First detection of 7,8-diazabicyclo[4.2.0]octa-1,3,5-triene produced from 3,5-cyclohexadiene-1,2-diimine in an argon matrix

1,2-Diaminobenzene in an argon matrix was exposed to UV light coming from a superhigh-pressure mercury lamp without optical filters, resulting in production of three isomers around the CN bonds of 3,5-cyclohexadiene-1,2-diimine by elimination of hydrogen atoms. When the matrix sample was subsequently irradiated by a UV photon of lower energy (λ > 290 nm), an infrared spectrum of a new species produced from trans,trans-3,5-cyclohexadiene-1,2-diimine was observed, in addition to the spectral change due to photoisomerization from cis–trans to cis–cis. The new species was identified as 7,8-diazabicyclo[4.2.0]octa-1,3,5-triene by a comparison with the predicted spectral pattern obtained by density-functional-theory calculation.