Planar, Stair-stepped, and Twisted: Modulating Structure and Photophysics in Pyrene- and Benzene-Fused N-Heterocyclic Boranes.

Because of their rigidity, polycyclic aromatic hydrocarbons (PAHs) have become a significant building block in molecular materials chemistry. Fusion or doping of boron into PAHs is known to improve the optoelectronic properties by reducing the LUMO energy level. Herein, we report a comprehensive study on the syntheses, structures, and photophysical properties of a new class of fused  N -heterocyclic boranes (NHBs), pyrene- and benzene-linked in a "Janus-type" fashion ( 2-4 ,  6-9,  and 11 ). Remarkably, these examples of fused NHBs display fluorescent properties, and collectively their emission spans the visible spectrum. The  pyrene-fused NHBs all display blue fluorescence, as the excitations are dominated by the pyrene core. In notable contrast, the emission properties of the benzene-fused analogues are highly tunable and is dependent on the electronics of the NHB fragments (i.e., the functional group directly bound to the boron atoms).  Pyrene-fused  2 - 4 and  11 represent the only molecules in which the K-region of pyrene is functionalized with NHB units , and while they exhibit distorted (twisted or stair-stepped) pyrene cores, benzene-fused 6 - 9 are planar.  The electronic structure and optical properties of these materials were probed by computational studies, including an evaluation of  aromaticity, electronic transitions, and molecular orbitals.