Unconventional, Gram-Scale Synthesis of a Molecular Dimer Organic Luminogen with Aggregation-Induced Emission

Organic luminogens have been widely
used in optoelectronic devices,
bioimaging, and sensing. Conventionally, the synthesis of organic
luminogens requires sophisticated, multistep design, reaction, and
isolation procedures. Herein, the products of the melt-phase condensation
of benzoguanamine (BG; 2,4-diamino-6-phenyl-1,3,5-triazine) at 370–410
°C display interesting reaction-condition-dependent luminescence
properties, including photoluminescence (PL) at a variety of wavelengths
in the visible spectrum and quantum efficiencies (PLQE) of up to 58%
in the powder form. With a simple and straightforward solvent washing
procedure, the prominent blue luminescent component BG dimer was obtained
in gram scale with >93% purity (96.5% purity after fractional sublimation).
The BG dimer exhibited distinct aggregation-induced emission (AIE)
properties. PL measurements indicate that the electronically excited
state of the BG dimer undergoes efficient intramolecular nonradiative
deactivation in room-temperature solution, leading to a significantly
reduced PLQE (<0.1%) in solution. These nonradiative processes
are substantially inhibited when the dimer existed in the form of
crystals, solid aggregates in solution or being fixed in a rigid polymer
film, resulting in a significant increase in the PLQE and lifetime.
This work not only provided a new understanding for PL properties
of self-condensation luminescent products but also represented an
unconventional strategy for large-scale preparation of organic luminogens
with high purity.