Chiroptics of In-Plane Excitons

The exciton chirality method relates chiroptical activity to absolute stereochemistry, but because the procedure focuses on the geometry of coupled transition electric dipole moments only, the essential roles of magnetic dipole and electric quadrupole transition moments are nonobvious in many applications. These moments become apparent when analyzing in-plane, achiral coupled excitons. In silico C₁₈H₂ polyyne dimers spanning geometries between optically inactive H- and J-aggregate extrema are used to investigate the multipolar response of coupled excited states through the gyration and rotatory strength tensors. As anticipated, the chiroptical response is dominated by the exciton states. The responses are largest in some directions for intermediate geometries and can be reconciled simply with the transition moments derived from structure. The connection between the in-plane moments and the exciton chirality method is established by a minimal symmetry-breaking perturbation that generates circular dichroism couplets while preserving the electronic and pedagogic relationship to the planar systems.