Hydrodynamic Effects in Soft-matter Self-assembly: The Case of J-Aggregates of Amphiphilic Porphyrins

Physical Chemistry Models for Chemical Research in the XXth and XXIst Centuries

Thermodynamic hypotheses and models are the touchstone for chemical results, but the actual models based on time-invariance, which have performed efficiently in the development of chemistry, are nowadays invalid for the interpretation of the behavior of complex systems exhibiting nonlinear kinetics and with matter and energy exchange flows with the surroundings. Such fields of research will necessarily foment and drive the use of thermodynamic models based on the description of irreversibility at the macroscopic level, instead of the current models which are strongly anchored in microreversibility.

Biased Symmetry Breaking in the Formation of Intercalated Layered Double Hydroxides: toward Control of Homochiral Supramolecular Assembly

Homochiral supramolecular assembly (HSA) based on achiral molecules has provided important clues to understand the origin of biological homochirality from the aspect of symmetry breaking. However, planar achiral molecules still face the challenge of forming HSA due to the lack of driving force for twisted stacking, which is a prerequisite for homochirality. Here, with the benefit of the formation of 2D intercalated layered double hydroxide (LDH, host-guest nanomaterials) in vortex motion, planar achiral guest molecules can form the chiral units with spatially asymmetrical structure in the confinement space of LDH. Once the LDH is removed, these chiral units are in a thermodynamic non-equilibrium state, which can be amplified to HSA by self-replicating. Especially, the homochiral bias can be predicted in advance by controlling the vortex direction. Therefore, this study breaks the bottleneck of complicated molecular design and provides a new technology to achieve HSA made of planar achiral molecules with definite handedness.

Kinetic Controlled Chirality Transfer and Induction in 2D Hydrogen-Bonding Assemblies of Glycylglycine on Au(111)

Chirality transfer is of vital importance that dominates the structure and functionality of biological systems and living matters. External physical stimulations, e.g. polarized light and mechanical forces, can trigger the chirality symmetry breaking, leading to the appearance of the enantiomeric entities created from a chiral self-assembly of achiral molecule. Here, several 2D assemblies with different chirality, synthesized on Au(111) surface by using achiral building blocks - glycylglycine (digly), the simplest polypeptide are reported. By delicately tuning the kinetic factors, i.e., one-step slow/rapid deposition, or stepwise slow deposition with mild annealing, achiral square hydrogen-bond organic frameworks (HOF), homochiral rhombic HOF and racemic rectangular assembly are achieved, respectively. Chirality induction and related symmetry broken in assemblies are introduced by the handedness (H-bond configurations in principle) of the assembled motifs and then amplified to the entire assemblies via the interaction between motifs. The results show that the chirality transfer and induction of biological assemblies can be tuned by altering the kinetic factors instead of applying external forces, which may offer an in-depth understanding and practical approach to peptide chiral assembly on the surfaces and can further facilitate the design of desired complex biomolecular superstructures.

Enantiomeric Resolution and Enantiomer Isolation of H2 TPPS4 J-Aggregate from Aqueous Solution Is Enabled by Vortexes

Protonated achiral H₂ TPPS4 spontaneously self-arranges at acids pH and high ionic strength to build mesoscopic J-aggregates that are intrinsically chiral. According to the symmetry rule aggregation leads to a racemate that, however, can be unbalanced by chemical (chiral pollutants) or physical stimuli (as vortexing the solution). Vortexing the title racemate, in principle, might either induce chiral separation or chiral enrichment. Indeed, herein it is shown that vortices enable the resolution of this racemic solution exploiting the tendency to deposit, onto the quartz cuvette walls, of the enantiomer favored by the stirring sense. Simultaneously, over time, it was found that the opposite chiral conformation becomes prevalent in solution realizing a significant enantiomeric resolution. Therefore, after removing all stirring-favored chiral J-aggregate from the solution, the recovering and isolating of the desired enantiomers from the cuvette walls was successfully obtained without complex procedures. In this sense, it has been demonstrated that the stirring forces are executively able to fulfil the chiral separation in H₂ TPPS4 J-aggregates, employed as model of a self-assembled system in aqueous solution.

Stereospecific Self-Assembly Processes of Porphyrin-Proline Conjugates: From the Effect of Structural Features and Bulk Solvent Properties to the Application in Stereoselective Sensor Systems

Conjugating the porphyrin ring with an amino acid via amide linkage represents a straightforward way for conferring both amphiphilicity and chirality to the macrocycle. Proline residue is a good choice in this context since its conformational rigidity allows for porphyrin assembling where molecular chirality is efficiently transferred and amplified using properly honed aqueous environments. Herein, we describe the evolution of the studies carried out by our group to achieve chiral systems from some porphyrin-proline derivatives, both in solution and in the solid state. The discussion focuses on some fundamental aspects reflecting on the final molecular architectures obtained, which are related to the nature of the appended group (stereochemistry and charge), the presence of a metal ion coordinated to the porphyrin core and the bulk solvent properties. Indeed, fine-tuning the mentioned parameters enables the achievement of stereospecific structures with distinctive chiroptical and morphological features. Solid films based on these chiral systems were also obtained and their recognition abilities in gaseous and liquid phase are here described.

Vortex Flow-controlled Circularly Polarized Luminescence of Achiral Pt(II) Complex Aggregates Assembled at the Air-Water Interface

Circularly polarized luminescence (CPL) has been researched for various applications by control of characteristics such as chirality and magnitude. Supramolecular chirality has been prepared by vortex motion as a mechanical stimulus; however, CPL has yet to be controlled precisely and reproducibly. In this work, the first precise control of CPL under vortex flow conditions at an air-water interface is reported. The supramolecular chirality of aggregates consisting of an achiral trans-bis(salicylaldiminato)Pt(II) complex bearing hexadecyl chains is induced and controlled with vortex flow at the air-water interface, whereas the complex naturally forms an achiral amorphous solid with non-chiroptical properties under non-vortex conditions. The CPL direction and magnitude (g_lum value) of the Pt(II) complex aggregates can be adjusted precisely according to the vortex conditions, including the rotatory direction and flow rate. Vortex-flow-induced emission enhancement is also observed upon an increase in the rate of the vortex flow.

Chirality transfer from a 3D macro shape to the molecular level by controlling asymmetric secondary flows

Homochirality is a fundamental feature of living systems, and its origin is still an unsolved mystery. Previous investigations showed that external physical forces can bias a spontaneous symmetry breaking process towards deterministic enantioselection. But can the macroscopic shape of a reactor play a role in chiral symmetry breaking processes? Here we show an example of chirality transfer from the chiral shape of a 3D helical channel to the chirality of supramolecular aggregates, with the handedness of the helical channel dictating the direction of enantioselection in the assembly of an achiral molecule. By combining numerical simulations of fluid flow and mass transport with experimental data, we demonstrated that the chiral information is transferred top-down thanks to the interplay between the hydrodynamics of asymmetric secondary flows and the precise spatiotemporal control of reagent concentration fronts. This result shows the possibility of controlling enantioselectively molecular processes at the nanometer scale by modulating the geometry and the operating conditions of fluidic reactors.

External physical forces can bias a spontaneous symmetry breaking process but whether the shape of a reactor plays a role in chiral symmetry breaking processes is an open question. Here, the authors demonstrate chirality transfer from the chiral shape of a 3D helical channel to chiral supramolecular aggregates whereby the handedness of the helical channel dictates the direction of enantioselection.

Supramolecular Chirality in Porphyrin Self-assembly Systems in Aqueous Solution

Abstract:

The self-assembly process appears as a powerful and attractive strategy for constructing complex supramolecules by the spontaneous organization of appropriate building blocks. In this scenario, water-soluble porphyrinoids lend themselves as ideal paradigms to disclose the self-assembly phenomenon by exploiting their well-known tendency to build aggregates in aqueous media via weak non-covalent forces. Nevertheless, the spontaneous organization of achiral porphyrins can result in a final chiral superstructure moving away from single-molecule behaviour to supramolecular chirality. Therefore, over the years numerous attempts have been implemented to investigate how a porphyrin aggregate, made up of achiral monomers, becomes not-symmetric and which processes govern the bias for a certain enantiomeric assembly rather than another. Thus, in this mini-review, we exclusively discuss the main strategies for designing and building chiral aggregates in water from achiral porphyrin monomers, with particular regard to their chiroptical features.

Static Retention of Dynamic Chiral Arrangements for Achiral Shear Thinning Metal-Organic Colloids

Chiral compounds are known to be important not only because they are the fundamental components of living organisms, but also for their unique chiroptical properties. In recent years, scientists have fabricated several chiral organic supramolecular aggregates by using chiral physical fields, such as vortex flow. Herein, the relationship between dynamic chiroptical properties and rheological nature is discussed, suggesting the shear thinning properties of non-Newtonian fluids might help colloidal particles adopt a chiral arrangement in vortices. Furthermore, the storage modulus of colloids could be increased by adding a linking agent, which successfully kept the dynamic chiroptical properties in the static state. Moreover, the salt effect on the host-guest interaction involved in the colloids was studied, the results suggested a significant enhancement of the transferred dynamic circular dichroism for the achiral guest molecule.

Circularly Polarized Luminescence of Achiral Metal-Organic Colloids and Guest Molecules in a Vortex Field

Recently, scientists have reported a range of chiral fluorescence materials or chiral composites that can emit circularly polarized luminescence. Herein, two achiral metal-organic colloidal solutions were studied, showing active circularly polarized luminescence, which is observed in vortex stirring. The absolute values for g_lum are 0.05 and 0.03 and the plus or minus sign of g_lum depends on the colloidal structure and stirring direction, which make the property easy to manipulate. Further, the host-guest interaction study suggests both electrostatic interactions and coordination bonding may influence the chiroptical property from the colloidal solution to the guest molecule. Rhodamine 6G and its carboxylic acid derivative exhibit good quantum yields and acceptable g_lum values in the colloidal solution.

Vortex-Induced Harmonic Light Scattering of Porphyrin J-Aggregates

An understanding of macroscopic vortex-induced chirality can provide insights into the origin of the homochirality of life. While circular dichroism measurements in stirred solutions are useful for the analysis of chiral supramolecular structures induced by vortex motion, there are no reports on the application of other spectroscopic methods. To obtain a deeper understanding of macroscopic vortex-induced chirality, it is essential to develop novel in situ spectroscopic methods that provide information about changes in both the size and chirality in stirred solutions. Here, we report the first observation by harmonic light scattering of the mirror-symmetry-breaking process of porphyrin J-aggregates under the rotation of a magnetic stirrer. The chiral supramolecular structure observed during stirring is likely due to the formation of a chiral aggregate that consists of porphyrin J-aggregates. The dissociation of the structure proceeds in two steps (a fast step and a slow step), as indicated by the signal decay rate when stirring was stopped. This novel method is useful for analyzing the supramolecular structural changes of chiral aggregates induced by external stimuli.

Symmetry Breaking in Self-Assembled Nanoassemblies

The origin of biological homochirality, e.g., life selects the L-amino acids and D-sugar as molecular component, still remains a big mystery. It is suggested that mirror symmetry breaking plays an important role. Recent researches show that symmetry breaking can also occur at a supramolecular level, where the non-covalent bond was crucial. In these systems, equal or unequal amount of the enantiomeric nanoassemblies could be formed from achiral molecules. In this paper, we presented a brief overview regarding the symmetry breaking from dispersed system to gels, solids, and at interfaces. Then we discuss the rational manipulation of supramolecular chirality on how to induce and control the homochirality in the self-assembly system. Those physical control methods, such as Viedma ripening, hydrodynamic macro- and micro-vortex, superchiral light, and the combination of these technologies, are specifically discussed. It is hoped that the symmetry breaking at a supramolecular level could provide useful insights into the understanding of natural homochirality and further designing as well as controlling of functional chiral materials.

Chemical Basis of Biological Homochirality during the Abiotic Evolution Stages on Earth

Spontaneous mirror symmetry breaking (SMSB), a phenomenon leading to non-equilibrium stationary states (NESS) that exhibits biases away from the racemic composition is discussed here in the framework of dissipative reaction networks. Such networks may lead to a metastable racemic non-equilibrium stationary state that transforms into one of two degenerate but stable enantiomeric NESSs. In such a bifurcation scenario, the type of the reaction network, as well the boundary conditions, are similar to those characterizing the currently accepted stages of emergence of replicators and autocatalytic systems. Simple asymmetric inductions by physical chiral forces during previous stages of chemical evolution, for example in astrophysical scenarios, must involve unavoidable racemization processes during the time scales associated with the different stages of chemical evolution. However, residual enantiomeric excesses of such asymmetric inductions suffice to drive the SMSB stochastic distribution of chiral signs into a deterministic distribution. According to these features, we propose that a basic model of the chiral machinery of proto-life would emerge during the formation of proto-cell systems by the convergence of the former enantioselective scenarios.

Towards homochiral supramolecular entities from achiral molecules by vortex mixing-accompanied self-assembly

Achieving homochirality is challenging both at the molecular and the supramolecular levels. While the origin of molecular homochirality still remains mysterious, the fabrication of homochiral assemblies from achiral molecules has attracted considerable interest since it provides many clues to understand the origin of molecular chirality. Here, by using a vortex mixing-accompanied self-assembly strategy, we obtained near-unity homochiral entities with controlled handedness from supramolecular gels that consist of exclusively achiral molecules without any chiral additives. The common supramolecular gelation process via heating and cooling of the achiral molecules only resulted in racemic gels. However, if vortex mixing is applied during the self-assembly, near-unity homochiral assemblies with uncontrolled handedness were obtained. Vortex mixing during the nucleation stage was found to be crucial in this case. On the other hand, if a small amount of the above vortex mixing produced assemblies was added as chiral seeds into the racemic gels, the racemic gels turned into near-unity homochiral suspensions with controlled handedness via a ripening process. Our studies provide an intriguing approach for achieving homochiral supramolecular assemblies from achiral molecules.

Control over the emerging chirality in supramolecular gels and solutions by chiral microvortices in milliseconds

The origin of homochirality in life is a fundamental mystery. Symmetry breaking and subsequent amplification of chiral bias are regarded as one of the underlying mechanisms. However, the selection and control of initial chiral bias in a spontaneous mirror symmetry breaking process remains a great challenge. Here we show experimental evidences that laminar chiral microvortices generated within asymmetric microchambers can lead to a hydrodynamic selection of initial chiral bias of supramolecular systems composed of exclusively achiral molecules within milliseconds. The self-assembled nuclei with the chirality sign affected by the shear force of enantiomorphic microvortices are subsequently amplified into almost absolutely chirality-controlled supramolecular gels or nanotubes. In contrast, turbulent vortices in stirring cuvettes fail to select the chirality of supramolecular gels. This study reveals that a laminar chiral microflow can induce enantioselection far from equilibrium, and provides an insight on the origin of natural homochirality.

Symmetry breaking and chiral amplification are fundamental principles in chemistry and biology but the control of initial chiral bias remains a great challenge. Here the authors show that chiral microvortices can lead to a selection of initial chiral bias of supramolecular systems composed of achiral molecules.

Structures and properties of porphyrin-based film materials part I. The films obtained via vapor-assisted methods

This review is devoted to porphyrin-based film materials. Various technological and scientific applications of ones are close to surface and interface related phenomena. In the part I of review the following topics are discussed the recent progress in field of submonolayers, monolayers and multilayers films on the vapor-solid interfaces, including results on (i) conformational behavior of adsorbed molecules, (ii) aggregation and surface phases formation, (iii) on-surface coordination networks, and (iv) on-surface chemical reactions. The examples of combined approaches to developing materials and porphyrin-based film materials application are also presented.

Spontaneous mirror symmetry breaking and origin of biological homochirality

Recent reports on both theoretical simulations and on the physical chemistry basis of spontaneous mirror symmetry breaking (SMSB), that is, asymmetric synthesis in the absence of any chiral polarizations other than those arising from the chiral recognition between enantiomers, strongly suggest that the same nonlinear dynamics acting during the crucial stages of abiotic chemical evolution leading to the formation and selection of instructed polymers and replicators, would have led to the homochirality of instructed polymers. We review, in the first instance, which reaction networks lead to the nonlinear kinetics necessary for SMSB, and the thermodynamic features of the systems where this potentiality may be realized. This could aid not only in the understanding of SMSB, but also the design of reliable scenarios in abiotic evolution where biological homochirality could have taken place. Furthermore, when the emergence of biological chirality is assumed to occur during the stages of chemical evolution leading to the selection of polymeric species, one may hypothesize on a tandem track of the decrease of symmetry order towards biological homochirality, and the transition from the simple chemistry of astrophysical scenarios to the complexity of systems chemistry yielding Darwinian evolution.