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

Cis–trans photoisomerisation of four azo dyes has been studied by 1H 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 1H 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 1H 1D, 1H–1H 2D COSY and 1H–1H 2D EXSY/NOESY, were used to assign 1H 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.