Manipulating the singlet–triplet energy gaps of arene-fused bis(1,2,3-dithiazoles): A computational study

Benzo- and azino-fused bis(1,2,3-dithiazoles) were investigated computationally using density functional theory (DFT) in an attempt to relate structure to ground state multiplicity preference. Structural changes on the central arene by substitution with electron donating (EDG) and electron withdrawing groups (EWG) or replacement of the central arene by pyridino or pyrazino rings, were studied by optimizing the structures as singlet and triplet ground states. The aromaticity of each ring was probed using nucleus independent chemical shift (NICS) calculations. Molecular orbital analysis identified the substituent effects on the energy of the frontier orbitals. Calculations show that the ground state multiplicity can be effectively controlled with strategic substitutions. EWG directly attached on the negative cyanine of the central arene stabilize zwitterionic singlet states whereas EDG attached at the same position favor a triplet ground state. NICS calculations indicate that the central arenes sacrifice their aromaticity and become non-aromatic for molecules with zwitterionic ground states as an effective way to avoid their overall 4n π antiaromaticity.