Excited Charge Transfer Promoted Electron Transfer in all Perylenediimide Derived, Wide-Band Capturing Conjugates: A Mimicry of the Early Events of Natural Photosynthesis.

Fundamental discoveries in electron transfer advance scientific and technological advancements. It is suggested that in plant and bacterial photosynthesis, the primary donor, a chlorophyll or bacteriochlorophyll dimer, forms an initial excited symmetry-breaking charge transfer state (CT*) upon photoexcitation that subsequently promotes sequential electron transfer (ET) events. This is unlike monomeric photosensitizer-bearing donor-acceptor dyads where ET occurs from the excited donor or acceptor (D* or A*). In the present study, we successfully demonstrated the former photochemical event using an excited charge transfer molecule as a donor. Electron-deficient perylenediimide (PDI) is functionalized with three electron-rich piperidine entities at the bay positions, resulting in a far-red emitting CT molecule (D). Further, this molecule is covalently linked to another PDI (A) carrying no substituents at the bay positions, resulting in wide-band capturing D-A conjugates. Selective excitation of the CT band of D in these conjugates leads to an initial D* that undergoes subsequent ET involving A resulting in D -A charge separation product (k10 s). Conversely, when A was directly excited, ultrafast energy transfer (ENT) from A* to D (k10 s) followed by ET from D* to PDI is witnessed. While increasing solvent polarity improved k rates, for a given solvent, the magnitude of the k values was almost the same, irrespective of the excitation wavelengths. The present findings demonstrate ET from an initial CT state to an acceptor is key to understanding the intricate ET events in complex natural and bacterial photosynthetic systems possessing multiple redox- and photoactive entities.