Department of Chemistry, Biology and Biotechnology, "Centro di Eccellenza Materiali Innovativi Nanostrutturati" (CEMIN), University of Perugia, Via Elce di Sotto 8, 06123, Perugia, Italy.
Department of Organic Chemistry, Faculty of Chemical Engineering and Technology, University of Zagreb, 10000, Zagreb, Croatia.
Photochem Photobiol Sci. 2022 Jun;21(6):935-947. doi: 10.1007/s43630-022-00184-5. Epub 2022 Mar 1.
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(CH) 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(CH) 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.
带有推/拉取代基的 2,6-二取代吡啶（2,6-DStP）衍生物的酸致变色和酸碱性质，研究了侧基取代基（即 2,6-双[(E)-2-(4-二甲氨基苯基)乙烯基]吡啶中两个二甲氨基、一个硝基和一个甲氧基，2-[(E)-2-(4-硝基苯基)乙烯基]，6-[(E)-2'-(4'-甲氧基苯基)乙烯基]吡啶和 2,6-双[(E)-2-(4-硝基苯基)乙烯基]吡啶中两个硝基基团）通过静态和时间分辨光谱进行了研究。发现吸收和发射光谱对介质酸度的敏感性相对于未取代的 2,6-DStP 在二甲氨基衍生物中增强，这也是由于 N(CH)基团的第二次质子化。分光光度滴定也通过全局拟合方法进行，给出了质子化中央吡啶的 pKa 值，对于具有供电子单元的衍生物，pKa 值更高，而对于具有吸电子基团的化合物，pKa 值更低。由于中性和质子化物种的荧光效率不可忽略，因此在二甲氨基衍生物的情况下进行了荧光滴定，揭示了从绿色到黄色的吡啶质子化后具有吸引力的裸眼酸（荧光）变色，然后通过涉及侧基 N(CH)取代基的第二次质子化变为紫色。由于飞秒瞬态吸收实验得出的激发态寿命极短，因此通过福斯特循环估计了激发态（pKa*）的 pKa 值，结果表明，二甲氨基衍生物的单质子化物种将在很宽的 pH 范围内成为唯一稳定的形式。
