Disclosing the molecular structure and dynamics of naphthalene diimide based organic semiconductors in the solid state

Naphthalene diimide (NDI) based molecules are soft organic functional materials investigated for their n-type semiconducting properties and, recently, for their coupled electronic and ionic transport properties. Such qualities make them potential candidates as mixed ionic-electronic conductors (OMIECs) for electrochemical energy storage devices and bioelectronic sensors. A key aspect is the rationalization of their solid-state structure and morphology, which ultimately controls their transport properties. Here, we introduce two newly synthesized NDIs, 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 characterization 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 gain detailed atomistic insights into both short- and long-range order, as well as to investigate the thermal disorder effects, highlighting the role of the functional side groups. Our study establishes a validated computational ground for modelling NDIs in condensed solid phases, benchmarking the procedure with respect to experimental XRD data, and extending it to amorphous films, thus paving the way for an in-depth understanding of the structure-property functions of small molecule n-type semiconductors.