Thermodynamically vs kinetically controlled self-assembly of a Naphthalenediimide-thiophene derivative: from crystalline, fluorescent, n-type semiconducting 1D needles to nanofibers

The control over aggregation pathways is a key requirement for present and future technologies, as it can provide access to a variety of sophisticated structures with unique functional properties. In this work, we demonstrate an unprecedented control over the supramolecular self-assembly of a semiconductive material, based on a naphthalenediimide core functionalized with phenyl-thiophene moieties at the imide termini, by trapping the molecules into different arrangements depending on the crystallization conditions. The control of the solvent evaporation rate enables the growth of highly elaborated hierarchical self-assembled structures: either in an energy-minimum thermodynamic state when the solvent is slowly evaporated forming needle-shaped crystals (polymorph alpha) or in a local energy-minimum state when the solvent is rapidly evaporated leading to the formation of nanofibers (polymorph beta). The exceptional persistence of the kinetically trapped beta form allowed the study and comparison of its characteristics with that of the stable alpha form, revealing the importance of molecular aggregation geometry in functional properties. Intriguingly, we found that compared to the thermodynamically stable alpha phase, characterized by a J-type aggregation, the beta phase exhibits (i) an unusual strong blue shift of the emission from the charge transfer state responsible for the solid-state luminescent enhancement, (ii) a higher work function with a "rigid shift" of the electronic levels, as shown by Kelvin probe force microscopy and cyclic voltammetry measurements, and (iii) a superior field-effect transistor mobility in agreement with an H-type aggregation as indicated by X-ray analysis and theoretical calculations.