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

Abstract

Helical bilayer nanographenes (HBNGs) are chiral π-extended 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.

Chart 1. HBNGs Endowed with Helicenes of Different Length, which Results in Different Degrees of π-Overlapping between the HBC Layers

Scheme 1. Synthesis of HBNGs [9]HBNG and [11]HBNG from Dichloro[5]helicene 1a and Dichloro[7]helicene 1b, Respectively

Figure 1

Molecular structures of compounds [9]HBNG, [10]HBNG, and [11]HBNG obtained by X-ray diffraction from single crystals (hydrogen atoms have been omitted for clarity). Lateral view (top), zenithal view (middle), and packing of the columns of molecules of [9]HBNG (along the [010] direction), [10]HBNG (along the [001]), and [11]HBNG (along [011]) (bottom).

Figure 2

Cyclic voltammograms of HBNGs [9]HBNG, [10]HBNG, and [11]HBNG vs Fc/Fc⁺ in toluene/CH₃CN (4:1).

Figure 3

UV–vis–NIR electronic absorption spectra obtained upon electrochemical oxidation of [9]HBNG in a 0.1 M Bu₄NPF₆ solution in CH₂Cl₂ at room temperature by using a thin-layer spectroelectrochemical cell. Black shadowed lines correspond to neutral species; blue/red shadowed lines correspond to the first/second oxidized species. UV–vis–NIR electronic absorption spectrum of the ^tBu-HBC radical cation is also shown (bottom, blue line) for reference.

Figure 4

Absorption spectra of ^tBu-HBC and of HBNGs [9]HBNG, [10]HBNG, and [11]HBNG in CHCl₃.

Figure 5

(a) Fluorescence spectra of ^tBu-HBC and of HBNGs [9]HBNG, [10]HBNG, and [11]HBNG in CHCl₃. (b) Samples of [9]HBNG, [10]HBNG, and [11]HBNG in CHCl₃ under 365 nm lamp irradiation (bottom).

Figure 6

(a) Solid-state Raman spectrum at room temperature of [10]HBNG (λ_exc = 1064 nm). (b) Raman spectra of (a) ^tBu-HBC (λ_exc = 1064 nm), (b) [9]HBNG (λ_exc = 325 nm), (c) [10]HBNG (λ_exc = 1064 nm), and (d) [11]HBNG in the solid state at room temperature.

Figure 7

Circular dichroism spectra of (+)-[9]HBNG and (−)-[9]HBNG (a), (+)-[10]HBNG and (−)-[10]HBNG (b), and (+)-[11]HBNG and (−)-[11]HBNG (c). The CD spectrum of (+)-[10]HBNG (including g_abs factor) is corrected to optical purity (i.e., scaled by a factor of ∼1.4).

Figure 8

CPL/PL spectra of (P) and (M) enantiomers of [9]HBNG, [10]HBNG, and [11]HBNG in THF at room temperature and at concentrations around 10^–5 M.

Figure 9

Directions and moduli of the ETDM (blue arrow) and MTDM (red arrows) in [9]HBNG.