Pentaporphyrin with flexible, chiral nucleosidic linkers: unexpected duality of the physico-chemical properties of its core

Nucleosides as organizing agents

In the present review, we discuss the results of the research we have done over the last decade on multi-porphyrinic architectures bearing a nucleotidic backbone, compare all the data, offer a novel overview, and emphasize common capabilities of nucleosides appearing in various systems, providing a new insight on the pre-organizing capabilities of nucleosidic backbones. The efficiency of nucleosides as pre-organizing agents was investigated through the synthesis and study of various bis-porphyrins bearing nucleosidic linkers, as well as through the investigation of the conformation of linear and arborescent multi-porphyrins constructed on a nucleosidic backbone. The capacity of these molecules to complex guests with a high association constant was used as a tool to evaluate their degree of pre-organization, as well as the investigation of the electronic coupling existing between their chromophores and their photo-chemical capacities. Such an overview of one decade of scientific investigations documents the fact that rigid linkers between chromophores are not necessary for their spatial pre-organization, opening new routes to the faster synthesis of flexible highly pre-organized molecular architectures avoiding the long and tedious synthesis of rigid tweezers, especially for the preparation of rigid linkers which may bring solubility and stability problems.

Selective anion binding by a cofacial binuclear zinc complex of a Schiff-base pyrrole macrocycle

The synthesis of the new cofacial binuclear zinc complex [Zn(2)(L)] of a Schiff-base pyrrole macrocycle is reported. It was discovered that the binuclear microenvironment between the two metals of [Zn(2)(L)] is suited for the encapsulation of anions, leading to the formation of [K(THF)(6)][Zn(2)(μ-Cl)(L)]·2THF and [Bu(n)(4)N][Zn(2)(μ-OH)(L)] which were characterized by X-ray crystallography. Unusually obtuse Zn-X-Zn angles (X = Cl: 150.54(9)° and OH: 157.4(3)°) illustrate the weak character of these interactions and the importance of the cleft preorganization to stabilize the host. In the absence of added anion, aggregation of [Zn(2)(L)] was inferred and investigated by successive dilutions and by the addition of coordinating solvents to [Zn(2)(L)] solutions using NMR spectroscopy as well as isothermal microcalorimetry (ITC). On anion addition, evidence for deaggregation of [Zn(2)(L)], combined with the formation of the 1:1 host-guest complex, was observed by NMR spectroscopy and ITC titrations. Furthermore, [Zn(2)(L)] binds to Cl(-) selectively in THF as deduced from the ITC analyses, while other halides induce only deaggregation. These conclusions were reinforced by density functional theory (DFT) calculations, which indicated that the binding energies of OH(-) and Cl(-) were significantly greater than for the other halides.

Cyclodipeptide-bridged porphyrin dimer supramolecular assemblies

A cyclodipeptide-bridged porphyrin dimer has formed fibrous and toroidal multi-porphyrin array systems by hydrogen bonding-mediated self-assembly.

Hydrogen-bonding induced cooperative effect on the energy transfer in helical polynorbornenes appended with porphyrin-containing amidic alanine linkers

Polynorbornenes appended with porphyrins containing a range of different linkers are synthesized. The use of bisamidic chiral alanine linkers between the pending porphyrins and the polymeric backbone has been shown to bring the adjacent porphyrin chromophores to more suitable orientation for exciton coupling owing to hydrogen bonding between the adjacent linkers. The hydrogen bonding between the adjacent pendants in these polymers may induce a cooperative effect and therefore render single-handed helical structures for these polymers. Such a cooperative effect is reflected in the enhancement of FRET efficiencies between zinc-porphyrin and free base porphyrin in random copolymers.

Synthesis and solvent driven self-aggregation studies of meso-"C-glycoside"-porphyrin derivatives

New types of porphyrin derivatives bearing "C-glycoside" moieties, either in 5,10,15,20- or in 5,15-meso-positions, were prepared and fully characterized. The presence of the glycosidic groups imparts to the title macrocycles, besides an amphiphilic character, a clear tendency to form chiral suprastructures upon solvent-driven self-aggregation in different aqueous-organic solvent mixtures. Supra-assembly phenomena, in terms of the size and morphology of the resulting structures, as well as their kinetics of aggregation, were studied by UV-visible, fluorescence, resonance light scattering (RLS), and CD spectroscopy, indicating that the morphology of the aggregates depends strongly on the structure of the porphyrin rings, and on the bulk conditions of aggregation.

Excitation-Energy Migration in Self-Assembled Cyclic Zinc(II)-Porphyrin Arrays: A Close Mimicry of a Natural Light-Harvesting System

The excitation-energy-hopping (EEH) times within two-dimensional cyclic zinc(II)-porphyrin arrays 5 and 6, which were prepared by intermolecular coordination and ring-closing metathesis reaction of olefins, were deduced by modeling the EEH process based on the anisotropy depolarization as well as the exciton-exciton annihilation dynamics. Assuming the number of energy-hopping sites N = 5 and 6, the two different experimental observables, that is, anisotropy depolarization and exciton-excition annihilation times, consistently give the EEH times of 8.0 +/- 0.5 and 5.3 +/- 0.6 ps through the 1,3-phenylene linkages of 5 and 6, respectively. Accordingly, the self-assembled cyclic porphyrin arrays have proven to be well-defined two-dimensional models for natural light-harvesting complexes.

Excitation Energy Migration in A Dodecameric Porphyrin Wheel

Intramolecular excitation energy hopping (EEH) time within a dodecameric porphyrin wheel C6ZA, in which six meso-meso linked zinc(II) diporphyrin (Z2) subunits are bridged by 1,3-phenylene spacers, is deduced by a Förster energy hopping model based on S(1)-S(1) exciton-exciton annihilation and anisotropy depolarization. Under the assumption that the energy hopping sites are six Z2 subunits, two different observables (e.g., exciton-exciton annihilation and anisotropy depolarization times) consistently give the EEH time of 4.0 +/- 0.4 ps via 1,3-phenylene spacer of C6ZA, which is faster than 9.4 ps of linear 2Z2 (1,3-phenylene-linked zinc(II) tetraporphyrin). As a consequence, C6ZA serves as a well-defined two-dimensional model for a light-harvesting complex.

Porphyrin Boxes Constructed by Homochiral Self-Sorting Assembly: Optical Separation, Exciton Coupling, and Efficient Excitation Energy Migration

meso-Pyridine-appended zinc(II) porphyrins Mn and their meso-meso-linked dimers Dn assemble spontaneously, in noncoordinating solvents such as CHCl3, into tetrameric porphyrin squares Sn and porphyrin boxes Bn, respectively. Interestingly, formation of Bn from Dn proceeds via homochiral self-sorting assembly, which has been verified by optical separations of B1 and B2. Optically pure enantiomers of B1 and B2 display strong Cotton effects in the CD spectra, which reflect the length of the pyridyl arm, thus providing evidence for the exciton coupling between the noncovalent neighboring porphyrin rings. Excitation energy migration processes within Bn have been investigated by steady-state and time-resolved spectroscopic methods in conjunction with polarization anisotropy measurements. Both the pump-power dependence on the femtosecond transient absorption and the transient absorption anisotropy decay profiles are directly associated with the excitation energy migration process within the Bn boxes, where the exciton-exciton annihilation time and the polarization anisotropy rise time are well described in terms of the Förster-type incoherent energy hopping model by assuming a number of hopping sites of N = 4 and an exciton coherence length of L = 2. Consequently, the excitation energy hopping rates between the zinc(II) diporphyrin units have been estimated for B1 (48 ps)(-1), B2 (98 +/- 3 ps)(-1), and B3 (361 +/- 6 ps)(-1). Overall, the self-assembled porphyrin boxes Bn serve as a well-defined three-dimensional model for the light-harvesting complex.

Molecular engineering of extended chiral and enantiopure multiporphyrinic architectures: Towards the control of their physico-chemical properties

We report our recent progress in the synthesis and study of extended chiral and enantiopure multiporphyrinic devices. We devoted particular efforts to the study of the influence of both structure and conformation of a molecule on its physicochemical properties, and in this perspective peptides and nucleosides have been chosen as interchromophore linkers. Concerned with the elaboration of molecular wires, we synthesized enantiopure peptides bearing pendant porphyrins. The synthesis of a peptide built according to a predetermined sequence of L-lysine derivative holding porphyrin-functionalized lateral chain allowed, beyond the preparation of homo-octaporphyrins with free-base or Zn(II) porphyrins, the elaboration of a pentaporphyrin with an assigned sequence of chromophores. A two-dimensional system, a star-shaped pentaporphyrin with chiral enantiopure linkers derived from uridine, was prepared. Electrochemical and photophysical investigations have shown a duality of the physicochemical properties of the core porphyrin.