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.