Fused Metallo-Carbaporphyrin-Embedded Graphene Nanoribbon Segments

Graphene nanoribbons (GNRs) may be considered as size-limited cutouts from graphene sheets or nanoscale polycyclic aromatic hydrocarbons (PAHs). These so-called "segments" serve as model systems for these larger constructs and are of interest in their own right because of the synthetic challenges they represent and the wide range of applications they might permit. One appealing approach to tune the electronic features of GNRs and segments involves introducing porphyrins into the backbone. To date, surface synthesis approaches have been extensively used to fuse porphyrins into GNRs in an atomically defined manner. We describe here the use of hexa-peri-hexabenzocoronene (HBC)-fused carbaporphyrin moieties to create GNR segments featuring two Pd^II-carbaporphyrin cores with multiple metal-carbon bonds, featuring a direct Pd-mediated triply fusion. One target was unambiguously characterized via single-crystal X-ray diffraction analysis, revealing a planar backbone length of over 3 nm. The Pd^II-carbaporphyrin-embedded GNR segment displays intense absorption features in the NIR-II spectral region and a ligand-to-metal charge transfer (LMCT) excited state that relaxes predominantly through nonradiative decay. The rigid Pd-incorporated backbone imparts photo- and thermal stability, effectively addressing the susceptibility to photodegradation typical of organic NIR-II dyes. It also benefits from an LMCT-driven relaxation pathway that allows high photothermal conversion efficiencies (PCEs) to be achieved under photoirradiation at either 980 or 1064 nm. The present study provides new insights into solution-phase syntheses of atomically precise porphyrinoid-GNR hybrids, particularly those that feature metal centers.