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Protonation equlibria and optical properties of benzo[b]thienopyridil-benzimidazoles studied by spectroscopic and computational methods

Three new amino substituted benzo[b]thieno[2, 3-b]pyrido[1, 2-a]benzimidazoles 1–3 were synthesized as potential fluorescent pH sensitive probes (Fig. 1). These dye molecules involve highly conjugated pentacyclic planar chromophores with remarkable optical properties including strong fluorescence in the visible spectrum and excellent photostability [1, 2]. To determine their analytical features, 1–3 were followed over a range of pH values by UV/Vis and fluorescence spectroscopies, which revealed that the protonation/deprotonation introduces drastic photophysical changes. For example, the fluorescence intensity of 3 strongly increases upon the exposure to acidic conditions [3] (Fig. 2). Determination of the protonated forms responsible for the observed spectroscopic trends was attempted by combining factorization of the titration spectra with computation of the protonation equilibria constants and electronic spectra. The preliminary results indicate that this requires titration spectra recorded at greater resolution (0.2 - 0.3 pH units). While they are still lacking, here we present the preliminary results obtained at resolution of 1 pH. In view of their analytical applicability, we propose the synthesis of two additional promising systems (4, 5), characterized by improved acid/base (pKa = 7–9) and spectroscopic features.

pH sensors ; optical properties ; UV/Vis spectra ; pKa values ; TDDFT calculations

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