Cyclodextrins and surfactants as modifiers of the chemiluminiscence of phthalhydrazide derivates

Resumen

The production of light during the course of a chemical reaction is known as chemiluminescence (CL). This property is own by, for example, phthalhydrazide (PHY) derivatives, whose alkaline oxidation either in water or organic solvents generates blue light. For this investigation we have focused on a family of PHY derivatives that can produce CL with good yields in both media: luminol (LUM), isoluminol (ISOL), N-(4-aminobutyl)-N-ethylisoluminol (ABEI) and N-(6-aminohexyl)-N-ethylisoluminol (AHEI). The interest for these compounds lies on their applications in diverse fields such as molecular biology, chemical analysis and environmental chemistry, among others. Certain molecules like cyclodextrins (CDs) and surfactants are known to modify some physical-chemical properties of the molecules they interact with. This fact represents the starting hypothesis for the aim of this research: to find out which of these media is the most suitable for increasing the intensity and duration of the CL emission and the mechanisms involved in each case. This Thesis has centred on the alkaline oxidation in aqueous media of these PHY derivatives with hydrogen peroxide (H2O2) and different catalysts, but specially cobalt (II). The natural oligosaccharides Alfa, Beta and Gamma-CD are more efficient boosters of the CL than their derivatives. The highest enhancement in the CL intensity of LUM and ISOL are achieved with Beta-CD (x16), whereas Gamma-CD is which produces the highest increases of the CL yields of ABEI and AHEI (x15). Studies of fluorescence, nuclear magnetic resonance (1H-NMR) and rotating frame Overhauser effect spectroscopy (ROESY) prove the formation of supramolecular associations between the CDs, the luminophores, and also their intermediates. In the case of LUM, its luminescent intermediate, 3-aminophthalate (3-AP), interacts with the CDs by the primary rim and this binding is related to the improvement of the CL yield. With ABEI and AHEI, the three dimensional structures of the 1:1 inclusion complexes proves that a major inclusion of the heterocyclic ring of these molecules implies a higher raise in the CL intensity. Unlike the CDs, the incorporation of three kinds of typical surfactants, sodium dodecyl sulphate (SDS), bromide cetyltrimethylammonium (CTAB) and pentaethylene glycol monododecyl ether (C12E5), either diminishes or does not improve the CL of this family of compounds. The non-substituted luminophores have poor affinity for the micelles in the case of SDS and C12E5, whereas the CTAB micelles incorporate the luminophores, but the charge compensation hinders the CL reaction and decreases the overall light emitted. The ionization constants (pKa) of natural CDs have been also studied by 1H-NMR and photon correlation spectroscopy. The secondary ¿OHs (position 2, 3) are the first in losing their protons, with pKa values between 13.7 and 15.0, while those of the primary rim (position 6) are ionized at pKa close to 15.5, measured in D2O. Dynamic light scattering (DLS) reveals that the ionization of the secondary belt of ¿OHs breaks the aggregates of b¿CD with a concomitant reduction in their size until its disappearance at pH above the pKa.