Bottom-up synthesis of graphene nanoribbons (GNRs) by surface-assisted polymerization and cyclodehydrogenation of specifically designed precursor monomers has been shown to yield precise edges and doping. Here we use a precursor monomer containing sulfur atoms to fabricate nanostructures on a Au(111) surface at different annealing temperatures. The nanostructures have distinct configurations, varying from sulfur-doped polymers to sulfur-doped chevron-type GNRs (CGNRs) and, finally, pristine graphene nanoribbons with specific edges of periodic five-member carbon rings. Non-contact atomic force microscopy provides clear evidence for the cleavage of C–S bonds and formation of pristine CGNRs at elevated annealing temperatures. First-principles calculations show that the CGNRs exhibit negative differential resistance.