Joaquim Crusats

Aggregation in water solutions of tetrasodium diprotonated meso-tetrakis(4-sulfonatophenyl)porphyrin

The title porphyrin shows non ideal cmc with formation of J-aggregates, due to the formation of intermolecularly stabilized zwitterions, which at high concentration also results in H-aggregates.

Frank Model and Spontaneous Emergence of Chirality in Closed Systems

In a closed system an irreversible enantioselective autocatalysis coupled to a mutual inhibition reaction, corresponding to a fast and low exergonic formation of the heterochiral dimer which reverts to the monomers in the final reaction work-up, yields absolute asymmetric synthesis even in the absence of chiral polarizations. This is due to the very high chiral amplifications of the initial small statistical deviations from the ideal racemic composition. Moreover, this system is sensitive to very small chiral polarizations (energy differences between transition states below the mJ mol(-1) range). This behaviour can also be observed in reversible exergonic reactions, because the racemization time scale is substantially longer than that of the transformation of the initial reagents. The effect of the presence of other reactions likely to occur (i.e. non-catalytic transformations, non-enantioselective catalysis and homodimer formation) is discussed. Even if these decrease the sensitivity of the network in several chemical scenarios, the emergence of kinetically controlled spontaneous symmetry breaking is not hindered. These features, together with the response of the system to a sequential reaction procedure, suggest that a similar type of network is at the heart of the Soai reaction.

SELF-ASSEMBLY IN WATER OF THE SODIUM SALTS OF MESO-SULFONATOPHENYL SUBSTITUTED PORPHYRINS

The aggregation in water of the free bases and the diprotonated forms of p-sulfonatophenyl and phenyl meso-substituted porphyrins [sodium salts of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS4), 5,10,15-tris(4-sulfonatophenyl)-20-phenylporphyrin (TPPS3), 5,10-bis(4-sulfonatophenyl)-15,20-diphenylporphyrin (TPPS2A), 5,15-bis(4-sulfonatophenyl)-10,20-diphenylporphyrin (TPPS2O), 5-(4-sulfonatophenyl)-10,15,20-diphenylporphyrin (TPPS1) and 5,15-bis(4-sulfonatophenyl)porphine (DPPS2O)] was studied by UV/Vis, 1H-NMR, fluorescence, polarized fluorescence and resonance light scattering. The presence of hydrophobic phenyl groups favours the aggregation. The free bases TPPS4 and TPPS3 aggregate into stacks of ill-defined geometry. TPPS2A and DPPS2O give H-aggregates. TPPS2O and TPPS1 give J-aggregates through interaction of the hydrophobic phenyl groups, that is their geometry corresponds to edge-to-edge arrangements. All these porphyrins give J-aggregates upon diprotonation, through electrostatic interactions between the positively charged porphyrin ring and the sulfonato group. These J-aggregates give different arrays depending on the meso-substitution pattern.

Point-dipole approximation of the exciton coupling model versus type of bonding and of excitons in porphyrin supramolecular structures.

The application of the exciton coupling model to interacting porphyrin chromophores is discussed. Covalently bonded systems and ionic or electrostatically bonded homoassociates require different orientations of the transition dipole moments in order to explain the experimental results: according to the symmetry of the assembly for covalently bonded porphyrins, and assuming isolated chromophores for ionic bonded porphyrins. Further, for covalently bonded systems, an extended exciton coupling has been demonstrated, but the ionic systems are in agreement with non-extended couplings. The relation of these facts to a molecular description of solid-state Wannier-Mott or Frenkel excitons is briefly discussed.

Emergence of Supramolecular Chirality by Flows

Hydrodynamic forces in stirred solutions induce chirality in some supramolecular species of J-aggregates, as detected at the level of the electronic transition. However, the mechanism that explains the phenomenon remains to be elucidated, although the basic effect of hydrodynamic gradients of the shear rate is most probably the folding or bending of the nanoparticles in solution. Herein, we demonstrate a correlation between chiral flows in different regions of circular and square stirred cuvettes and the emergence of true circular dichroism (CD). The results show that chaotic flows lead to a racemic mixture of chiral shaped supramolecular species, and vortical flows to scalemic mixtures. In a magnetically stirred flask the descending and ascending flows are of different chiral sign and the CD reading depends on the weighting of these two flows of inverse chiral sign. The effect of the gradient of shear rates of the flows leading to chiral shape objects depends on the shape of the cuvette, which suggests that the flask shape and the controlled addition of reagents in defined regions of the stirred solutions may exert a control in self-assembly processes.

On the Mechano‐Chiral Effect of Vortical Flows on the Dichroic Spectra of 5‐Phenyl‐10,15,20‐tris(4‐sulfonatophenyl)porphyrin J‐Aggregates

Phase-modulated ellipsometry of the J-aggregates of the title porphyrin shows that the material gives a true CD signal. This confirms that there is a real chiral transfer by mechanical forces, mediated by shear gradient flows, from the macroscopic to the electronic transition level. Dislocations in the structure of the aggregate could justify the formation of chirality at the level of the electronic transitions once the mesophases can be sculptured by hydrodynamic gradient flows.

Metastability in Supersaturated Solution and Transition towards Chirality in the Crystallization of NaClO3

Reports on spontaneous deracemization of crystal mixtures of achiral or racemizing compounds (Viedma deracemization) are arousing strong interest not only because of their potential applications but also for understanding spontaneous emergence of chirality in chemical scenarios. The systems that may undergo such deracemizations involve compounds that crystallize as enantiopure crystals (racemic conglomerates) but are achiral or racemize rapidly in solution. The experimental conditions that promote such deracemizations are wet grinding of a racemic conglomerate in contact with its saturated solution and, if necessary, addition of a catalyst to accelerate racemization of the compound in the liquid phase. The results cannot be explained by dynamic kinetic resolution, because the assumption of a single parent crystal does not apply when a mixture of enantiomeric crystals is ground. Moreover, they cannot be explained by a second-order dynamic kinetic resolution process, nor by enantioselective inhibition of crystal growth under the influence of a chiral minor component, because of the absence of a second chiral compound. For systems such as NaClO3 and 1,1’-binaphthyl, there are reports of crystallizations that, under the action of strong stirring, yield polycrystalline mixtures of composition near to homochirality. Such results have been interpreted by the one-single-parent-crystal assumption together with a crystal growth dominated by secondary nucleation processes; the strong stirring creates new seeds by fracturing crystals. However, a recent report that describes a homochiral outcome in the crystallization of NaClO3 from boiling solutions suggests that the deracemization might also occur at the level of subcritical clusters. Notice that chiral amplification by secondary nucleation by no means precludes amplification at the level of subcritical clusters. In fact, some results in these previous reports indicate that the enantiomeric excess of crystals increases with the initial supersaturation, which, as we will show, is consistent with a significant role of the metastable stage preceding primary nucleation. Several mechanisms have been proposed to explain Viedma deracemization. However, it is important to understand the thermodynamic aspects and constraints that allow such deracemizations to occur. As we have previously discussed, in the initial stagnant conditions the system is in a chemical equilibrium that is determined solely by interactions between individual crystals and the solvated chemical compound. Solid-to-solution interaction does not distinguish between the enantiomorphic crystals. Under these conditions the enantiomorphic solids are thermodynamically identical, and the system is defined by one component in addition to the solvent; there is only one solid phase. This is the long-accepted chemical interpretation in respect to the available degrees of freedom that explain the experimental chemical behavior of these systems. Under Viedma deracemization conditions, we proposed that erosion of the crystals would lead to clusters that, in their interaction with other crystals or with each other, would recognize each other as different thermodynamic phases, that is, an enantiomeric discrimination would occur between the two enantiomorphic solid phases. Consequently, under constant grinding the system is taken out of equilibrium and evolves towards a final stationary state that, according to the published reports, is homochiral or nearly homochiral. Note, however, that certain experimental conditions may lead to a racemic stationary state of different composition than the stagnant conditions, or even to oscillations. Customary failure to publish “negative” results inhibits evaluating the range of possibilities. We have now conducted experiments to determine whether, in the crystallization of NaClO3 from boiling solutions, deracemization can occur at the level of subcritical clusters during the metastable stage previous to crystal growth. Our starting assumption, according to a previous report, is that in these systems during the highly concentrated, supersaturated metastable stage, that is, before crystal growth, an enantiomeric excess of chiral subcritical clusters [*] Dr. Z. El-Hachemi, Dr. J. Crusats, Prof. J. M. Rib Department de Qu mica Org nica and Institute of Cosmos Science (ICC), Universitat de Barcelona (IEEC-UB) c. Mart i Franqu s 1, 08028-Barcelona Catalonia (Spain) Fax: (+ 34)93-339-7878 E-mail: zelhachemi@ub.edu jmribo@ub.edu

Chiral sign selection on the J‐aggregates of diprotonated tetrakis‐(4‐sulfonatophenyl)porphyrin by traces of unidentified chiral contaminants present in the ultra‐pure water used as solvent

Traces of biological contaminants that cannot be detected, but are expected to be present, in ultra-pure water suffice to select the emerging chiral sign in the spontaneous mirror symmetry breaking that takes place during the formation of the J-aggregates of the amphiphilic diprotonated tetrakis-(4-sulfonatophenyl)porphyrin (H(4)TPPS(4)(2-)). This is demonstrated by competition experiments with a chiral cationic surfactant. The sensitivity of the detection depends on the hierarchical control of the H(4)TPPS(4)(2-) self-aggregation.

Absolute asymmetric synthesis in enantioselective autocatalytic reaction networks: theoretical games, speculations on chemical evolution and perhaps a synthetic option.

The Soai reaction and the Viedma deracemization of racemic conglomerate crystal mixtures are experimental pieces of evidence of the ability of enantioselective autocatalytic coupled networks to yield absolute asymmetric synthesis. Thermodynamically open systems or systems with non-uniform energy distributions may lead to chiral final states and, in systems able to come into thermodynamic equilibrium with their surroundings, to kinetically controlled absolute asymmetric synthesis. The understanding of network parameters and of the thermodynamic scenarios that may lead to spontaneous mirror symmetry breaking (SMSB) could assist in the development of new methods for asymmetric synthesis and enantioselective polymerizations (e.g., replicators), and to frame reasonable speculations on the origin of biological homochirality.

Structure vs. properties — chirality, optics and shapes — in amphiphilic porphyrin J-aggregates

The structure of the meso-tetrakis(4-sulfonatophenyl)porphyrin (TPPS4) J-aggregates could be determined by X-ray and electron diffraction methods. A sheet-like architecture reveals the relationship between structure and chirality, optics and shapes of the J-aggregates of the meso 4-sulfonatophenyl- and phenyl-substituted porphyrins. The structure of the J-aggregates of H4TPPS4 belongs to the chiral space group P21 and includes four porphyrin molecules in its unit cell. The intermolecular stabilization of the zwitterionic units by hydrogen bonding and electrostatic interactions between the positively charged central NH groups and the periphery anionic sulfonato groups results in a structure of porphyrins sheets along the [01] plane direction. The structure of the sheet on the [01] plane is already chiral and its molecular architecture explains the simultaneous presence of H- and J-aggregate bands in their absorption spectra. This structure also accounts for the high similarity observed between the absorption spectra of different mesomorphs of the same substance and even between different members of the series of meso-4-sulfonatophenyl- and aryl-substituted diprotonated porphyrins. The possibility, or not, of the sheet-like structure on [01] to interact with other layers, either through ionic or hydrophobic interactions, depends on the substitution pattern at the meso-positions of the porphyrin ring. Thus, the different morphologies of the particles [mono- bi- and multilayered] of this series of J-aggregates are explained taking into account the role that the fourth meso-substituent plays in the interlayer stabilization. The results suggest that supramolecular helicity, previously detected in several J-aggregates, is not the explanation of their chirality but would be the expression of the intrinsic chirality of the packing between building blocks.