The PL emission of PDI-diPS and PDI-tetraPS with the same molecular weight (DP = 62) was first measured (Figure , B and C, respectively). Their absorption and emission spectra in DMF are similar to that of the PDI initiators, showing typical PDI features in a diluted state (Figure and). Upon aggregation by the addition of water to the DMF solution, both polymers showed high sensitivity to water (Figure , and D top). Notable red emission peaks at ~620-640 nm were observed at a water fraction of > 20%. The difference between PDI-diBr and PDI-tetraBr in aggregate emission was reflected again on the polymers, with PDI-tetraPS showing a 2.5-fold stronger emission intensity than PDI-diPS under the same condition (DMF/H2O 5/95, v/v). In the meantime, the emission peak of PDI-tetraPS exhibited an increased intensity with a slight blue shift when the water fraction was increased from 20% to 95%. This phenomenon was not observed in PDI-diPS. It is speculated that the four-arm structure may facilitate the displacement of PDI stacking in a more efficient way. This is also indicated by the absorption spectra of the polymers in the aggregate state showing more significant shoulder peaks at ~535 nm (Figure and).