Detecting photoinduced symmetry-breaking charge separation with near-zero driving force in a perylene diimide cage

Photoinduced symmetry-breaking charge separation (SB-CS) between two identical molecules has been reported with an increasing number of chromophores, but is sometimes difficult to detect when reversible. Using a perylenediimide-based molecular cage, we demonstrate that the combination of electronic transient absorption and time-resolved fluorescence enables the detection of SB-CS even when no clear spectral signature of the ionic product is visible in the transient spectra. This is possible thanks to the presence of the easily observable delayed fluorescence and the global target analysis of both transient absorption and time-resolved fluorescence data using a modified exciplex model. We found that SB-CS in this cage occurs reversibly in a broad range of solvent polarities, even in highly polar media. On the basis of molecular dynamics simulations, the weak driving force of SB-CS is explained by the limited mobility of the solvent molecules inside the cage, and the reduced solvation energy of the charge-separated state that results.