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

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 {S(1),S(2)(π → π*) → T(1),T(2)(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 S(1)(π(H) → π(L)*) state to the T(2)(π(H-1) → π(L)*) state via this channel is confirmed. The calculated ISC rate constant for this channel is k(ISC) ≈ 3.35 × 10(8) s(-1), which can compete with the radiative depopulation of the S(1)(π(H) → π(L)*) state via fluorescence (k(F) ≈ 1.66 × 10(8) s(-1)) in a vacuum. The S(1)(π(H) → π(L)*) → T(1)(π(H) → π(L)*) and {S(2)(π(H-1) → π(L)*) → T(1),T(2)(π → π*)} ISC channels have been estimated to be less efficient (k(ISC) ≈ 10(5)-10(6) 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 S(1)(π(H) → π(L)*) → T(2)(π(H-1) → π(L)*) channel with a computed rate constant of the order of 10(8)-10(9) s(-1) which is in accord with the experimental finding (k(ISC) = 2.8 × 10(9) s(-1)).