Non-Statistical Assembly of Donor-Acceptor Cages for Light-Induced Charge Separation

We report light-triggered charge separation in two discrete supramolecular architectures that self-assemble in a single step from donor (D) and acceptor (A) functionalized bridging ligands and Pd(II) cations. The "shape complementary assembly" (SCA) strategy allows for exclusive formation of the cis-[Pd₂D₂A₂]⁴⁺ cage isomer. Compared to previously reported statistical DA assemblies, lacking stoichiometry and stereo control, the number of possible electron transfer routes was reduced. This enables a better understanding and tunability of the excited state dynamics. Cage assembly was investigated by NMR, MS, and single crystal X-ray diffraction analysis. Steady-state absorption and electrochemical properties indicate that donor and acceptor moieties remain largely independent in the electronic ground state. Femtosecond pump-probe spectroscopy in the visible and infrared was applied to compare the fate of photoexcited states for pure ligands, donor- and acceptor-only assemblies, and the donor-acceptor heteroleptic cages. For the latter, ultrafast intracage ligand-to-ligand charge separation is followed by two back electron transfer pathways, occurring on timescales of hundreds of picoseconds and around one nanosecond, assignable to D/A ligands facing each other in cis- or trans-position. Our work shows that non-statistical modular self-assembly can be used for the precise positioning of photoredox-active components in defined distances on the nanoscale.