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Excited-State Proton Transfer in Indigo.

J Pina1, Daniela Sarmento1, Marco Accoto1,2

  • 1CQC, Department of Chemistry, University of Coimbra , 3004-535 Coimbra, Portugal.

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|February 22, 2017
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Excited-state proton transfer (ESPT) in indigo derivatives was studied using spectroscopy and TDDFT calculations. Results reveal a single ESPT mechanism, with NHxInd showing an additional nonradiative pathway competing with proton transfer.

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Area of Science:

  • Photochemistry
  • Physical Chemistry
  • Computational Chemistry

Background:

  • Excited-state proton transfer (ESPT) is a fundamental photochemical process.
  • Indigo derivatives are known to exhibit ESPT, but the detailed mechanisms and influencing factors require further elucidation.

Purpose of the Study:

  • To investigate the ESPT mechanism in indigo (Ind) and its monohexyl-substituted derivative (NHxInd) in solution.
  • To determine the activation energy and rate constants for ESPT.
  • To explore the influence of solvent viscosity, polarity, and temperature on the ESPT process.

Main Methods:

  • Time-correlated single photon counting (TCSPC) for fluorescence decay analysis.
  • Femtosecond (fs)-transient absorption spectroscopy for ultrafast dynamics.
  • Time-dependent density functional theory (TDDFT) calculations for theoretical insights.

Main Results:

  • Fluorescence decays were biexponential, indicating the presence of excited-state keto and enol species.
  • TDDFT calculations confirmed a single ESPT mechanism with Arrhenius-type activation and tunneling.
  • Activation energies were determined to be ~11 kJ/mol for Ind and ~5 kJ/mol for NHxInd, closely matching TDDFT predictions.
  • Rate constants for ESPT in dimethyl sulfoxide were found to be 9.24 × 10^10 s^-1 (Ind) and 7.12 × 10^10 s^-1 (NHxInd).
  • NHxInd exhibited an additional nonradiative pathway, likely due to rotation around the central C-C bond leading to a conical intersection.

Conclusions:

  • The ESPT mechanism in indigo and NHxInd is similar, involving a single proton transfer.
  • NHxInd possesses an additional nonradiative decay channel competing with ESPT, influenced by molecular rotation and conical intersections.
  • The study provides a comprehensive understanding of ESPT dynamics in indigo derivatives, integrating experimental and theoretical approaches.