From red to blue shift: switching the binding affinity from the acceptor to the donor end by increasing the π-bridge in push–pull chromophores with coordinative ends

A series of homologous push–pull compounds, in which identical donor (a dimethylamino) and acceptor (a malonate ester) functionalities endcap crescent PPV fragments, exhibit striking differences in their supramolecular recognition of cations acting as Lewis acids. The shorter conjugated compound (one phenyl ring) coordinates a wide variety of lanthanide cations (Eu3+, Yb3+ and Er3+) in MeCN solutions to the 1,3-dicarbonyl acceptor end, resulting in an overall supramolecular polarization of the system (red shift of the intramolecular charge-transfer ICT band). With the “hard” cation Sc3+, recognition switches to the tertiary amine donor end, turning the conjugated system from D–π–A to A–π–A, and resulting in a blue shift of the ICT band upon complexation. Interestingly, increasing the conjugation by means of the insertion of sequential p-phenylenevinylene units into the ligand results in coordination to the donor end regardless of cation “hardness” (Sc3+, Eu3+ and Er3+), suggesting a relative change in the nucleophilicity of the two coordinating ends when increasing the length of the conjugated π-bridge. Such a hypothesis is supported by quantum chemical calculations on the ligands and subsequent atomic charges determination using two independent approaches (QTAIM and CHelpG). The characterization of the thermodynamic stabilities and the dimensionalities of the ligand–cation complexes in solution reveals striking differences from case to case, yet increasing affinities (from log Kav = 2.5 to log Kav = 4.9) are recorded with the increase of the π-conjugated bridge

A chiroptical molecular sensor for ferrocene

A chiral molecular sensor is used to recognize ferrocene, with the chiroptical readout used selectively in the presence of competing analytes

Kinetics and Mechanism of Chromate Reduction with Hydrogen Peroxide in Base

CrVIO4]2- is reduced to CrV(O2)4]3- by hydrogen peroxide in strongly basic media where the acid dissociation of H2O2 (pKa = 11.65) is appreciable. The reaction is first order in chromium(VI) and inhibited by hydroxide. The hydrogen peroxide dependence is defined by the form of the effective pseudo-first-order rate constant:? keff = H2O2]3/(K1 + K2H2O2] + K3HO2-]) with K1 = 175(43) s·M3, K2 = 403(18) s·M2, and K3 = 1422(34) s·M2. Hydrogen peroxide anion initially attacks chromate, and subsequent equilibrium steps that exchange oxo groups for three peroxo groups precede a rate-determining, one-electron, intramolecular reduction step. CrVIO4]2- is reduced to CrV(O2)4]3- by hydrogen peroxide in strongly basic media where the acid dissociation of H2O2 (pKa = 11.65) is appreciable. The reaction is first order in chromium(VI) and inhibited by hydroxide. The hydrogen peroxide dependence is defined by the form of the effective pseudo-first-order rate constant:? keff = H2O2]3/(K1 + K2H2O2] + K3HO2-]) with K1 = 175(43) s·M3, K2 = 403(18) s·M2, and K3 = 1422(34) s·M2. Hydrogen peroxide anion initially attacks chromate, and subsequent equilibrium steps that exchange oxo groups for three peroxo groups precede a rate-determining, one-electron, intramolecular reduction step

Dynamic Switching between Binding Sites in the Complexation of Macrocyclic "Push-Pull" Chromophores to Lanthanides

The introduction of a 1,3-diacetylpyridine moiety, as an additional binding niche in a macrocyclic receptor ligand containing a conjugated, push–pull malonate functionality, leads to, in addition to the 1:1 binding stoichiometry, the equilibrium formation of multiple complexes, specifically 1:2, 2:1, and 3:1 ligand:metal complexes with lanthanide trications. Various binding modes elicit distinctive responses in the visible region, which enhance the potential recognition of lanthanide ions. This paper also highlights the usefulness of a full factor analysis in the elucidation of complex binding phenomena

Modeling Methylene Blue Aggregation in Acidic Solution to the Limits of Factor Analysis

Methylene blue (MB⁺), a common cationic thiazine dye, aggregates in acidic solutions. Absorbance data for equilibrated solutions of the chloride salt were analyzed over a concentration range of 1.0 × 10^–3 to 2.6 × 10^–5 M, in both 0.1 M HCl and 0.1 M HNO₃. Factor analyses of the raw absorbance data sets (categorically a better choice than effective absorbance) definitively show there are at least three distinct molecular absorbers regardless of acid type. A model with monomer, dimer, and trimer works well, but extensive testing has resulted in several other good models, some with higher order aggregates and some with chloride anions. Good models were frequently indistinguishable from each other by quality of fit or reasonability of molar absorptivity curves. The modeling of simulated data sets demonstrates the cases and degrees to which signal noise in the original data obscure the true model. In particular, the more mathematically similar (less orthogonal) the molar absorptivity curves of the chemical species in a model are, the less signal noise it takes to obscure the true model from other potentially good models. Unfortunately, the molar absorptivity curves in dye aggregation systems like that of methylene blue tend to be sufficiently similar so as to lead to the obscuration of models even at the noise levels (0.0001 ABS) of typical benchtop spectrophotometers

Electroactive Anion Receptor with High Affinity for Arsenate

Herein, we present the synthesis and characterization of a macrocyclic polyamide cage that incorporates redox-active 1,4-dithiin units. UV/vis titration experiments with eight anions in acetonitrile revealed high affinity for H2AsO4- (log β2 = 10.4-0.4+0.4) and HCO3- (log β2 = 8.3-0.4+0.3) over other common anionic guests, such as Cl- (log K1:1 = 3.20-0.02+0.03), HSO4- (log K1:1 = 3.57-0.03+0.02), and H2PO4- (log K1:1 = 4.24-0.04+0.05), by the selective formation of HG2 complexes. The recognition of arsenate over phosphate is rare among both proteins and synthetic receptors, and though the origin of selectivity is not known, exploiting the difference in the binding stoichiometry represents an underexplored avenue toward developing receptors that can differentiate between the two anions. Additional analysis by 1H NMR in 1:3 CD2Cl2/MeCN-d3 found a strong dependence of anion binding stoichiometry with the solvent employed. Finally, titration experiments with cyclic voltammetry provided varying and complex responses for each anion tested, though reaction between the anion and receptors was observed in most cases. These results implicate 1,4-dithiins as interesting recognition moieties in the construction of supramolecular receptors.National Science Foundation (Grant DC-2004005)U.S. Army Engineer Research and Development Center (Contract W912HZ-17-2-0027

Nesting complexation of C60 with large, rigid D2 symmetrical macrocycles

A series of four chiral D2 symmetrical macrocycles, in which two 3,3¢-disubstituted Binol units are bridged by conjugated organic spacers of differing lengths and/or electronic properties, have been synthesized and characterized. The four different bridges consist of either ether or ester linkages in combination with either short biphenyl spacers or long diethynylphenyl spacers. NMR, CD spectroscopy, and molecular modeling help rationalize the shape of the cyclic scaffolds and even subtle modifications in the bridging units lead to drastic changes in conformation. The three macrocycles with longer bridging units and/or ester linkages form stable 1 : 1 complexes with C60 in toluene. The one with a short spacer and ether linkage does not. The binding constants have been determined with a high degree of accuracy via equilibrium-restricted factor analysis; with long spacers and ester linkages log Ka = 4.37(2); with short spacers and ester linkages log Ka = 3.498(4); with long spacers and ether linkages log Ka = 3.509(2)