Termination-Controlled Fractional Spin Modes at Organic Topological Spin-Chain Interfaces

Organic radical chains provide an atomically precise platform for realizing interacting quantum spin nanostructures, but controlling the local fate of fractional boundary degrees of freedom at internal interfaces remains challenging. Here, we investigate interfaces between a dimerized spin-1/2 sector and a Hund-coupled effective Haldane spin-1 sector within a trioxoazatriangulene-based parent chain. Although the constituent fractional boundary moments are individually established, their many-body response at the heterojunction depends sensitively on the bond-level termination registry. In one geometry, adjoining spin-1/2-like boundary contributions locally compensate and quench the interfacial response, whereas a parity-shifted registry leaves an uncompensated localized spin-1/2-like mode. Two such internal modes of a finite embedded Haldane domain hybridize with an approximately exponentially decreasing splitting. The active and quenched junctions therefore exhibit distinct local low-energy spin responses. These results identify termination parity and bond-level interface registry as practical control parameters for activating, locally quenching, and coupling fractional spin modes in organic quantum spin nanostructures.