Kunsagi-Maté Sandor, Szabó Kornélia, Desbat Bernard, Bruneel Jean-Luc, Bitter Istvan, Kollar Laszló
Department of General and Physical Chemistry, University of Pécs, Pécs, Hungary.
J Phys Chem B. 2007 Jun 28;111(25):7218-23. doi: 10.1021/jp068567a. Epub 2007 Jun 5.
To improve the selectivity of sensing, the thermodynamics of the complex formation of some calix[4]arene hosts with neutral phenol guests was studied in carbon tetrachloride as nonpolar solvent. The molecular shape of calixarenes was varied by the selective functionalization with tBu and O-CH2-Ph (O-benzyl) or OPr groups at the upper and lower rim, respectively. To vary the electron density on the guest's aromatic rings, the parent phenol was functionalized in the para position with electron-withdrawing Cl, as well as H, and electron-releasing CH3 and tBu groups. To study the interaction between calixarene and the guests, PL and quantum-chemical methods were applied. The results revealed an overall 1:1 complex stoichiometry except for the parent dibenzyloxycalix[4]arene, where 1:2 host-guest stoichiometries were observed irrespective of the quality of phenol. In the latter case, the complex formation shows a self-switched character: the first phenol molecule is included in the calixarene cavity, and only afterward, a second guest molecule is bound by the two benzyloxy aromatics. Although the enthalpy change predicts strong interaction between the host and the guest, the Gibbs free energy change of the complex formation is small, resulting in a relatively low complex stability. The solvent-relaxation measurements support that the unexpected entropy change could be the consequence of the reorientation of solvent molecules around the calixarene building block. The reorientation is assisted by dispersive forces between solute and solvent molecules. IR and RAMAN analysis of the complexes exclude a considerable participation of the phenolic OH group in the stabilization of the complex. This result is in agreement with earlier findings where deterministic role of pi-pi interaction in the complex stability was assumed.
为提高传感的选择性，研究了一些杯[4]芳烃主体与中性酚客体在作为非极性溶剂的四氯化碳中形成配合物的热力学。通过分别在上缘和下缘用叔丁基和O-CH₂-Ph（邻苄基）或OPr基团进行选择性官能化，改变了杯芳烃的分子形状。为改变客体芳环上的电子密度，母体酚在对位用吸电子的Cl以及H，和供电子的CH₃和叔丁基进行了官能化。为研究杯芳烃与客体之间的相互作用，应用了荧光光谱（PL）和量子化学方法。结果表明，除了母体二苄氧基杯[4]芳烃外，总体配合物化学计量比为1:1，在二苄氧基杯[4]芳烃中，无论酚的性质如何，均观察到1:2的主客体化学计量比。在后一种情况下，配合物形成表现出自切换特性：第一个酚分子被纳入杯芳烃空腔，只有在此之后，第二个客体分子才被两个苄氧基芳环结合。尽管焓变预示着主体与客体之间有强相互作用，但配合物形成的吉布斯自由能变化很小，导致配合物稳定性相对较低。溶剂弛豫测量结果表明，意外的熵变可能是杯芳烃结构单元周围溶剂分子重新取向的结果。溶质与溶剂分子之间的色散力有助于这种重新取向。配合物的红外和拉曼分析排除了酚羟基对配合物稳定性有显著贡献。这一结果与早期假设π-π相互作用在配合物稳定性中起决定性作用的研究结果一致。
