C-Halogenated 9,10-Diboraanthracenes: How the Halogen Load and Distribution Influences Key Optoelectronic Properties.

9,10-Dihydro-9,10-diboraanthracenes (DBAs) have low-energy LUMOs and narrow HOMO-LUMO gaps and are thus attractive electron-transporting and light-emitting materials in optoelectronic devices. A systematic series of ten C-halogenated 9,10-(Mes)2 -DBAs was synthesized and studied by cyclic voltammetry, UV/Vis absorption and emission spectroscopy, and quantum-chemical calculations (Mes=mesityl). We probed the influence of the nature of the halogen atoms and the halogen substitution patterns on key optoelectronic properties of the DBAs. All 9,10-(Mes)2 derivatives can be reversibly reduced at the DBA cores and at electrode potentials between E1/2 Red1 =-1.84 and -1.26 V (vs. FcH/FcH+ ). The most bathochromic UV/Vis absorption and the fluorescence emission of each DBA correspond to an ICT transition between the Mes rings and the DBA core. Br substituents lower the DBA LUMO energy and narrow the energy gap to the highest degree along the series F<Cl<Br. Halogen atoms located at 1,4,5,8-positions are more influential than those at 2,3,6,7-positions. An increasing fluorine load continuously decreases the LUMO levels and HOMO-LUMO gaps. Colorless octafluoro- and tetrafluoro-DBAs form colored and fluorescent π-stacked hetero dimers with anthracene in C6 H6 solutions and in the solid state. The sterically congested 1,5-(ThMe)2 -9,10-(Mes)2 -DBA was prepared in 74 % yield from 1,5-(Br)2 -9,10-(Mes)2 -DBA and Me3 Sn-ThMe through a Stille-type C-C coupling reaction (ThMe=5-Me-2-thienyl).