Rational Construction and Efficient Regulation of Stable and Long-Lived Charge-Separation State in Fullerene Materials

Abstract

Photoinduced charge separation (CS) ensures efficient light-energy conversion. The stable and long-lived charge-separation state (CSs) is beneficial for suppressing charge recombination and facilitating the separation of highly reduction-active electrons or highly oxidation-active holes to participate in subsequent photoreactions. Accordingly, the construction of stable and long-lived charge-separation states has been an important goal for researchers. These results highlighted the importance of fullerene materials. Characterized by their well-defined and stable structures and exceptional electronic properties, fullerenes have emerged as prominent electron acceptors. The energy levels and excited-state electron transfer features can be modulated by altering the carbon cage (selecting diverse carbon-cage configurations), embedding clusters (metallofullerenes), or modifying the functional groups on the carbon cage (fullerene additive reactions). Importantly, the low electron reorganization energy of fullerenes makes them promising materials for constructing long-lived CSs. Therefore, researchers commonly employ fullerenes as acceptors to design photoelectric materials or investigate their fundamental charge separation mechanisms. The primary task is to construct stable CSs through system design and extend the lifetime of CSs according to appropriate regulations. However, critical challenges stem from the inadequate comprehension of CS patterns, unsuitable system choices, and lack of simple and efficient strategies for CS regulation. Therefore, our research approach, which originates from the inherent principles of CS, aims to explore strategies for constructing and regulating stable and long-lived CSs in fullerene derivatives.

In this Account, we systematically summarize the following three aspects of charge separation in fullerene materials. (1) Construction of thermodynamically stable CSs. We established a mathematical correlation between the external modifying groups and the HOMO energy levels, enabling a rapid and straightforward prediction of the stability of CSs. Stable CSs can be successfully constructed by increasing the HOMO of the donor, lowering the LUMO of the acceptor, or altering the direction of the CSs. (2) Develop kinetic regulation strategies to extend CSs lifetime. We found that the meta- or ortho-substituted configuration determines the excited-state charge localization, effectively slowing charge recombination and thus prolonging the CSs lifetime. Additionally, our findings indicate that restricting molecular conformational changes can extend the CSs lifetime. Strategies for conformational regulation, including redox regulation and the introduction of steric hindrance, were subsequently designed. (3) Potential applications of stable and long-lived CSs. We primarily elucidated the applications of CSs in photovoltaics and photocatalysis (hydrogen production and NAD⁺ regeneration). Stable and long-lived CSs ensures effective charge separation and photogenerated carrier transport, which are important for efficient photovoltaic and photocatalytic reactions. Finally, aiming for a prolonged lifetime, more universal and efficient regulation strategies, and broader applications of the charge-separation state, we propose some perspectives that can be further applied to fullerene-based materials.