Detection of a diradical intermediate in the cis–trans isomerization of 5,5′-bis(4,5-diphenyl-2H-imidazol-2-ylidene)-5,5′-dihydro--bithiophene

5,5′-Bis(4,5-diphenyl-2H-imidazol-2-ylidene)-5,5′-dihydro- Δ 2 , 2 ′ -bithiophene provides a rare example of the detection of a diradical intermediate in the cis–trans isomerization. The reversibility and the energy profiles for both the cis–trans isomerization and the diradical formation indicate that the observed diradical is an intermediate in the cis–trans isomerization.