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, T-1, 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 So state shift to the low-wavenumber side by 172 and 251 cm(-1) in the T-1 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 T-1 states. (C) 2003 Elsevier Science B.V. All rights reserved.