Diastereoisomerism, Stability, and Morphology of Substituted meso-4-Sulfonatophenylporphyrin J-Aggregates

Abstract

The comparison between nanoparticle morphologies of the J-aggregates of different meso-4-sulfonatophenylporphyrins showing non-sulfonato groups at some of the meso-positions constitutes an ultimate proof of the 2D crystal-like character of the basic self-assembly motif of this family of J-aggregates. Diastereoisomerism stemming from the tacticity of the relative configurations in relation to the J-aggregate bidimensional sheet is the key factor that determines both the striking monolayer in solution and also the hierarchical pathways leading to different nanoparticle morphologies upon further growth. The unexpected stability of such large monolayered sheets made up of porphyrin units is probably caused by the support originated at both surface faces by the double layer potentials of the peripheral ionic substituents. These double layer potentials play a driving role in the subsequent 3D growth of the monolayers, as deduced herein from the determining role of tacticity both in the stability of the J-aggregate sheet and in its evolution either to monolayered or to bilayered nanoparticles. The stabilizing role of the forces at the electrical double layer of the particle suggests a relationship between these forces and the previously reported detection of racemic biases when shear hydrodynamic forces are in action during the aggregation process.

Scheme 1. Primordial Chiral Sheet of H₂TPPS₄^2– J Aggregates (One Enantiomorph Represented)

The different colors of the porphyrin units, symbolized here as parallelepipedes, indicate two conformational enantiomers of the porphyrin monomer. This basic sheet structure evolves toward mono- or multilayers nanoparticles of different shapes.^, Adapted with permission from ref (12).

Scheme 2. Formula Scheme of Diprotonated meso-4-Sulfonatophenyl-substituted Porphyrins Discussed in This Work and on the Diastereoisomerism Arising from the Relative configuration of the Out-of-Plane 10- and 20-Meso Substituents of the J-Aggregate Primordial Sheet

Figure 1

UV/vis absorption spectra of H₂TPHS₃^– (4.5 μM and 0.8 mM) and H₂DPH₂S₂ (0.5 μM and 0.9 mM) J-aggregates UV/vis (mm cuvettes path length 1 cm and 0.01). At the μM concentration range (solid traces), no J-aggregates are formed. At the mM range (dotted traces), H₂TPHS₃^– does not form J-aggregates, but H₂DPH₂S₂ yields J-aggregates of ill-defined geometry (broad absorption bands).

Figure 2

Cryo-TEM and SPM of H₂TPCS₃^– J-aggregates (mica substrate). The first-formed single-walled nanotubes [peak force microscopy (PFM) on mica] evolve after few days toward large plates, which do not show enough contrast to be observed by CryoTEM, but can be observed by PFM.

Scheme 3. Scheme Showing How Isotacticity in H₂TPCS₃^– J-Aggregates Yields the Adequate Geometry for the Formation of a Polymeric Hydrogen Bonding Interface between Two Sheets

Eventually, anionic sulfonato groups (partial isotacticity) acting as hydrogen bond acceptors could replace some carboxylic acid groups. Different tacticity domains would explain the ability to form plates composed of more than two sheets (Figure 2)

Figure 3

CryoTEM and PFM imaging of H₂TPPS₃^– J-aggregate nanotubes obtained by the capillary injection of the mother porphyrin solution into a stirred acidic solution. In contrast with the J-aggregate nanotubes of other porphyrins, their walls show voids and defects from linear growth and even the change of the curvature.

Figure 4

CryoTEM imaging of H₂TPPS₃^– J-aggregate nanotubes obtained by dropping a porphyrin mother solution into the acidic HCl solution (the black drops correspond to the initial phase of the transformation of vitrified water to crystallized water). The defects observed in the case of the capillary injection preparation method (Figure 3) are now enhanced yielding arborescent structures of irregular single-walled nanotubes. By SPM these samples show, after their deposition on dry substrates, a collapse (ref (7)) to the characteristic helical bundles of ribbons previously reported (e.g., ref (27)).

Figure 5

CryoTEM images of monolayer sheets of H₂TPF₅S₃^– J-aggregates. Their thickness, measured by PFM, agrees with monolayers of slightly different thickness as expected for different tacticities.

Figure 6

UV/vis absorption spectra of H₂TPPyS₃ (≈1 mM: cuvette path length = 0.01 mm) J-aggregates showing strongly shifted H- and J-aggregate excitonic bands (410 and 501 nm respectively compared to the B-band of the diprotonated monomer 437). The presence of bands with lower red shifts (shoulders at 467 and 488 nm) indicates the presence of shorter sheets along the x axis (see Scheme 1).

Figure 7

PFM image of H₂TPPyS₃ aggregates (HOPG). The relatively small nanoplates are all bilayered, but some of them show that monomeric porphyrin deposit on the top of the plate.