See http://pubs.rsc.org/EN/content/articlelanding/2017/ra/c6ra28147h#!divAbstract
 
Shao, Y.; Yin, G.-Z.; Ren, X.; Zhang, X.; Wang, J.; Guo, K.; Li, X.; Wesdemiotis, C.; Zhang, W.-B.; Zhu, M.;* Sun, B.* Engineering π-π Interactions for Enhanced Photoluminescent Properties: Unique Discrete Dimeric Packing of Perylene Diimides. RSC Adv. 2017, 7, 6530-6537.
 

Abstract
 
This article reports the enhanced photoluminescent properties observed in the crystals of a cube-plane-cube shape amphiphile composed of perylene diimide (PDI) tethered with polyhedral oligomeric silsesquioxanes (POSS) at the imide position via either rigid or flexible linkages (POSS-PDI-POSS). The fluorescence quantum yields (Φ_f) of the conjugates in crystal (∼0.48) are much higher than that of the more amorphous samples (∼0.17) and that of the reference compound without pendant POSS cage (∼0.12). The enhancement was understood in three ways. First, their self-assembly behaviors in gaseous phase were studied by tandem mass spectrometry coupled with traveling wave ion mobility separation; second, their self-assembly behaviors in solution were revealed by concentration- and solvent-dependent experiments using UV/vis absorption and fluorescence spectrometry; and third, the molecular packing in crystals was determined by wide angle X-ray diffraction and transmission electron microscope. The results demonstrate the conjugates' strong tendency toward dimer formation in solution and show a unique molecular packing of discrete dimeric motifs in the solid states. It suggests that the high Φ_f of these conjugates can be attributed to the discontinuous π–π stacking, which results in a weaker electron interaction between the dimers, and the relatively fixed position of the PDI, which prohibits the common structural relaxation of PDIs π-planes.