Benzothiazole merocyanine dyes as middle pH optical sensors

• Published in: Dyes and pigments 2020-05, Vol.176, p.108193, Article 108193
• Main Authors: Coelho, Felipe Lange, da Costa Duarte, Rodrigo, de Ávila Braga, Cláudia, Toldo, Josene Maria, Bruno Gonçalves, Paulo Fernando, da Silveira Santos, Fabiano, Rodembusch, Fabiano Severo
• Format: Article
• Language: English
• Subjects: Benzothiazole; Colorimetric sensor; ESIPT; Merocyanine; Optical sensor
• ISSN: 0143-7208; 1873-3743
• DOI: 10.1016/j.dyepig.2020.108193

Substituted merocyanine dyes were obtained by Knoevenagel condensation between formylated arylbenzothiazole and quaternary 2-alkylbenzothiazoles. The condensations were performed in the presence of sodium methoxide and the products were obtained with excellent yields after simple purification. The merocyanines presented absorption maxima in the range 576–605 nm (green to orange-red region). The calculated Strickler-Berg parameters indicated fully spin and symmetry allowed electronic transitions with 1π-π* character. These compounds show significant fluorescence emission at the orange red region with a very small Stokes shift. Surprisingly, although the benzothiazole core possesses a structural moiety (o-hydroxyaryl moiety) which should facilitate proton transfer in the excited state, no evidence was found of this phototautomerism in the compounds studied. TD-DFT calculations show that ESIPT is not probable to happen due to the high barriers to transfer the proton. In addition, the aliphatic chain does not affect the absorption and emission maxima location. For both, ground and excited states, a negative solvatochromic effect was observed. Theoretical calculations also show that this effect is due to the larger dipole moment in the S0 than in the S1 state. In addition, these compounds presented large shifts on the absorption spectra changing the pH (yellow to violet color), which allowed studying successfully as optical sensor for middle pH values. [Display omitted] •Merocyanines obtained by Knoevenagel condensation between arylbenzothiazole and 2-methylbenzothiazoles.•Intense fluorescence emission at the orange red region with very small Stokes shift.•No evidence of proton transfer in the excited state.•Absorption spectra tailored by pH (yellow to violet color).