Infrared spectrum and molecular structure of lowest electronically excited triplet state of 1,4-dicyano-2,3,5,6-tetrafluorobenzene in low-temperature matrices

Abstract

An infrared spectrum of the T 1 state for 1,4-dicyano-2,3,5,6-tetrafluorobenzene in a low-temperature argon matrix was measured with a Fourier transform infrared spectrophotometer. The density-functional-theory calculation was performed to assign the observed bands. The optimized geometrical structure in the T 1 state is found to have two stable forms, planar ( D 2 h ) and non-planar ( C 2 v ), the former being more stable than the latter by 35.1 kJ mol −1. The T 1 state is identified as the planar form by a comparison of the observed matrix spectrum with the calculated spectral patterns. This assignment is confirmed by an analysis of the matrix-isolation phosphorescence spectra, where the observed vibronic bands are assigned to the fundamental and/or combination bands of the a g vibrational modes in analogy with those of 1,4-dicyanobenzene, where the T 1 structure belongs to D 2 h symmetry.