For even-numbered segments, more pronounced spectral changes were observed in the lowenergy range of 18,000-22,000 cm -1 . At n = 2 or 4, two positive peaks are observed in this range, but as n increases to 6 and 8, only one major positive peak remains and the lowestenergy peak nearly disappears. Engle et al. also found in their calculations that PDI dimers and trimers exhibit different spectral characteristics. This odd-even pattern in the vibronic ECD spectra, driven by aggregate length, is of significant importance for the identification of aggregate size. The observed trends highlight the nuanced interaction between vibronic coupling and aggregate length in shaping the optical properties of these materials. It is noted that this odd-even characteristic gradually becomes blurred as the number of molecules in aggregate increases as Figure shows.