Tunable Topological Phases in an Organic One-Dimensional Mott Chain: Exchange-Alternating S=12 and Haldane S=1

Establishing symmetry-protected topological (SPT) phases with interactions in chemically realistic systems remains an open challenge. We show that a single, synthetically plausible organic one-dimensional chain, tunable via chemical modification of its radical sites, hosts two such phases: an even-Haldane phase of a dimerized $S=\frac{1}{2}$ Heisenberg chain and a Haldane phase of an S = 1 chain realized when Hund coupling locks two $S=\frac{1}{2}$ spins per monomer into S = 1. Density-functional theory places the active manifold deep in the Mott regime (U/t > 100), justifying a spin-only Heisenberg description; a compact (t,U) → J mapping then fixes exchange couplings. Exact diagonalization and DMRG reveal a consistent SPT fingerprint across both phases, including a quantized many-body Zak phase, even-degenerate entanglement spectrum, protected edge spins, and characteristic triplon-Haldane features in S^+-(q,ω). Our results identify a chemically programmable molecular platform for interacting SPT physics in one dimension and suggest concrete spectroscopic routes to organic Haldane spin chains for nanoscale quantum devices.