Strong Acceptor Incorporated Phenothiazine-C60 Multi-Redox Push-Pull Conjugates: Demonstration of C60’s Superior Electron Acceptor Characteristics

Abstract

Among the several exceptional properties of fullerene, C60, its electron acceptor property is a highly studied topic. This work demonstrates the superior electron acceptor property of C60, even in the presence of a stronger electron acceptor(s) in multi-modular donor-acceptor constructs. For this, novel bis-phenothiazine-C60 donor-acceptor conjugates incorporating strong electron acceptors, tetracyanobutadiene (TCBD) or dicyanoquinodimethane (DCNQ), have been newly synthesized. In this molecular design, the TCBD and DCNQ electron acceptors were placed between the two phenothiazine entities, while the C60 was in the peripheral position of one of the phenothiazine entities. After establishing their molecular structure, intramolecular charge transfer in these systems was probed through optical and electrochemical measurements, while time-dependent DFT studies initially probed the ground and excited-state charge transfer. These studies established the role of C60 as an acceptor compared to TCBD and DCNQ due to the sandwiching of the latter electron acceptors between two phenothiazine electron donors, which modulates their overall electron-acceptor abilities. Femtosecond pump-probe studies, covering broad spatial and temporal scales, provided experimental evidence that C60 serves as the terminal electron acceptor, wherein the electron transfer product of C60 was spectrally possible to identify. These unprecedented findings present new opportunities for designing multi-redox entities featuring push-pull systems, paving the way for the next generation of efficient energy harvesting, photocatalytic, and optoelectronic applications.