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• Photocatalysis has advanced rapidly owing to its unique advantage in activating substrates and reshaping reaction pathways. Photocatalysts activate substrates mainly via two modes, single-electron transfer (SET) and energy transfer (EnT). While SET-based photoredox catalysis has greatly expanded the frontiers of synthetic chemistry [1], the full potential of EnT catalysis remains underexplored [2]. In the realm of asymmetric EnT photocatalysis, several strategies have been developed, including direct excitation strategy, hybrid catalytic systems, and dual catalytic systems. Among these, the dual catalytic approach combining a photosensitizer with a chiral catalyst offers both the advantages of visible-light compatibility and a tunable chiral environment, enabling the asymmetric photocycloaddition of various alkenes (Fig. 1a) [3].

  Figure 1

  

  Figure 1.  (a) Research background. (b) Relay EnT strategy and design of new catalysts.

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  According to Dexter's theory, the triplet energy transfer rate decays exponentially with increasing distance between the photosensitizer and the acceptor [3]. In dual catalytic systems, the chiral catalyst inevitably introduces spatial segregation between the photosensitizer and the substrate, thereby imposing an EnT barrier (Fig. 1b). The Tan group identified this barrier as a critical factor that negatively impacts the efficiency and even the feasibility of EnT photocatalysis, which has not received sufficient attention in previous reports. Addressing this spatial constraint is a key scientific challenge for expanding the boundaries of asymmetric EnT photocatalysis. The Tan group proposed a relay energy transfer photocatalytic strategy, which involves integrating an energy carrier into the catalyst structure to build a bridge for energy transfer from the photosensitizer to the substrate [4]. The paramount challenge lies in designing organocatalysts that seamlessly incorporate the three essential components, an energy carrier, a catalytic center, and a stereogenic element, while preserving the individual tunability of each component for pursuing optimal performance (Fig. 2a).

  Figure 2

  

  Figure 2.  (a) Design and (b) application of the new CETA catalyst.

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  Having established a library of CETA catalysts, the group applied it to asymmetric EnT photocatalysis to validate the effectiveness of the relay EnT concept. In 2021, Glorius and co-workers reported the dearomative cycloaddition (DAC) of quinolines and alkenes via acid-promoted substrate activation, offering a highly efficient route to valuable 3D molecules from simple starting materials [5]. However, this transformation suffered from the reliance on stoichiometric acidic activator, as well as limited regioselectivity and stereoselectivity control. Conventional Brønsted acids such as phosphoric acids and phosphoramides showed modest catalytic activity (~25% yield), but the introduction of a chiral scaffold led to a significant loss of activity. Mechanistic studies revealed that the EnT barrier caused by spatial isolation from the catalyst side arms was the primary cause. CETA exhibited remarkably high catalytic activity in this transformation. Its scaffold-assisted energy transfer ensures that the side arms do not negatively impact catalytic activity. Under optimized conditions, only 5 mol% of CETA was sufficient to achieve full conversion, while the flexible adjustment of the chiral side arms enabled excellent control over regio- and stereoselectivity (Fig. 2b).

  The catalytic system demonstrated broad substrate scope. Various alkyl and halogen-substituted quinolines, as well as alkyl, electron-rich, and electron-deficient alkenes, all delivered the desired products with satisfactory yields and stereoselectivities. Moreover, CETA showed excellent tolerance toward a wide range of functional groups, including alkyl, halide, cyano, hydroxyl, aryl, carboxyl, and ester groups.

  The team conducted extensive mechanistic investigations using steady-state phosphorescence quenching, NMR titration experiments, time-resolved Stern-Volmer analysis, low-temperature phosphorescence, cyclic voltammetry (CV), and nanosecond transient absorption spectroscopy (ns-TA). These studies confirmed that the catalyst side arms can hinder EnT efficiency from the photosensitizer to the substrate by up to 80% (Fig. 3). Importantly, the relay energy transfer enabled by CETA significantly improved EnT efficiency by as much as threefold. Most notably, CETA not only facilitates energy transfer but also acts as an energy storage unit, extending the excited-state lifetime of the substrate through reversible energy transfer, thereby increasing the probability of productive transformations. This study establishes a new paradigm in energy transfer photocatalysis, expands the boundaries of organocatalysis and EnT photocatalysis, and offers new insights for the design of chiral catalysts.

  Figure 3

  

  Figure 3.  Mechanistic studies and excited-state lifetime modulation.

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  CRediT authorship contribution statement

  Lixin Duan: Writing – original draft, Investigation. Yajing Shen: Validation, Investigation. Xin He: Validation, Investigation. Qingjie Zhao: Writing – review & editing, Validation. Tao Tu: Writing – review & editing, Project administration, Funding acquisition.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  Financial support from the National Natural Science Foundation of China (No. 22271060), Zhejiang Provincial Key R & D Program (No. 2024C03109) and Key Research and Development Program of Ningxia Hui Autonomous Region (No. 2026BEE02007). The Center for Basic Research and Innovation of Medicine and Pharmacy (MOE), School of Pharmacy at Naval Medical University, and the College of Chemistry and Chemical Engineering at Ningxia University are gratefully acknowledged.

  
  
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  Figure 1  (a) Research background. (b) Relay EnT strategy and design of new catalysts.

  下载: 全尺寸图片 幻灯片
  

  Figure 2  (a) Design and (b) application of the new CETA catalyst.

  下载: 全尺寸图片 幻灯片
  

  Figure 3  Mechanistic studies and excited-state lifetime modulation.

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