Influence of the Alkyl Side Chain Length on the Assembly and Thermochromic Solid-State Properties in Symmetric and Asymmetric Monoalkoxynaphthalene–Naphthalimide Donor–Acceptor Dyads

We have been investigating monoalkoxynaphthalene-naphthalimide (MAN-NI) donor-acceptor dyads that undergo a unique and dramatic orange-to-yellow thermochromic transition in the solid state. This study reports a new series of these stimuli-responsive solids, based on dyads with symmetric and relatively long alkyl side chains, as well as asymmetric derivatives. The length of the alkyl side chains influenced the structures, properties, and dynamic behavior of the resulting solids. All of the symmetric alkyl side chain derivatives can crystallize with identical packing (allowing for alkyl chain length differences) in a lasagna like fashion with alternating alkyl and stacked aromatic layers. The fully extended alkyl side chains are aligned in an interdigitated, "cork and bottle" fashion. The aromatic layers feature head-to-head stacking geometries of the aromatic dyad cores. The crystalline solids derived from each of these derivatives undergo dramatic thermochromic orange-to-yellow transition in the solid state. Crystal structures were obtained for three of the four asymmetric dyad derivatives. In each case, layered structures were observed, although the mismatch in alkyl side chain lengths results in a unique hybrid alkyl/aromatic layer. Two of the asymmetric derivatives exhibit almost identical head-to-head dyad core stacking geometries, while one exhibits a head-to-tail dyad core stacking geometry. Of the four asymmetric dyads studied, only two asymmetric derivatives with the longer alkyl side chains exhibit dynamic solid-state behavior. Overall, our results demonstrate the relatively general nature of the thermochromic orange-to-yellow color-changing mechanism of the MAN-NI dyads and verify the link between the alkyl side chain interactions and this interesting stimuli-responsive behavior in the solid-state.