Details

Autor(en) / Beteiligte

• Rodriguez-Serrano, Angela
• Rai-Constapel, Vidisha
• Daza, Martha C.
• Doerr, Markus
• Marian, Christel M.

Titel

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

Ist Teil von

• Photochemical & photobiological sciences, 2012-12, Vol.11 (12), p.1860-1867

Ort / Verlag

Cham: Springer International Publishing

Erscheinungsjahr

2012

Quelle

MEDLINE

Beschreibungen/Notizen

• 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 ).