Formation of different photodimers of isoquinolinone by irradiation of solid molecular compounds

[4 + 4] Photodimerization of isoquinolin-3(2H)-one (isoquinolinone) (1) may yield 12 different isomers depending on the relative geometry of the two monomers prior to the reaction. Irradiation of the solid neat compound shows that of all possible dimers only the dimer between the two hetero-rings having an inversion center is produced. On the other hand, exposure to UV light of solid molecular compounds composed of isoquinolinone as the guest molecule and different host molecules shows to produce few other isomers of the dimer. The use of different host molecules affects the packing of the molecules in the crystal. As a result, the relative geometries between two monomer molecules are varied enabling photochemical dimerization to yield different isomers. When 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol (I) is used as the host molecule, two polymorphic forms of the molecular compound are obtained. Irradiation of single-crystal of the two polymorphs yields a mixture of isomers of the dimer. When the host molecule is cyclohexanol-1,2,4,5-tetracarboxylic acid (II) three isomers are expected, and when the host molecule is 1,3-benzenediol (III) a single isomer is formed which is identical to that obtained from the neat isoquinolinone.