Unique contrast patterns from resonance-enhanced chiral SHG of cell membranes

Examination of aggregation-induced enhanced emission in a propeller-shaped chiral nonconjugated blue emitter from restricted intramolecular rotation and J-type ππ stacking interactions

Herein, to get rid of aggregation-caused quenching (ACQ) in the solid phase, which is generally observed in π-conjugated planar molecules, a new chiral nonconjugated molecule with the propeller-shaped blue emitter property was prepared and characterized, and its photoluminescence behavior in the solid and solution phases were investigated through single crystal X-ray crystallography and photophysical studies. To further understand the aggregation-induced enhanced emission process (AIEE), concentration effect and solvent effect on the fluorescence properties of the solution phase and fluorescence quantum yield measurements in the solid and solution states were performed. In addition, thermal gravimetric analysis (TGA), cyclic voltammogram measurements (CV) and theoretical analysis of the molecule were carried out. According to the results of the experimental work done, the molecule showed aggregation-induced enhanced blue emission (AIEE) in the crystal state, which arose from the strong J-type ππ stacking interactions between the adjacent rings of the molecules, and the restricted intramolecular rotation process (RIR). The solvent polarity-dependent emission also verified the AIEE process. Therefore, combining the advantages of nonconjugation and chirality properties in the synthesis of the molecule afforded a new, unusual AIEE material and provided a new way for the design of a nonconjugated chiral fluorescent optoelectronic material.

Helical Nanofibers Formed by Palladium-Mediated Assembly of Organic Homochiral Macrocycles Containing Binaphthyl and Pyridyl Units

Herein, we report the synthesis and characterization of homochiral macrocycles, in which molecular rigidity, combined with the presence of multiple functional groups, allow for the assembly of helical nanostructures. 1,1'-bi-2-naphthol (Binol) units are used as robust chirality inducers, and pyridyl units embedded within the molecular frameworks allow the assembly, upon coordination with Pd(II) metal ions, of the macrocyclic building blocks. CD and NMR spectroscopies show the formation of ordered 1D assembly in solution. AFM studies indicate that the molecular systems are capable of forming nanoscale structures. The effective transfer of chiral information results in helical nanofibers, with lengths ranging from a few hundreds of nanometers to some micrometers. AFM line profiles reveal a helical longitudinal period of about 50 nm and a transverse width of 25 to 45 nm after deconvolution.

Aggregation-Induced Circularly Polarized Luminescence: Chiral Organic Materials for Emerging Optical Technologies

Chirality is becoming increasingly important in the design of organic materials with functional properties, when bulk anisotropy is needed. In the past decades, a plethora of chiral organic materials have been studied and developed. Nanostructures have brought substantial advancement to the realization of organic-molecule-based devices, and the possibilities for solid-state light emission are very promising in view of potential applications. Scientific approaches to the realization of chiral emissive materials are indeed growing exponentially. The chiral nanostructures discussed are related both to the way in which luminescence is generated and the way in which it is detected. As to the former, the focus will be on organic chromophores with aggregation-induced emission properties, so that emission is present, or at least largely amplified, when the molecules are in the aggregated state. As to the latter, the focus will be on the ability and a quantitative comparison of organic nanostructures capable of circularly polarized emission.

Switch-ON Near IR Fluorescent Dye Upon Protonation: Helically Twisted Bis(Boron Difluoride) Complex of π-Extended Corrorin

A novel helically twisted π-extended corrorin derivative having two boron dipyrrin (BODIPY) moieties at the periphery, a BODIPY DYEmer (6-BF₂ ) cross-bridged with π-conjugated dipyrrinylidene unit, was synthesized and characterized. The neutral 6-BF₂ is nonfluorescent due to the internal photoinduced charge transfer (CT) character in the excited state as inferred from the femtosecond transient absorption spectroscopy. However, upon protonation, the CT process is suppressed and the species H6-BF₂⁺ becomes near infrared (IR) emissive. With the aid of rigid helical conformations in 6-BF₂ , the helical isomers (P- and M-forms) were resolved and their chiroptical property was investigated. The distinct circular dichroism (CD) spectral changes of the enantiomers were observed in the presence of acids, which demonstrates that 6-BF₂ can be utilized for potential acid-responsive chiroptical materials.

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 λ_max ≈ 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 β₀ 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 β₀ 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 β_tot 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.

Ferrocenyl helquats: unusual chiral organometallic nonlinear optical chromophores

Three new dipolar cations have been synthesised, containing ferrocenyl (Fc) electron donor groups attached to helquat (Hq) acceptors. These organometallic Hq derivatives have been characterised as their TfO^- salts by using various techniques including NMR and electronic absorption spectroscopies and electrochemical measurements. UV-vis spectra show multiple intense low energy absorptions attributable to intramolecular charge-transfer (ICT) excitations. Each compound displays a reversible Fc^+/0 redox process, together with two reversible one-electron reductions of the Hq fragment. Molecular quadratic nonlinear optical (NLO) responses have been determined by using hyper-Rayleigh scattering at 1064 nm, and Stark (electroabsorption) spectroscopic studies on the visible absorption bands. The obtained first hyperpolarizabilities β are moderate, consistent with the relatively short π-conjugation lengths between the Fc and attached pyridinium group. A single-crystal X-ray structure has been solved for one of the complexes as its PF₆^- salt, revealing a centrosymmetric packing in the triclinic space group P1[combining macron]. Density functional theory (DFT) and time-dependent DFT calculations indicate that the lowest energy absorption bands have mainly metal-to-ligand charge-transfer character. The donor orbitals involved in the electronic transitions forming the next lowest energy ICT band also have substantial contributions from the Fe atom. Good agreement between the simulated and experimental UV-vis absorption spectra is achieved by using the PBE0 functional with the 6-311++G(d)/LANL2DZ mixed basis set, and the theoretical β values are reasonably large. Oxidation of the Fc unit is predicted to cause the β_tot value to decrease by more than 80% in one of the complexes.

Quantifying pulsed electric field-induced membrane nanoporation in single cells

Plasma membrane disruption can trigger a host of cellular activities. One commonly observed type of disruption is pore formation. Molecular dynamic (MD) simulations of simplified lipid membrane structures predict that controllably disrupting the membrane via nano-scale poration may be possible with nanosecond pulsed electric fields (nsPEF). Until recently, researchers hoping to verify this hypothesis experimentally have been limited to measuring the relatively slow process of fluorescent markers diffusing across the membrane, which is indirect evidence of nanoporation that could be channel-mediated. Leveraging recent advances in nonlinear optical microscopy, we elucidate the role of pulse parameters in nsPEF-induced membrane permeabilization in live cells. Unlike previous techniques, it is able to directly observe loss of membrane order at the onset of the pulse. We also develop a complementary theoretical model that relates increasing membrane permeabilization to membrane pore density. Due to the significantly improved spatial and temporal resolution possible with our imaging method, we are able to directly compare our experimental and theoretical results. Their agreement provides substantial evidence that nanoporation does occur and that its development is dictated by the electric field distribution.

Thiophene-based dyes for probing membranes

We report the synthesis of four new cationic dipolar push–pull dyes, together with an evaluation of their photophysical and photobiological characteristics pertinent to imaging membranes by fluorescence and second harmonic generation (SHG). All four dyes consist of an N,N-diethylaniline electron-donor conjugated to a pyridinium electron-acceptor via a thiophene bridge, with either vinylene (–CH=CH–) or ethynylene (–C≡C–) linking groups, and with either singly-charged or doubly-charged pyridinium terminals. The absorption and fluorescence behavior of these dyes were compared to a commercially available fluorescent membrane stain, the styryl dye FM4-64. The hyperpolarizabilities of all dyes were compared using hyper-Rayleigh scattering at 800 nm. Cellular uptake, localization, toxicity and phototoxicity were evaluated using tissue cell cultures (HeLa, SK-OV-3 and MDA-231). Replacing the central alkene bridge of FM4-64 with a thiophene does not substantially change the absorption, fluorescence or hyperpolarizability, whereas changing the vinylene-links to ethynylenes shifts the absorption and fluorescence to shorter wavelengths, and reduces the hyperpolarizability by about a factor of two. SHG and fluorescence imaging experiments in live cells showed that the doubly-charged thiophene dyes localize in plasma membranes, and exhibit lower internalization rates compared to FM4-64, resulting in less signal from the cell cytosol. At a typical imaging concentration of 1 μM, the doubly-charged dyes showed no significant light or dark toxicity, whereas the singly-charged dyes are phototoxic even at 0.5 μM. The doubly-charged dyes showed phototoxicity at concentrations greater than 10 μM, although they do not generate singlet oxygen, indicating that the phototoxicity is type I rather than type II. The doubly-charged thiophene dyes are more effective than FM4-64 as SHG dyes for live cells.

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.

Second Harmonic Imaging of Membrane Potential

The non-linear optical effect known as second harmonic generation (SHG) has been recognized since the earliest days of the laser. But it has only been in the last 20 years that it has begun to emerge as a viable microscope imaging contrast mechanism for visualization of cell and tissue structure and function. This is because only small modifications are required to equip a standard laser scanning 2-photon microscope for second harmonic imaging microscopy (SHIM). SHG signals from certain membrane-bound dyes are highly sensitive to membrane potential, indicating that SHIM may become a valuable probe of cell physiology. However, for the current generation of dyes and microscopes, the small signal size limits the number of photons that can be collected during the course of a fast action potential. Better dyes and optimized microscope optics could ultimately lead to the ability to image neuronal electrical activity with SHIM.

Synthesis, chiroptical and SHG properties of polarizable push–pull dyes built on π-extended binaphthyls

Novel binaphthyl push–pull dyes extend their π-bridge and incorporate as acceptors pyridine units, useful for further supramolecular polarization.

Nanostructuring with chirality: binaphthyl-based synthons for the production of functional oriented nanomaterials

Chirality is a powerful tool for the generation of order, directionality, and, as such, of function, in assembled nanoscale chemical devices. Axially chiral binaphthyls have been widely used in organic synthesis; the stability of the enantiomers enables their use as robust chirality inducers and catalysts in asymmetric reactions, and they are nowadays industrially applied in a variety of organic transformations. Applications of these compounds in the field of nanosciences are more recent, and not yet fully explored. The integration of such a robust class of chiral compounds, capable of efficient transfer of stereochemical information, into functional aggregates and nanoarchitectures is of great current interest. We will discuss preeminent examples of applications of these synthons in several fields of nanoscience, such as reticular chemistry, non-linear optical materials and imaging, and liquid crystals.

Synthesis and anion recognition properties of shape-persistent binaphthyl-containing chiral macrocyclic amides

We report on the synthesis and characterization of novel shape-persistent, optically active arylamide macrocycles, which can be obtained using a one-pot methodology. Resolved, axially chiral binol scaffolds, which incorporate either methoxy or acetoxy functionalities in the 2,2' positions and carboxylic functionalities in the external 3,3' positions, were used as the source of chirality. Two of these binaphthyls are joined through amidation reactions using rigid diaryl amines of differing shapes, to give homochiral tetraamidic macrocycles. The recognition properties of these supramolecular receptors have been analyzed, and the results indicate a modulation of binding affinities towards dicarboxylate anions, with a drastic change of binding mode depending on the steric and electronic features of the functional groups in the 2,2' positions.

Mechanism for the reaction of 2-naphthol with N-methyl-N-phenyl-hydrazine suggested by the density functional theory investigations

For the first time the computed mechanisms for the novel reaction of 2-naphthol with N-methyl-N-phenylhydrazine, leading to 1-amino-2-naphthol (Tang et al., J Am Chem Soc 2008, 130, 5840), have been investigated using the density functional theory. Four distinct possible pathways were evaluated: two amination mechanisms with the attack of NH(2) group respectively at the α-position C1 and β-position C3 atoms of 2-naphthol (pathways 1 and 2) as well as two rearrangement processes with displacement of the phenolic hydroxyl group followed by the benzidine-like rearrangement at the α-position C1 and β-position C3 atoms of 2-naphthol, respectively (pathways 3 and 4). Solvent effect has been tested based on the optimized geometries of the stationary points in solution at the B3LYP/PCM/6-31+G(d,p) level of theory with an averaged dielectric constant of binary solvent. Single-point energies of the optimized structures have been calculated using three hybrid density functionals, B3LYP, MPW3LYP, and B3PW91 with the 6-311++G(3df,2p) basis set. Our computed results clearly manifest that pathway 1 (α-amination) has the highest possibility to occur, with the Gibbs free energies being lower by 6 to 20 kcal/mol compared with the other three pathways, which leads to 1-amino-2-naphthol and N-methylaniline as products. It is in excellent agreement with the experimental observation.

Chiral vibrational structures of proteins at interfaces probed by sum frequency generation spectroscopy

We review the recent development of chiral sum frequency generation (SFG) spectroscopy and its applications to study chiral vibrational structures at interfaces. This review summarizes observations of chiral SFG signals from various molecular systems and describes the molecular origins of chiral SFG response. It focuses on the chiral vibrational structures of proteins and presents the chiral SFG spectra of proteins at interfaces in the C-H stretch, amide I, and N-H stretch regions. In particular, a combination of chiral amide I and N-H stretches of the peptide backbone provides highly characteristic vibrational signatures, unique to various secondary structures, which demonstrate the capacity of chiral SFG spectroscopy to distinguish protein secondary structures at interfaces. On the basis of these recent developments, we further discuss the advantages of chiral SFG spectroscopy and its potential application in various fields of science and technology. We conclude that chiral SFG spectroscopy can be a new approach to probe chiral vibrational structures of protein at interfaces, providing structural and dynamic information to study in situ and in real time protein structures and dynamics at interfaces.

Amino(oligo)thiophene-based environmentally sensitive biomembrane chromophores

There is a growing need for cellular imaging with fluorescent probes that emit at longer wavelengths to minimize the effects of absorption, autofluorescence, and scattering from biological tissue. In this paper a series of new environmentally sensitive hemicyanine dyes featuring amino(oligo)thiophene donors have been synthesized via aldol condensation between a 4-methylpyridinium salt and various amino(oligo)thiophene carboxaldehydes, which were, in turn, obtained from amination of bromo(oligo)thiophene carboxaldehyde. Side chains on these fluorophores impart a strong affinity for biological membranes. Compared with benzene analogues, these thiophene fluorophores show significant red shift in the absorption and emission spectra, offering compact red and near-infrared emitting fluorophores. More importantly, both the fluorescence quantum yields and the emission peaks are very sensitive to various environmental factors such as solvent polarity or viscosity, membrane potential, and membrane composition. These chromophores also exhibit strong nonlinear optical properties, including two-photon fluorescence and second harmonic generation, which are themselves environmentally sensitive. The combination of long wavelength fluorescence and nonlinear optical properties make these chromophores very suitable for applications that require sensing or imaging deep inside tissues.

Nonlinear optical potentiometric dyes optimized for imaging with 1064-nm light

Nonlinear optical phenomena, such as two-photon fluorescence (2PF) and second harmonic generation (SHG), in combination with voltage sensitive dyes, can be used to acquire high-resolution spatio temporal maps of electrical activity in excitable cells and tissue. Developments in 1064-nm fiber laser technology have simplified the generation of high-intensity, long-wavelength, femtosecond light pulses, capable of penetrating deep into tissue. To merge these two advances requires the design and synthesis of new dyes that are optimized for longer wavelengths and that produce fast and sensitive responses to membrane potential changes. In this work, we have systematically screened a series of new dyes with varying chromophores and sidechains that anchor them in cell membranes. We discovered several dyes that could potentially be used for in vivo measurements of cellular electrical activity because of their rapid and sensitive responses to membrane potential. Some of these dyes show optimal activity for SHG; others for 2PF. This regulated approach to dye screening also allows significant insight into the molecular mechanisms behind both SHG and 2PF. In particular, the differing patterns of sensitivity and kinetics for these two nonlinear optical modalities indicate that their voltage sensitivity originates from differing mechanisms.