A theoretical study of thionine: spin-orbit coupling and intersystem crossing

Area of Science:

• Photochemistry
• Quantum Chemistry
• Spectroscopy

Background:

• Thionine exhibits a high triplet quantum yield, suggesting efficient intersystem crossing (ISC) from singlet to triplet states.
• Understanding ISC mechanisms is crucial for applications in photodynamic therapy and organic electronics.
• Previous studies have not fully elucidated the specific pathways responsible for thionine's efficient triplet state population.

Purpose of the Study:

• To investigate the intersystem crossing (ISC) mechanisms in thionine (3,7-diamino-phenothiazin-5-ium).
• To identify the dominant ISC pathways contributing to the high triplet quantum yield.
• To computationally determine the rate constants for various ISC channels.

Main Methods:

• Theoretical calculations of intersystem crossing (ISC) mechanisms.
• Examination of radiationless deactivation channels, specifically {S(1),S(2)(π → π*) → T(1),T(2)(π → π*)}.
• Analysis of vibronic spin-orbit coupling between low-lying singlet and triplet (π → π*) states.

Main Results:

• The direct ISC mechanism does not fully account for the observed high triplet quantum yield.
• Vibronic spin-orbit coupling facilitates efficient population transfer from the S(1)(π(H) → π(L)*) state to the T(2)(π(H-1) → π(L)*) state.
• Calculated ISC rate constant for the S(1) → T(2) channel is approximately 3.35 × 10^8 s⁻¹, competitive with fluorescence.
• Other ISC channels, S(1) → T(1) and S(2) → T(1),T(2), are significantly less efficient (k(ISC) ≈ 10⁵–10⁶ s⁻¹).

Conclusions:

• The efficient triplet quantum yield of thionine in water is primarily attributed to the S(1)(π(H) → π(L)*) → T(2)(π(H-1) → π(L)*) ISC channel.
• The computed ISC rate constant for this dominant channel (10⁸–10⁹ s⁻¹) aligns with experimental findings (2.8 × 10⁹ s⁻¹).
• Vibronic spin-orbit coupling is the critical factor enabling efficient triplet state population in thionine.