Highly Deformed o‐Carborane Functionalised Non‐linear Polycyclic Aromatics with Exceptionally Long C−C Bonds

Abstract The effect of substituting o‐carborane into the most sterically hindered positions of phenanthrene and benzo(k)tetraphene is reported. Synthesised via a Bull–Hutchings–Quayle benzannulation, the crystal structures of these non‐linear acenes exhibited the highest aromatic deformation parameters observed for any reported carborane compound to date, and among the largest carboranyl C−C bond length of all organo‐substituted o‐carboranes. Photoluminescence studies of these compounds demonstrated efficient intramolecular charge‐transfer, leading to aggregation induced emission properties. Additionally, an unusual low‐energy excimer was observed for the phenanthryl compound. These are two new members of the family of carborane‐functionalised non‐linear acenes, notable for their peculiar structures and multi‐luminescent properties.


Experimental Procedures
All solvents, reactants, and reagents were purchased from Acros Organics, Alfa Aesar, Apollo Scientific, Sigma Aldrich, TCI Europe, or VWR, and used without further purification. Reactions were carried out under BOC Pureshield Argon unless otherwise stated. Proton ( 1 H), Boron ( 11 B), and carbon ( 13 C) solution-state NMR spectra were obtained from a Bruker Avance-400 (400 MHz) spectrometer, at room temperature using CDCl3 as the deuterated solvent. Chemical shift data was obtained using residual CDCl3 internal standards ( 1 H: 7.26 ppm; 13 C: 77.36 ppm); singlet = s, doublet = d, triplet = t, quarter = q, multiplet = m, broad = br, with combinations for complex multiplets. High resolution mass spectrometry (HRMS) data was obtained from an Agilent HP6890 GC (EI); Dinoex Ultimate 3000 (APCI); and ultrafleXtreme (MALDI-ToF). Infra-red (ATR-IR) spectroscopy measurements were obtained using a PerkinElmer FT-IR Spectrum 100 fitted with an ATR (Ge/Ge) accessory. X-ray crystallographic data was obtained with an Agilent Xcalibur PX Ultra A, at 173 K. Cyclic Voltammetry data was obtained using a Metrohm Autolab PGSTAT101 potentiostat in dry DCM containing 0.1M NBu4PF6 with a platinum working electrode, Pt counter electrode and an Ag/AgCl (Ag/Ag + ) reference electrode with ferrocene as internal reference.
Density functional theory (DFT) calculations were carried out using B3LYP [1,2] functional and 6-31G(d,p) [3,4] basis set, with no symmetry constraints, in the gas phase. Dihedral scans were obtained using OPT(Z-MAT) optimisation, starting from the calculated global ground state geometry and proceeding with 1° increments around the bond connecting the carborane and the substituent.
Warning: Exposure to polycyclic aromatic hydrocarbon (PAH) containing substances increases the risk of cancer in humans. [5] Replication of these syntheses should be proceeded with caution. [6] Allyl bromide (17.99 mL, 208 mmol) was added to a suspension of 1-naphthol (15.00 g, 104 mmol) and K2CO3 (43.13 g, 312 mmol) in dry acetonitrile (300 mL) and stirred overnight at room temperature. The inorganic components were separated by vacuum filtration and washed with diethyl ether (300 mL). The filtrate was washed with water (5 x 200 mL), brine (2 x 200 mL) and dried over MgSO4. Solvent was removed in vacuo yielding the title compound as a yellow oil (17.63 g, 95.7 mmol, 92%), which was subsequently used without further purification. 1  2-Allyl-1-(2,2,2-trichloroacetyl)naphthalene [6] 1-(Allyloxy)naphthalene (15.00 g, 81.4 mmol) was heated under argon, with stirring, for 2 h at 210 °C, after which the heating was removed and the brown oil was allowed to cool to room temperature. Dry diethyl ether (300 mL) and pyridine (9.00 mL, 111 mmol ) were added, the solution was cooled to 0 °C and trichloroacetyl chloride (12.50 mL, 111 mmol) was added dropwise. The mixture was stirred for 2 h then allowed to warm to room temperature. The brown solution was poured into a slurry of ice-water (200 g) and NaHCO3 (5 g), washed with water (2 x 100 mL), saturated aq. NH4Cl (3 x 100 mL), water (3 x 100 mL), brine (2 x 100 mL), and dried over MgSO4. Solvent was removed in vacuo yielding the title compound as an orange solid (22.81 g, 69.2 mmol, 85%), which was subsequently used without further purification. 1

4-(2-phenyl-1,2-dicarbadodecaboran-1-yl)phenanthrene (1)
4-(Phenylethynyl)phenanthrene (286 mg, 1.03 mmol) and decaborane (377 mg, 3.09 mmol) were added to a vigorously stirring flask charged with a biphasic mixture of bmim(Cl) (162 mg, 0.93 mmol) and dry toluene (10 mL), and the mixture was heated to reflux for 48 h. After cooling the solvent was removed in vacuo and the crude product was subjected to column chromatography with hexane eluent, yielding the title compound as bright green crystals (131 mg, 0.33 mmol, 32%). Crystals suitable for X-ray diffraction analysis were grown by slow evaporation of a biphasic solution of DCM and hexane. 1 13 [6] Note: the target compound is a lachrymator and should be treated with caution.

2-Bromo-1-tetralone
To a stirring solution of 1-tetralone (18.20 mL, 137 mmol) in dry diethyl ether (100 mL) cooled to 0 °C, was added bromine (7.02 mL, 137 mmol) dropwise over 5 mins, and the mixture was stirred for a further 30 mins. Ice water (100 mL) was added to the reaction, followed by sat. sodium sulfite solution (100 mL). The organic layer was separated, washed with water (3 x 100 mL), brine (100 mL), dried over MgSO4. Solvent was removed in vacuo to yield the title compound as a brown oil (28.37 g, 126 mmol, 92%) which was subsequently used without further purification. 1

2'-(Naphthalene-1'-yloxy)-1-methylene-2,3,4-trihydronaphthalene [6]
To a vigorously stirring suspension of 2-(Naphthalene-1'-yloxy)-1-tetralone (15.00 g, 52.0 mmol) in dry diethyl ether (200 mL) was added a ylide solution freshly prepared from the addition of methyltriphenylphosphonium bromide (28.17 g, 78.9 mmol) and KO t Bu (8.85 g, 78.9 mmol) to a solution of dry diethyl ether (200 mL) at 0 °C, which had been left stirring for 30 mins. The resulting bright yellow mixture was left stirring overnight, then the inorganic material was removed via suction filtration through celite. The filtration was concentrated in vacuo and dry loaded onto a silica plug which was eluted with Pet. ether until a yellow colour appeared in the filtrate, after which eluent was changed to 1:9 DCM:Pet. ether. The yellow filtrate fractions were combined and left to evaporate slowly yielding the title compound as bright yellow crystals (7.89 g, 27.6 mmol, 53%). 1 13