing a layered structure, composed of stacked planar graphene molecules joined by rigid [ n ]­helicene units (Figure a), could exhibit unique intramolecular π–π interaction among layers and superior chiroptical properties compared to non-π-extended [ n ]­helicenes. − For the synthesis of helically layered NGs, the formation of a helicene-based oligophenylene precursor followed by a Scholl-type oxidative cyclodehydrogenation is a widely used strategy. , For example, in 2018, Martín et al reported the first helical bilayer NG bearing [10]­helicene with two hexabenzocoronene (HBC) as terminal units . Recently, they have expanded upon this strategy, synthesizing a range of helical bilayer NGs, wherein the interlayer interaction could be modulated by altering the size of the inner helicene linker . Very recently, helical trilayer NGs were synthesized by Tan et al starting from a prefused HBC core, revealing their enhanced interlayer coupling compared to bilayer NGs.…”

CPL calculations of [7]helicenes with alleged exceptional emission dissymmetry values

“…The situation is common among [6]helicenes or smaller analogues 14,37,38,44,45,48,50,51,77 and may be tackled by different structural designs, such as double-helicenes, S-shaped or 8-shaped helical conjugated systems, elongated or extended helicenes. 46,77–84 Interestingly enough, in several of the just mentioned reports, the molecular design was steered by a preliminary computational screening, which confirms the demand for accurate computational approaches for property predictions.…”