Crystallization-Induced Emission Enhancement or Quenching? Elucidating the Mechanism behind Using Single-Molecule-Based Versatile Crystals

It is challenging to predict optical properties of fluorescent dyes, especially in the crystalline state, owing to the uncertainty in conformation, packing, and coupling. Herein, we elucidate the decisive role of molecular conformation and molecular packing in the fluorescence emissions of some crystalline materials based on experimental results and theoretical calculations. Two homologous fluorophores (Ph-MP and Ph-HP) were synthesized, and they both exhibited interesting crystallization-induced emission enhancement and quenching. Although the homologues show almost the same fluorescence behavior in the solid state, on–off emission of their crystals depends upon different factors. Emission of the Ph-MP crystals is governed by the twisted intramolecular charge transfer effect, while emission of the Ph-HP crystals relied on π–π stacking. Based on this understanding, application of single-molecule-based versatile crystals in information encryption was demonstrated. It is believed that the evidence and unveiled mechanism for the effect of crystallization on emission will contribute to development in high-performance luminescent materials.