π-magnetism of finite-sized nanoribbon occurring on the topological molecular interfaces remains largely unexplored due to limited experimental examples. Herein, we report rational design, solution synthesis and systematical characterization of a novel type of stair-like aza-nanographene (ANG) ANG-a~b with precise N-doping on the interfacial cove-edges. Within the same molecular π-backbone, ANG-a had a closed-shell structure due to the electronic perturbation of cove-edge substitution; while ANG-b hosted a spin-polarized interface state, and impressively its open-shell singlet diradicaloid structure produced a combined optoelectronic, magnetic and physicochemical characteristics. Besides, dicationic ANG-b was also synthesized and characterized as a ground-state diradicaloid, again closely associated with the interfacial spin-polarization in the charged π-system. Our studies might provide insights into future structural engineering of topological open-shell materials with robust yet exotic spin-polarized interface states.