π-Extension of 2-aryl-phenanthroimidazoles via rhodium(III)-catalyzed C-H activation and alkyne annulation is developed. This method enables rapid, practical and modular assembly of diverse ring-fused phenanthroimidazoles, including an unusual rearrangement product using aryl-alkyl asymmetric alkyne and a thiophene fused product which could serve as a Fe(3+) fluorescent probe. The feasibility of the one-pot synthesis and ruthenium(II)-catalyzed versions of this reaction was also verified.