Multicolor Emission of Small Molecule-Based Amorphous Thin Films and Nanoparticles with a Single Excitation Wavelength

Fluorescent organic nanoparticles (FON) made of small molecules recently arouse considerable interest in biomedical imaging and opto-electronics as potential microlasers or emitting displays.2 Their collective emission largely enhanced with respect to dilute media and their reduced sensitivity to external bleaching and quenching effects make them very attractive when powerful excitation is required as in multiphoton fluorescence spectroscopy. Fluorescence in the solid state is however facing two deleterious effects: competitive non-radiative quenching processes caused by strong intermolecular aggregation, and emission reabsorption owing to the inherent high chromophore density. The main strategies consist in introducing bulky substituents to prevent the formation of non-emissive π−π stacked assemblies as well as elaborating materials with a large optical Stokes shift. Hydrophobic materials are often obtained and considered of little use in bioimaging. We want to report herein the synthesis and the properties of a new family of push-pull amorphous compounds which form strong one-and two-photon emitting nanoparticles that can efficiently be utilized to image lipophilic cell membranes despite their water insolubility. The present series of fluorescent compounds consists in polar triphenylamino derivatives substituted by bulky tert-butylphenyl groups following a Suzuki aromatic cross coupling reaction as a final step. Both the twisted star-shaped triphenylamino-core and peripheral hindered substituents prevent deleterious π−π stacking and ensure high solubility in polar and apolar organic solvents.