M. Haggmark, G. Gate, S. Boldissar
Nov 1, 2018
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Influential Citations
6
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Journal
Chemical Physics
Abstract
Abstract Indigo is a blue dye molecule that has been used since antiquity, although it is better known today for its use in blue jeans. Indigo has previously been shown to exhibit remarkable photostability due to fast excited state dynamics mediated by an excited state intramolecular proton transfer. Study of this process is complicated by the fact that the photophysics of indigo is very sensitive to the environment. In order to disentangle the intrinsic photodynamics of indigo from the effects contributed by the environment, we studied indigo in a molecular beam using resonance enhanced multiphoton ionization. We obtained excited state lifetimes of individual vibronic bands with pump-probe spectroscopy ranging from 60 ps to 24 ns. We have mapped a barrier to relaxation at about 700 cm−1, beyond which fast excited state dynamics dominate. Below this barrier two decay processes compete and mode-specific relaxation occurs with certain vibronic bands near the origin relaxing faster than others, or exhibiting different partitioning between the two relaxation channels. Computational studies at the ADC(2)/MP2/cc-pVDZ level indicate that two low-barrier reaction paths exist in the S1 state of indigo, one corresponding to proton transfer, the other to hydrogen transfer. In both cases the charge distribution changes drastically upon de-excitation. These data provide a sensitive probe of the potential energy landscape, responsible for the response to absorption of light. The results may help in understanding the photostability that preserves the blue color of indigo dyes.