Cyclopentadienylidene. A matrix isolation study exploiting photolysis with unpolarized and plane-polarized light

Diazocyclopentadiene (1) was photolyzed in N(2), CO, and other low-temperature matrices. The resulting carbene, cyclopentadienylidene (2), was characterized by its UV and IR spectra, and its thermal dimerization and reaction with CO were observed. Photolysis of 1 with plane-polarized light gave matrices exhibiting linear dichroism. Comparison of dichroic IR and UV spectra revealed that the photolysis proceeds via an excited A(l) state of the diazo compound. Plane-polarized irradiation of the corresponding ketene (4) in CO matrices resulted in photoreorientation of the molecules of 4 without significant loss.