This thesis describes the synthesis and design of novel benzo[e][1,2,4]triazine crystals (known as F-C8-BTriaz, Cl-C8-BTriaz, and Br-C8-BTriaz), in which selective change of a halogen atom at the C8 position results in mechanical flexibility. Through developmental understanding of organic plastic and elastic crystals, the rational and explanation of flexibility and brittleness of F-C8-BTriaz, Cl-C8-BTriaz, and Br-C8-BTriaz is elucidated. F-C8-BTriaz and Cl-C8-BTriaz display mechanical flexibility with differing packing arrangements, whilst Br-C8-BTriaz mainly exhibits brittleness as a crystal. Alongside the synthesis of novel benzo[e][1,2,4]triazine crystals, attempts were made at synthesizing a novel Blatter radical (classified as O-Blatter-Me-BTriaz) that was postulated to exhibit mechanical flexibility as a single crystal. A discussion into the synthetic protocols and issues in synthesizing O-Blatter-Me-BTriaz is presented as O-Blatter-Me-BTriaz was never obtained in large quantities.