Phenanthroline-Fused Pyrazinacenes: One-Pot Synthesis, Tautomerization and a Ru^II(2,2′-bpy)₂ Derivative

We report the one-pot synthesis of a phenanthroline-fused pyrazinacene derivative (6,13-dihydrodipyrido[3,2-a:2′,3′-c]-5,6,7,8,11,12,13,14-octaazapentacene-9,10-dicarbonitrile) and its behaviour under alkylating conditions used to improve solubility. Tautomerization of the starting pyrazinacene due to the presence of a reduced pyrazine ring contained within an octaazatetracene chromophore led to mixtures of isomers, and factors affecting the relative yields of these isomers were considered. Isomer population can be described by a two-step reaction model where initial N-alkylation affects the reactivity of the remaining nitrogen atoms available for subsequent alkylation. A discrete soluble non-isomerizable phenanthroline-fused pyrazinacene was also prepared and the activity of its Ru(bpy)₂ complex as a photosensitizer for dye-sensitized solar cell application was investigated. The compounds reported illustrate the unusual reactivity of reduced pyrazinacenes and also their potential as photosensitizers.