Atomically precise edge chlorination of nanographenes and its application in graphene nanoribbons

Abstract

Chemical functionalization is one of the most powerful and widely used strategies to control the properties of nanomaterials, particularly in the field of graphene. However, the ill-defined structure of the present functionalized graphene inhibits atomically precise structural characterization and structure-correlated property modulation. Here we present a general edge chlorination protocol for atomically precise functionalization of nanographenes at different scales from 1.2 to 3.4 nm and its application in graphene nanoribbons. The well-defined edge chlorination is unambiguously confirmed by X-ray single-crystal analysis, which also discloses the characteristic non-planar molecular shape and detailed bond lengths of chlorinated nanographenes. Chlorinated nanographenes and graphene nanoribbons manifest enhanced solution processability associated with decreases in the optical band gap and frontier molecular orbital energy levels, exemplifying the structure-correlated property modulation by precise edge chlorination.

Figure 1. Chlorinated nanographenes (1–7).

(a) Structural formulae; (b) mass spectra; (c) photos of the toluene solution of 1–7.

Figure 2. Crystal structures of chlorinated nanographenes (1–5).

(a) C₄₂Cl₁₈ (1); (b) C₄₈Cl₁₈ (2); (c) C₆₀Cl₂₂ (3); (d) C₆₀Cl₂₄ (4); and (e) C₉₆Cl₂₇H₃ (5). Front- and side-view representations of the structures of 1–5 are shown. The ORTEP drawings show thermal ellipsoids at a 50% probability level. The carbon and chlorine atoms are represented as grey and green balls, respectively. The peripheral hexagons flip up and down with respect to the inner rings, which are highlighted in blue and red, respectively. The hexagons in 2 and 4, which adopt the twisted configuration, are highlighted in yellow.

Figure 3. Crystal packing of 1–5.

(a) C₄₂Cl₁₈ (1); (b) C₄₈Cl₁₈ (2); (c) C₆₀Cl₂₂ (3); (d) C₆₀Cl₂₄ (4); and (e) C₉₆Cl₂₇H₃ (5). The Cl–π short contacts are represented as blue dashed line. The π–π short contacts in dimmer of 5 are represented as red dashed line.

Figure 4. Optical properties of chlorinated nanographenes.

(a) Ultraviolet–visible–near-infrared spectra of 1–7 were acquired in toluene. The respective spectra are indicated by the corresponding Arabic numeral of the chlorinated nanographenes. (b) The calculated and experimental highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) gaps of 1–7 are represented by green squares and black triangles, and those of their hydrogen-terminated counterparts are represented by blue squares and red triangles, respectively.

Figure 5. Chlorination of bottom-up synthetic GNRs.

(a) Scheme of chlorination of bottom-up synthetic GNRs. (b) Infrared spectrum of GNRs shows the vibrations of the tert-butyl group at 2,953, 1,364 and 866 cm⁻¹. (c) Infrared spectrum of chlorinated GNRs shows the vibration peaks of C=C in chlorinated aromatic carbon at 1,299 and 1,225 cm⁻¹ and vibration peaks of C–Cl bonds at 810, 770 and 608 cm⁻¹. (d) C 1s XPS of chlorinated GNRs. The XPS signal of C 1 s at 284.6 and 285.9 eV. (e) Cl 2p XPS of chlorinated GNRs. The Cl 2p peaks at 200.2 eV (2p3/2) and 201.8 eV (2p1/2). (f) Ultraviolet–visible–near-infrared spectra of GNRs (blue) and chlorinated GNRs (red) in 1,2,4-trichlorobenzene (TCB). Insert images are photos of GNRs and chlorinated GNRs solution in TCB, respectively.