Ground and excited state dipole moments of pyranthione and xanthione by the electro-optic method

Electric field-induced changes in the absorption spectrum (electrochromism) have been used to obtain the ground (S₀) and second excited (S₂) state dipole moments of two sulphur-containing heteroaromatics, pyranthione and xanthione, which have C_2v symmetry. The magnitude of the change in the dipole moment (Δμ = |μ(S₂) - μ(S₀)|) was found to be nearly 2 D for both compounds, which is consistent with results reported previously based on microscopic solvent shifts. The transition dipole moment for the S₀-S₂ transition is parallel to the direction of the ground state dipole moment (μ_g), i.e. along the C₂ axis which contains the CS bond. For comparison the ground state dipole moments of both compounds have also been calculated using the AM1 method will full geometry optimization.