A more quantitative comparison of the crystal structures and the intermolecular interactions taking place can be obtained using Hirshfeld surfaces and their corresponding fingerprint plots. An explanation of the generation of Hirshfeld surfaces is given in the experimental section and in the literature citations.34–36 Hirshfeld surfaces for EPI and the two halogenated derivatives, 2F-EPI and 2Cl-EPI, are shown in Fig. 6 mapped over a d norm range of –0.6 to 1.2 Å. The strongest intermolecular interactions between neighboring molecules, shown as the red areas on the Hirshfeld surfaces, occur for the N–H···O hydrogen bonds, blue areas correspond to areas with no close contacts (shorter than the sum of the van der Waals radii). Comparing the fingerprint plots of the different structures, again confirms the similarity of the EPI structure with that of 2F-EPI. The main difference between the fingerprint plot of the chlorinated derivative and those of the other structures is the presence of two wings, marked by asterisks in Fig. 6, at (d i, d e) positions of (1.8 Å, 1.2 Å) and (1.2 Å, 1.8 Å). These arise from C–H···Cl interactions, which are clearly absent in the other two structures. For the 2F-EPI structure, the analagous C–H···F interactions occur over similar (d i, d e) positions to the H···H interactions, showing that the addition of larger chlorine atoms to the conjugated backbone disrupts the packing of the hydrogenated EPI structure more than the addition of smaller fluorine atoms. A quantitative analysis of the contributions of different interactions to the Hirshfeld surface area shows that, for EPI and 2F-EPI, the H···H interactions are dominant, with contributions to the Hirshfeld surface area of 42.7% (EPI) and 20.5% (2F-EPI). For all three structures, contributions are also made by the O···H interactions arising from the hydrogen bonds (15.9% for EPI, 16.3% for 2F-EPI and 14.9% for 2Cl-EPI). In the case of 2Cl-EPI, the dominant interactions are Cl···H (23.6%), closely followed by H···H interactions (18.8%), while for 2F-EPI a significant contribution is also made by F···H interactions (18.8%). Generally, the Hirshfeld surfaces show the slight dominance of interactions involving the substituents to the conjugated backbone (involving H, F and Cl), as opposed to π interactions, C···C (mostly π–π) and C···H (mostly C–H···π) interactions (30.2% in EPI, 27.2% in 2F-EPI and 25.8% in 2Cl-EPI). A graphical representation of the relative contributions of different interactions to the Hirshfeld surfaces is given in the ESI† (Fig. S5).