Ultrafast Symmetry-Breaking Charge Separation in Covalent Boron-Dipyrromethene Dimers Modulated by Coulombic and Charge Transfer Coupling

To mimic the light capture and charge generation in natural light-harvesting systems, photoinduced symmetry-breaking charge separation (SBCS) in various multichromophoric systems has been widely investigated. Yet, the thermodynamic and kinetic constraints of SBCS hinder its further development and application. Therefore, a comprehensive understanding of the SBCS mechanism is still needed. Herein, photoinduced dynamics in a series of boron dipyrromethene (BODIPY) dimers were investigated by femtosecond time-resolved transient absorption spectroscopy and quantum chemical calculations. Two key criteria pertinent for ultrafast SBCS are unveiled: limited excitonic coupling and effective charge transfer (CT) coupling in excited-state geometry. The former establishes the thermodynamic foundation for SBCS, making the process thermodynamically feasible. Once this feasibility is established, enhancing CT coupling becomes critical as it enables the effective coupling between the environmental fluctuations (such as intramolecular nuclear motion and solvation) and CS reaction, which drives SBCS on a picosecond time scale. Our work highlights the ultrafast SBCS mechanism in multichromophoric systems, suggests the factors responsible for the ultrafast and complete CS, and thereby establishes a potential strategy for the design of optoelectronic materials.