B-N-fused dianthracenylpyrazine derivatives are synthesized to generate new low gap chromophores. Photophysical and electrochemical, crystal packing, and theoretical studies have been performed. Two energetically similar conformers are identified by density functional theory calculations, showing that the core unit adopts a curved saddle-like shape (<i>x-isomer</i>) or a zig-zag conformation (<i>z-isomer</i>). In the solid state, the <i>z-isomer</i> is prevalent according to an X-ray crystal structure of a C<sub>6</sub>F<sub>5</sub>-substituted derivative (<b>4-Pf</b>), but variable-temperature nuclear magnetic resonance studies suggest a dynamic behavior in solution. B-N fusion results in a large decrease of the HOMO-LUMO gap and dramatically lowers the LUMO energy compared to the all-carbon analogues. <b>4-Pf</b> in particular shows significant absorbance at greater than 700 nm while being almost transparent throughout the visible region. After encapsulation in the biodegradable polymer DSPE-mPEG<sub>2000</sub>, <b>4-Pf</b> nanoparticles (<b>4-Pf-NPs</b>) exhibit good water solubility, high photostability, and an excellent photothermal conversion efficiency of ∼41.8%. <b>4-Pf-NPs</b> are evaluated both in vitro and in vivo as photothermal therapeutic agents. These results uncover B-N Lewis pair functionalization of PAHs as a promising strategy toward new NIR-absorbing materials for photothermal applications.