Thioxanthone Dioxide Triplet States Have Low Oxygen Quenching Rate Constants

Area of Science:

• Photochemistry
• Organic Chemistry
• Physical Chemistry

Background:

• Triplet excited states (³M₁) of organic molecules are usually quenched by ground state molecular oxygen (O₂(X³Σ_g^-)) with high rate constants (k_q > 10⁹ M^-1 s^-1).
• This quenching can sensitize the production of singlet oxygen (O₂(a¹Δ_g)) if the triplet state energy exceeds 94 kJ/mol.
• The efficiency of quenching and singlet oxygen yield are influenced by mixing with the molecule-oxygen charge-transfer state.

Purpose of the Study:

• To investigate triplet excited states of thioxanthen-9-one-10,10-dioxide derivatives.
• To determine the quenching rate constants (k_q) and singlet oxygen quantum yields for these compounds.
• To explore the relationship between molecular properties and oxygen quenching dynamics.

Main Methods:

• Synthesis of thioxanthen-9-one-10,10-dioxide derivatives.
• Measurement of triplet state quenching rate constants (k_q) by molecular oxygen.
• Determination of quantum yields for photosensitized singlet oxygen production.
• Measurement of oxidation potentials of the studied molecules.

Main Results:

• Several thioxanthen-9-one-10,10-dioxide derivatives exhibited unusually low k_q values (down to 1 × 10⁸ M^-1 s^-1).
• These compounds achieved near-unity quantum yields for singlet oxygen (O₂(a¹Δ_g)) production.
• The molecules showed high oxidation potentials (around 3.5 V vs SCE).

Conclusions:

• Reduced charge-transfer character in the excited triplet state-oxygen encounter complex is proposed to explain the low k_q.
• This reduced charge transfer allows for efficient singlet oxygen generation.
• The findings provide experimental validation for theoretical models of organic molecule excited state quenching by oxygen.