Conformationally Locked BODIPY Donor–Acceptor Dyads: Charge Separation and Recombination across a [2.2.2]Bicyclooctane Spacer

Abstract

Controlling the excited-state dynamics of charge-separated (CS) states in donor-acceptor (D-A) systems is challenging due to geometry changes from intramolecular rotations and vibrations. A key question is how molecular geometry governs the formation, stability, and recombination of CS states, and whether these processes can be decoupled from spin-related transitions such as intersystem crossing (ISC). Here, we investigate a rigid design strategy where BODIPY and anthracene chromophores are fused to a bicyclo[2.2.2]octane scaffold, fixing the D-A geometry and eliminating conformational flexibility. This setup enables direct evaluation of how geometric restriction affects charge-transfer dynamics. Femtosecond transient absorption spectroscopy shows efficient charge separation in polar solvents (2–9 ps), followed by rapid recombination (15–205 ps), with no detectable triplet state formation. Nanosecond transient absorption, 77 K luminescence, and singlet oxygen measurements confirm suppression of spin-orbit charge-transfer intersystem crossing (SOCT-ISC). These findings demonstrate that rigid molecular architecture can block ISC but does not inherently extend CS state lifetimes. Our results provide new insight into the fate of CS states and emphasize that geometric rigidity alone does not ensure long-lived charge separation in D-A systems.