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 (ϕ~F~) and singlet oxygen formation (ϕ~Δ~) 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 ϕ~F~ and ϕ~Δ~ values were found to be very low ≤10^−3^. Because of the absence of transient triplet–triplet signal, the intersystem crossing yields (ϕ~T~) were estimated by assuming that all the triplet states formed give rise to singlet oxygen formation, that is, ϕ~Δ~≈ϕ~T~ . It was then possible from ϕ~IC~=1−ϕ~F~−ϕ~T~ to estimate the S~1~∼∼→S~0~ 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, k~NR~, clearly dominate over the radiative rate constants, k~F~, (and processes). Secondly, the main deactivation channel for the compounds in their keto form is the radiationless S~1~∼∼→S~0~ 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.