Photophysical and Spectroscopic Studies of Indigo Derivatives in Their Keto and Leuco Forms

Abstract

A comprehensive spectroscopic and photophysical study of the keto and leuco forms of indigo and three other ring-substituted derivatives in solution was performed. The characterization involves absorption, fluorescence, and triplet-triplet absorption spectra, making it possible to obtain the quantum yields for fluorescence (φ F ), singlet-triplet intersystem crossing (φ ISC ), internal conversion (φ IC ), and lifetimes for fluorescence (τ F ) and triplet decay (τ T ). For the case of the keto forms, pulse radiolysis experiments have revealed the existence of a triplet acceptor (from energy transfer from different donors) for the indigo, purple, and indirubin compounds. It is shown that with the keto form the major deactivation pathway involves internal conversion from the lowest singlet excited state to the ground state whereas with the leuco form there is competition between internal conversion, triplet formation, and fluorescence deactivation processes. Furthermore, leuco forms present much higher Stokes shifts compared with keto ones, suggesting an excitedstate geometry different from the ground-state geometry, possibly involving rotational photoisomerization.

Photophysics of an Indigo Derivative (Keto and Leuco Structures) with Singular Properties

“…Moreover, the maximum is similar to those for other leuco-indigo species. 9 The emission spectrum is also similar in intensity and relative position of bands to those for indigo and indigo-related molecules. 9 If the absorption maximum is taken as that from the excitation spectrum, the Stokes shift (∆ SS ) of the reduced form of Cibalackrot is similar to that for reduced indigo (see Table 1).…”

“…With indigo, for the keto form, where hydrogen bonding occurs, the Stokes shift is 1080 cm -1 , whereas for the leuco form, where the absence of hydrogen bonding allows rotation around the central C-C bonds, it is 3504 cm -1 (Table 1). 9 For preCiba and indigo there is no vibrational structure, in either the absorption or fluorescence spectra, in clear contrast to Cibalackrot. From the marked Stokes Shifts displayed by preCiba, it can be clearly inferred that significant structural variations occur between the ground and lowest excited state.…”

“…9 Although not perfect, the match between the absorption and the fluorescence excitation spectra is reasonable. Moreover, the maximum is similar to those for other leuco-indigo species.…”

“…This is particularly pro- nounced with fluorescence, which increases by more than 2 orders of magnitude relative to indigo. 9 This contrasts with the behavior of keto forms of other indigo derivatives, where internal conversion is always the major singlet deactivation pathway. 9 Moreover, the low singlet oxygen yield (φ ∆ ) (see Table 1) reveals that this is not efficiently produced by the compound.…”

“…9 Two different explanations for the predominance of IC are particularly relevant: (1) intramolecular excited-state proton transfer (ESIPT) between adjacent hydrogen-bonded carbonyl and N-H groups; (2) intermolecular hydrogen-bonding between the dye and the solvent. 6,10,11 The presence of hydrogen bonding in indigo has also been suggested as the reason this compound exists in the stable trans form, precluding the competing cistrans photoisomerization.…”