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

Abstract 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.