Supramolecular Predisposition Promotes Intramolecular Heavy-Atom Effects for Self-Sensitized Oxidation

Area of Science:

• Supramolecular Chemistry
• Photochemistry
• Organic Synthesis

Background:

• Controlling excited-state reaction pathways, especially spin-forbidden processes like intersystem crossing (ISC), is challenging in supramolecular chemistry.
• The heavy-atom effect, crucial for ISC, is usually an intrinsic property, not a geometry-dependent supramolecular parameter.
• Supramolecular confinement's potential to direct these pathways remains largely unexplored.

Purpose of the Study:

• To investigate if supramolecular predisposition can enforce intramolecular heavy-atom effects to activate spin-forbidden transitions.
• To demonstrate efficient self-sensitized oxidation using this approach.
• To establish supramolecular predisposition as a design principle for switchable photocatalysts.

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 under white-light irradiation, supported by scavenger and electron paramagnetic resonance (EPR) studies.

Main Results:

• CB[8] encapsulation enforced a folded guest geometry, juxtaposing the bromine heavy atom and the aldehyde.
• The predisposed complex underwent selective aldehyde oxidation to a carboxylic acid upon irradiation.
• Evidence supported singlet oxygen generation via triplet-oxygen energy transfer as the operative pathway.
• Catalytic turnover was achieved with substoichiometric host loadings due to preferential host binding to the substrate.

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 strategy enables efficient self-sensitized oxidation by controlling heavy-atom effects and ISC.