The crystalline conformations of perylene polyimides (PPIs), with alkyl spacers varying in length from C-3 to C-12 are discussed. Although modeling of single chains would suggest the possibility that those with odd number of CH2 groups adopt a flat helical shape, X-ray diffraction suggests that the chains with odd and even spacers have the same conformation, due to the necessity to pack the perylene units. Upon annealing, the UV-vis absorption maximum red-shifts by about 10 nm, indicating enhanced π-stacking between the perylenes. Several changes in the spectra are seen with an increase in annealing time, at a given temperature. With an increase in annealing time, X-ray diffraction patterns show changes in the number of reflections, indicative of a crystal to mesogenic, transition. The time required for this transition increases with a decrease in the spacer length, and annealing temperature, suggesting that the dynamics of this transition is very slow. © 2005 Elsevier Ltd. All rights reserved.