Derivatized malachite green leuco dyes for use in radio-chromic systems are described. The dyes have the following structure:




in which Ar is a substituted phenyl or thiophene ring in which at least one substitution of the Ar ring is not ortho to the bond between the ring and the linking carbon, and in which R1, R2, R3, and R4 are independently selected from methyl, alkyl, or alkyl halide. The systems include the dyes in conjunction with an activator, e.g., a halogenated activator, and a carrier, e.g., a fluid carrier or an encapsulating polymeric matrix.