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The acidochromism and acid–base properties of 2,6-distyrylpyridine (2,6-DStP) derivatives bearing on the sides push/pull substituents (namely two dimethylamino, one nitro, and one methoxy and two nitro groups in the case of 2,6-bis[(E)-2-(4-dimetylaminophenyl)ethenyl]pyridine, 2-[(E)-2-(4-nitrophenyl)ethenyl],6-[(E)-2′-(4′-methoxyphenyl)ethenyl]pyridine and 2,6-bis[(E)-2-(4-nitrophenyl)ethenyl]pyridine, respectively) were investigated by stationary and time-resolved spectroscopies. The sensitivity of the absorption and emission spectrum to the medium acidity was found to enhance in the dimethylamino-derivative relative to the unsubstituted 2,6-DStP, also because of the second protonation by the N(CH3)2 group. Spectrophotometric titrations, also processed by a global fitting approach, gave pKa values, for the protonation of the central pyridine, higher in the derivatives with electron-donor unities and lower in compounds bearing electron-acceptor groups. A fluorometric titration was performed in the case of the dimethylamino-derivative thanks to non-negligible emission efficiencies for both neutral and protonated species, unveiling an attractive naked-eye acido(fluoro)chromism from green to yellow upon pyridine protonation, and then to purple with the second protonation involving the lateral N(CH3)2 substituent. Due to the extremely short excited-state lifetimes, as resulted from femtosecond transient absorption experiments, the pKa values for the excited state (pKa*) were estimated through the Förster cycle, revealing that the monoprotonated species of the dimethylamino-derivative would become upon excitation the only stable form in a wide range of pH. Graphical abstract

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A simple and efficient protocol is utilized for the transformation studies of a thiophene analog of E-resveratrol by photocatalytic oxygenation using an anionic and a cationic free-base porphyrin, as well as their manganese(III) complexes. The starting substrate was chosen as a representative of heterostilbenes with proven good antioxidant activity. The experiments were carried out in two photoreactor types (batch and microflow reactor) to investigate the impact of the reactor type and design on conversion and photoproduct composition. NMR spectroscopy and UHPLC/MS analyses were applied for the identification and quantification of four photoproducts (Z-1, 2, 3, and 4), results of isomerization, dimerization, cyclization, and oxygenation. Different yields of photoproducts were obtained in a batch reactor and microflow reactor. In the experiments performed in a microflow reactor, Z-1 was most dominant because it was constantly removed from the reaction mixture. Therefore, the formation of other products (2, 3, 4, and undefined) whose precursor is Z-1 was avoided. This was not the case in the experiments performed in a batch reactor. Additionally, all the reactions tested were significantly accelerated in a microflow reactor, making it the preferred reactor type and design for the photocatalytic transformation of resveratrol derivative.

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