Synthesis of Helical and Planar Extended-Phenanthridinium Salts

The investigation of new synthetic routes towards positively charged N-heterocycles is of high importance for the further development of medicinal chemistry, functional materials, catalysis and other areas of application. For accessing acridinium dyes, we previously reported an approach based on an aryne-imine-aryne coupling followed by the subsequent oxidation of the acridane intermediates. Herein, we now present an unusual reaction outcome when phenanthryne is used as aryne component. Under optimized conditions, this two-step synthetic methodology led to the formation of a helical tetrabenzophenanthridinium derivative. Furthermore, the susceptibility of this product to photoinduced cyclodehydrogenation was observed, providing a highly fluorescent planar polycyclic aromatic hydrocarbon with a positively charged nitrogen. The photophysical and electrochemical properties of the mesityl-phenyltetrabenzophenanthridinium tetrafluoroborate were also determined.