Abstract Herein, we report the synthesis and inclusion of a new series of azo-dyes into first generation Frechet type dendrimers. The incorporated dyes are amino-nitro, amino-methoxy and amino-butyl substituted azobenzenes bearing a well defined oligo(ethylene glycol) side chain. The optical properties of the dendrimers were studied by absorption spectroscopy. Dendrimer bearing amino-nitro substituted azobenzenes (λ = 480 nm in CHCl 3 ) behaved as a pseudostilbene type azobenzene whereas the others (λ = 409 nm in CHCl 3 ) showed the typical behaviour of aminoazobenzenes. Moreover, the trans–cis photoisomerization of the dendrimers was studied by UV–vis spectroscopy by irradiating at two different wavelengths (254 and 365 nm). In chloroform, the appearance of an intense red-shifted band revealed the presence of a photoprotonation effect in the azobenzene moiety. This phenomenon was also studied by absorption spectroscopy in function of time. The results were compared to those predicted by molecular modelling using Density Functional Theory calculations.