Isoenergetic Symmetry Breaking Charge Separation in Far-Red Absorbing Orthogonal BODIPY Dimer -a Classic Case of No Energy Loss During the Process of Light Capture and Conversion

Abstract

Symmetry-breaking charge separation (SB-CS) in a far-red capturing, orthogonally linked BODIPY dimer, 2, revealing minimal electronic coupling, is demonstrated under isoenergetic conditions (with little or no energy loss), thus helping to maximize the process of solar light capture and conversion. The orthogonal design of the dimer and proximity resulted in poor orbital overlaps between the chromophores, promoting a long-lived SB-CS state without the need for a thermodynamic driving force -a crucial factor for increasing solar device efficiency. Multiple techniques were employed to establish and prove this phenomenon. Steady-state and time-resolved emission studies revealed substantial quenching of the dimer in both nonpolar and polar solvents compared to the BODIPY monomer, 1, providing initial evidence of SB-CS. The redox gap, measured to assess thermodynamic feasibility through electrochemical studies, confirmed the event as a barrierless process (∆G ET ~ 0.0 eV). TD-DFT calculations supported this realization by illustrating the generation of excited-state electron density and hole-electron distribution, revealing an unsymmetrical dipolar distribution. Short-range and long-range electronic coupling calculations yielded negligible values, confirming weak excitonic coupling, reducing the Coulombic interactions between the hole and electron, thereby facilitating the formation of radical ion pairs with minimal energy loss. Transient absorption spectroscopy further provided conclusive evidence of SB-CS and allowed the extraction of kinetic parameters.Finally, Marcus's theory of electron transfer was applied, yielding a low electronic coupling (V) value of as little as 7.6 meV.These findings indicate that electron transfer can occur even under weak-coupling (null-exciton) conditions without an energy barrier -a step forward in maximizing solar energy harvesting.