Association Constants In Solution Research Articles

Supramolecular complex of a fused zinc phthalocyanine-zinc porphyrin dyad assembled by two imidazole-C60 units: ultrafast photoevents.

A new zinc phthalocyanine-zinc porphyrin dyad (ZnPc-ZnP) fused through a pyrazine ring has been synthesized as a receptor for imidazole-substituted C60 (C60Im) electron acceptor. Self-assembly via metal-ligand axial coordination and the pertinent association constants in solution were determined by 1H-NMR, UV-Vis and fluorescence titration experiments at room temperature. The designed host was able to bind up to two C60Im electron acceptor guest molecules to yield C60Im:ZnPc-ZnP:ImC60 donor-acceptor supramolecular complex. The spectral data showed that the two binding sites behave independently with binding constants similar in magnitude. Steady-state fluorescence studies were indicative of an efficient singlet-singlet energy transfer from zinc porphyrin to zinc phthalocyanine within the fused dyad. Accordingly, the transient absorption studies covering a wide timescale of femto-to-milli seconds revealed ultrafast energy transfer from 1ZnP* to ZnPc (kEnT ∼ 1012 s-1) in the fused dyad. Further, a photo induced electron transfer was observed in the supramolecularly assembled C60Im:ZnPc-ZnP:ImC60 donor-acceptor complex leading to charge separated states, which persisted for about 200 ns.

Conjugated Porphyrin Dimers: Cooperative Effects and Electronic Communication in Supramolecular Ensembles with C60.

Two new conjugated porphyrin-based systems (dimers 3 and 4) endowed with suitable crown ethers have been synthesized as receptors for a fullerene-ammonium salt derivative (1). Association constants in solution have been determined by UV–vis titration experiments in CH2Cl2 at room temperature. The designed hosts are able to associate up to two fullerene-based guest molecules and present association constants as high as ∼5 × 108 M–1. Calculation of the allosteric cooperative factor α for supramolecular complexes [3·12] and [4·12] showed a negative cooperative effect in both cases. The interactions accounting for the formation of the associates are based, first, on the complementary ammonium-crown ether interaction and, second, on the π–π interactions between the porphyrin rings and the C60 moieties. Theoretical calculations have evidenced a significant decrease of the electron density in the porphyrin dimers 3 and 4 upon complexation of the first C60 molecule, in good agreement with the negative cooperativity found in these systems. This negative effect is partially compensated by the stabilizing C60–C60 interactions that take place in the more stable syn-disposition of [4·12].

Advantages of organic halogen bonding for halide recognition

The synthesis of a bidentate halogen bonding receptor and a nearly isostructural hydrogen bonding analogue is described. Crystal structures reveal the interactions of each receptor with anions in the solid state, while NMR titrations elucidate bidentate binding and association constants in solution. Of the two, the halogen bonding receptor demonstrates stronger, water resistant halide binding in competitive solvents.

Charge-displacement analysis as a tool to study chalcogen bonded adducts and predict their association constants in solution.

The secondary interaction between a polarized chalcogen atom and different Lewis bases, either anionic or neutral, has been studied by charge displacement analysis. Using charge displacement analysis, the charge rearrangement in the adduct upon the formation of the interaction has been quantified and described in great detail. By comparing the theoretical results with the experimental association constants, two linear correlations can be found for anionic and neutral bases. Such correlations can be used to reliably predict the association constants of adducts for which experimental data are not available yet.

Solvent‐Dependent Truxene‐Based Nanostructures

Three truxene derivatives functionalized with alkyl chains, either attached directly or distanced by linking phenyl or ethynyl groups, self-assemble in solution and induce the gelation of different solvents in spite of not being endowed with groups able to establish strong directional interactions. A (1)H NMR study points to face-to-face alternating π-stacked motifs at the origin of nucleation. Solvents play an important role in modulating the aggregation of these derivatives giving rise to fibrous or spherical superstructures. Analysis of the influence of different solvents on the morphology of the aggregates provides a better understanding of the various stages of the hierarchical self-assembly. The way in which alkyl chains are attached to the central core also strongly affects the self-assembling properties and gelation ability of this series. Phenyl spacers present the highest association constants in solution and give rise to gelation in a broader range of solvents. This behavior has been rationalized by means of (1)H NMR spectroscopy, X-ray powder diffraction, SEM, and photophysical measurements. Interestingly, it was found that these compounds in the gel state exhibit unusual emission properties most likely arising from the formation of excimers, which evidences that π-π interactions also occur in the excited state.

Correlation between Hydrogen Bonding Association Constants in Solution with Quantum Chemistry Indexes: The Case of Successive Association between Reduced Species of Quinones and Methanol

The functional M05-2X together with the SMD solvent model have been used to calculate hydrogen bonding association constants in dimethylsulfoxide (DMSO) solution. Data of equilibrium constants in DMSO for the case of electrochemically generated dianions interacting with methanol have been considered to test the reliability of the chemistry theoretical approach. From this approach, it was found that the successive association constants involved in the formation of the complexes depend on a linear combination of three quantum chemistry indexes which are the ionization energy, the electron affinity, and the charge on the oxygen atom receiving the methanol molecule. Under this perspective, the stoichiometry of all the dianion-methanol complexes was explained on the basis of the relative strength of the hydrogen bonding interaction compared to that of the methanol-DMSO and methanol dimer complexes. This linear combination seems to be valid regardless of the nature of the dianion structure and the number of methanol molecules in the complex, which is a relevant finding to generalize the applicability of both the functional M05-2X and the SMD solvent model, to calculate association constants for any other neutral or anionic molecules interacting by hydrogen bonding with proton donors.

Anthracene-9,10-diones as potential anticancer agents. Synthesis, DNA-binding, and biological studies on a series of 2,6-disubstituted derivatives.

A series of 2,6-bis(omega-aminoalkanamido)anthracene-9,10-diones (9,10-anthraquinones), of general formula Ar(NHCO(CH2)nNR2)2, where Ar = anthracene-9,10-dione and n = 1 or 2, have been synthesized by treatment of the corresponding bis(omega-haloalkanamido) derivatives with appropriate secondary amines. The DNA-binding properties of these compounds were evaluated by thermal denaturation studies, unwinding of closed-circular DNA, determination of association constants in solution, and examined by molecular modeling. A representative compound in the series has been examined by X-ray crystallography. In vitro cytotoxicity data is reported for the compounds and some indications of structure-activity relationships have been discerned. In particular, those compounds with two methylene links (n = 2) in each side chain separating the amide and terminal amine moieties have superior activity and, in general, enhanced DNA binding characteristics. It is postulated that the mode of reversible binding of these compounds to DNA involves the side chains occupying both major and minor grooves and, further, that this may confer cytotoxic properties which are distinct from those of previously reported anthracene-9,10-dione cytotoxins.

The Evaluation of Thermodynamic Association Constants in Solutions with an Application to Molten Salt Solutions

The evaluation of thermodynamic association constants in solution is discussed and is applied to molten salt systems. The calculated association constants K/sunb 1/, K/sub 2/, and K/sup 12/ for the formation of the species AgCl, AgCl/sub 2//bar/, and Ag/sub 2/Cl, in molten KNO/sub 3/ at 385 deg are 460 plus or minus l5, l69 plus or minus 20, and 20 plus or minus 10 (moles/mole KNO/sub 3/)/sup -1/, respectively. A recalculation of the association constants, K/sub 1/, for the species CdCl/sup +/ and PbCl/sup +/ from cryoscopic, polarographic and from solubility data in nitrate meits incidates that the greater part of the very large discrepancies in the published values of K/sub 1/ is a result of the incorrect analysis of the data. (auth)