[Arylazobenzene–BF3] dyes: Electronic absorption and NMR spectroscopic evidence for a novel class of dyes stable in aprotic solvents

Treatment of a number of 4-substituted arylazobenzenes (–H, –Me, –CF3, –Br, –F and –OMe) with BF3·OEt2 in dry chloroform solution afforded chromophores that exhibited significant hyper- (log ɛ 4.41–4.60) and bathochromic shifts (λmax 416–473) in electronic spectra, together with significant shifts in their 1H NMR and 13C NMR spectra, indicating the formation of novel chemical species. It is proposed that complexes are formed between the azo dyes and the Lewis acid, similarly to what happens for the well-known protic acids. The equilibrium constants (log K) of the proposed complexes have been calculated from the experimental data. Numerical modelling has also been performed to investigate the nature of the new systems, revealing a correlation between the electronic levels of the azo dye and the ones of the BF3·OEt2.