twisted conformation to arrange themselves as a bulk lattice. The destruction of confined structures should have caused the quantum
yield to decrease, which is common in many systems [22,41,42]. However, considering the confined packing modes are also twisted, the
molecular structure could be tuned planar in response to external stress if the molecules are symmetric and with proper size. In that case,
the destruction of their previous confined structures was counteracted by the formation of new kinds of dense packing modes, so the
effective intramolecular conjugation can be expanded and emissions are therefore enhanced [43], which is the case of BPDPA, BP2DPA,
and BP2TPA. As in BPTPA, its molecular structure is neither symmetric nor small enough, making it hard to form the above new packing
modes, so its quantum yield suffers a decrease by the collapse of crystalline lattice.
It is also noted that the ground solids of BPDPA and BP2TPA own comparable efficiencies (~35%), whereas those of BP2DPA and
BPTPA (~12%) are also approaching. The fluorescence lifetimes of the recrystallized solids before/after grinding are 1.50/5.04,
1.18/1.52, 2.73/4.35, and 2.48/4.15 ns for BPDPA, BP2DPA, BPTPA, and BP2TPA, respectively, which highly indicate the presence of
unfavorable strong exciton interactions in the ground solids. The final efficiencies, however, should be determined by multiple factors,
which may be associated with chemical and electronic structures, as well as the excimer-like interactions.
In summary, a group of D-A structured twisting compounds based on BP and D(T)PA were prepared and studied in view of their
photophysical properties. These heavy-atom free pure organic luminogens exhibit typical CIP characteristics, generating fluorescence-
phosphorescence dual emission at crystalline states. Such intrinsically dual emissive pure organics might be useful in revealing the
underlying spin correlations of organic semiconductors within the lifetime of the excitons [44]. Meanwhile, upon manual grinding, they
undergo distinct phase transition from crystalline to amorphous states, accompanying conformation planarization and destruction of
intermolecular interactions. Such changes induce obvious variations in emission color, wavelength, as well as efficiency, thus
demonstrating remarkable mechanochromism.
Acknowledgment
This work was financially supported by the National Natural Science Foundation of China (No. 51473092).
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