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Supramolecular confinement can control chemical reactions by enforcing specific molecular geometries. This study demonstrates how macrocycle encapsulation activates spin-forbidden reactions, enabling efficient photocatalysis.

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Area of Science:

  • Supramolecular Chemistry
  • Photochemistry
  • Organic Chemistry

Background:

  • Supramolecular confinement is key for molecular control, yet its role in directing excited-state reactions, especially spin-forbidden processes like intersystem crossing (ISC), is underexplored.
  • The heavy-atom effect typically relies on intrinsic properties, not geometry, limiting its supramolecular regulation for ISC.
  • Directing ISC via supramolecular design remains a significant challenge in chemistry.

Purpose of the Study:

  • To investigate supramolecular control over spin-forbidden excited-state reactions.
  • To demonstrate the use of macrocycle encapsulation to enforce intramolecular heavy-atom effects.
  • To enable efficient self-sensitized oxidation through designed supramolecular complexes.

Main Methods:

  • Encapsulation of a flexible aldehyde- and bromine-substituted guest within cucurbit[8]uril (CB[8]).
  • Structural characterization using solution studies and single-crystal X-ray diffraction.
  • Photochemical oxidation studies under white-light irradiation with control experiments and mechanistic investigations (scavenger, EPR).

Main Results:

  • CB[8] encapsulation enforces a folded guest geometry, juxtaposing the heavy atom (bromine) and reactive aldehyde.
  • The predisposed complex undergoes efficient, selective aldehyde oxidation to carboxylic acid under white-light irradiation.
  • Mechanistic studies confirm singlet oxygen generation via triplet-oxygen energy transfer, driven by CB[8]-enforced spatial arrangement.

Conclusions:

  • Macrocyclic encapsulation can transform spatial geometry into a switch for spin-forbidden pathways.
  • Supramolecular predisposition is a versatile principle for designing switchable photocatalysts.
  • This approach enables efficient catalysis with reduced product inhibition due to preferential host-guest binding.