Disclosing the structure and dynamics of naphthalene-diimide based organic mixed ionic-electronic conductors at the molecular scales

Naphthalene diimide (NDI) based molecules are soft organic functional materials investigated for their coupled electronic and ionic transport properties. Such quality makes them potential candidates as mixed ionic-electronic conductors (OMIECs) for electrochemical energy storage devices and bioelectronic sensors. A key aspect of OMIECs is the rationalization of their solid- state structure and morphology, ultimately controlling the bulk charge and ion transport. Here, we introduce two newly synthesized NDI based OMIECs, namely NDI-TEG, functionalised with linear triethylene glycol side chains, and NDI-crown, with 15-crown-5 rings. Their bulk structural properties, investigated via experimental characterizations and state-of-the-art molecular dynamics (MD) simulations, are compared with those of the parental species NDI- C10, featuring symmetrical n-decyl side chains. Our MD simulations reproduce remarkably well the experimental crystal structures of NDI-C10 and NDI-TEG. At the same time, we compute ex novo the amorphous morphology of NDI-crown, by simulating the experimental film casting conditions. Structural order parameters and correlation functions allow us to provide detailed atomistic insights into both short- and long-range order, as well as to disclose the thermal disorder effects for each NDI species, highlighting the role of the functional side groups. Our joint experimental and computational investigation paves the way for an in-depth understanding of the structure-property functions of small molecule based OMIECs.