Chiral Dye Research Articles

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.

Circularly polarized laser emission induced in isotropic and achiral dye systems.

The production of efficient, tunable, and switchable circularly polarized laser emission would have far reaching implications in optical communications or biophotonics. In this work, it is demonstrated the direct generation of circularly polarized (CP) laser emission in achiral and isotropic dye laser systems without the use of extracavity polarizing elements, and without resorting to chiral dyes, chiral liquid crystal matrices, or interferometric methods. The origin of this ellipticity arises from the dynamic birefringence induced by the strong and polarized laser pumping and the subsequent orientation anisotropy of the excited molecular dipoles. A complete polarimetric characterization of the polarization state of conventional dye laser oscillators as a function of different experimental parameters is performed and it is shown that the generated light always possesses a certain level of circularity that changes in a distinctive way with pump energy and polarization. These results demonstrate that it is possible to generate and modulate CP laser light from efficient and photostable conventional laser dyes.

Synthesis of Binaphthyl-Based Push-Pull Chromophores with Supramolecularly Polarizable Acceptor Ends

We report on the design and synthesis of new enantiopure binaphthyl derivatives in which electron-donating and electron-withdrawing substituents are placed in direct conjugation, to create push-pull dyes potentially active for NLO applications. The dyes, unprecedentedly, extend theirπ-bridge from the 3,3′ positions of the binaphthyl units and incorporate as acceptors 1,3-dicarbonyl and tetrafluorobenzene units, useful for further supramolecular polarization of the chiral dyes.

Silica Colloidal Membranes with Enantioselective Permeability

AbstractRobust mesoporous membranes composed of silica spheres were surface‐modified with chiral selector moieties, including small molecules, macrocycles, and polymers. Diffusion rates of enantiomers of a chiral dye through the resulting asymmetrically modified colloidal membranes were measured and the corresponding permselectivities were calculated. The membranes showed enantioselectivities in the range of 1.2–1.8, which were not significantly affected by the structure of the surface‐immobilized chiral electors. This selectivity is on par with most reported polymer‐based solid membranes and bulk liquid membranes. The enantioselectivity results from the surface‐facilitated mechanism of transport of enantiomers through the mesopores.

Synthesis and Stability of N,N′‐Dialkyl‐1,13‐dimethoxyquinacridinium (DMQA+): A [4]Helicene with Multiple Redox States

AbstractN,N′‐Dialkyl‐1,13‐dimethoxyquinacridinium (DMQA+) is a configurationally locked [4]helicene, a stable carbenium ion and a potent near‐infrared emitter. The versatile synthesis and resolution into M and P enantiomers has made DMQA+ and its congener helicenium derivatives attractive chiral dyes and building blocks. In the present study, electrochemical and spectroscopic studies showed that several redox states of DMQA+ are accessible and stable. The cation stability towards nucleophiles is determined and it is shown that the addition of OH− proceeds with a surprisingly low rate of reaction. The synthetic methodologies leading to DMQA+ are described, and the chemistry occurring from the different redox states of DMQA is identified. This [4]helicene is shown to be a unique molecular framework that is stable as a neutral radical and very stable as a carbenium ion.

Helical Assembly Induced by Hydrogen Bonding from Chiral Carboxylic Acids Based on Perylene Bisimides

The control over self-assembly behavior becomes absolutely critical because it is dependent on the orientation and morphology. The motivation is focused on borrowing the help of O-H···O hydrogen bonding interactions to realize the control in chiral self-assembly. A series of perylene bisimide (PBI) dyes 3a-3d bearing chiral amino acid derivatives on the imide N atoms and four phenoxy-type substituents at the bay positions of the perylene core were synthesized. Optical properties and aggregation behavior of PBIs were investigated by absorption, fluorescence, circular dichroism (CD), and (1)H NMR spectroscopy. Except for the chiral ester 3c and achiral 3d, chiral dyes 3a and 3b show bisignated CD signals, indicating that the chiral carboxylic acid-functionalized PBI systems are found to be spontaneously self-assembled into supramolecular helices via intermolecular hydrogen bonding rather than π-π stacking. Furthermore, the chirality-controlled helical superstructures are strongly dependent on several factors, such as solvent polarity, concentration, and temperature. The supramolecular helical chirality can be well-controlled by the chiral amino acid residues in the PBI system; that is, the assembled clockwise (plus, P) or anticlockwise (minus, M) helices can be induced by L- or D-isomers, respectively.

Amphiphilic and chiral unsymmetrical perylene dye for solid-state dye-sensitized solar cells

A novel amphiphilic, unsymmetrically substituted N-((2S)-2-aminohexanoic acid)-N'-[(S)-1-phenylethyl]-3,4,9,10- perylenetetracarboxydiimide with 2 different chiral centers was synthesized and characterized. Chiral amphiphilicity, which can be tunable between hydrophilic and hydrophobic parts, is responsible for controlled self-assembly through intermolecular hydrogen bonding (N-H \cdots O). The product showed high thermal stability but partial solubility in common organic solvents. We observed 2 isosbestic points at 533 and 611 nm, confirming the presence of overlapped monomer and excimer emissions in the temperature range of 10-80 °C. Fluorescence quenching in m-cresol was attributed to charge-transfer interactions. Importantly, the excited state of the chiral dye can decay only by fluorescence in the solid state, mainly due to O-H \cdots N hydrogen bonds. Novel red-light-emitting (\lambda_{em} = 658 nm) chiral perylene dye has a great potential for solid-state lighting technologies.

Chiral Water‐Soluble Perylenediimides

AbstractFor the first time the synthesis of covalently linked very water‐soluble chiral perylenediimides (PDIs) is presented. The PDIs carry the amino acids alanine or lysine as imide substituents, respectively, which are coupled to Newkome‐type dendrimers acting as hydrophilic groups. The tert‐butyl‐protected precursors 5 as well as its water‐soluble free acid compounds 6 were investigated by absorption, fluorescence and circular dichroism (CD) spectroscopy. Except for the sterically most crowded 2nd generation alanine compounds 5f and 6f all chiral dyes show bisignate CD effects which indicate helical aggregation. The 1st generation alanine compound 6b shows the greatest CD effects in buffer solution and its aggregation behaviour was also directly proven by the use of cryogenic transmission electron microscopy. Furthermore, the formation of chiral superstructures of 6b was found to be pH‐ and concentration‐dependent. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Self‐Assembly of Tripodal Squaraines: Cation‐Assisted Expression of Molecular Chirality and Change from Spherical to Helical Morphology

Let's do the twist: Tripodal squaraines self-assemble from acetonitrile to form hollow spheres, the complexation of which with Ca2+ or Mg2+ results in extended networks. An analogous chiral dye exhibits a bisignate CD couplet and a helical morphology upon Ca2+ binding (see figure). Thus, the molecular chirality of a functional dye is expressed through specific cation binding and manifested in the form of supramolecular helicity.

Unique Contrast Patterns from Resonance-Enhanced Chiral SHG of Cell Membranes

Chirality can produce novel nonlinear optical effects that may form the basis for new imaging contrast agents. In this paper, we developed a new chiral chromophore 2, which is the dimer of a known voltage sensitive dye, monomer 1, with the chirality originating from the twisted orientation between two subunits. Racemic dimer and monomer 1 were used as the references to study the effect of chirality in SHG microscopy of live cells. All these dyes selectively stain the outer leaflets of cell membranes, producing strong resonance-enhanced SHG images. At the symmetric junction between two adherent cells, monomer or racemic dimer SHG is forbidden due to centrosymmetry, and indeed little SHG was observed (10 +/- 1% relative to nonjunction). When stained with the chiral dimer, the junction is no longer centrosymmetric and much stronger SHG was observed (39 +/- 4% relative to nonjunction). Plane polarized light produces highly polarized images of spherical cells stained with racemic dye, but for the chiral dye, the polarized pattern is largely eliminated by the chiral SHG emanating from the subresolution membrane convolutions.

Role Reversal in a Supramolecular Assembly: a Chiral Cyanine Dye Controls the Helicity of a Peptide−Nucleic Acid Duplex

A new dicarbocyanine dye bearing branched, chiral N-alkyl substituents was synthesized and its ability to form helical aggregates on peptide nucleic acid (PNA) double-helical templates was studied. The dye aggregates less effectively than an analogous dye bearing linear, achiral substituents, presumably due to steric problems with packing the branched substituents compared with the linear substituents. When the PNA duplex has a left-handed helicity, addition of the achiral dye leads to formation of a left-handed dye aggregate. However, when the chiral dye aggregates in the presence of this duplex, a right-handed structure is formed, suggesting that the dye alters the helicity of the underlying template. When a racemic PNA duplex (i.e., equal amounts of right- and left-handed helices) is used, no chirality is observed for the dye aggregate formed by the achiral dye but a right-handed helical aggregate is once again formed by the chiral dye. These results indicate that chirality is transferred from the dye to the PNA, as opposed to other examples of polymer-templated dye aggregation where chirality is transferred from the template to the dye.

Chiral separation of fluorescamine-labeled amino acids using microfabricated capillary electrophoresis devices for extraterrestrial exploration

Chiral separations of fluorescamine-labeled amino acids are characterized and optimized on a microfabricated capillary electrophoresis (CE) device. A standard mixture of acidic and neutral amino acids is labeled with fluorescamine in less than 5 min and the hydroxypropyl-β-cyclodextrin (HPβCD) concentration, temperature, and pH are optimized (15 mM HPβCD, 6 °C, pH<9) to achieve high-quality and low background chiral separations in less than 200 s. All four stereoisomers formed in the labeling reaction of the chiral dye with the chiral amino acids are typically resolved. At pH>9, isomerization of the dye chiral center is observed that occurs on the time scale of the chip separation. Typical limits of detection are ∼50 nM. These results demonstrate the feasibility of combining fluorescamine labeling of amino acids with microfabricated CE devices to develop low-volume, high-sensitivity apparatus and methods for extraterrestrial exploration.

Chiral determination of amino acids by capillary electrophoresis and laser-induced fluorescence at picomolar concentrations

In this publication we present results on the determination of enantiomers of amino acids at very low concentrations. A fluoresceine-based chiral dye was synthesized to allow the separation of diastereoisomers of d- and l-amino acids. We used capillary electrophoresis with different non-ionic surfactants (Brij). The separation parameters were optimized and separations of d- and l-isovaline, an unusual terrestrial amino acid, were obtained. The sensitivity limits were also determined using a commercial laser-induced fluorescence detector. The quantitation of these amino acids is very important to understand the process of chiral selection on Earth.

High-Resolution Nonlinear Optical Imaging of Live Cells by Second Harmonic Generation

By adapting a laser scanning microscope with a titanium sapphire femtosecond pulsed laser and transmission optics, we are able to produce live cell images based on the nonlinear optical phenomenon of second harmonic generation (SHG). Second harmonic imaging (SHIM) is an ideal method for probing membranes of living cells because it offers the high resolution of nonlinear optical microscopy with the potential for near-total avoidance of photobleaching and phototoxicity. The technique has been implemented on three cell lines labeled with membrane-staining dyes that have large nonlinear optical coefficients. The images can be obtained within physiologically relevant time scales. Both achiral and chiral dyes were used to compare image formation for the case of single- and double-leaflet staining, and it was found that chirality plays a significant role in the mechanism of contrast generation. It is also shown that SHIM is highly sensitive to membrane potential, with a depolarization of 25 mV resulting in an approximately twofold loss of signal intensity.

Chiral polymethine dyes. Part 6. Synthesis, absolute configuration, UV/Vis spectroscopic, and chiroptical properties of Chiral Tri- and Pentamethinium Cyanine Dyes with 1,2,3,4-tetrahydro-3,6-dimethylquinolyl end groups

Starting with monochiral 1,2,3,4-tetrahydro-3,6-dimethyl quinoline 14a,b, (Schemes 1 and 2), we have synthsized the new chiral, symmetrical and unsymmetrical, tri- and pentamethinium streptocyanine dyes 2a, (Scheme 8), 4a, (Scheme 5), and 5a,b, (Scheme 4), resp. 9a, (Scheme 6), 10a,b, (Scheme 4), and 11a, (Scheme 7) with one or two stereogenic centers in the two heterocyclic end groups. The absolute configuration of 14a,b, and thus the absolute configuration of all monochiral polymethinium dyes derived from 14a,b, has been determined by a single-crystal X-ray analysis of its 4-bromobenzenesulfonyl derivative 17a, (Scheme 1 and Fig. 1). The UV/Vis spectroscopic and chiroptical properties of the new polymethinium dyes have been studied for the first time and compared with that of similar streptocyanine dyes synthesized earlier (Tables 1 and 2) in order to find possible correlations between chiroptical properties and molecular structure.

Evidence for the cyclodextrin mediated aggregation of cyanine dyes into oligomers.

Based on the analysis of UV/Vis-and CD spectra of several cyanine dyes in β- and γ-cyclodextrin a case is made for the aggregation of the dyes into oligomers. The criteria for this new type of aggregates are: (i) the appearance of a concentration and temperature dependent shoulder on the high energy side of the dimer band in the UV/Vis-spectra; (ii) two oppositely signed high intensity CD absorptions, one centered at the shoulder, the other at about the energy of the monomer absorption. Calculations based on coupled-oscillator theory support the contention that chiral dye aggregates are formed in the presence of cyclodextrins, most probably trimers and tetramers. From UV/Vis- and CD-data a helically twisted sandwich structure is suggested for these aggregates. In contrast to the reported inclusion complexes these aggregates must form — at least partly — outside the chiral host.

ChemInform Abstract: Pyridinium N‐Phenoxide Betaines and Their Application for the Characterization of Solvent Polarities. Part 14. Attempted Synthesis of a Chiro‐Solvatochromic Pyridinium N‐Phenoxide Betaine Dye.

AbstractWith regard to the well known standard betaine dye (VIII) (which is solvato‐, thermo‐, piezo‐, and halochromic) it is suggested that a related but chiral dye might be chiro‐solvatochromic as well, i.e., that its absorption band is shifted in opposite directions by a chiral (R)‐ vs.