The origin of the color of 1,2-dithiins - Interpretation by Kohn-Sham orbitals

According to calculations by time-dependent density-functional theory (TD-DFT) the wine-red color of the half-chair (C-2) 1,2-dithiin (1) is mainly due to a one-electron transition between the frontier orbitals. These orbitals are characterized by local two-center CC pi bonds. The symmetric HOMO of the twisted structure 1 correlates with the fully delocalized HOMO of the planar reference structure with C-2v symmetry. The anti-symmetrical LUMO correlates both with the essentially localized sigma* (SS) of the LUMO and the pi* of the LUMO+1 of the planar compound. Thus the dominating one-electron transition of 1 has some pi-->sigma* character in the terminology of the planar system with a strong admixture of the anti-bonding combination of the CC bond orbitals to the sigma*-type LUMO. The color of 1,2-dithiins are unique because of the simultaneous presence of a non-coplanar C=C bond system and the SS bond. For the sake of comparison calculations were also performed at ab initio levels of theory (TD-RHF, SCI, EOM-CCSD and CASPT2). Although the absorption wavelengths are strongly underestimated by SCI and TD-RHF the order and the nature of the lowest-excited states are the same as those found by TD-DFT. The electron excitation of 1,2-dithiin is compared with that of 1,2-dithiane (3). In addition, the structure and the energetics of 1,2-dithiin are compared with 1,4-dithiin (4) and thiepin (5).