The lowest triplet state of tetramethyl-1,3-cyclobutanedithione. I. Single-crystal polarized absorption spectrum

Abstract

The single-crystal polarized absorption spectrum of tetramethyl-1,3-cyclobutanedithione (TMCBDT) has been measured in the region of the lowest triplet state. All bands are polarized exclusively perpendicular to the tetragonal unit cell c axis. The 0-0 band which lies at 594.3 nm is the most intense feature with active vibrations at 309, 329, 600, 913, and 969 cm −1 of lower intensity. The electronic transition is assigned to 3A u ← 1A g (n∼ =) in the D 2h molecular point group. Weak, low-frequency vibrations (< 100 cm −1) observed near the origin band (and several other vibronic bands) have been analyzed using a model in which the excited triplet and ground electronic states are described by double-minimum potentials (DMPs). The parameters for these states were found by fitting the low-temperature (1.6 K) absorption and emission crystal spectra using the Coon, Naugle, and McKenzie method. A 23 cm −1 potential barrier is found for the ground state and a 95 cm −1 one for the triplet state. Using these results the temperature dependence of the absorption spectrum was simulated and it is shown that the essential features of the observed temperature behavior are accounted for. Finally, it is suggested that the DMP in both states results from either an intramolecular torsional motion and/or a particular lattice mode.