Synthesis, crystal structures and Hirshfeld surface studies of chalcone derivatives: (2 E )-1-(4-2, 4-Dichlorophenyl)-3-[4-(propan-2-yl)phenyl]prop-2-en-1-one and (2 E )-1-(4-Fluorophenyl)-3-[4-(propan-2-yl) phenyl] prop-2-en-1-one