On-Surface Synthesis of Chevron-Shaped Conjugated Ladder Polymers Consisting of Benzo[a]azulene Units

Conjugated ladder polymers (CLPs) have attracted broad interest due to their intriguing optical and electronic properties. While many of these CLPs have been synthesized using solution-based reactions, on-surface synthesis under high vacuum conditions has gradually gained prominence in recent decades. This new approach holds promise for overcoming some of the limitations of conventional solution-based methods, such as the low solubility and stability of newly formed large π-conjugated systems. Azulene derivatives are attractive precursors for on-surface synthesis of CLPs that incorporate non-benzenoid moieties. The use of alkyl-substituted azulene precursors shows potential in providing CLPs with more complex backbone structures and modulated electronic properties compared to traditional CLPs that containing solely of six-membered rings. However, this strategy has been scarcely explored to date. In this study, we report on the thermal reactions of 3,3’-dibromo-2,2’-dimethyl-1,1’-biazulenyl (DBMA) on Au(111) surfaces. At room temperature, we observed that the deposited molecules formed amorphous aggregates in the fcc (face center cubic) regions of the reconstructed Au(111) surface, remaining unchanged below 100 °C. Debromination of DBMA was induced above 150 °C, leading to the formation of 1,1’-biazulenyl-2,2’-dimethyl-3,3’-diyls-Au organometallic polymers. These polymers exhibited complex stereostructures and distinct imaging features. At higher temperatures, the organometallic polymer underwent C―C coupling, followed by dehydrocyclization between the methyl groups and the adjacent azulene units, resulting in the ladder polymer containing benzo[a]azulene units. Interestingly, we observed that the formation of hexagonal rings between the methyl groups and the adjacent azulene units caused the polymeric chain to bend, increasing the distance between the corresponding reaction sites (methyl group and azulene) on the other side of the polymer chain. Due to the ring strain, the second ring closure did not occur within the azulene dimer as expected. Instead, this methyl group cyclized toward the other azulene unit, resulting in CLPs with a chevron shape and the absence of long-range periodicity. The evolution of related chemical species and the structures of CLPs were analyzed using scanning tunneling microscopy (STM) and bond-resolved atomic force microscopy (BR-AFM), and the reaction mechanism was discussed. This study thus demonstrates the feasibility of util