Raimon Rubires

Stirring effects on the spontaneous formation of chirality in the homoassociation of diprotonated meso-tetraphenylsulfonato porphyrins.

Homoassociates of the achiral title porphyrins in acid solutions show spontaneous symmetry breaking, which can be detected by circular dichroism (CD). The CD spectra are due to differential scattering and differential absorption contributions, the relative significance of which is related to the shape and size of the homoassociate. When an earlier model, designed for the association of these diprotonated porphyrins (J aggregates with the geometry of stepped sheets of intramolecular-stabilised zwitterions), was applied to an exciton-coupling point-dipole approximation, the folding of the one-dimensional homoassociates explained the CD signals detected. An effect of the vortex direction, caused by stirring or rotary evaporation, upon the exciton chirality sign was detected. In the case of H2TPPS3, the number of experiments performed gave a statistical significance to this effect. This vortex effect can be attributed to enhancement of the chirality fluctuations that originate in the diffusion-limited aggregation to high-molecular-weight homoassociates. In this sense, the phenomenon could be general for supramolecular systems that are obtained under kinetic control, and its detection would be possible when inherent chiral chromophores were being generated in the association process.

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.

Chiral shape and enantioselective growth of colloidal particles of self-assembled meso-tetra(phenyl and 4-sulfonatophenyl)porphyrins

The self-assembly of diprotonated phenyl and 4-sulfonatophenyl meso-tetrasubstituted porphyrins gives a spontaneous chiral symmetry breaking, but only for H2TPPS3−, which forms helicoidal colloidal particles; the selection of the resulting chirality sign by the hydrodynamic forces of a stirring vortex can be demonstrated.

Self-assembly to ordered films of the homoassociate solutions of the tetrasodium salt of 5,10,15,20-tetrakis(4-sulfonatophenyl) porphyrin dihydrochloride

Abstract Films were obtained from the evaporation on flat glass substrates of homoassociate water solutions of the title porphyrin. The films of 1 were characterized by UV–VIS and FT-IR spectroscopy, microscopy, X-ray diffraction, thermogravimetry (TG) and differential scanning calorimetry (DSC). The films are composed by a birefrigent organic phase and NaCl crystallites; the organic phase shows an orientated structure. The water content of the film depends directly on the water-vapour pressure of the surrounding atmosphere; a change of structure occurs for pH2O>10 Torr and there is a transition to gel-like material for pH2O>20 Torr. In the organic phase, water, sodium, and chloride ions are placed in layers or channels parallel to the evaporation surface with thickness depending on the water content.

A kinetic study by NMR of the benzoin condensation catalyzed by thiazolium salts in mild basic conditions: a second order process in both aldehyde and pre-catalyst.(§)

Abstract A kinetic study of the benzoin condensation catalyzed by thiazolium cations in mild basic conditions monitored by 1 H-NMR shows that the reaction is a complex process of second order in both aldehyde and thiazolium cation. These data strongly support our previous mechanistic proposal that the actual catalytic species in that reaction are the bis(thiazolin-2-ylidene)s. Consequences for the biochemical processes mediated by thiamin are also discussed.

Structure versus electrical behavior in films of homoassociates of diprotonated meso-tetrakis(4-sulfonato- phenyl)porphyrin

The gel-like films obtained from the homoassociation of diprotonated meso-tetrakis(4-sulfonatophenyl)porphyrin show structural changes depending on their water content, which is determined by the affinity for water of the ionic species segregated from the homoassociation process. These materials, in which NaCl or H2SO4 are segregated, were studied and their electrochemical behavior at several water contents was examined. The behavior of humid films was clearly different from dry films. The pH2O value, which determines the separation between the dry and humid behavior, depends on the type of the ionic species segregated in the homoassociation. This is briefly discussed in relation to the design of humidity sensors.

Solution Structure of Mesobilirubin XIIIa Bridged Between the Propionic Acid Substituents [1]

Summary. The effect of cyclization of the propionic acid residues in mesobilirubin XIIIα propan-1,3-diyl ester on the angle between the dipyrrinone halves has been studied. This compound is shown to dimerize in chloroform solutions, and a conformation is proposed which is similar to that of the corresponding dimethyl ester. The results point to the contribution of π–π interactions, besides hydrogen bonding, to the geometry of the dimer.Zusammenfassung. Es wird der Einfluß der Zyklisierung von Mesobilirubin XIIIα– Propan-1,3-diylester über die Propionsäurereste auf den Winkel zwischen den beiden Dipyrrinonhälften untersucht. Die Verbindung liegt in CHCl3 als Dimeres vor. Eine Konformation analog zum entsprechenden Dimethylester wird vorgeschlagen. Neben H-Brücken sind offenbar auch π–π-Wechselwirkungen für die Struktur des Dimeren ausschlaggebend.

Solution Structure of Mesobiliverdin XIIIα Bridged Between the Propionic Acid Substituents [1]@@@Zur Struktur des über die Propionsäurereste überbrückten Mesobiliverdins XIIIα in Lösung

Summary. The solution structure of two intramolecular diesters (methylene and 1,3-propylidene) of mesobiliverdin-XIIIα was studied and compared with that of the corresponding dimethyl ester. The UV/Vis absorption spectra, chiral discrimination with ethyl (S)-(−)-lactate, and the 1H NMR spectra (ROESY) show that the cyclization of the propionate substituents of biliverdins does not significantly affect the helix structure or its (P) ⇌ (M) interconversion. The internal methylene diester does not show conformational heterogeneity of the propionate substituents and probably exists only in one diastereomeric form. In this case, the results point to a simultaneous racemization of the tetrapyrrole helix and the bridge cycle. The methylene diester of mesoprotoporphyrin was also synthesized. In this case, the geometry of the propionate chains is probably similar to that present in some hemoproteins.Zusammenfassung. Die Struktur zweier zyklischer Ester (Methylendioxy- und 1,3-Dipropylenoxy-) des Mesobiliverdin XIIIα in Lösung wurde untersucht und mit der des entsprechenden Dimethylesters verglichen. Die UV/Vis-Spektren, die chirale Diskriminierung mit Äthyl-(S)-(−)-Laktat und die 1H-NMR-Spektren (ROESY) zeigen, daß durch interne Zyklisierung über die Propionsäure die Helixstruktur oder die (P)=(M) Konvertierung kaum beinflußt wird. Beim zyklischen Methylendioxyester weisen die Propionsäuresubstituenten verschiedene Konformationen auf. Wahrscheinlich existiert nur eine einzige diastereoisomere Form unter gleichzeitiger Racemisierung der Tetrapyrrolhelix und des Ringes der Brückenester. Der Methylendioxyester des Mesoprotoporphyrins wurde ebenfalls dargestellt. Die Geometrie seiner Propionatreste entspricht wahrscheinlich derjenigen, die in einigen Hämoproteinen gefunden wurde.