Investigating the Kinetics and Structural Effects of Azo Dye Photochemistry Using NMR With In Situ Laser Irradiation and Ab Initio (DFT) Calculations
Date
2007Author
Gibson, David
Metadata
Abstract
Reversible cis-trans isomerisation of a series of commercially interesting yellow azo
dyes has been studied using the technique of Nuclear Magnetic Resonance (NMR)
spectroscopy with in situ laser irradiation. Photostationary state (PSS) spectra of the
azo dyes, provided by coupling laser irradiation into the sample within the probe of
the NMR spectrometer, have allowed observation of azo cis isomer species that would
otherwise elude detection and characterisation by NMR due to their rapid thermal
decay times.
The NMR results have been combined with geometry optimised structures obtained
through ab initio (DFT) calculations in order to allow visualisation of the trans and
cis isomer species, and explain NMR spectral features. In the majority of cases, these
in vacuo calculated structures show a great deal of correlation with NMR observations
of asymmetric, cis-trans isomerisation-induced chemical shift changes for protons adjacent
to the azo bond. The cis isomer spectral pattern for substituted naphthyl group
protons can hence be used as a diagnostic tool in determining the correct cis isomer
conformation in molecules where more than one conformation may exist.
In addition to the aforementioned characterisation studies, in situ irradiation has provided
the opportunity to undertake a thorough investigation of the kinetics of photoand
thermal isomerisation for the same yellow dye series. The results of these studies
have been combined with previous work on similar systems to provide an extensive
data set, and conduct analysis in a systematic fashion. Adding fibre-reactive groups,
varying phenyl and chlorotriazine ring substituents, and altering naphthyl group sulfonation
patterns have a profound effect on both the photochemical and thermal rates
of isomerisation in these systems. In certain cases, the same structural calculations
noted earlier have proved useful in rationalising the identified kinetic differences.
The presence of phenyl substituents ortho to the azo bond has been shown to increase
the rate of thermal cis isomer decay. Additionally, substituents bonded to a fibrereactive
group distant from the azo bond have an appreciable effect on the barrier to
thermal cis-trans isomerisation, but little effect on the photochemical characteristics of
each isomer. Several sulfonated naphthyl group patterns have been studied, leading to an observation that sulfonate groups positioned ortho to the azo bond assist in retarding
thermal isomerisation, with sulfonate groups in other positions having a much smaller
effect.
One particular molecule, a component of a currently available commercial dye, was
studied for its interesting and previously unexplained behaviour, both photochemical
and chemical. The dye demonstrated photoisomerisation at low concentrations only,
with aggregation preventing formation of the cis isomer at higher concentrations. The
trans isomer was found to undergo degradation to a product which did not photoisomerise.
This product was identified as a benzotriazinium compound by multinuclear
2D correlation NMR spectroscopy, formed by a reversible cyclisation reaction involving
the azo bond.