Universidade Estadual de Maringá, Campus Regional de Umuarama, Departamento de Tecnologia, 87506-370 Umuarama, PR, Brazil.
Universidade de São Paulo, Instituto de Física, 05508-090 São Paulo, SP, Brazil.
Spectrochim Acta A Mol Biomol Spectrosc. 2022 Mar 5;268:120664. doi: 10.1016/j.saa.2021.120664. Epub 2021 Nov 29.
The charge-transfer complexes (CTC) between electron donor iodide and pyridinium dimers and monomers as acceptors have been studied spectrophotometrically in acetonitrile, DMSO and water. The dimers were: N,N'-alkyldiyl-bis(4-cyanopyridinium) and N,N'-alkyldiyl-bis(2-bromopyridinium), and the monomers were: N-alkyl-4-cyanopyridinium and N-alkyl-2-bromopyridinium, bridged by n methylene spacers. The formation constant (K), molar absorptivity (ε), fluorescence-quenching constant (K) were determined. The results indicate that the stoichiometry of the CTC for dimers and monomers is 1:1 (equimolar ratio), and its formation is favored for dimers more than for monomers, especially dimers with short methylene spacers forming a "sandwiched type-complex" in which the iodide is close to the two pyridinium rings. Solvents with low polarity favored the complex, which was destroyed by the presence of water. The experimental studies were complemented with theoretical studies with quantum mechanics (QM) calculations using density functional theory (DFT) and molecular mechanics (MM) simulations with Molecular Dynamics for identify the most stable conformers in acetonitrile solution. The electronic excitations were calculated with sequential QM/MM hybrid method, showing a charge-transfer wavelength in agreement with the UV-Vis absorption spectra obtained experimentally. It confirms the "sandwiched type-complex" conformers favoring the interaction of the iodide with one pyridinium rings and simultaneously forming one, or two, hydrogen bonds with the alkyl chain and additionally, for the case of dimers, with the other pyridinium ring.
电子给体碘化物与吡啶鎓二聚体和单体作为受体的电荷转移配合物（CTC）已在乙腈、DMSO 和水中通过分光光度法进行了研究。二聚体为：N,N'-亚烷基双（4-氰基吡啶鎓）和 N,N'-亚烷基双（2-溴代吡啶鎓），单体为：N-烷基-4-氰基吡啶鎓和 N-烷基-2-溴代吡啶鎓，由 n 亚甲基间隔基桥接。确定了形成常数（K）、摩尔吸光率（ε）和荧光猝灭常数（K）。结果表明，二聚体和单体的 CTC 的化学计量比为 1:1（等摩尔比），其形成有利于二聚体而不是单体，尤其是具有短亚甲基间隔基的二聚体形成“夹心型配合物”，其中碘化物靠近两个吡啶鎓环。低极性溶剂有利于配合物的形成，而水的存在会破坏配合物。实验研究辅以量子力学（QM）计算的理论研究，使用密度泛函理论（DFT）和分子力学（MM）模拟分子动力学，以确定乙腈溶液中最稳定的构象。通过顺序 QM/MM 混合方法计算电子激发，显示出与实验获得的紫外-可见吸收光谱一致的电荷转移波长。这证实了“夹心型配合物”构象有利于碘化物与一个吡啶鎓环相互作用，同时与烷基链形成一个或两个氢键，此外，对于二聚体，还与另一个吡啶鎓环形成氢键。
