Is Magnetic Bistability of Carbenes a General Phenomenon? Isolation of Simple Aryl(trifluoromethyl)carbenes in Both Their Singlet and Triplet States.

p-Tolyl(trifluoromethyl)carbene and the related fluorenyl(trifluoromethyl)carbene were synthesized in solid argon and characterized by IR, UV-vis, and electron paramagnetic resonance spectroscopy as well as by quantum mechanical calculations. The carbenes can be generated in both their triplet and singlet states, and both states coexist under the conditions of matrix isolation. According to our calculations, the singlet and triplet states of these carbenes are energetically nearly degenerate in the gas phase. Warming of matrices containing pure triplet p-tolyl(trifluoromethyl)carbene from 3 to 25 K leads to an interconversion of up to 20% of the triplet into the singlet state. This interconversion is thermally irreversible, and cooling back to 3 K does not change the singlet to triplet ratio. Irradiation at 365 nm results in a complete singlet to triplet interconversion, whereas 450 nm irradiation produces again up to 20% of the singlet state. An alternative way to generate the singlet carbene is the reaction of the triplet with water molecules by annealing water-doped matrices at 25 K. This results in the irreversible formation of a hydrogen-bonded complex between the singlet carbene and water. For fluorenyl(trifluoromethyl)carbene, very similar results are obtained, but the yield of the singlet state is even higher. Magnetic bistability of carbenes seems to be a general phenomenon that only depends on the singlet-triplet gap rather than on the nature of the carbene.