Trans to cis photoisomerization of N,N′-disubstituted indigo dyes via excited singlet states; a laser flash photolysis and steady state irradiation study

Abstract

The relaxation pathways of excited trans-N,N′-diacylindigo dyes (trans-1–trans-4) and of the rigid trans (5) and cis (6) indigo analogues have been studied by nanosecond flash photolysis and steady state measurements. In fluid solution the trans to cis photoisomerization of 1–4 occurs via a singlet excited state mechanism on direct excitation, and the triplet state appears to be reached only by sensitized reaction. The lower quantum yield for trans → cis isomerization of 2–4 at room temperature, as compared to 1, is due to a thermally activated internal conversion step leading to the trans ground state. This pathway competes with fluorescence and twisting of the molecule around the central double bond in the excited singlet state. Increasing the viscosity of the medium decreases the twisting of the flexible molecules and, as a consequence, increases the population of the lowest triplet state. A triplet transient, the intensity of which increases with the viscosity of the medium, could be characterized at low temperature under direct excitation. A transient species, which can be assigned to a triplet state on the basis of quenching experiments and on the fact that the same transient could be found under sensitized conditions, also has been observed at room temperature for the rigid 5 and 6 molecules. Quenching measurements of triplet sensitizers by 5, trans-1, and trans-2 enables the localization of the triplet energy at around 40 kcal/mol.

Red-Purple Photochromic Indigos from Green Chemistry: Mono-tBOC or Di-tBOC N-Substituted Indigos Displaying Excited State Proton Transfer or Photoisomerization

“…Excited-State Isomerization (Photoisomerization) of N,N'tBOCInd in Nonpolar Solvents. Whereas in disubstituted indigo derivatives, there are several reports of trans−cis photoisomerization, 5,12,15,16,19,60,61 with monosubstituted indigo derivatives this has not been reported until very recently when Huber et al 42 described that monoarylated indigo derivatives with relatively neutral substituents undergo trans− cis photoisomerization despite the possibility for de-excitation via ESPT.…”

A Computational Study of the Protoisomerization of Indigo and Its Imine Derivatives

“…Treatment of the protonated isomerized structure with a base regenerates the original trans structure, indicating a fully reversible process. This process for the indigo mono- and diimines differs from those for thioindigo , and N,N′-disubstituted indigo derivatives − in that light is not required in the former. The similar protoisomerization was however not observed for indigo .…”

N,N′-Disubstituted Indigos as Readily Available Red-Light Photoswitches with Tunable Thermal Half-Lives

“…In strong contrast, N,N ′-disubstituted indigos, carrying no amide protons, are capable of undergoing efficient E–Z photoisomerization (Scheme ). ,− …”

“…To date, symmetrical N,N ′-diacyl, N,N ′-dimethyl, and N,N ′-di­( tert -butyloxycarbonyl) indigos are among the only reported photoswitches in this family. Their absorption maxima (λ max ) range from 530 nm all the way to 670 nm, which is strongly red-shifted when compared to conventional E–Z photoswitches, such as stilbenes and azobenzenes .…”