Impact of pyrene orientation on the electronic properties and stability of graphene ribbons

We report the synthesis and study of trialkylsilylethyne-substituted “oblique” pyrene-fused acenes, carbon nanoribbons demonstrating near-IR absorption with reversible oxidation and reduction, and acene-like evolution of electronic properties upon extension of the aromatic core. Their electronic structures are investigated through DFT studies, which support the more delocalized nature of their frontier molecular orbitals compared to more common “vertical” pyrene systems. Despite a longer aromatic core, the more extended of the two examples demonstrates enhanced photostability compared to the shorter derivative, running counter to the trend in linear acenes. The unusual stability of the longer core is ultimately linked to its relatively low T₁ energy inhibiting the generation of reactive O₂ species. The byproduct generated upon photooxidation of the shorter nanoribbon appears to catalyze the generation of ¹O₂ due to its large T₁ energy, leading to its relatively decreased stability.