Two BN Isosteres of Anthracene: Synthesis and Characterization

The synthesis of two parental BN anthracenes, <b>1</b> and <b>2</b>, was developed, and their electronic structure and reactivity behavior were characterized in direct comparison with all-carbon anthracene. Gas-phase UV-photoelecton spectroscopy studies revealed the following HOMO energy trend: anthracene, −7.4 eV; BN anthracene <b>1</b>, −7.7 eV; bis-BN anthracene <b>2</b>, −8.0 eV. The λ<sub>max</sub> of the lower energy band in the UV–vis absorption spectrum is as follows: anthracene, 356 nm; BN anthracene <b>1</b>, 359 nm; bis-BN anthracene <b>2</b>, 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<sub>max</sub> values for the three investigated anthracene compounds are at 403 nm. The p<i>K</i><sub>a</sub> values of the <i>N</i>-H proton for BN anthracene <b>1</b> and bis-BN anthracene <b>2</b> were determined to be approximately 26. BN anthracenes <b>1</b> and <b>2</b> do not undergo heat- or light-induced cycloaddition reactions or Friedel–Crafts acylations. Electrophilic bromination of BN anthracene <b>1</b> with Br<sub>2</sub>, 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 cycloaddition and Friedel–Crafts reactions, and (2) the HOMO orbital coefficients 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.