The effect of lateral π-extension on azulene-based molecules on surface studied by LT-STM

Azulene-based systems offer a robust platform for exploring molecular self-assembly and electronic properties on surfaces due to their non-benzenoid topology and dipolar structure. Here, we report the low-temperature scanning tunnelling microscopy investigation of two structurally related azulene-based molecules, namely cyclopenta[cd]azulene trimers (CPAT) and its phenyl extended analogue cyclopenta[cd]azulene trimers (CPAT-Ph). The latter has lateral extensions in the form of three phenyl rings, designed to investigate the influence of functionalization on supramolecular behaviour. We demonstrate that while CPAT self-assembles in homochiral domains on Au(111), CPAT-Ph is weakly adsorbed and mobile on Au(111) and on Cu(110), exhibiting no ordered domains. Spatially resolved scanning tunnelling spectroscopy (STS) reveals that lateral π-extension primarily modifies electronic level alignment through altered molecule-substrate coupling, while leaving intermolecular electronic interactions weak. Upon thermal activation, both molecules undergo disordered intermolecular linkages on Au(111). However, on Cu(110) enhanced interaction between the substrate and the CPAT-Ph molecules enables intramolecular cyclodehydrogenation through multiple competing ring-closure pathways. These findings indicate lateral π-extension as a design parameter for tuning adsorption-dominated electronic structure and surface reactivity in non-benzenoid π-systems.