The photophysical properties of PDI-diBr and PDI-tetraBr were then characterized. When dissolved in N,Ndimethylformamide (DMF), both molecules showed three typical absorption peaks (458, 489, 525 nm), alongside a shoulder peak (~425 nm) corresponding to 0-0, 0-1, 0-2, and 0-3 electronic transitions of PDI in a diluted state, respectively (Figures and, dark green curves). Meanwhile, nearly mirror images of the absorption spectra were observed for the fluorescence emission of PDI-diBr and PDI-tetraBr (Figures and, dark green curves). To evaluate their photophysical properties in an aggregate state, both initiators were dispersed in DMF/H2O (5/95, v/v). PDI-diBr exhibited a broadened peak losing the vibronic structures with a clear hypsochromic shift of the absorption maxima to ~470 nm (Figure , B, orange curve). This feature usually originates from the co-facial aggregation of PDI (typically Haggregation), which was further confirmed by the single-crystal x-ray diffraction and spectrophotometry showing a ππ packing distance of 6.14 Å (Figure ). On the other hand, PDI-tetraBr showed the absorption maximum at ~500 nm and a significant shoulder peak at ~550 nm (Figure D, orange curve), in the aggregate state (DMF/H2O 5/95, v/v).