Symmetry-Breaking Charge Separation in Phenylene-Bridged Perylenediimide Dimers

Perylenediimides (PDIs) are important molecular building blocks that are being investigated for their applicability in optoelectronic technologies. Covalently linking multiple PDI acceptors at the 2,5,8,11 (headland) positions adjacent to the PDI carbonyl groups is reported to yield higher power conversion efficiencies in photovoltaic cells relative to PDI acceptors linked at the 1,6,7,12 (bay) positions. While the photophysical properties of PDIs linked via the bay positions have been investigated extensively, those linked at the headland positions have received far less attention. We showed previously that symmetry-breaking charge separation (SB-CS) in PDIs hold promise as a strategy for increasing photovoltaic efficiency. Here we use transient absorption and emission spectroscopies to investigate the competition between SB-CS, fluorescence, and internal conversion in three related PDI dimers linked at the headland positions with o-, m-, and p-phenylene moieties: o-PDI, m-PDI, and p-PDI, respectively. It is found that o-PDI supports SB-CS yielding PDI^•+-PDI^•-, which is in equilibrium with the o-PDI₂ first excited state in a polar solvent (CH₂Cl₂) while m-PDI and p-PDI exhibit accelerated internal conversion due to the motion of the linker along with subnanosecond intersystem crossing (ISC). Electronic coupling and structural dynamics are shown to play a significant role, with o-PDI being the only member of the series that exhibits significant through-bond interchromophore coupling. The pronounced o-PDI steric congestion prevents the free internal rotation that leads to rapid deactivation of the excited state in the other dimers.