Abstract: The X-ray structure analysis show that the radical dimerization of 8-(p-tolyl)-8H-acenaphtho [1, 2-d] imidazole leads to 1, 2'-isomer structure. The estimated geometrical parameters are in excellent agreement with the experimental crystal structural peers. 8-(p-tolyl)-8H-acenaphtho [1, 2-d] imidazolyl dimer (TAID) exhibited excellent photochromism. The colored species of TAID are stable in solutions with good fatigue resistance. Its kinetic studies show that the photocoloration obey first order kinetics and the thermal bleaching processes obey second order. The photochromic dye shows decrease of fluorescence intensity with the photochromic reaction, due to the formation of nonemissive photoproduct and with the thermal recombination reaction in the dark the fluorescence intensity recovers.