Understanding and Controlling Short- and Long-Range Electron/Charge-Transfer Processes in Electron Donor–Acceptor Conjugates

We probed a series of multicomponent electron donor2–donor1–acceptor1 conjugates both experimentally and computationally. The conjugates are based on the light harvester and primary electron-donor zinc-porphyrin (ZnP, donor1) to whose β positions a secondary electron-donor ferrocene (Fc, donor2) and the primary electron-acceptor C60-fullerene (C60, acceptor1) are attached. Linking all of them via p-phenylene-acetylene/acetylene bridges of different lengths to gain full control over shuttling electrons and holes between C60, ZnP, and Fc is novel. Different charge-separation, charge-transfer, and charge-recombination routes have been demonstrated, both by transient absorption spectroscopy measurements on the femto, pico-, nano-, and microsecond time scales and by multiwavelength and target analyses. The molecular wire-like nature of the p-phenylene-acetylene bridges as a function of C60–ZnP and ZnP–Fc distances is decisive in the context of generating distant and long-lived C60•––ZnP–Fc•+ charge-separated states. For the first time, we confirm the presence of two adjacent charge-transfer states, a C60–ZnP•––Fc•+ intermediate in addition to C60•––ZnP•+–Fc, en route to the distant C60•––ZnP–Fc•+ charge-separated state. Our studies demonstrate how the interplay of changes in the reorganization energy and the damping factor of the molecular bridges, in addition to variation in the solvent polarity, affect the outcome of the charge-transfer and corresponding rate constants. The different regions of the Marcus parabola are highly relevant in this matter: The charge recombination of, for example, the adjacent C60•––ZnP•+–Fc charge-separated state is located in the inverted region, while that of the distant C60•––ZnP–Fc•+ charge-separated state lies in the normal region. Here, the larger reorganization energy of Fc relative to ZnP makes the difference.