Spectral and Photophysical Studies of Substituted Indigo Derivatives in Their Keto Forms

Abstract

The spectral and photophysical properties of indigo derivatives with di‐, tetra‐, and hexa‐substitution in their neutral (keto) form are investigated in solution. The study comprises absorption and emission spectra, together with quantitative measurements of quantum yields of fluorescence (ϕF) and singlet oxygen formation (ϕΔ) and fluorescence lifetimes. The energy difference between the HOMO and LUMO orbitals is dependent on the degree (number of groups) and relative position of substitution. The ϕF and ϕΔ values were found to be very low ≤10−3. Because of the absence of transient triplet–triplet signal, the intersystem crossing yields (ϕT) were estimated by assuming that all the triplet states formed give rise to singlet oxygen formation, that is, ϕΔ≈ϕT . It was then possible from ϕIC=1−ϕF−ϕT to estimate the S1∼∼→S0 internal conversion yields and thus, with the other data, to determine the rate constants for all decay processes. From these, several conclusions are drawn. Firstly, the radiationless rate constants, kNR , clearly dominate over the radiative rate constants, kF , (and processes). Secondly, the main deactivation channel for the compounds in their keto form is the radiationless S1∼∼→S0 internal conversion process. Finally, although the changes are relatively small, internal conversion yield seems to be independent of the overall pattern of substitution. A more detailed investigation of the decay profiles with collection at the blue and red emission of the fluorescence band of indigo and one di‐substituted indigo reveals the decays to be bi‐exponential and that at longer emission wavelengths these appear to be associated with both rise and decay times indicating that two excited species exist, which is consistent with a keto‐excited form giving rise (by fast proton transfer) to the enol‐form of indigo. Evidence is presented which supports the idea that intramolecular (and possibly some intermolecular) proton transfer can explain the high efficiency of internal conversion in indigo.

Characterization of the Excited States of Indigo Derivatives in their Reduced Forms

“…[15,17] In Table 1, it can be seen that the Stokes shift (SS, D SS ) display values of~90 nm, which contrasts with the smaller values found for the keto forms,~40 nm. [17] This suggests that, in the leuco form, the ground-and excited-state structures of the investigated indigo derivatives are significantly different and that this is presumably related to a photoisomerization process occurring with the leuco forms.…”

“…[15,17] In Table 1, it can be seen that the Stokes shift (SS, D SS ) display values of~90 nm, which contrasts with the smaller values found for the keto forms,~40 nm. [17] This suggests that, in the leuco form, the ground-and excited-state structures of the investigated indigo derivatives are significantly different and that this is presumably related to a photoisomerization process occurring with the leuco forms. Moreover, with the leuco form, the hydrogen bonds that keep indigo (and indigo-like structures) in a trans planar configuration, and is responsible for the fast and dominant internal conversion deactivation channel (through intra or intermolecular proton transfer), [18] are no longer present.…”

“…This clearly indicates that a light-induced process (nonexistent in the keto structures) is present, and that it is compatible with a photoisomerization process. [15,17] whereas with the leuco forms (Table 1) they increase by about two orders of magnitude. Moreover, the same applies to the fluorescence lifetimes for the keto and leuco forms whose major component fall, respectively, in the picosecond and nanosecond time domains.…”

“…Moreover, the same applies to the fluorescence lifetimes for the keto and leuco forms whose major component fall, respectively, in the picosecond and nanosecond time domains. [15,17] This shows that, in addition to internal conversion (dominant with the keto forms), [15,17] with the leuco forms fluorescence (and singlet-triplet intersystem crossing) now becomes an efficient deactivation pathway for the excited state of the indigo derivatives.…”

“…Data for leuco indigo and leuco Tyrian purple were previously obtained in dimethylformamide (DMF), [15] and for the correspondent keto forms of the compounds in Scheme 2 in dioxane. [17] In Figure 1 the tail observed at longer wavelengths (in the absorption spectra) is due to scattering promoted by particles of the reduced agent, which partially precipitates when added to the solution. Indeed, the presence of this tail in the absorption does not change the pale yellow color of the solution.…”