The triplet state of indigo.

Indigo is one of the most emblematic molecules for the color blue, and is believed in many cultures to be charged with unique talismanic properties. The vivid color and high stability of this dye have led to an almost mythical status, from the “tecklet” of the Hebrews (believed to be a mixture with Tyrian Purple), through the blue of Napoleon Bonaparte(s army uniforms, to its current application as the dye for denim in blue jeans. Indigo is also associated with the genesis of the German chemical industry. However, in spite of the numerous reports on the properties and applications of indigo (and analogues) some mysteries remain to be solved. Of particular importance is the characterization of the triplet state. Although triplet states of thioindigo derivatives have been characterized and shown to be involved in cis–trans photoisomerization, and both thioindigo and indigo quench the triplet state of tin(IV) porphyrins, no sensitized processes or evidence of a long-lived triplet are seen in the latter case. We have therefore characterized this elusive indigo species in terms of its energy and quantum yield of intersystem crossing using two different and complementary techniques, namely, energy-transfer pulse radiolysis and timeresolved photoacoustic calorimetry (PAC). We believe this has important implications for the properties and applications of the compound.