Packing Density-Promoted Emission Strategy toward Tunable Photoluminescence and Room-Temperature Phosphorescence

The
method of enhancing the photoluminescence (PL) of nonconventional
luminogens, especially the room-temperature phosphorescence (RTP)
by tableting, has been recognized, but the underlying mechanism has
not been elucidated. To clarify this, the specific law between the
changing emission center and its emission should be studied precisely.
Herein, a packing density-promoted emission (PDE) strategy is proposed
using three solid states (film, powder, and tablet) of cellulose nanocrystals
with a well-defined crystal structure, in which their average molecular
packing density (MPD) is regulated. The PL and RTP emission enhanced
with increasing MPD. The emission mechanism of the PDE strategy, the
photophysical change process, and the effects of detailed molecular
interaction changes in the crystal on the formation of cluster emission
centers are elucidated by constructing crystal structures in three
states combined with theoretical calculations. These findings shed
a new light on the origin and facile achievement of tunable PL in
pure organics and afford strong implications to the emission mechanism
of nonconventional luminophores in turn.