  Carbon-based nanostructures possessing {\pi}-electron magnetism have
attracted tremendous interest due to their great potential for nano
spintronics. In particular, quantum chains with magnetic molecular units
synthesized by on-surface reactions provide an ideal playground for
investigating magnetic exchange interactions between localized spin components.
Here, we present an extensive study of antiferromagnetic nanographene chains
with the diazahexabenzocoronene molecule as the repeating unit. A combination
of bond-resolved scanning tunneling microscopy, density functional theory and
quantum spin models revealed their detailed structures and electronic and
magnetic properties. We found that the antiferromagnetic chains host a
collective state featuring gapped excitations for an even number of repeating
units and one featuring a Kondo excitation for an odd number. Comparing with
exact many-body quantum spin models, our molecular chains provide the
realization of an entangled quantum Heisenberg model. Coupled with the
tunability of the molecular building blocks, these systems can act as an ideal
platform for the experimental realization of topological spin lattices.
