Bis(9-Boraphenanthrene) and Its Stable Biradical
Samir Kumar Sarkar, Kimberly K. Hollister, Andrew Molino, Akachukwu D. Obi, Chun‐Lin Deng, Bi Youan E. Tra, Brennan M. Stewart, Diane A. Dickie, David J. D. Wilson, Robert J. Gilliard
Abstract
Selective and site-specific boron-doping of polycyclic aromatic hydrocarbon frameworks often give rise to redox and/or photophysical properties that are not easily accessible with the analogous all-carbon systems. Herein, we report ligand-mediated control of boraphenanthrene closed- and open-shell electronic states, which has led to the first structurally characterized examples of neutral bis(9-boraphenanthrene) ( 2 – 3 ) and its corresponding biradical ( 4 ). Notably, compounds 2 and 3 show intramolecular charge transfer absorption from the 9-boraphenanthrene units to p -quinodimethane, exhibiting dual (red-shifted) emission in solution due to excited state conjugation enhancement (ESCE). Moreover, while boron-centered monoradicals are ubiquitous, biradical 4 represents a rare type of open-shell singlet compound with 95% biradical character, among the highest of any reported boron-based polycyclic species with two radical sites.