Semiconductor layer homogeneity and uniform crystallinity are crucial in achieving reliable reproducibility in organic electronic devices such as OFETS, OLEDS and Organic Solar Cells. The formation of polymorphic domains is detrimental in solution-processed organic active layer materials, especially in small molecular weight materials. Particularly in OFETS, the device parameters exhibit large deviations due to the film inhomogeneity. Barrierless multiple crystallisation pathways are the origin of the formation of polymorphic domains in the active layer films, which need special attention to circumvent. Particularly in rigid molecular structures, competing non-covalent interactions leading to such polymorphic forms are known. Here, we investigate the formation and the consequences of polymorphic forms in a rigid-end group substituted naphthalene diimide-derived n-type molecular semiconductor. Considering the need for rigid backbone and rigid-end groups for efficient charge transport under minimal structural reorganisation, picolyl end groups were chosen. Three distinct forms of crystals (P-NDI, B-NDI and R-NDI) were isolated with distinct colours and other optical features. The SCXRD analysis revealed distinct supramolecular packing and the existence of conformational polymorphs. The torsional angle with respect to the bridging carbon was considered for the primary analysis and was found to be 62°, 99° and 134° for P-, R- and B-NDI polymorphs. This minor change in the structure contributed to major variations in the intermolecular interactions. A distinct excimer band in the solid-state emission spectra revealed the structure-optical property relationship. For P-NDI, due to its low centroid-to-centroid packing distance, moderate pi-pi overlap area, larger excimer-to-monomer ratio and higher LUMO overlap between the dimers, a high charge transfer integral was evident. A detailed structure, optical, and charge transport features of these polymorphs are summarised in the manuscript.