12b,24b‐Diborahexabenzo[a,c,fg,l,n,qr]pentacene: A Low‐LUMO Boron‐Doped Polycyclic Aromatic Hydrocarbon

Abstract Herein we devise and execute a new synthesis of a pristine boron‐doped nanographene. Our target boron‐doped nanographene was designed based on DFT calculations to possess a low LUMO energy level and a narrow band gap derived from its precise geometry and B‐doping arrangement. Our synthesis of this target, a doubly B‐doped hexabenzopentacene (B 2 ‐HBP), employs six net C−H borylations of an alkene, comprising consecutive hydroboration/electrophilic borylation/dehydrogenation and BBr3/AlCl3/2,6‐dichloropyridine‐mediated C−H borylation steps. As predicted by our calculations, B 2 ‐HBP absorbs strongly in the visible region and emits in the NIR up to 1150  nm in o‐dichlorobenzene solutions. Furthermore, B 2 ‐HBP possesses a very low LUMO level, showing two reversible reductions at −1.00  V and −1.17  V vs. Fc+/Fc. Our methodology is surprisingly selective despite its implementation of unfunctionalized precursors and offers a new approach to the synthesis of pristine B‐doped polycyclic aromatic hydrocarbons.

Aluminium (III) chloride, 2,6-dichloropyridine, bis(trifluoromethylsulfonyl)imide and 2,2,6,6-tetramethylpiperidine-1-oxyl free radical (TEMPO radical) were obtained from TCI and used without further purification. Anhydrous chlorobenzene (CB) and 1,2dichlorobenzene (o-DCB) were obtained from Sigma Aldrich and dried over 4 Å molecular sieves before use. DMSO was dried over 4 Å molecular sieves before use. THF and dichloromethane were purified with a Grubbs-type column system manufactured by Innovative Technology. Deuterated solvents were obtained from commercial sources and used without further purification. Anhydrous hexane was obtained from Sigma Aldrich and used without further purification. All other solvents for spectroscopic measurements were spectroscopic grade and used without further purification. Column chromatography was performed with commercial glass columns using silica gel 60M (particle size 0.04-0.063 mm). All other reagents and solvents were obtained from commercial sources and used without further purification.
UV/Vis absorption spectra were recorded on a Jasco V-670 or Jasco V-770 spectrophotometer for solution phase measurements.
Fluorescence spectra were recorded on an Edinburgh Instruments FLS980 fluorescence spectrometer. Relative fluorescence quantum yields were determined using the comparative method at four excitation wavelengths with respect to standards: rhodamine 101 in EtOH and 1,1',3,3,3',3'-hexamethylindotricarbocyanine iodide in EtOH. [4] Time-resolved measurements were performed with Edinburgh S3 Instruments picosecond pulsed laser diodes and a TCSPC detection unit.
NMR spectra were recorded on Bruker Avance III HD 400 or Bruker Avance III HD 600 spectrometers. Chemical shifts are listed in parts per million and are given relative to SiMe4 and referenced to a residual solvent signal ( 1 H, 13 C). Coupling constants (J) are quoted in Hertz (Hz). 11 B signals for boron-containing compounds could not be observed due to broadening and/or poor solubility.
High resolution mass spectrometry was carried out on a Bruker Daltonics micrOTOF focus or on a Bruker Daltonics ultrafleXtreme instrument.
Cyclic voltammetry was carried out using a standard commercial electrochemical analyzer (EC epsilon; BAS Instruments, UK) with a three-electrode single-compartment cell. The supporting electrolyte tetrabutylammonium hexafluorophosphate ((n-Bu)4NPF6) was prepared according to the literature, [5] and recrystallized from ethanol/water. The measurements were carried out using ferrocene (Fc) as an internal standard for the calibration of the potential. Potentials of irreversible redox events were determined by square 3 wave voltammetry experiments. An Ag/AgCl reference electrode was used. A Pt disc and a Pt wire were used as working and auxiliary electrodes, respectively.
Single crystal X-ray diffraction data were collected at the P11 beamline at DESY. The diffraction data were collected by a single 360° scan ϕ sweep at 100 K. The diffraction data were indexed, integrated, and scaled using the XDS program package. [6] The structure were solved using SHELXT, [7] expanded with Fourier techniques and refined using the SHELX software package. [8] Hydrogen atoms were assigned at idealized positions and were included in the calculation of structure factors. All non-hydrogen atoms were refined anisotropically. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 2122085 (B2-HBP). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.ac.uk/data.request/cif. Computational details. Geometry optimizations were performed by the density functional theory (DFT) calculations employing B3LYP as functional [9] and 6-311G** as basis set [10] as implemented in the Gaussian 09 program package. [11] The optimized geometries were confirmed to have no negative frequency by frequency calculations. Time-dependent density functional theory (TD-DFT) calculations were performed on the geometry-optimized structures employing the same basis set and functional as for the geometry optimizations. The absorption spectra were simulated by the GaussView 5 visualization software package. [12] NICS(1)zz values were calculated using the NMR chemical shift simulated by the GIAO method. The visualization of AICD was performed by the program package provided by Prof. R. Herges.

2) Synthetic Procedures
Scheme S1. Reaction scheme for the synthesis of precursor 2.
After cooling to 80 °C, the TEMPO solution was added and the mixture was stirred at 80 °C for another 38 h. The reaction was cooled to room temperature and the solvent was removed under reduced pressure. Purification with column chromatography (eluent: toluene/dichloromethane 3/2) yielded 2 as a black solid (61.7 mg, 105 µmol, 14%).           The peaks between 2 and 3 ppm correspond to impurities in C6D4Cl2, which could be confirmed by measuring the solvent only (see Figure S12).