Coverage-dependent self-organization of chlorophyll a at graphene interfaces: A Molecular dynamics study

Understanding the interaction between photosynthetic pigments and graphitic nanomaterials is important for the design of hybrid bio-nanostructured interfaces. Here we investigate the adsorption and coverage-dependent organization of chlorophyll a confined between two graphene sheets using classical molecular dynamics simulations. Systems containing 1, 2, 4, 8, 12, and 16 chlorophyll molecules were simulated in methanol using a stepwise deposition protocol. Chlorophyll molecules adsorb onto graphene and remain predominantly associated with the interfaces throughout the microsecond simulations. As surface coverage increases, the adsorbed population distributes between the two graphene walls, while a small number of molecules adopt weakly adsorbed configurations because of crowding. Molecules directly adsorbed on graphene maintain a nearly parallel chlorin-ring orientation, with ring-normal angles of approximately 6 – 8 ° , whereas weakly adsorbed molecules exhibit broader orientations extending to approximately 30–40°. Increasing coverage produces progressively greater intermolecular connectivity and moderate conformational heterogeneity. Overall, increasing surface coverage reorganizes the adsorbed layer from relatively isolated molecules toward densely interconnected interfacial assemblies while preserving the characteristic face-on adsorption geometry of the chlorin macrocycle.