π-Extended Bodipy Self-Assembly as Supramolecular Photonic Security Ink and Optical Waveguide

Abstract

Iridescent photonic materials have wide-ranging applications in security printing, and optical devices. While the commonly used photonic materials are based on polymers or inorganic colloidal particles, self-assembled small molecules are rarely exploited. Herein, a Bodipy (Bodipy-PE-2) self-assembly is reported that forms a layered photonic structure upon thermal annealing, exhibiting angle-dependent iridescence of green and pink with a luminescence shift from yellow to red. The micro- and nano-periodicity associated with the layered structure and the phase transformation due to the annealing are responsible for the observed properties. The single-crystal analysis reveals the critical role of weak dispersive interactions (C–H···π, C–H···O, and N–H···F) in the crystal packing and a possible phase change in the formation of the layered structure. A stimuli-responsive ink prepared with the annealed powder using polyethylene glycol as a vehicle medium exhibits reversible emission change between red and yellow upon heating and solvent exposure, respectively. A composite gel of the annealed Bodipy-PE-2 and polystyrene displays angle-dependent color change and optical waveguiding properties. The self-assembled Bodipy-PE-2 obtained from toluene exhibits active and passive waveguiding properties when excited with 532 and 633 nm laser sources.