See:      http://pubs.rsc.org/en/content/articlelanding/2017/ra/c7ra00779e#!divAbstract

Yu Shao, Xinlin Zhang, Kai Liang, Jing Wang, Yuejian Lin, Shuguang Yang, Wen-Bin Zhang, Meifang Zhu, and Bin Sun. How Does the Interplay between Bromine Substitution at Bay Area and Bulky Substituents at Imide Position Effects the Photophysical Properties of Perylene Diimides? RSC Adv., 2017,7, 16155-16162. 

Abstract
This article reports a comparative study on the synthesis, self-assembly, and photophysical properties of perylene diimides (PDIs) symmetrically tethered with long alkyl chains or polyhedral oligomeric silsesquioxanes (POSS) at the imide position and/or bromo substitutions at 1,7-positions of the bay area. This series of samples include Dodecyl-PDI_H-Dodecyl (1), Dodecyl-PDI_Br-Dodecyl (2), POSS-PDI_H-POSS (3), and POSS-PDI_Br-POSS (4). In solution, the PDIs with bromine substitution at bay area (2, 4) exhibit red-shifted absorption maximum compared to those without (1, 3), which is consistent with a twisted perylene chromophore as revealed by molecular simulation. Similar bathochromatic shift was observed on the solid crystal state emission of 2 as compared to 1. However, in crystals, the emission spectrum of 4 exhibits a seemingly hypochromatic shift relative to that of 3, which could be rationalized by their packing in the crystals. The bromo substitution is believed to partially quench the fluorescence and the relatively loose packing of the twisted π-plane of 4 may not be able to confine π-plane in place, leaving multiple pathways for fluorescent quenching rather than red-shifted emission. While both 3 and 4 exhibit a unique dimer packing scheme, the dimers have quite different longitudinal offset and transverse offset of the π-plane. The longitudinal offset in dimers of 4 is so large that the naphthalene moieties in the dimer almost adopts a face-to-face arrangement and their mutual interactions are considered relatively independent. All these contribute to the less red-shifted fluorescent emission and the lower fluorescent yields in crystals of 4 relative to 3 as compared to that in solution. The study shall shed light into the complicated mutual interactions among intrinsic electronic structure, microscopic molecular packing, and the macroscopic optoelectronic properties.