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.