As n increases from 1 to 3, the spectral lineshape changes dramatic. However, the normalized absorption spectra become nearly saturated at n=7, implying that the exciton coherence length in this one-dimensional closely stacked molecular chain is finite. It is interestingly noted that the ECD spectral lineshapes exhibit a distinct odd-even alternation pattern, and the ECD spectra are much more sensitive to the oligomer's size. This characteristic odd-even pattern in oligomer's ECD spectra primarily arises from the relatively irregular geometric arrangement of PDIs. Taking the PDI-CN tetramer as an example, based on the centroid coordinates of four PDI-CN fragments listed in Table in SI, the separations between two nearest neighbouring fragments are 3.60, 4.39 and 3.60 Å, respectively. This quite different intermolecular stacking distance results in the different exciton couplings between adjacent fragments (the calculated couplings are 0.0428, 0.0711, and 0.0435 eV, respectively). In the end, the aggregate size ultimately affects the ECD spectral lineshape. Irrespective of the odd or even nature of the aggregates, an overall blue shift in the ECD spectra is observed with the increase of aggregate size.