Trifold anisotropy and strain-driven switching in octagon-pentagon graphene nanoribbons

Octagon-pentagon graphene nanoribbons (OPG-ZNRs) provide a non-hexagonal carbon platform in which edge topology can couple electronic, vibrational and mechanical properties. Here, five hydrogen-passivated OPG-ZNR families are investigated to establish how crystallographic direction, edge-ring sequence, uniaxial strain and lateral confinement determine their response. Distinct IR and Raman signatures are obtained for the pentagon, octagon and mixed pentagon-octagon edges, providing spectroscopic markers for the different ribbon families. Under uniaxial strain between -5% and +5%, the ribbons separate into qualitatively different electronic regimes. acOPGZNR-O exhibits the largest equilibrium PBE gap, 0.420 eV, with a nearly linear strain dependence of approximately 35 meV %^-1, whereas acOPGZNR-P develops a compression-activated gap and acOPGZNR-P2 remains close to a metallic state except for a threshold-like gap opening at +5% strain. The PO-edge families remain gapless while showing pronounced strain-dependent redistribution of density of states, including flat-band features in zzOPGZNR-PO. The mechanical response is equally topology dependent, with axial rigidities ranging from 75.2 to 229.0 nN and auxetic behaviour in acOPGZNR-O and acOPGZNR-P2. Increasing the width from W = 1 to W = 2 qualitatively redistributes the electronic and mechanical response, producing a 0.602 eV PBE gap in acOPGZNR-P2 at zero strain, while M-graphene ribbons remain metallic and non-auxetic under the same conditions. These results show that the coupled anisotropy of OPG-ZNRs is governed by their specific octagon-pentagon connectivity rather than by lateral confinement or non-hexagonal topology alone.