Donor-Controlled Intramolecular Charge Transfer in Multi-Modular Push-Pull Conjugates Containing Competing Electron Acceptors.

The superior electron-accepting behavior of fullerene (C60), even in the presence of strong electron acceptor entities, is demonstrated in a series of multi-modular push-pull systems carrying a donor, phenothiazine, and C60, coupled with strong electron acceptors, tetracyanobutadiene or dicyanoquinodimethane, between the donor and phenothiazine entities. Owing to the strong push-pull nature of the closely spaced entities, strong intramolecular charge transfer was observed, extending the optical coverage into the near-infrared region. Electrochemical studies revealed multiple redox processes (up to ten) owing to the presence of four redox-active molecular entities. DFT and TD-DFT studies helped assess the electronic structures and the excited states responsible for charge transfer, and, importantly, the role of molecular segment(s) within the multi-modular system as electron push or pull groups. Femtosecond pump-probe spectroscopy confirmed the occurrence of excited-state charge transfer and separation, with C60 as the terminal electron acceptor. Additionally, the terminal electron donor is shown to influence the lifetime of the final charge-separated state. The unique findings reported here provide new opportunities for modular systems for the next generation of efficient energy-harvesting, photocatalytic, and optoelectronic applications.