Two fluorescent molecules, 4-aminophthalimide (AP) and 4-(N, N-dimethyl)amino-N-methylphthalimide (DMP) have been used as the building blocks to fabricate fluorescent organic nano particles. DMP, the analogue of AP, has been synthesized by substituting all the amine hydrogens of AP with methyl groups to get an idea about the effect of intermolecular hydrogen bonding interactions (N-H…) on the aggregation behavior of these molecules. All the systems have been characterized by field emission scanning electron microscopy (FESEM). Photophysical behavior of these well characterized systems has been investigated in molecular as well as aggregated forms. Interestingly, while the AP-aggregates exhibit a blue-shifted absorption band (as compared to AP in its molecular form), DMP-aggregates exhibit a red-shifted absorption band (as compared to DMP in its molecular form). These absorption data indicate the formation of H and J aggregates for AP and DMP, respectively. The intermolecular interactions that are responsible for the molecular self assembly of AP and DMP are studied by using X-ray crystallography. X-ray analysis demonstrates the presence of strong intermolecular hydrogen bonding interactions in AP, but only weak interactions (C-H…O, C-H…π) in the case of DMP. X-ray analysis also demonstrates that varying the nature of intermolecular interactions leads to different modes of aggregation. Theoretical studies (DFT and TD-DFT) have been carried out to investigate how different modes of aggregation lead to changes in the optical (UV-VIS spectra) properties of these systems.