Puccioni Stefano, Bazzicalupi Carla, Bencini Andrea, Giorgi Claudia, Valtancoli Barbara, De Filippo Greta, Lippolis Vito, Salvi Pier Remigio, Pietraperzia Giangaetano, Chelli Riccardo, Gellini Cristina
Dipartimento di Chimica "Ugo Schiff", Università di Firenze , Via della Lastruccia 3, 50019 Sesto Fiorentino, Firenze, Italy.
J Phys Chem A. 2013 May 9;117(18):3798-808. doi: 10.1021/jp4015756. Epub 2013 May 1.
Synthesis and characterization of a new macrocyclic compound, composed by a triethylentetraamine chain linking the 4 and 5 positions of an acridine moiety, are reported. The molecule, devised as a fluorescent chemosensor for anions, has revealed an intriguing pH-dependent spectroscopic behavior, whose features are the specific object of this article. Ligand protonation in aqueous solution has been analyzed by means of potentiometric, (1)H NMR, UV-vis, and fluorescence emission measurements. The molecule binds up to four protons in the pH range 2-11. Protonation takes place on the aliphatic tetraamine chain, while the acridine nitrogen does not participate to proton binding even at very low pH. Differently from acridine, the UV-vis spectra are almost unaffected by the pH. On the opposite, the emission spectra are strongly pH-dependent. In fact, at low pH values, the spectra show a blue-shifted emission, resembling that of unprotonated acridine, while at slightly acidic and alkaline pH the fluorescence features a red-shifted band similar to that of acridinium cation. This unusual behavior occurs in the mono-, bi-, and triprotonated forms of the compound and is interpreted as due to an excited state proton transfer from an aliphatic ammonium group adjacent to the acridine moiety to the acridine nitrogen. In the fully protonated state, this process is prevented owing to unfavorable molecular arrangements mainly determined by electrostatic repulsions. This interpretation is supported by quantum mechanical calculations as well as molecular dynamics simulations.
报道了一种新型大环化合物的合成与表征，该化合物由连接吖啶部分4位和5位的三乙烯四胺链组成。该分子被设计为一种用于阴离子的荧光化学传感器，其展现出了有趣的pH依赖性光谱行为，本文将具体探讨其特性。通过电位滴定法、（1）H NMR、紫外可见光谱和荧光发射测量对水溶液中的配体质子化进行了分析。该分子在pH值为2至11的范围内可结合多达四个质子。质子化发生在脂肪族四胺链上，而即使在非常低的pH值下，吖啶氮也不参与质子结合。与吖啶不同，紫外可见光谱几乎不受pH值影响。相反，发射光谱强烈依赖于pH值。实际上，在低pH值下，光谱显示出蓝移发射，类似于未质子化的吖啶，而在略呈酸性和碱性的pH值下，荧光呈现出与吖啶鎓阳离子相似的红移带。这种异常行为发生在该化合物的单质子化、双质子化和三质子化形式中，并被解释为由于从与吖啶部分相邻的脂肪族铵基团到吖啶氮的激发态质子转移。在完全质子化状态下，由于主要由静电排斥决定的不利分子排列，该过程被阻止。这种解释得到了量子力学计算以及分子动力学模拟的支持。
