Clustering-Triggered Ultralong Room-Temperature Phosphorescence of Organic Crystals through Halogen-Mediated Molecular Assembly

To achieve efficient room-temperature
phosphorescence of organic
materials with ultralong lifetime, it is imperative to resolve the
dilemma that the introduction of heavy atoms simultaneously improves
emission efficiencies and shortens the emission lifetimes. Herein,
we report a new molecular design approach for halogenated luminogens
with a methylene bridge to avoid the lifetime shortening induced by
heavy halogens and propose a general molecular engineering strategy
to realize efficient and ultralong room-temperature phosphorescence
via halogen-mediated molecular clustering. The halogenated N-benzylcarbazole
derivatives show distinct photophysical behaviors depending on different
physical states, including single-molecule state and cluster state.
Their crystals demonstrate the halogen-dependent emission duration
of room-temperature phosphorescence upon excitation. Experimental
data and theoretical analysis indicate that halogen-regulated molecular
clustering in the crystal is responsible for the generation of efficient
ultralong room-temperature phosphorescence, and halogen-dominated
molecular engineering favors the promotion of the intersystem crossing
process and the following triplet emissions.