The effect of axial and helical chirality on circularly polarized luminescence: lessons learned from tethered twistacenes.

The effect of axial and helical twisting on the circularly polarized luminescence of acenes was studied both experimentally and computationally, using four series of tethered twisted acenes. We find that the combination of axial and helical chirality yields the highest anisotropy factors, and that the ratio between the absorption and emission anisotropy factors is an intrinsic property for twistacenes.

Circularly polarized and total luminescence as probes of nucleation and growth in chiral nanocrystals.

Nucleation of crystals as well as their growth is difficult to study experimentally. We have recently demonstrated that chiral Eu3+ -doped terbium phosphate nanocrystals are an interesting system for studying nanocrystal formation mechanisms and chiral symmetry breaking, occurring during their formation, directed by chiral ligands, such as tartaric acid. In this paper, we show how simultaneous, in situ monitoring of both total emission intensity and circularly polarized luminescence magnitude and sign versus time during nanocrystal formation provides considerable information on the mechanisms of nanocrystal nucleation and growth. Specifically, we show that the presence of tartaric acid leads to the formation of chiral prenucleation clusters, which deterministically transform into nanocrystals of a specific handedness. Additionally, we demonstrate that both unseeded and seeded nanocrystal syntheses behave differently mechanistically and that the addition of seed nanocrystals catalyses both enantio-specific (also called secondary nucleation) as well as nonspecific nucleation.

A Kinetic Isotope Effect in the Formation of Lanthanide Phosphate Nanocrystals.

Mechanisms of nucleation and growth of crystals are still attracting a great deal of interest, in particular with recent advances in experimental techniques aimed at studying such phenomena. Studies of kinetic isotope effects in various reactions have been useful for elucidating reaction mechanisms, and it is believed that the same may apply for crystal formation kinetics. In this work, we present a kinetic study of the formation of europium-doped terbium phosphate nanocrystals under acidic conditions, including a strong H/D isotope effect. The nanocrystal growth process could be quantitatively followed through monitoring of the europium luminescence intensity. Hence, such lanthanide-based nanocrystals may serve as unique model systems for studying crystal nucleation and growth mechanisms. By combining the luminescence and NMR kinetics data, we conclude that the observed delayed nucleation occurs due to initial formation of pre-nucleation clusters or polymers of the lanthanide and phosphate ions, which undergo a phase transformation to crystal nuclei and further grow by cluster attachment. A scaling behavior observed on comparison of the H2O and D2O-based pre-nucleation and nanocrystal growth kinetics led us to conclude that both pre-nucleation and nanocrystal growth processes are of similar chemical nature.

Time-resolved circularly polarized luminescence of Eu3+ -based systems.

Chiral Eu3+ -based systems are frequently studied via circularly polarized luminescence spectroscopy. The emission lifetimes of each circular polarization, however, are virtually always ignored, because in a homogeneous sample of emitters, there should be no difference between the two. However, we show that in less robust Eu3+ complex structures, as in the chiral complex Eu (facam)3 , a difference in the lifetimes of the two circularly polarized emission components arises due to heterogeneity of the complexes. In this case, each species within the sample could have different degrees of circularly polarized luminescence and decay rates at certain emission lines. The superposition of the emission components of the various chiral species leads to an overall difference in decay rate between the two circular polarizations. Such a difference is also shown for Eu3+ -doped chiral TbPO4 ·D2 O nanocrystals. We believe that this kind of measurement could be a unique tool for determining the homogeneity of a lanthanide-based chiral system, where other methods might fail in this task.

Enantiomeric Control of Intrinsically Chiral Nanocrystals.

The chiral aspect of inorganic crystals that crystallize in chiral space groups has been largely ignored until recently, partly due to difficulties in characterizing the chiroptical properties of bulk crystals, and also due to the difficulty in separating (sub)micrometer-scale chiral crystal enantiomers. In recent years, the colloidal synthesis of intrinsically chiral nanocrystals (NCs) of several chiral inorganic compounds with significant enantiomeric excess has been demonstrated. This is achieved through the use of chiral molecular ligands, which bind to the atomic/ionic components of the crystals, preferentially forming one crystal enantiomorph. Here, recent progress on several aspects of these NCs is described, including the connection between ligand structure and its ability to direct NC handedness, chiral amplification in the synthesis leading to enantiopure NC samples, spontaneous symmetry breaking, the formation of NCs with chiral shapes, the connection between lattice and shape chirality and mixed contributions of atomic-scale and shape chirality to the chiroptical properties.

Spontaneous and directed symmetry breaking in the formation of chiral nanocrystals.

Symmetry plays a crucial part in our understanding of the natural world. Mirror symmetry breaking is of special interest as it is related to life as we know it. Studying systems which display chiral amplification, therefore, could further our understanding of symmetry breaking in chemical systems, in general, and thus also of the asymmetry in Nature. Here, we report on strong chiral amplification in the colloidal synthesis of intrinsically chiral lanthanide phosphate nanocrystals, measured via circularly polarized luminescence. The amplification involves spontaneous symmetry breaking into either left- or right-handed nanocrystals below a critical temperature. Furthermore, chiral tartaric acid molecules in the solution direct the amplified nanocrystal handedness through a discontinuous transition between left- and right-handed excess. We analyze the observations based on the statistical thermodynamics of critical phenomena. Our results demonstrate how chiral minerals with high enantiopurity can form in a racemic aqueous environment.

Determination of Handedness in a Single Chiral Nanocrystal via Circularly Polarized Luminescence.

The occurrence of biological homochirality is attributed to symmetry-breaking mechanisms which are still debatable. Studies of symmetry breaking require tools for monitoring the population ratios of individual chiral nano-objects, such as molecules, polymers, or nanocrystals. Moreover, mapping their spatial distributions may elucidate on their symmetry-breaking mechanism. While luminescence is preferred for detecting single particle chirality due to its high signal-to-noise ratio, the typical low optical activity of chromophores limits its applicability. Here, we report on handedness determination of single chiral lanthanide-based luminescent nanocrystals with a total photon count of 2 × 104. Due to the large emission dissymmetry, we could determine the handedness of individual particles using only a single circular polarization component of the emission spectrum, without polarization modulation. A machine learning algorithm, trained to several spectral line shape features, enabled us to determine and spatially map the handedness of individual nanocrystals with high accuracy and speed. This technique may become invaluable in studies of symmetry breaking in chiral materials.

Tuning the molecular weight of polymeric amphiphiles as a tool to access micelles with a wide range of enzymatic degradation rates.

Enyzme-responsive polymeric assemblies hold great potential for biomedical applications due to the over-expression of disease-associated enzymes, which can be utilized to activate such systems only in afflicted tissues. Herein we demonstrate that the overall molecular weight of polymeric amphiphiles, which have the same hydrophilic/hydrophobic ratio, can be tuned to create polymeric micelles with an extreme range of degradation rates. This approach expands the available set of molecular parameters that can be adjusted to tune the degradation rate of polymeric assemblies, paving new possibilities for rational design of polymeric systems with controlled degradation rates.

Reversible Dimerization of Polymeric Amphiphiles Acts as a Molecular Switch of Enzymatic Degradability.

Enzyme-responsive polymeric micelles have great potential as drug delivery systems due to the high selectivity and overexpression of disease-associated enzymes, which could be utilized to trigger the release of active drugs only at the target site. We previously demonstrated that enzymatic degradation rates of amphiphilic PEG-dendron hybrids could be precisely tuned by gradually increasing the hydrophobic to hydrophilic ratio. However, with the increase in hydrophobicity, the micelles rapidly became too stable and could not be degraded, as often encountered for many other amphiphilic assemblies. Here we address the challenge to balance between stability and reactivity of enzymatically degradable assemblies by utilizing reversible dimerization of diblock polymeric amphiphiles to yield jemini amphiphiles. This molecular transformation serves as a tool to control the critical micelle concentration of the amphiphiles in order to tune their micellar stability and enzymatic degradability. To demonstrate this approach, we show that simple dimerization of two polymeric amphiphiles through a single reversible disulfide bond significantly increased the stability of their micellar assemblies toward enzymatic degradation, although the hydrophilic to hydrophobic ratio was not changed. Reduction of the disulfide bond led to dedimerization of the polymeric hybrids and allowed their degradation by the activating enzyme. The generality of the approach is demonstrated by designing both esterase- and amidase-responsive micellar systems. This new molecular design can serve as a simple tool to increase the stability of polymeric micelles without impairing their enzymatic degradability.

Simple and cost-effective fluorescent labeling of 5-hydroxymethylcytosine.

The nucleobase 5-hydroxymethylcytosine (5-hmC), a modified form of cytosine, is an important epigenetic mark related to regulation of gene expression. 5-hmC levels are highly dynamic during early development and are modulated during the progression of neurodegenerative disease and cancer. We describe a spectroscopic method for the global quantification of 5-hmC in genomic DNA. This method relies on the enzymatic glucosylation of 5-hmC, followed by a glucose oxidation step that results in the formation of aldehyde moieties that are covalently linked to a fluorescent reporter by oxime ligation. The fluorescence intensity of the labeled sample is directly proportional to its 5-hmC content. We show that this simple and cost-effective technique is suitable for quantification of 5-hmC content in different mouse tissues.