Photochemical Mechanisms in Electron Donor–Acceptor Complexes

Electron donor–acceptor (EDA) complexes arise from the reversible association of electron-rich donors and electron-deficient acceptors through weak interactions such as dipole–dipole or halogen bonds. Upon irradiation, EDA complexes absorb light in their charge-transfer bands, prompting a photoinduced electron transfer (PET) that generates radical ion pairs. These radicals engage in diverse bond-forming reactions—ranging from C–C and C–S couplings to hydrogen-atom transfer and stereoselective additions—under mild, catalyst-free conditions. Mechanistic pathways often proceed via radical chain propagation or involve intramolecular charge-transfer within covalently linked donor–acceptor systems, enabling exquisite control over reactivity and selectivity. Recent work has extended EDA photochemistry to aqueous media, aerobic oxidation and enantioselective transformations, highlighting its potential for sustainable synthesis and scalable processes utilising visible-light sources.

Research from Nature Portfolio

A novel photocatalytic system has been devised to overcome oxygen sensitivity in traditional light-driven EDA processes. By engineering a stable donor–acceptor pair that transfers electrons to molecular oxygen under visible light, superoxide radicals form in situ, facilitating the aerobic oxidation of boronic acids to their corresponding phenols at ambient conditions. In foundational work on enantioselective radical conjugate additions, an intramolecular iminium-ion-based EDA complex was employed to generate radicals for stereocontrolled additions to cyclic enones. Visible-light activation of the chiral iminium precursor yields enantiomerically enriched quaternary centres, demonstrating how embedded charge-transfer interactions can drive asymmetric photoreactions.

Photochemical Mechanisms in Electron Donor–Acceptor Complexes publication trend

The graph below shows the total number of articles in photochemical mechanisms in electron donor–acceptor complexes across all publications each year (not limited to Nature Index journals).

Technical terms

Electron donor–acceptor (EDA) complex: A reversible, non-covalent assembly of an electron donor and acceptor that exhibits a charge-transfer absorption band upon light irradiation.

Charge-transfer band: A spectral feature corresponding to electronic transition from donor to acceptor within an EDA complex, initiating photoexcitation.

Photoinduced electron transfer (PET): The light-driven movement of an electron from donor to acceptor, generating radical ion pairs.

Radical chain mechanism: A propagation process where radicals produced in the initiation step react to regenerate new radical species, sustaining the reaction.

Halogen bonding: A non-covalent interaction between an electron donor (e.g., thiolate) and the electropositive region of a halogen atom in an acceptor molecule.

References

1. Synthetic reactions driven by electron-donor–acceptor (EDA) complexes. Beilstein Journal of Organic Chemistry (2021).
2. Dioxygen compatible electron donor-acceptor catalytic system and its enabled aerobic oxygenation. Nature Communications (2024).
3. Enantioselective radical conjugate additions driven by a photoactive intramolecular iminium-ion-based EDA complex. Nature Communications (2018).
4. Enforced Electronic‐Donor‐Acceptor Complex Formation in Water for Photochemical Cross‐Coupling. Angewandte Chemie International Edition (2023).
5. A photoexcited halogen-bonded EDA complex of the thiophenolate anion with iodobenzene for C(sp 3 )–H activation and thiolation. Chemical Science (2021).
6. The Photochemical Activity of a Halogen-Bonded Complex Enables the Microfluidic Light-Driven Alkylation of Phenols. Organic Letters (2022).