Self-organized porphyrinic materials

Abstract

The self-assembly and self-organization of porphyrins and related macrocycles enables the bottom-up fabrication of photonic materials for fundamental studies of the photophysics of these materials and for diverse applications. This rapidly developing field encompasses a broad range of disciplines including molecular design and synthesis, materials formation and characterization, and the design and evaluation of devices. Since the self-assembly of porphyrins by electrostatic interactions in the late 1980s to the present, there has been an ever increasing degree of sophistication in the design of porphyrins that self-assemble into discrete arrays or self-organize into polymeric systems. These strategies exploit ionic interactions, hydrogen bonding, coordination chemistry, and dispersion forces to form supramolecular systems with varying degrees of hierarchical order. This review concentrates on the methods to form supramolecular porphyrinic systems by intermolecular interactions other than coordination chemistry, the characterization and properties of these photonic materials, and the prospects for using these in devices. The review is heuristically organized by the predominant intermolecular interactions used and emphasizes how the organization affects properties and potential performance in devices.

Figure 1. Porphyrin tweezers for C60

X-ray crystal structure of a porphyrin dyad with encapsulated C₆₀. The Pd2···C24F distance is 2.856(10) Å. A combination of π-π interactions and electrostatic forces between the porphyrins in the tweezers allow the complexation of the fullerene.

Figure 2. Rotaxane binding

Intramolecular hydrogen bonds help to pre-organize a porphyrin dimer to bind and cap a rotaxane as well as stabilize the final structure.

Figure 3. Self-complementary H-bonding dimer

Intermolecular self-complementary hydrogen bonding groups mediate the assembly of an open porphyrin dimer. Note that intramolecular hydrogen bonds help to orient the recognition motifs, and the atropisomerization is minimized by the strap between the meso aryl groups.

Figure 4. Goldberg structure

Mono-(3-pyridyl)-tris-(4-carboxyphenyl)porphyrin forms a non-centrosymmetric space group P21 with a twofold screw axis running in the vertical direction driven by the COOH…N(py) hydrogen bonds wrapped around in a helical manner. (Courtesy of I. Goldberg).

Figure 5. Capsule with calixerene

Reversible self-assembly of host capsule formed by hydrogen bonding to a calixerane allows it to discriminate between possible guests.

Figure 6. Melamine barbituric structures

Atropisomers of uracyls on the 5 and 15 positions of a porphyrin dictate the formation of closed, face-to-face dimers, or open zig-zag chains when the complementary melamine is added.

Figure 7. H-bonding square array

Self-assembly of square helix by self-complementary hydrogen bonding motifs. The recognition groups are attached directly to the meso positions of the porphyrins. Thus, the optimal positioning minimizes the dynamics that would decrease the structural fidelity of the assembled systems.

Figure 8. NLO film

Electrostatic self-assembly of multilayers on quartz can result in porphyrinic films with good NLO properties.

Figure 9. Porphyrin-POM films

Films fabricated by electrostatic interaction from tetra-N-methylpyridiniumporphyrin⁴⁺ and [EuPW₁₁O₃₉]⁴⁻ polyoxometalate using a sequential dipping method on an ITO surface: an example of multilayered architecture created from small molecules.

Figure 10. Por-M-POM ternary complexes

Crystal packing of the [(TPyP)Hf(PW₁₁O₃₉)]⁻⁵ complex shows formation of the zig-zag pattern along the a-axis, solvent omitted, where the top surface of one porphyrin approaches the side of the POM of an adjacent complex. The structure is reinforced by H-bonds between water and the pyridyl moeieties.

Figure 11. Hupp layers

Hierarchically organized thin film materials: films of molecular squares are formed by layer-by-layer deposition (Courtesy of J. T. Hupp).

Figure 12. Hipps STM studies

STM constant current image of Co(II)TPP/Co(II)Pc (top) and Co(II)TPP/F₁₆Co(II)Pc (bottom) on gold illustrates how weak intermolecular hydrogen bonding between the components in the system with the fluorinated dye result in significantly different 2-dimensional order.

Figure 13. MnPor on Au electrode

Molecular structures of sulfide-linked manganese porphyrin derivatives with different methylene spacer lengths. SAMs based on protoporphyrin IX can be used as a model to study the electron transfer to the electrode as a function of the different tethers, length and molecular orientation.

Figure 14. Lindsey triple decker

Triple decker sandwich compounds have been proposed as building blocks for molecular electronics because of their multiple reversible oxidation/reduction process. (A) Schematic representation of the camshaft rotation of the triple decker with surface attachment group (SAG). (B) Example of molecular structure of a phthalocyanine-phthalocyanine-porphyrin triple decker sandwich.

Fig. 15. porphyrin dyad with spacer: 1.2 nm separation

The correct separation of two porphyrins in a dyad should be about 1.2 nm in order to host a fullerene C₆₀.

Fig. 16. Highly fluorinated porphyrin and C60

Films of a highly fluorinated porphyrin and C₆₀ form when cast on ITO due to both π-π interactions and the additional van der Waal’s interactions between the fluorous alkanes and the fullerene.

Figure 17. D’Souza por C60

Self-assembly of porphyrin-fullerene materials can be mediated by synergic interactions using metal ion coordination and ionic recognition. First the tetra-crown ethers substituted porphyrin (a) pre-assembles into a cofacial arrangement where the potassium ion is sandwiched between crown ethers (b). Second, a fullerene, bis-substituted with a pyridyl and alkylammonium groups, coordinates to the Zn and one of the crown ether entities yielding the formation of a triad if the ratio is 1:1 (c), or tetrad if the ratio is 1:2 (d).

Figure 18. TPPF20 C60 cocrystals -- Hosseini

Honeycomb array of C₆₀ co-crystallized with perfluorophenylporphyirn. The porphyrin-C₆₀ arrangement is dictated by π-π and electrostatic interactions, whereas the proximity of the porphyrins is due to C-F···H-C interactions between some F atoms to the pyrrole βH on a neighboring molecule.

Figure 19

Wasielewski Zinc-TPP surrounded by four perylene-3,4,9,10-bis(dicarboximide) (ZnTPP-PDI)₄. Left: Molecular structure. Right: Side view of MM+ geometry optimized structure. The perylene units are electron-acceptors and promote the electronic communication between the adjacent porphyrin allowing energy or charge transfer. Self-assembly of the (ZnTPP-PDI)₄ is driven primarily by π-π interactions between the adjacent PDI and yields columnar stacks of an average of five cofacial molecules

Figure 20

Schawb’s porphyrin nanorods AFM image of H₄TPPS₄2- bundles of nanorods formed from acidic solution deposited on MICA. The nanorods form by self-assembly of J-aggregates of porphyrins in HCl aqueous solution. The substrate was immersed in 5 μM solution of porphyrin in 0.3 M HCl. The density of the nanorods bundles increase with immersion time.

Figure 21. dodecaphenyl + C60

Saddle-distorted porphyrins and metalloporphyrins are tectons for self-assembly with curved surfaces so are useful in the fabrication of tubular and circular structure nanostructures based on non-covalent interactions. For example, Mo(V)dodecaphenylporphyrin has an ideal shape to yield curved structures.

Figure 22

Nolte liquid crystal figure Porphyrin liquid crystal patterning: both the presence of nine dodecyl groups and intramolecular hydrogen-bonding contribute to the organization of a porphyrin trimer on MICA. (A) Molecular structure, (B) schematic representation, (C) columnar stack of the liquid crystal, (D) AFM image (scan size = 25 × 25 μm²) of a pattern formed on MICA, (E) zoomed AFM image (scan size = 10 × 10 μm²).

Figure 23. Sessler’s LC

Hydrazinoporphyrin liquid crystal; n=1,6,10,14. This is the first example of an expanded porphyrin as a core of a liquid crystal.

Figure 24. Nolte catalyst on polymer

The direction of a supramolecular catalyst threaded on a butadiene polymeric substrate is driven by the differences in binding affinities for the diene compared to the epoxide product (courtesy of R.J.M. Nolte).

Scheme 1

Different types of porphyrinoid structures.

Scheme 2

Left: porphyrin macrocycles ‘P’ can be attached to a metal substrate ‘M’ through a spacer ‘S’ having a reactive linking group ‘X’ that is matched to the chemical reactivity of the surface. Right: the number, direction, and relative positions of the linking groups can determine the relative geometry of the macrocycles on a surface.