Determination of the binding constants and ionic mobilities of diquat complexes with randomly sulfated cyclodextrins by affinity capillary electrophoresis.

The enantiomers of diquats (DQs), a new class of functional organic molecules, were recently separated by capillary electrophoresis (CE) with high resolution up to 11.4 within 5-7 min using randomly sulfated α-, ß-, and γ-cyclodextrins (CDs) as chiral selectors. These results indicated strong interactions between dicationic diquats and multiply negatively charged sulfated CDs (S-CDs). However, the binding strength of these interactions was not quantified. For that reason, in this study, affinity CE was applied for the determination of the binding constants and ionic mobilities of the complexes of DQ P- and M-enantiomers with CD chiral selectors in an aqueous medium. The non-covalent interactions of 10 pairs of DQ enantiomers with the above CDs were investigated in a background electrolyte (BGE) composed of 22 mM NaOH, 35 mM H3PO4, pH 2.5, and 0.0-1.0 mM concentrations of CDs. The average apparent binding constant and the average actual ionic mobility of the DQ-CD complexes were determined by nonlinear regression analysis of the dependence of the effective mobility of DQ enantiomers on the concentration of CDs in the BGE. The complexes were found to be relatively strong with the averaged apparent binding constants in the range 13 600-547 400 L/mol.

Regular Arrays of Rod-Shaped Molecular Photoswitches: Synthesis, Preparation, Characterization, and Selective Photoswitching within Mono- and Bilayer Systems.

We assembled photoresponsive mono- and bilayer systems with well-defined properties from rod-shaped molecules equipped with different photoswitches. Using properly chosen chromophores (diarylethene-based switch and unidirectional light-driven molecular motor), we then selectively targeted layers made of the same types of photoswitches using appropriate monochromatic light. UV-vis analysis confirmed smooth and unrestricted photoisomerization. To achieve this, we synthesized a new class of triptycene-based molecular pedestals adept at forming sturdy Langmuir-Blodgett films on a water-air interface. The films were smoothly transferred to gold and quartz surfaces. Repeated deposition afforded bilayer systems: one layer containing diarylethene-based photoswitches and the other a unidirectional light-driven molecular motor. Structural analysis of both mono- and bilayer systems revealed the molecules to be tilted with carboxylic functions pointing to the surface. At least two different polymorphs differing in monolayer thickness and tilt angle (~40° and ~60°) were identified on the gold surface.

The separation of the enantiomers of diquats by capillary electrophoresis using randomly sulfated cyclodextrins as chiral selectors.

Diquats, derivatives of the widely used herbicide diquat, represent a new class of functional organic molecules. A combination of their special electrochemical properties and axial chirality could potentially result in their important applications in supramolecular chemistry, chiral catalysis, and chiral analysis. However, prior to their practical applications, the diquats have to be prepared in enantiomerically pure forms and the enantiomeric purity of their P- and M-isomers has to be checked. Hence, a chiral capillary electrophoresis (CE) method has been developed and applied for separation of P- and M-enantiomers of 11 new diquats. Fast and better than baseline CE separations of enantiomers of all 11 diquats within a short time 5-7 min were achieved using acidic buffer, 22 mM NaOH, 35 mM H3 PO4 , pH 2.5, as a background electrolyte, and 6 mM randomly sulfated α-, ß-, and γ-cyclodextrins as chiral selectors. The most successful selector was sulfated γ-cyclodextrin, which baseline separated the enantiomers of all 11 diquats, followed by sulfated ß-cyclodextrin and sulfated α-cyclodextrin, which baseline separated enantiomers of 10 and nine diquats, respectively. Using this method, a high enantiopurity degree of the isolated P- and M-enantiomers of three diquats with a defined absolute configuration was confirmed and their migration order was identified.

Determination of binding constants of multiple charged cyclodextrin complexes by ACE using uncorrected and ionic strength corrected actual mobilities of the species involved.

In this study, the apparent binding constants and limiting mobilities of the multiply charged complexes of the Δ- and Λ-enantiomers of Ru(II)- and Fe(II)-polypyridyl associates ([Ru(2,2'-bipyridine)3 ]2+ , [Ru(1,10-phenanthroline)3 ]2+ , and [Fe(1,10-phenanthroline)3 ]2+ ) with single-isomer 2,3-diacetylated-6-sulfated-cyclodextrins (CDs) (12Ac-6S-α-CD, 14Ac-7S-ß-CD, and 16Ac-8S-γ-CD) were determined by ACE using uncorrected and ionic strength corrected actual mobilities of the species involved. Two limiting models were tested for the ionic strength correction of the actual mobilities based on an empirical relation for the ionic strength correction of multivalent ionic species. In model 1, the nominal values of the charge numbers (zS,nom ) and analytical concentrations (cS,nom ) of the above CD selectors in the BGEs were applied for calculation of the BGE ionic strength, as usual. In model 2, the CD selectors were considered as singly charged species (zS  = -1) with |zS,nom |-times higher concentrations in the BGE than their analytical concentrations (cS  = |zS,nom | × cS,nom ) in the calculation of the BGE ionic strength. In all three cases-with uncorrected actual mobilities as well as with actual mobilities corrected according to the two limiting models-the measured effective mobilities of the above enantiomers fit well the theoretical curves of their mobility dependences on the CD selectors concentrations in the BGE, with high average coefficients of determination (R2  = 0.9890-0.9995). Nevertheless, the best physico-chemically meaningful values of the apparent binding constants and the limiting mobilities of the enantiomer-CDs complexes with low RSDs were obtained using the actual mobilities of the species involved corrected according to model 2.

Diquats with Robust Chirality: Facile Resolution, Synthesis of Chiral Dyes, and Application as Selectors in Chiral Analysis.

Diquats with extremely high racemization barriers with ΔG≠theor of 233 kJ mol-1 at 180 °C are described. Reported configurational robustness is due to a combination of two structural features: the rigid o-xylylene tether connecting the nitrogen atoms and the presence of two substituents in the bay region of the bipyridinium scaffold. The straightforward synthesis of diquats, plus facile resolution and derivatization make them attractive for chiral application studies. This is demonstrated by: 1) synthesis of the first non-racemic diquat dyes with pronounced chiroptical properties, and 2) capability of diquats to interact stereospecifically with chiral molecules. This suggests potential for diquat derivatives to be used as chiral selectors in separation methods.

Dutch Resolution of a configurationally stable [5]helquat.

Synthesis and nontrivial optical resolution of a helicene-like dication, helquat 1, has been accomplished. Starting with gram scale of the racemic helquat 1 sample, Dutch Resolution using family of 3 tartrate anions was key to achieve successful separation of M and P helical enantiomers of 1. Hundreds of milligrams of each enantiomer of this configurationally stable C2 -symmetric helquat have been obtained. Racemization barrier of 1 has been determined. To our knowledge this is the first report on Dutch Resolution performed with a helicene-like compound. Moreover, there are no literature precedents for Dutch Resolution of chiral quaternary ammonium cations.

Tunable Chiral Second-Order Nonlinear Optical Chromophores Based on Helquat Dications.

Fourteen new dipolar cations have been synthesized, containing methoxy or tertiary amino electron donor groups attached to helquat (Hq) acceptors. These Hq derivatives have been characterized as their TfO- salts by using various techniques including NMR and electronic absorption spectroscopies. UV-vis spectra show intense, relatively low energy absorptions with λmax ≈ 400-600 nm, attributable to intramolecular charge-transfer (ICT) excitations. Single-crystal X-ray structures have been solved for two of the chromophores, one as its PF6- salt, revealing centrosymmetric packing arrangements (space groups Pbca and P1̅). Molecular quadratic nonlinear optical (NLO) responses have been determined directly by using hyper-Rayleigh scattering (HRS) with a 800 nm laser, and indirectly via Stark (electroabsorption) spectroscopy for the low energy absorption bands. The obtained static first hyperpolarizabilities ß0 range from moderate to large: (9-140) × 10-30 esu from HRS in MeCN and (44-580) × 10-30 esu from the Stark data in PrCN. The magnitude of ß0 increases upon either extending the π-conjugation length or replacing a methoxy with a tertiary amino electron donor substituent. Density functional theory (DFT) and time-dependent DFT calculations on selected tertiary amino chromophores confirm that the low energy absorptions have ICT character. Relatively good agreement between the simulated and experimental UV-vis absorption spectra is achieved by using the CAM-B3LYP functional with the 6-311G(d) basis set. The ßtot values predicted by using DFT at the same level of theory are large ((472-1443) × 10-30 esu in MeCN). Both the theoretical and experimental results show that para-conjugation between Hq and electron donor fragments is optimal, and enlarging the Hq unit is inconsequential with respect to the molecular quadratic NLO response.

Chiral analysis of α-diimine Ru(II) and Fe(II) complexes by capillary electrophoresis using sulfated cyclodextrins as stereoselectors.

CE using randomly highly sulfated α-, ß-, and γ-CDs (S-α-CD, S-ß-CD, S-γ-CD), sulfobutylether-ß-CD (SBE-ß-CD), single isomer (6-O-sulfo) α-, ß-, and γ-CDs, and their derivatives as stereoselectors was applied to chiral analysis of polypyridyl complexes of [Ru(bpy)3 ]2+ , [Ru(phen)3 ]2+ , and [Fe(phen)3 ]2+ (bpy = 2,2'-bipyridine; phen = 1,10 phenanthroline). The best separations of Δ- and Λ-enantiomers of the these complexes with high resolution (up to R1,2  = 7.0) and short analysis times (10-20 min) were achieved in the BGE composed of 22 mM NaOH/35 mM H3 PO4 , pH 2.4, containing 1.5-6.0 mM S-α-CD or S-ß-CD, or SBE-ß-CD as chiral selectors. The developed method was applied to the assessment of enantiomeric purity of several samples of [Ru(bpy)3 ]2+ catalyst. CE experiments were performed in a homemade analyzer equipped with bare or hydroxypropylcellulose-coated fused-silica capillaries (total/effective length 40/29 cm, id/od 50/375 µm) and an UV absorption detector operating at 206 nm. In addition to chiral analysis, apparent binding constants of the complexes of [Ru(bpy)3 ]2+ , [Ru(phen)3 ]2+ , and [Fe(phen)3 ]2+ enantiomers with five sulfated CDs (S-α-CD, S-ß-CD, S-γ-CD, SBE-ß-CD, and 16Me-8S-γ-CD) were determined from the dependence of their effective electrophoretic mobilities on the concentration of the CDs in the BGE by nonlinear regression analysis. Calculated apparent binding constants of these complexes were found to be in the (1.10-4.66) × 103  L/mol range. Moreover, it was shown that at selected concentrations of some S-CDs and suppressed or very low electroosmotic flow, the exceptional enantioseparations with infinite resolution could be achieved.

Helquat Dyes: Helicene-like Push-Pull Systems with Large Second-Order Nonlinear Optical Responses.

Helquat dyes combine a cationic hemicyanine with a helicene-like motif to form a new blueprint for chiral systems with large and tunable nonlinear optical (NLO) properties. We report a series of such species with characterization, including determination of static first hyperpolarizabilities ß0 via hyper-Rayleigh scattering and Stark spectroscopy. The measured ß0 values are similar to or substantially larger than that of the commercial chromophore E-4'-(dimethylamino)-N-methyl-4-stilbazolium. Density functional theory (DFT) and time-dependent DFT calculations on two of the new cations are used to probe their molecular electronic structures and optical properties. Related molecules are expected to show bulk second-order NLO effects in even nonpolar media, overcoming a key challenge in developing useful materials.

Single-Crystal-to-Single-Crystal Transition in an Enantiopure [7]Helquat Salt: The First Observation of a Reversible Phase Transition in a Helicene-Like Compound.

Here it is reported that crystals of an enantiopure [7]helquat salt undergo reversible thermal solid-solid phase transition at 404 K. Differential scanning calorimetry (DSC), capillary electrophoresis (CE), and X-ray diffraction analysis were used to unravel the mechanistic details of this process. The single-crystal-to-single-crystal course enabled direct monitoring of the structural changes by in situ variable-temperature X-ray diffraction, thus providing the first direct evidence of a solid phase transition in a helicene-like compound.

Functional helquats: helical cationic dyes with marked, switchable chiroptical properties in the visible region.

Helquat dyes are the first helicene-like cationic styryl dyes obtained as separate enantiomers. Their remarkable chiroptical properties are due to the unique combination of a cationic hemicyanine chromophore and a helicene-like motif. The magnitude of the ECD response and the pH switching along with their positioning in the visible region are unprecedented among helicenoids.

Search for conglomerate in set of [7]helquat salts: multigram resolution of helicene-viologen hybrid by preferential crystallization.

Investigation of a set of 12 [7]helquat salts by X-ray crystal diffraction led to identification of conglomerate behavior in bis(trifluoroacetate) salt [2][CF(3)CO(2)](2). This is to demonstrate that a systematic search for conglomerates can be performed for a given helicenoid enabling straightforward multigram resolution via preferential crystallization. Subsequently, preferential crystallization of this chiral helicene-viologen hybrid has been established to obtain pure P and M enantiomers on a multigram scale, 5 g each. Furthermore, preparation of nonracemic samples of [7]helquat 2 via diastereomeric (R,R)-dibenzoyltartrate salts is described, and determination of absolute configuration and racemization barrier is also reported.

Helquat-induced chiroselective aggregation of Au NPs.

Au nanoparticles (NPs) are functionalized with chiral (R) or (S) binaphthol phenylboronic acid ligands, (1a) or (1b). The (R)- or (S)-binaphthol phenylboronic acid ligands form donor-acceptor complexes with the chiral dicationic helicene, helquat (P)-HQ(2+) or (M)-HQ(2+), (2a) or (2b). The association constants between (1a)/(2a) and (1a)/(2b) correspond to (7.0 ± 0.5) × 10(5) M(-1) and (2.5 ± 0.3) × 10(5) M(-1), respectively, whereas the association constants between (1b)/(2b) and (1b)/(2a) correspond to (4.0 ± 0.5) × 10(5) M(-1) and (1.8 ± 0.3) × 10(5) M(-1), respectively. Chiroselective aggregation of chiral binaphthol phenylboronic acid-capped Au NPs triggered by the chiral helquats, is demonstrated.

Chiral analysis of helquats by capillary electrophoresis: resolution of helical N-heteroaromatic dications using randomly sulfated cyclodextrins.

Enantiomers of helical N-heteroaromatic dications, helquats, were separated by CE. An acidic 22/35 mM sodium/phosphate background electrolyte, pH 2.4, with addition of randomly sulfated α-, ß- and γ- cyclodextrins allowed enantioresolution of a series of helquats, which comprised 5, 6 and 7 fused rings participating in the helical backbone. In general, at least one of the chiral selectors was found to provide baseline separation for 22 out of 24 helquats and partial separation for the remaining two. Individually, the sulfated γ-cyclodextrin turned out to separate 79% of the helquats, followed by the ß- and α-congeners with 54 and 42% of the resolved compounds, respectively. Migration order of enantiomers was inspected for selected helquats and a relation of molecular size of the analytes to a cavity of the cyclodextrin selectors is discussed.

Helquats, helical extended diquats, as fast electron transfer systems.

Helicene-viologen structural hybrids, like [5]helquat, 6,7,10,11-tetrahydrodipyrido[2,1-a:1',2'-k][2,9]phenanthrolinediium, and its four methylated derivatives, are characterized by electrochemical admittance and EPR spectroscopy. All compounds are reversibly reduced in two one-electron steps. Formal redox potentials correlate with the calculated LUMO energies. The electron transfer is coupled with a weak adsorption of the reactants. The analysis of the frequency dependence of the electrode admittance is used for the separation of Faradaic and double layer contributions and finally to the estimation of heterogeneous rate constants. Heterogeneous rate constants determined this way are in the range 0.1 to 3 cm s(-1). In all cases the second electron transfer is faster than the first redox step by a factor of three. The Frumkin correction for the acceleration by the double layer potential further amplifies this difference. The heterogeneous rate constants of derivatives correlate with the solvent reorganization energy estimated from the Marcus model. EPR spectra confirm the radical cation formation. The radical of [5]helquat participates in an extremely fast self-exchange process with the parent dication characterized by the self-exchange rate constant k(ET) = (2.4 +/- 0.5) x 10(9) M(-1) s(-1).