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

A study of the possible intersystem crossing (ISC) mechanisms (S hooked right arrow 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(pi -> pi*) hooked right arrow T-1,T-2(pi -> pi*)} 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 (pi -> pi*) states of interest. An efficient population transfer from the S-1(pi(H) -> pi(L)*) state to the T-2(pi(H-1) -> pi(L)*) state via this channel is confirmed. The calculated ISC rate constant for this channel is k(ISC) approximate to 3.35 x 10(8) s(-1), which can compete with the radiative depopulation of the S-1(pi(H) -> pi(L)*) state via fluorescence (k(F) approximate to 1.66 x 10(8) s(-1)) in a vacuum. The S-1(pi(H) -> pi(L)*) hooked right arrow T-1(pi(H) -> pi(L)*) and {S-2(pi(H-1) -> pi(L)*) hooked right arrow T-1,T-2(pi -> pi*)} ISC channels have been estimated to be less efficient (k(ISC) approximate to 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(pi(H) -> pi(L)*) hooked right arrow T-2(pi(H-1) -> pi(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 x 10(9) s(-1)).