How Boron Doping Shapes the Optoelectronic Properties of Canonical and Phenylene‐Containing Oligoacenes: A Combined Experimental and Theoretical Investigation

• Published in: Chemistry : a European journal 2017-04, Vol.23 (21), p.5104-5116
• Main Authors: Kirschner, Sven, Mewes, Jan‐Michael, Bolte, Michael, Lerner, Hans‐Wolfram, Dreuw, Andreas, Wagner, Matthias
• Format: Article
• Language: English
• Subjects: Boron; Chemistry; computational chemistry; Deactivation; density functional calculations; Fluorescence; luminescence; Mathematical analysis; Optoelectronics; organoboranes; Photoluminescence; polycyclic aromatic hydrocarbons; Quantum chemistry; Trends
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201700056

Optimized syntheses of 6,13‐dimesityl‐6,13‐dihydro‐6,13‐diborapentacene (DBP) and a related compound (DBI) featuring two biphenylene‐2,3‐diyl units in place of naphthalene‐2,3‐diyl moieties are reported. Striking differences between the optoelectronic properties of DBP and DBI have been experimentally observed, and explained by quantum chemical calculations. DBP is a member of the oligoacene family, DBI is a linear [N]phenylene derivative. The yellow DBP shows blue photoluminescence, the deep red DBI is nonfluorescent. Both compounds give rise to two reversible redox transitions at E1/2 =−2.03 V, −2.75 V (DBP) and −1.52 V, −2.30 V (DBI; THF, vs. FcH/FcH+). The higher electron affinity of DBI agrees with a lower calculated LUMO energy level [−0.57 eV for DBI with respect to DBP @HF//SCS‐MP2/def2‐TZVP] and a higher Lewis acidity of its boron centers, which is reflected in the trend of adduct formation with small Lewis bases (MeCN, F−). The thermochemistry underlying this trend, as well as the mechanism of fluorescence quenching in DBI, are revealed by state‐of‐the‐art quantum chemical calculations. It is suggested that the nonradiative deactivation occurs via a low‐lying, doubly excited state. Doubly exciting: Twofold benzannulation of diboraanthracene boosts the fluorescence but weakens the Lewis acidity. In contrast, attachment of two phenylene moieties via formal σ bonds renders the resulting borane a stronger Lewis acid, but completely quenches its photoluminescence. The nonradiative deactivation likely proceeds via an energetically low‐lying doubly excited state.