Symmetry-Breaking Charge Separation and Null Aggregates
Frank C. Spano
Abstract
The relationship between symmetry-breaking charge separation (SB-CS) and null aggregate (NA) formation in molecular dimer complexes is investigated theoretically, with applications mainly to dimers based on PDI chromophores. Both processes derive from null points, which are defined by degeneracies in either the upper or lower adiabatic exciton bands. The Hamiltonian contains intermolecular coupling derived from the interacting molecular transition dipole moments (Coulomb coupling), which gives rise to Frenkel excitons, as well as the coupling between the Frenkel excitons and charge-transfer (CT) excitons, driven by the electron ( t e ) and hole ( t h ) hopping integrals. A recipe for efficient SB-CS is presented, which, beyond the Weller condition─which holds that the solvent-stabilized CT state should appear below the optical gap -includes two additional conditions: the presence of a null point and, just as important, lopsided electron and hole transfer integrals, | t e | ≫ | t h | or | t h | ≫ | t e |. When such conditions are met, the developing charge asymmetry is optimized in response to small fluctuations in the polar solvent-induced electric field, which eventually leads to the solvent-stabilized charge-separated state. It is shown that SB-CS can also occur in H-dimers with significant cofacial overlap, consistent with the observations made by several groups.