Grupo de Bioquímica Teórica, Universidad Industrial de Santander, Bucaramanga, Colombia.
Photochem Photobiol Sci. 2012 Dec;11(12):1860-7. doi: 10.1039/c2pp25224d.
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)).
本文研究了硫堇（3,7-二氨基吩噻嗪-5-鎓）中可能的系间交叉（ISC）机制（S→T），这有利于三重态态的有效布居。研究了无辐射去活化通道{S(1)，S(2)(π→π*)→T(1)，T(2)(π→π*)}。由于直接 ISC 不能解释该体系中高三重态量子产率，因此注意力集中在感兴趣的低 singlet 和三重态（π→π*）态之间的振动态旋轨耦合上。通过此通道，从 S(1)(π(H)→π(L))态有效地将电子布居转移到 T(2)(π(H-1)→π(L))态得到确认。该通道的ISC 速率常数 k(ISC)≈3.35×10(8)s(-1)，可以与真空荧光（k(F)≈1.66×10(8)s(-1)）通过辐射衰减来竞争S(1)(π(H)→π(L))态的去布居。S(1)(π(H)→π(L))→T(1)(π(H)→π(L))和{S(2)(π(H-1)→π(L))→T(1)，T(2)(π→π*)}ISC 通道被估计为效率较低（k(ISC)≈10(5)-10(6)s(-1)）。基于计算的 ISC 速率常数和激发态溶剂位移，建议硫堇在水中的高效三重态量子产率主要归因于 S(1)(π(H)→π(L))→T(2)(π(H-1)→π(L))通道，其计算速率常数约为 10(8)-10(9)s(-1)，与实验发现（k(ISC)=2.8×10(9)s(-1)）一致。
