Spectrophotometric studies of charge-transfer complexes formed with ions N,N'-alkyldiyl-bis(pyridinium) derivatives and iodide.

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 (KCTC), molar absorptivity (εCTC), fluorescence-quenching constant (KSV) 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.

pKa determinations of xanthene derivates in aqueous solutions by multivariate analysis applied to UV-Vis spectrophotometric data.

Xanthenes form to an important class of dyes which are widely used. Most of them present three acid-base groups: two phenolic sites and one carboxylic site. Therefore, the pKa determination and the attribution of each group to the corresponding pKa value is a very important feature. Attempts to obtain reliable pKa through the potentiometry titration and the electronic absorption spectrophotometry using the first and second orders derivative failed. Due to the close pKa values allied to strong UV-Vis spectral overlap, multivariate analysis, a powerful chemometric method, is applied in this work. The determination was performed for eosin Y, erythrosin B, and bengal rose B, and also for other synthesized derivatives such as 2-(3,6-dihydroxy-9-acridinyl) benzoic acid, 2,4,5,7-tetranitrofluorescein, eosin methyl ester, and erythrosin methyl ester in water. These last two compounds (esters) permitted to attribute the pKa of the phenolic group, which is not easily recognizable for some investigated dyes. Besides the pKa determination, the chemometry allowed for estimating the electronic spectrum of some prevalent protolytic species and the substituents effects evaluation.