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

Abstract

A study of the possible intersystem crossing (ISC) mechanisms (S ⇝ T) in thionine (3,7-diamino-phenothiazin-5-ium), which is conducive to the efficient population of the triplet manifold, is presented. The radiationless deactivation channels {S₁,S₂(π → π*) ⇝ T₁,T₂(π → π*)} have been examined. Since the direct ISC does not explain the high triplet quantum yield in this system, attention has been centered on the vibronic spin–orbit coupling between the low-lying singlet and triplet (π → π*) states of interest. An efficient population transfer from the S₁(π_H → π_L*) state to the T₂(π_H−1 → π_L*) state via this channel is confirmed. The calculated ISC rate constant for this channel is k_ISC ≈ 3.35 × 10⁸ s⁻¹, which can compete with the radiative depopulation of the S₁(π_H → π_L*) state via fluorescence (k_F ≈ 1.66 × 10⁸ s⁻¹) in a vacuum. The S₁(π_H → π_L*) ⇝ T₁(π_H → π_L*) and {S₂(π_H−1 → π_L*) ⇝ T₁,T₂(π → π*)} ISC channels have been estimated to be less efficient (k_ISC ≈ 10⁵–10⁶ s⁻¹). Based on the computed ISC rate constants and excited-state solvent shifts, it is suggested that the efficient triplet quantum yield of thionine in water is primarily due to the S₁(π_H → π_L*) ⇝ T₂(π_H−1 → π_L*) channel with a computed rate constant of the order of 10⁸–10⁹ s⁻¹ which is in accord with the experimental finding (k_ISC = 2.8 × 10⁹ s⁻¹).