Self-assembly induced tunable multiple fluorescence output from a white light-emitting functionalized single π-conjugated molecule and implication in VOC sensing applications

Abstract

Self-assembled peptide-based nanostructures comprised of naturally occurring amino acids display excellent fluorescence properties. Nanostructures obtained from small organic molecules or generated from the supramolecular assembly of various functional π-conjugated molecules have stimulated significant interest due to their potential applications in colour tunable displays, optoelectronic devices, chemical sensors, and many related fields. Soft materials with tunable fluorescence output are imperative for developing multiple-colour-emitting devices. These π-conjugated molecules with extended π-conjugation have proven their potential in several organic electronic applications. In this context, for the improved performance of an organic electronic application, the fabrication of monomeric organic probe molecules into an ordered superstructure is of supreme significance. To this end, herein, we have designed and synthesized two isomeric functional π-conjugated molecules, L1 and L2, and studied their self-assembling properties. Under identical conditions, these two building blocks self-assemble into different supramolecular architectures with distinct morphologies. However, by varying the polarity of the self-assembly medium, both L1 and L2 generated various superstructures and generated multiple tunable luminescence colours (blue–green–yellow–white). More importantly in a highly polar solvent medium (9 : 1 H₂O/THF), they exhibited aggregation-induced white light emission. To the best of our knowledge, this is a unique example of a π-conjugated molecule as a single-component white light emitter, which can also be utilized for generating multiple luminescence colours. We have also shown the role of halogen substitutions in the π-conjugated molecules, affecting the morphologies of the self-assembled structures in comparison to those without halogen substitutions. These substantial morphology differences may be attributed to the existence of additional strong intermolecular interactions, possibly halogen bonding in balance with other non-covalent intermolecular forces responsible for self-assembly. Scanning Kelvin probe with surface photovoltage measurements on the surfaces of L1 and L2 films were carried out both in the dark and under visible light illumination in different vapours of volatile organic compounds. As per the results, under visible light illumination, the thin film of the L1 surface showed highly selective adsorption towards 1-hexanol vapours.