Single-crystal-to-single-crystal photodimerization of 4-chlorocinnamoyl-O, O-dimethyldopamine

[2+2]Photodimerization of the title compound, C₁₉H ₂₀ClNO₃, has been observed in situ by single-crystal X-ray diffraction. Pairs of monomers related by centers of symmetry have parallel C=C bonds at a C...C distance of 4.155 (17) Å. Irradiation of a single crystal with a mercury lamp achieves 100% conversion to the dimer. Redetermination of the structure during the course of the reaction revealed a linear correlation between the percent conversion to the dimer and the decrease in the cell volume. The displacement parameters for the pure dimer structure are substantially smaller than those for the pure monomer structure. The dimerization reaction is also induced by irradiation with X-rays, the induction being stronger with Cu Kα than with Mo Kα radiation.