Lowest excited triplet states of 1,2- and 1,4-dicyanobenzenes by low-temperature matrix-isolation infrared spectroscopy and density-functional-theory calculation

Infrared spectra of transient species produced from 1,2- and 1,4-dicyanobenzenes in low-temperature argon matrices during UV irradiation were measured by Fourier transform infrared (FTIR) spectroscopy. The observed bands were assigned to the lowest excited triplet, T1, states of the dicyanobenzenes by comparison with the spectral patterns obtained by hybrid density-functional-theory calculations at the B3LYP/6-31++G** level. The CN stretching bands in the S0 state shift to the low-wavenumber side by 172 and 251 cm−1 in the T1 state for 1,2- and 1,4-dicyanobenzenes, respectively. These findings suggest that the quinoide-type structure similar to that in o- and p-benzoquinones contributes to the T1 states.