Molecular bilayer graphene

Abstract

Bilayer graphene consists of two stacked graphene layers bound together by van der Waals interaction. As the molecular analog of bilayer graphene, molecular bilayer graphene (MBLG) can offer useful insights into the structural and functional properties of bilayer graphene. However, synthesis of MBLG, which requires discrete assembly of two graphene fragments, has proved to be challenging. Here, we show the synthesis and characterization of two structurally well-defined MBLGs, both consisting of two π-π stacked nanographene sheets. We find they have excellent stability against variation of concentration, temperature and solvents. The MBLGs show sharp absorption and emission peaks, and further time-resolved spectroscopic studies reveal drastically different lifetimes for the bright and dark Davydov states in these MBLGs.

Fig. 1

Mass spectra of molecular bilayer graphenes. The mass spectra of 1 (a) and 2 (b) with different desorption laser powder. The insert figures represent the isotopic distribution for the mass peak of 1 and 2

Fig. 2

Nuclear magnetic resonance (NMR) characterization of molecular bilayer graphene. a, e The top views of 1 and 2, respectively. The asymmetric units in 1 and 2 are highlighted in blue. In and out represents the inward-spacing and outward-spacing methyl of the mesityl group. b, f The side views of 1 and 2. c, g Numeration of hydrogen atoms in the asymmetric units of 1 and 2, respectively. d, h ¹H NMR spectra of 1 and 2. The theoretical NMR spectra of 1 and 2 are represented as orange lines. All of the proton resonances are assigned with the assistance of two-dimensional NMR spectroscopy (Supplementary Figs. 21–32 and Supplementary Note 2)

Fig. 3

H···H proximity in 2 and crystal structure of 2’. a Typical H···H proximity found in 2. The interlayer and intralayer H···H proximity are represented as red and cyan dashed lines, respectively. The rest mesityl groups are omitted for clarity. b Expanded two-dimensional nuclear overhauser effect spectroscopy of 2, showing interlayer proton coupling. The detailed H···H distances are labeled inside. b. c Top view of the crystal structure of 2’. For clarity, one layer is illustrated with the spacefill model and the other layer with the ellipsoid model. d Side view of the crystal structure of 2’. The thermal ellipsoids are set at 50% probability level

Fig. 4

Optical properties of molecular bilayer graphenes. a Absorption (solid line) and emission (dashed line) spectra of 1 (blue) and 2 (red). b Time-resolved fluorescence kinetics of 1 (blue, monitored at 767 nm) and 2 (red, monitored at 683 nm). The average lifetime of 1 and 2 is 28.5 and 118 ns. c, d Transient absorption kinetics of c 1 and d 2 monitored at their respective first vibronic peak position