Morphology and Emission Tuning in Fluorescent Nanoparticles Based on Phenylenediacetonitrile

Abstract

Organic nanoparticles exhibiting tunable emission properties in response to morphology changes are attractive for application in low-cost fluorescence sensors, e.g., for sensing vapors, temperature, etc., and therefore, convenient ways for altering nanoparticle morphology are highly desired. In this work, phenylenediacetonitrile-based molecules featuring aggregation-induced emission enhancement which are suitable for the realization of morphology-tunable nanoparticles by precipitation method have been designed. The morphology tuning was enabled by rational functionalization of the molecular backbone with pyrazole moieties and adjacent alkoxy/cyclic side-groups, which were varied in size and branchiness. The longer alkoxy chains generally caused formation of crystalline nanostructures, while shorter ones resulted in preferably amorphous nanoaggregates. Remarkably, the sharp tuning of the nanoparticle morphology (crystalline → amorphous → crystalline) with the subsequent high-contrast emission switching (emissive → nonemissive → emissive) was achieved for phenylenediacetonitrile bearing dihexylmethoxy-type side-groups by adjusting solvent/nonsolvent ratio (1/1 → 1/4 → 1/9) in the mixture with the dissolved compound. Electron and polarized optical microscopy data confirmed the intermediate nonemissive state (ΦF = 2%) emanates from amorphous spherical aggregates, whereas two highly emissive states (ΦF up to 70%) originate from nanowire-like crystalline particles, which were attributed to different phenylenediacetonitrile polymorphs. Finally, the applicability of the phenylenediacetonitrile-based fluorescent nanoparticles for organic vapor sensing was demonstrated.