Narrow Results

Our recent progresses in the synthesis and the study of supramolecular porphyrin-fullerene conjugates are reported. In particular, we show that π-stacking of the two chromophores can have a dramatic effect on recognition interactions in non-covalent C₆₀-porphyrin ensembles. Specifically, a supramolecular complex has been obtained from a porphyrin-crown ether conjugate and a methanofullerene derivative bearing an ammonium function. In addition to the ammonium-crown ether interaction, intramolecular stacking of the fullerene moiety and the porphyrin subunit has been evidenced in the supramolecular system. Due to this additional recognition element, the association constant (K_a) for the complex is increased by two orders of magnitude when compared to K_a values previously found for complexation of the fullerene-ammonium derivative with other crown ethers. Finally, non-covalent systems resulting from the association of the fullerene-ammonium derivative with porphyrins bearing two crown-ether subunits have been investigated. Intramolecular C₆₀-porphyrin interactions have also been observed within these supramolecular arrays. As a result, the 2:1 complexes are very stable.

A fullerene derivative bearing two pyridine subunits has been designed to allow assembly with a bis-Zn(II)-phorphyrin receptor with two equivalent Zn binding sites separated by about 20 Å. Owing to the complementarity between the bis-Zn(II)-porphyrin receptor and a fullerene ligand bearing two pyridine substituents, the substrate can be clicked on the ditopic receptor, thus leading to a stable non-covalent macrocyclic 1:1 complex.

We report herein the binding studies of three bis-porphyrinic tweezers bearing nucleosidic linkers and a bis-porphyrinic dipeptide with two bidentate bases of different size, DABCO and 4,4' -bipyridine. In the nucleosidic series, the nucleoside confers, for two dimers out of three, sufficient preorganization to the dimers to enhance the association constants, documenting the fact that rigid dimers are not necessary for preorganization, and opening up new routes to the faster synthesis of flexible tweezers capable of complexing guests with a high association constant. Comparison of these results with those obtained for rigid tweezers shows a better efficiency of the flexible nucleosidic dimers. We thus document the fact that the choice of rigid spacers is not the only way to pre-organize bis-porphyrins, and that some well-chosen nucleosidic linkers offer an interesting option for the synthesis of such devices.

We report herein the synthesis of a dinucleotide bearing pendant porphyrins dedicated to adopt a pre-organized coformation with face-to-face porphyrins, and capable to self-organize in a stable sandwich type complexe with bidentate base such as DABCO. Earlier studies demonstrated that a peptidic linker does not provide sufficient pre-organization to enhance significantly the association constant with bidentate bases such as DABCO on the contrary of some other flexible linkers such as uridine or 2′-deoxyuridine. We document herein that the gain in stability for the formation of sandwich type host–guest complex with DABCO can be even greater when a dinucleotide linker is used. Such pre-organization increases the association constants by one to two orders of magnitude when compared to the association constants of the same bidentate ligands with a reference Zn(II) porphyrin. Comparison of these results with those obtained for rigid tweezers shows a better efficiency of the flexible nucleosidic dimers. We thus document the fact that the choice of rigid spacers is not the only way to pre-organize bis-porphyrins, and that some well-chosen nucleosidic linkers offer an interesting option for the synthesis of such devices. Furthermore, the chirality and enantio-purity of the nucleosidic linkers paves the way toward the selective complexation of enantio-pure bidentate guests and the resolution of racemates.

We report herein that chiral and enantiopure compounds such nucleosides and peptides can pre-organize multi-porphyrinic systems and influence their properties. The first example given concerns star-shaped mutli-porphyrins with chiral and enantiopure nucleosidic linkers. If the configuration is indeed a star-shaped nanomolecule, it appears that the induced conformation is nothing as expected. The four peripheral Zn(II) porphyrins collapse over the free-base central one, inducing totally different photo-physical properties. Despite a minor expected light energy harvesting behavior, the principal capability of this system is to quench the collected light energy and convert it from radiative to non-radiative de-activation. The second example concerns polypeptides with pendant porphyrins. The peptidic backbone confers to the systems, after a certain degree of oligomerization, a 3${}_{\mathrm{\text{10}}}$ right handed helical conformation which induces cavities within the multi-porphyrinc architecture, ready to welcome guests and render, for example, the complexation of C${}_{\mathrm{\text{60}}}$ much easier. We thus have constructed novel organic photovoltaic systems using supramolecular complexes of porphyrin–peptide oligomers with fullerene clusters. The composite cluster OTE/SnO${}_{\mathrm{\text{2}}}$ electrode prepared with (P(ZnP)${}_{\mathrm{\text{16}}}+$ C${}_{\mathrm{\text{60}}}{)}_{\mathrm{\text{m}}}$, exhibits an impressive incident photon-to-photocurrent efficiency (IPCE) with values reaching as high as 56%. The power conversion efficiency of the (P(H${}_{\mathrm{\text{2}}}$P)${}_{\mathrm{\text{16}}}+$ C${}_{\mathrm{\text{60}}}{)}_{\mathrm{\text{m}}}$ modified electrode reaches 1.6%, which is 40 times higher than the value (0.043%) of the porphyrin monomer (P(H${}_{\mathrm{\text{2}}}$P)${}_{\mathrm{\text{1}}}$$+$ C${}_{\mathrm{\text{60}}}{)}_{\mathrm{\text{m}}}$ modified electrode. Thus, the organization approach between porphyrins and fullerenes with polypeptide structures is promising, and may make it possible to further improve the light energy conversion properties by using a larger number of porphyrins in a polypeptide unit.