Indigo derivatives and their colour

Abstract

This thesis focuses upon the spectroscopic properties of indigo and its derivatives.
The first chapter introduces the background of indigo and derivatives that are found in nature and
used in industry. The historical literature and works were reviewed to understand the reason for
indigo’s blue colour from both a spectroscopic and computational perspective.
Using Suzuki and Sonogashira coupling methods, this research prepared a range of indigo
derivatives appended with aromatic and arylethynyl substituents. Also dibromoindirubin, and N,N’-
diBoc-indigo were prepared. UV-Visible spectroscopy was used to characterise the indigo
derivatives prepared herein, and DTF-based molecular modelling (Gaussian-16) calculations were
done on the di(p-tolyl)-indigos (B3LYP/6-31+G(d)).to investigate the effects of substituent position.
The synthetic methods and results are reported.
This research has found that the absorption spectra of indigo derivatives with aryl or arylethynyl
substituents around the benzene ring of the indole unit (positions 4-7) are dominated by the indigo
core and have an absorption maximum of around 600 nm.
However, although indigo itself is very stable towards light and oxygen, it has been noticed that a
few of the indigo derivatives prepared here rapidly degrade and lose their intense blue colour. This
is particularly noticeable when they are dissolved, a degradation would occur with a colour change
from a deep blue to a colourless/yellow solution as a result of oxidation to the isatins.