Lowest Electronically Excited Triplet States of 1,2,4,5-Tetracyanobenzene and Tetracyanopyrazine by Matrix-Isolation Infrared Spectroscopy Combined with a Density-Functional-Theory Calculation

The infrared spectra of photoexcited transient species produced during UV irradiation from 1,2,4,5-tetracyanobenzene (TCNB) and tetracyanopyrazine (TCNP) in low-temperature argon matrices were measured with a Fourier-transform infrared (FTIR) spectrophotometer. The produced transient species were identified as the lowest electronically excited triplet states, T1, by a comparison of the observed infrared spectra with the calculated spectral patterns obtained by the hybrid densityfunctional-theory (DFT) method. The vibrational wavenumbers of the asymmetric C(identical)N stretching modes in the T1 states were found to be shifted to the lowwavenumber side by about 150 cm^-1, indicating that the bonds of the cyano groups, -C(identical)N, in the T1 states are close to cumulative double bonds, C=C=N. This observation is supported by calculations of the optimized geometrical structures and the Mulliken spin density obtained by the DFT method.