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Excited-state isomerization of leuco indigo
R Rondão1, J Seixas de Melo, M J Melo
1Department of Chemistry, University of Coimbra, P3004-535 Coimbra, Portugal.
The Journal of Physical Chemistry. A
|February 23, 2012
Summary
UV light causes leuco indigo and its derivatives to undergo photoisomerization. This photoreaction changes their fluorescence properties over time, with quantum yields varying based on the specific indigo compound.
Area of Science:
- Photochemistry
- Spectroscopy
- Organic Chemistry
Background:
- Indigo and its derivatives are important dyes with unique photochemical properties.
- Understanding the photoreaction mechanisms of reduced indigo (leuco indigo) is crucial for applications in materials science and photochemistry.
- Previous studies have explored indigo's photophysics, but detailed investigations into its leuco form's photoreaction dynamics are less common.
Purpose of the Study:
- To investigate the photoreaction of indigo and two derivatives in their leuco form.
- To analyze the changes in fluorescence quantum yield and reaction quantum yield under UV irradiation.
- To elucidate the photoisomerization mechanism from trans to cis forms.
Main Methods:
- Absorption and fluorescence spectroscopy (steady-state and time-resolved) were employed.
- UV irradiation was used to induce photoreactions.
- Kinetic analysis of fluorescence decay curves was performed.
- A model compound with a blocked central C-C bond was used for comparison.
Main Results:
- Fluorescence quantum yield (φ(F)) increased with UV irradiation time for indigo and DBMNI.
- A model compound with restricted rotation showed a constant φ(F), indicating the role of bond rotation.
- Time-resolved fluorescence showed initial biexponential decays transitioning to monoexponential decays.
- Quantum yields for isomerization (φ(R)) were 0.9 for indigo and 0.007 for DBMNI.
Conclusions:
- The photoreaction involves photoisomerization from trans to cis forms of leuco indigo in the first excited singlet state.
- The observed changes in fluorescence are directly linked to the photoisomerization process.
- Structural modifications in indigo derivatives significantly impact their photoreaction efficiency and fluorescence behavior.
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