Self-assembled D–π–A multifunctional systems with tunable stimuli-responsive emission and optical waveguiding behaviour

Abstract

Smart materials with stimuli-responsiveness are a subject of great attention nowadays. In this work, we describe two D–π–A naphthalenimide (NI) derivatives, with different N-functionalization prepared by an environmentally benign process. They self-assemble into fluorescent crystals that display optical waveguiding behaviour with low optical loss coefficients. In addition, they present thermal/mechanical stimuli-responsiveness, which is tuned upon substitution at the molecular main core as a result of changes in crystal packing. A cold-crystallization process, as confirmed by DSC and power X-ray analysis, was identified as the underlying cause of the colour change upon heating, which can be reverted due to an amorphization process upon shearing. The combination of the reversible coloured amorphous-crystalline phase transition, together with the light guiding, holds significant promise for practical applications including use in security inks, rewritable materials or modern optics.