Two BN Isosteres of Anthracene: Synthesis and Characterization

摘要

The synthesis of two parental BN anthracenes,1 and 2, was developed, and their electronic structure and reactivity behavior were characterized in direct comparison with all-carbon anthracene. Gas-phase UV-photoelecton spec- troscopy studies revealed the following HOMO energy trend: anthracene, -7.4 eV; BN anthracene 1, -7.7 eV; bis-BN anthracene 2, -8.0 eV. The λ_(max) of the lower energy band in the UV-vis absorption spectrum is as follows: anthracene, 356 nm; BN anthracene 1, 359 run; bis-BN anthracene 2, 357 nm. Thus, although the HOMO is stabilized with increasing BN incorporation, the HOMO-LUMO band gap remains unchanged across the anthracene series. The emission λ_(max) values for the three investigated anthracene compounds are at 403 nm. The pK_a values of the N-H proton for BN anthracene 1 and bis-BN anthracene 2 were determined to be approximately 26. BN anthracenes 1 and 2 do not undergo heat- or light-induced cydoaddition reactions or Friedel-Crafts acylations. Electrophilic bromination of BN anthracene 1 with Br_2, however, occurs regioselectively at the 9-position. The reactivity behavior and regioselectivity of bromination of BN anthracenes are consistent with the electronic structure of these compounds; i.e., (1) the lower HOMO energy levels for BN anthracenes stabilize the molecules against cydoaddition and Friedel-Crafts reactions, and (2) the HOMO orbital coeffidents are consistent with the observed bromination regioselectivity. Overall, this work demonstrates that BN/CC isosterism can be used as a molecular design strategy to stabilize the HOMO of acene-type structures while the optical band gap is maintained.