2,3,7,8-Tetramethoxythianthrene. A novel ground state triplet dication, the neutral photogenerated triplet, and the radical cation

The dication of 2,3,7,8-tetramethoxythianthrene was studied in solid nitromethane at 77 K by computer-controlled electron spin resonance. The Δm = 2 transition was observed, yielding a value of D* = 0.106 cm^-1. For comparison the values of D* for thianthrene and 2,3,7,8-tetramethoxythianthrene photogenerated triplets determined were 0.135 and 0.121 cm^-1, respectively. Lifetimes of these triplet states were also determined. Frozen solutions of the tetramethoxythianthrene dication exhibited a very broad Δm = 1 signal that could not be resolved into components D and E of the dipolar tensor. The free energy separation between the triplet and singlet states is between 90 and 190 cm^-1. The ESR spectrum of the cation was redetermined. The hyperfine splittings at 23°C in nitromethane were a_S = 7.30 ± 0.05 G, a_H(OCH₃) = 0.28 ± 0.02 G, and a_H(1,4,6,9) = 0.87 ± 0.04 G, and the g factor is 2.00732 ± 0.00006.