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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
).