Low pH optical sensor based on benzothiazole azo dyes

2-Amino-6-substituted benzothiazoles were obtained by a one-pot reaction of different anilines with thiocyanogen and used to prepare the respective 2-phenylazo-6-substituted-benzothiazoles in good yields by the classical diazotization reaction followed by coupling with two N,N-dialkylanilines. The aminobenzothiazoles 2a–d present absorption in the UV region, although the azo dyes 4a–e present absorption maxima in the visible region. In the aminobenzothiazoles and the respective azo dyes, changes in the donor and acceptor character of the substituents affect the absorption maxima. All electronic transitions were fully spin- and symmetry-allowed, with 1π-π* character. The aminobenzothiazoles 2a–d do not show significant fluorescence emission, while the azo dyes 4a–e presented very low fluorescence emission in the green-orange region. The azo dyes present large changes in the absorption spectra with changing pH (blue to red colour), suggesting their use as optical sensors for low pH values. Theoretical calculations were also performed to study the charge distribution and photophysical properties of the azo dyes in their ground and excited states, utilizing Density Functional Theory (DFT) and Time Dependent Density Functional Theory (TD-DFT). All calculations were carried out using PBE1PBE and ωB97XD functionals at the cc-pVDZ level for geometrical optimization, and at the jun-cc-pVTZ level for vertical transition energies and electronic property computations. Solvent effects were included by PCM formalism using acetonitrile and dichloromethane. Results were in good agreement with experimental data.