Investigation of the origin of a dimeric pair structure of the title compound molecules in a crystal was extensively conducted. Molecular dynamics calculations suggested that conventional van der Waals and electrostatic estimations did not accurately reproduce the temperature dependence of the interatomic distance of the dimeric pair, as observed in X-ray structure analyses. A dimeric pair formation of the monomer in a solution was also observed by spectroscopic studies. X-ray structure analysis of a photodimer crystal revealed that the packing motif of the monomer crystal is quite similar to the dimer crystal. It is speculated that, on crystallisation from a solution, the monomer molecule form a dimeric structure near the crystal surface and make a stack similar to the dimer crystal.