Photoisomerization and photocyclization of 3,5-cyclohexadiene-1,2-diimine and its methyl-substituted derivatives in low-temperature argon matrices

3,5-Cyclohexadiene-1,2-diimine was produced from 1,2-diaminobenzene in a low-temperature argon matrix by UV irradiation (λ<270 nm). Conformational changes from cis-cis to cis-trans and trans-trans were induced upon irradiation of λ>410 nm, while the shorter-wavelength irradiation of 410>λ>350 nm resulted in photoisomerization from cis-trans to cis-cis in addition to photocyclization to yield 7,8-diazabicyclo[4.2.0]octa-1,3,5-triene. This photocyclization mainly occurred upon irradiation of 350>λ>290 nm. Similar spectral changes due to photoisomerization among the four isomers of 4-methyl-3,5-cyclohexadiene-1,2-diimine and among the three of 4,5-dimethyl-3,5-cyclohexadiene-1,2-diimine were observed, but not due to photocyclization. The wavelength dependence for the photoisomerization and the methyl-substitution effect for the photocyclization were elucidated in terms of the π-π* and n-π* vertical transition energies and oscillator strengths obtained by the time-dependent density functional theory (DFT) calculation.