Warren J. Ebenezer

The novel use of NMR spectroscopy with in situ laser irradiation to study azo photoisomerisation

Abstract Cis–trans photoisomerisation of four azo dyes has been studied by 1 H nuclear magnetic resonance (NMR) spectroscopy, with in situ laser irradiation of the sample. The laser radiation was coupled into the sample within the NMR magnet via an optical fibre. This enabled the first 1 H NMR measurements to be made of the transient cis isomers of 4-(4-nitrophenylazo)aniline (CI Disperse Orange 3) and 4-[N-ethyl-N-(2-hydroxyethyl)amino]-4′-nitroazobenzene (CI Disperse Red 1). Typical NMR methods, including 1 H 1D, 1 H – 1 H 2D COSY and 1 H – 1 H 2D EXSY/NOESY, were used to assign 1 H resonances of the trans and cis isomers. Ab initio calculations were used to predict the chemical shifts of cis- and trans-azobenzene, in good agreement with the experimental results, and to rationalise the chemical shift changes observed on isomerisation.

Observation of a Fluorescent Dimer of a Sulfonated Phthalocyanine

With very few exceptions, phthalocyanine dimers are found to be nonfluorescent. We report here the observation of a fluorescent dimer of a tetrasulfonated copper phthalocyanine in ethanol and water. Fluorescence excitation and emission spectra at room temperature and at 77 K are presented. These are consistent with the conventional model of exciton coupling in a cofacial dimer.

Direct synthesis of sulfonated azacalixarenes in water

Successive treatment of cyanuric chloride with two aromatic diamines, at least one of them sulfonated, yields water-soluble sulfonated azacalix[4]arenes which may be isolated by crystallisation. Functionalised azacalixarenes may be made by first displacing two chloro substituents from the cyclisation precursor. Attempted formation of an azacalix[6]arene led to a dimeric species for which two structures may be proposed, one of them an azacalixarene catenane.

Comparison of experimental and calculated 1H NMR chemical shifts of geometric photoisomers of azo dyes

Abstract Quantum chemical calculations based on Density Functional Theory have been used to predict 1H NMR chemical shifts of the cis and trans isomers of three model azo-dye compounds. Calculated absolute chemical shift values, and differences between isomers, were in good agreement with experimentally assigned 1H NMR data. Simulated NMR spectra based on calculated chemical shifts proved useful in identifying differences between predicted and experimental data. The technique could prove valuable in assisting assignment of NMR spectra of more complex dyes of commercial interest, and preliminary investigations on larger systems indicate this is feasible.

Structural characterisation of the photoisomers of reactive sulfonated azo dyes by NMR spectroscopy and DFT calculations

1H NMR spectroscopy coupled with in situ laser irradiation has been used together with density functional theory (DFT) computation to examine the structures of the photoisomers of a series of sulfonated reactive azo dyes. Assignment of 1H NMR spectra acquired at the photostationary state has allowed, for the first time, NMR characterisation of unstable cis isomers of commercially relevant water-soluble azo dyes. Structural features of the two isomeric forms predicted by DFT calculations are clearly reflected in the experimental NMR data. The trans-cis photoisomerisation process could be unambiguously identified in each case, based on the large chemical shift change observed for resonances associated with aromatic protons adjacent to the azo linkage.