Investigating the Kinetics and Structural Effects of Azo Dye Photochemistry Using NMR With In Situ Laser Irradiation and Ab Initio (DFT) Calculations

摘要

Reversible cis-trans isomerisation of a series of commercially interesting yellow azouddyes has been studied using the technique of Nuclear Magnetic Resonance (NMR)udspectroscopy with in situ laser irradiation. Photostationary state (PSS) spectra of theudazo dyes, provided by coupling laser irradiation into the sample within the probe ofudthe NMR spectrometer, have allowed observation of azo cis isomer species that wouldudotherwise elude detection and characterisation by NMR due to their rapid thermaluddecay times.udThe NMR results have been combined with geometry optimised structures obtainedudthrough ab initio (DFT) calculations in order to allow visualisation of the trans andudcis isomer species, and explain NMR spectral features. In the majority of cases, theseudin vacuo calculated structures show a great deal of correlation with NMR observationsudof asymmetric, cis-trans isomerisation-induced chemical shift changes for protons adjacentudto the azo bond. The cis isomer spectral pattern for substituted naphthyl groupudprotons can hence be used as a diagnostic tool in determining the correct cis isomerudconformation in molecules where more than one conformation may exist.udIn addition to the aforementioned characterisation studies, in situ irradiation has providedudthe opportunity to undertake a thorough investigation of the kinetics of photoandudthermal isomerisation for the same yellow dye series. The results of these studiesudhave been combined with previous work on similar systems to provide an extensiveuddata set, and conduct analysis in a systematic fashion. Adding fibre-reactive groups,udvarying phenyl and chlorotriazine ring substituents, and altering naphthyl group sulfonationudpatterns have a profound effect on both the photochemical and thermal ratesudof isomerisation in these systems. In certain cases, the same structural calculationsudnoted earlier have proved useful in rationalising the identified kinetic differences.udThe presence of phenyl substituents ortho to the azo bond has been shown to increaseudthe rate of thermal cis isomer decay. Additionally, substituents bonded to a fibrereactiveudgroup distant from the azo bond have an appreciable effect on the barrier toudthermal cis-trans isomerisation, but little effect on the photochemical characteristics ofudeach isomer. Several sulfonated naphthyl group patterns have been studied, leading to an observation that sulfonate groups positioned ortho to the azo bond assist in retardingudthermal isomerisation, with sulfonate groups in other positions having a much smallerudeffect.udOne particular molecule, a component of a currently available commercial dye, wasudstudied for its interesting and previously unexplained behaviour, both photochemicaludand chemical. The dye demonstrated photoisomerisation at low concentrations only,udwith aggregation preventing formation of the cis isomer at higher concentrations. Theudtrans isomer was found to undergo degradation to a product which did not photoisomerise.udThis product was identified as a benzotriazinium compound by multinuclearud2D correlation NMR spectroscopy, formed by a reversible cyclisation reaction involvingudthe azo bond.