Molecular mechanisms of the photostability of indigo

Abstract

The photophysics of indigo as well as of bispyrroleindigo, the basic chromophore of indigo, has been investigated with ab initio electronic-structure calculations. Vertical electronic excitation energies and excited-state potential-energy profiles have been calculated with the CASSCF, CASPT2 and CC2 methods. The calculations reveal that indigo and bispyrroleindigo undergo intramolecular single-proton transfer between adjacent N-H and C=O groups in the (1)ππ* excited state. The nearly barrierless proton transfer provides the pathway for a very efficient deactivation of the (1)ππ* state via a conical intersection with the ground state. While a low-lying S(1)-S(0) conical intersection exists also after double-proton transfer, the latter reaction path exhibits a much higher barrier. The reaction path for trans→cis photoisomerization via the twisting of the central C=C bond has been investigated for bispyrroleindigo. It has been found that the twisting of the central C=C bond is unlikely to play a role in the photochemistry of indigo, because of a large potential-energy barrier and a rather high energy of the S(1)-S(0) conical intersection of the twisted structure. These findings indicate that the exceptional photostability of indigo is the result of rapid internal conversion via intramolecular single-proton transfer, combined with the absence of a low-barrier reaction path for the generation of the cis isomer via trans→cis photoisomerization.

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“…It has been found that the twisting of the central C C bond is unlikely to play a role in the photochemistry of indigo, because of a large potential-energy barrier and a rather high energy of the S1-S0 conical intersection of the twisted structure. These findings indicate that the exceptional photostability of indigo is the result of rapid internal conversion via intramolecular single-proton transfer, combined with the absence of a lowbarrier reaction path for the generation of the cis isomer via trans → cis photoisomerization [33] .…”

Effects of resonant excitation, pulse duration and intensity on photoelectron imaging of a dianion

“…We have recently studied the excited state photophysics of InC 2-in the gas-phase using time-resolved PE imaging and showed that the mechanism for decay involves an excited state intramolecular proton transfer reaction followed by rapid internal conversion. 27,28 Our current focus is on the PE 10 imaging of InC 2-.…”

On the intrinsic photophysics of indigo: a time-resolved photoelectron spectroscopy study of the indigo carmine dianion

“…There are three possible intra-molecular mechanisms that can lead to the rapid internal conversion. Firstly, trans → cis isomerisation corresponds to the promotion of an electron from a π to a π* orbital, which leads to a weakening of the central C=C bond 8,9 and could in principle result in free rotation around this central carbon bond. Many other dyes with a C=C (stilbenes) or N=N bond (azobenzenes) show considerable trans-cis photo-5 isomerisation yields.…”

“…Ab initio calculations by Yamazaki et al agree with experiment and indicate that the single ESIPT is the more likely relaxation mechanism. 9 A negligible barrier was calculated for 40 ESIPT on the S 1 potential energy surface and a conical intersection (CI) with the ground state was identified near the enol form that is energetically accessible. A viable CI was also identified along the trans-cis isomerisation coordinate; however, a large barrier to isomerisation inhibits this process, in agreement 45 with measurements that indicate that no isomerisation occurs.…”