NBN‐ and BNB‐Phenalenyls: the Yin and Yang of Heteroatom‐doped π Systems

NBN‐ and BNB‐doped phenalenyls are isoelectronic to phenalenyl anions and cations, respectively. They represent a pair of complementary molecules that have essentially identical structures but opposite properties as electron donors and acceptors. The NBN‐phenalenyls 1 – 4 considered here were prepared from N , N’‐ dimethyl‐1,8‐diaminonaphthalene and readily available boron‐containing building blocks (i. e., BH 3 ⋅SMe 2 ( 1 ), p ‐CF 3 ‐C 6 H 4 B(OH) 2 ( 2 ), C 6 H 5 B(OH) 2 ( 3 ), or MesBCl 2 / i Pr 2 NEt ( 4 )). Treatment of 1 with 4‐Me 2 N‐2,6‐Me 2 ‐C 6 H 2 Li gave the corresponding NBN derivative 5 . The BNB‐phenalenyl 6 was synthesized from 1,8‐naphthalenediyl‐bridged diborane(6), PhNH 2 , and MesMgBr. A computational study reveals that the photoemission of 1 , 4 , and 5 originates from locally excited (LE) states at the NBN‐phenalenyl fragments, while that of 2 is dominated by charge transfer (CT) from the NBN‐phenalenyl to the p ‐CF 3 ‐C 6 H 4 fragment. Depending on the dihedral angle θ between its Ph and NBN planes, compound 3 emits mainly from a less polar LE ( θ >55°) or more polar CT state ( θ <55°). In turn, the energetic preference for either state is governed by the polarity of the solvent used. An equimolar aggregate of the NBN‐ and BNB‐phenalenyls 3 and 6 (in THF/H 2 O) shows a distinct red‐shifted emission compared to that of the individual components, which originates from an intermolecular CT state.