Modelling the acidochromism of pyridylazulenes

The structure of six recently synthesised neutral or charged 1-pyridylazulenes and 1,3-dipyridylazulene have been investigated with the PCM–PBE0/6-311++G(2d,2p) approach. Our calculations demonstrate that several conformers of similar energies might coexist in the experimental conditions. The absorption spectra of all derivatives have also been simulated with TD-DFT, and a very good agreement between the experimental and theoretical shifts is attained for both acidochromic and auxochromic phenomena. The origin of colour changes upon pH modification is rationalized.