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The elusive triplet state of indigo (see picture) has been fully characterized by energy-transfer pulse radiolysis and photoacoustic calorimetry. The triplet energy is (1.04±0.10) eV and the singlet-to-triplet intersystem crossing yield in solution is ϕISC=0.0066. Inefficient intersystem crossing, low triplet energy, and fast internal conversion of the singlet state likely contribute to the high stability of indigo.
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Melo et al. (2007) studied this question.
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