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

Abstract

AbstractNBN‐ 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., BH3⋅SMe2 (1), p‐CF3‐C6H4B(OH)2 (2), C6H5B(OH)2 (3), or MesBCl2/iPr2NEt (4)). Treatment of 1 with 4‐Me2N‐2,6‐Me2‐C6H2Li gave the corresponding NBN derivative 5. The BNB‐phenalenyl 6 was synthesized from 1,8‐naphthalenediyl‐bridged diborane(6), PhNH2, 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‐CF3‐C6H4 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/H2O) shows a distinct red‐shifted emission compared to that of the individual components, which originates from an intermolecular CT state.