peri-Acenoacene Ribbons with Zigzag BN-Doped Peripheries

Abstract

Here, we report the synthesis of BN-doped graphenoid nanoribbons, in which peripheral carbon atoms at the zigzag edges have been selectively replaced by boron and nitrogen atoms as BN and NBN motifs. This includes high-yielding ring closure key steps that, through N-directed borylation reaction using solely BBr₃, allow the planarization of meta-oligoarylenyl precursors, through the formation of B-N and B-C bonds, to give ter-, quater-, quinque-, and sexi-arylenyl nanoribbons. X-ray single-crystal diffraction studies confirmed the formation of the BN and NBN motifs and the zigzag-edged topology of the regularly doped ribbons. Steady-state absorption and emission investigations at room temperature showed a systematic bathochromic shift of the UV-vis absorption and emission envelopes upon elongation of the oligoarylenyl backbone, with the nanoribbon emission featuring a TADF component. All derivatives displayed phosphorescence at 77 K. Electrochemical studies showed that the π-extension of the peri-acenoacene framework provokes a lowering of the first oxidative event (from 0.83 to 0.40 V), making these nanoribbons optimal candidates to engineer p-type organic semiconductors.

Figure 1

(a) (m,n)-peri-Acenes and (m,n)-peri-acenoacenes nanographenes: armchair/zigzag (left) and zigzag/zigzag (right) topologies; (b) example of an O-doped (2,7)-peri-acenoacene derivative reported by our group.

Figure 2

Representative BN-doped molecular graphenoids featuring zigzag peripheries.

Scheme 1. Envisaged Synthetic Strategy towards BN-Doped Zigzag Graphenoid Nanoribbons Using N-Directed Borylation As the Planarization Reaction (PG = Protecting Group)

Scheme 2. Synthesis of BN-Doped Zigzag Graphenoid Nanoribbons

Reagents and conditions: (a) Na₂CO₃, [Pd(PPh₃)₄], toluene/EtOH/H₂O, 95 °C; (b) 6 M aq. HCl, dioxane, 100–120 °C; (c) MesBr, t-BuONa, [Pd₂(dba)₃], rac-BINAP, toluene, 100–110 °C; (d) BBr₃, o-DCB or TCB, 170–330 °C; (e) B₂pin₂, KOAc, [Pd(dppf)Cl₂], DMF, 60–95 °C.

Figure 3

HR LD MS spectra for the BN-doped nanoribbons 1^Me, 1^Ph, 2, and 3 and HR MALDI MS (Matrix: DCTB) spectra for 6 and 4. Inset: experimental (black) and simulated (red) isotopic pattern of the M⁺ peak. Satellite high-mass peaks correspond to the [M+O]⁺ and [M+2O]⁺ adducts, presumably formed during the ionization.

Figure 4

Single-crystal X-ray structures for 6, 1^Ph, 2, and 3. (a) Top view; (b) short-side view; (c) long-side view. Space groups: P2₁/n (6), P1 (1^Ph), P1 (2), P1 (3). Crystallization solvents: CHCl₃/MeOH (6), toluene/MeOH (1^Ph), CH₂Cl₂/CH₃CN (2), toluene/MeOH (3). Hydrogen atoms and solvent molecules are omitted for clarity. For compound 1^Ph, a single, crystallographically independent, molecule representative of the crystal is shown. Atom colors: gray, C; pink, B; blue, N.

Figure 5

Absorption (black), normalized fluorescence (red), and excitation (dotted) spectra of 5^Ph, 6, 1^Ph, 2, and 3 in 2-MeTHF at rt. Normalized phosphorescence (blue) at 77 K.

Figure 6

Steady-state emission profiles (a–c) of 1^Ph, 2, and 3, measured in 2-MeTHF at rt in air-equilibrated (blue traces) and in degassed (red traces) solutions. The quantum yield of the degassed solutions was estimated from the relative emission intensity and the previously determined absolute quantum yield for the air-equilibrated solutions. Fluorescence decays (d–f) of degassed solutions in 2-MeTHF, highlighting the PF component at rt (blue traces) and the delayed emission at rt (red traces) and 77 K (black traces). (g–i) Gated (200 μs) fluorescence emission spectra of 1^Ph, 2, and 3, measured in degassed 2-MeTHF at rt, attenuated with a series of neutral density filters (OD 0.03–1). Inset: Linear dependence of the integrated delayed fluorescence on the excitation power.

Figure 7

Cyclic voltammetry of molecules 6, 1^Ph, 2, and 3 (0.2 mM). Scan rate: 60 mV/s. Solvent: o-DCB/CH₃CN (4:1) for 6, 2, and 3; TCE for 1^Ph. Supporting electrolyte: TBAPF₆. Working electrode: 7 mm² platinum disk. Counter electrode: Platinum wire. DmFc is used as an internal reference standard. The E_1/2^ox for the Fc/Fc⁺ (—) couple is shown for comparison purposes.

Figure 8

UV–vis–NIR absorption spectra measured during the electrochemical oxidation of 6 (a), 1^Ph (b), 2 (c), and 3 (d). 1^Ph was measured in TCE, while 6, 2, and 3 were measured in a 4:1 mixture of o-DCB/CH₃CN. Potentials vs Ag/AgCl.

Figure 9

ESP mapped on the van der Waals surface up to an electron density of 0.001 electron·bohr^–3.

Figure 10

(a) Calculated frontier orbital energy levels for molecules 5^Ph, 6, 1^Ph, 2, 3, and 4 (with Mes groups substituted by H atoms) together with their HOMO and LUMO profiles at B3LYP/6-311+G** level of theory (Gaussian16); (b) comparison between the HOMO and LUMO energy levels as estimated experimentally from the CV and photophysical data (green circles, Table 4) and theoretically (blue squares); (c) calculated absorption maxima (TD-DFT, circle) and experimental optical bandgaps (square) as a function of 1/N, with N being the number of meta-linked aryls within the framework. Solid lines are fits using the Kuhn equation and dashed lines using a linear equation. Dotted lines represent the y-intercept (E_g^opt-∞) of the Kuhn fit. N_ECL^Ex and N_ECL^T are determined from the interception of the linear fit with the y-intercept of the Kuhn fit.