Valérie Heitz

Rotaxanes as new architectures for photoinduced electron transfer and molecular motions

Rotaxanes are molecular architectures ideally suited for building integrated, multicomponent modular systems, displaying novel chemical and physical properties. In this new field of functional rotaxanes, those incorporating transition metals, which are incidentally used as synthetic templates, are particularly attractive for their photophysical and electronic properties as well as their dynamic behaviour. It is believed that they will provide, in the future, the basic elements for constructing nanoscale machines and motors.

Diketopyrrolopyrrole‐Porphyrin Conjugates with High Two‐Photon Absorption and Singlet Oxygen Generation for Two‐Photon Photodynamic Therapy

Two-photon photodynamic therapy is a promising therapeutic method which requires the development of sensitizers with efficient two-photon absorption and singlet-oxygen generation. Reported here are two new diketopyrrolopyrrole-porphyrin conjugates as robust two-photon absorbing dyes with high two-photon absorption cross-sections within the therapeutic window. Furthermore, for the first time the singlet-oxygen generation efficiency of diketopyrrolopyrrole-containing systems is investigated. A preliminary study on cell culture showed efficient two-photon induced phototoxicity.

A [3]Rotaxane with Two Porphyrinic Plates Acting as an Adaptable Receptor

Following a multistep procedure, the copper(I)-templated strategy allowed preparation of a multifunctional [3]rotaxane. The dumbbell consists of a central two-bidentate chelate unit and two terminal stoppers. The two rings threaded on the rotaxane axis consist each of a 1,10-phenanthroline-incorporating macrocycle, rigidly connected to an appended zinc-complexed porphyrin. The copper(I) template can be removed, affording a free rotaxane whose two rings can glide freely along the axis and spin around it. The dumbbell being very long (approximately 85 A in its extended conformation from one stopper to the other), the porphyrin-porphyrin distance can be varied over a wide range. The two porphyrinic plates constitute the key elements of a receptor able to complex various guests between the plates. The ability of the threaded rings to move freely makes the host perfectly adjustable, allowing capture of geometrically very different guests. The copper(I)-complexed rotaxane also acts as an efficient receptor, although its adaptability is obviously more limited than that of its free rotaxane counterpart.

Templated Synthesis of Cyclic [4]Rotaxanes Consisting of Two Stiff Rods Threaded through Two Bis-macrocycles with a Large and Rigid Central Plate as Spacer

Two related cyclic [4]rotaxanes consisting of double macrocycles and rigid rods incorporating two bidentate chelates have each been prepared in high yield. The first step is a multigathering and threading reaction driven by coordination of two different bidentate chelates (part of either the rings or the rods) to each copper(I) center so as to afford the desired precursor. In both cases, the assembly step is done under very mild conditions, and it is quantitative. The second key reaction is the stopper-attaching reaction, based on click chemistry. Even if the quadruple stoppering reaction is not quantitative, it is relatively high-yielding (60% and 95%), and the copper-driven assembly process is carried out at room temperature without any aggressive reagent. The final copper-complexed [4]rotaxanes obtained contain two aromatic plates roughly parallel to one another located at the center of each bis-macrocycle. In the most promising case in terms of host-guest properties, the plates are zinc(II) porphyrins of the tetra-aryl series. The compounds have been fully characterized by various spectroscopic techniques ((1)H NMR, mass spectrometry, and electronic absorption spectroscopy). Unexpectedly, the copper-complexed porphyrinic [4]rotaxane could be crystallized as its 4PF(6)(-) salt to afford X-ray quality crystals. The structure obtained is in perfect agreement with the postulated chemical structure of the compound. It is particularly attractive in terms of symmetry and molecular aesthetics. The distance between the zinc atoms of the two porphyrins is 8.673 A, which is sufficient to allow insertion between the two porphyrinic plates of small ditopic basic substrates able to interact with the central porphyrinic Zn atoms. This prediction has been confirmed by absorption spectroscopy measurements in the presence of various organic substrates. However, large substrates cannot be introduced in the corresponding recognition site and are thus complexed mostly in an exo fashion, being located outside the receptor cavity. Noteworthy, the stability constants of the 1:1 host-guest complexes are high (10(7) M(-1)).

Cyclic [2]pseudorotaxane tetramers consisting of two rigid rods threaded through two bis-macrocycles: copper(I)-templated synthesis and X-ray structure studies.

Variously substituted coordinating rigid rods have been synthesized which incorporate a central 4,7-phenanthroline nucleus attached to two 2-pyridyl groups via its 3 and 8 positions, so as to yield bis-bidentate chelates, the two-coordinating axes of the chelates being parallel to one another. Regardless of the nature of the substituents borne by the rods, the copper(I)-induced threading reaction of two such rods through the rings of two bis-macrocycles affords in a quantitative yield the 4-copper(I) threaded assembly. The [2]pseudorotaxane tetramers thus obtained have been fully characterized in solution and, for one of them, an X-ray structure could be obtained, confirming the threaded nature of the complex and providing important structural information.

Photoinduced electron transfer in multiporphyrin clusters and rotaxanes

A zinc(I1) porphyrin and a gold(II1) porphyrin dimer bridged by a 2,9-diphenyl-l, 10-phenanthrolin e (dpp) spacer has been synthesized in order to mimic the array of tetrapyrrolic chromophores in the bacterial photosynthetic centres. The electronic properties of the dpp bridge have been tuned by complexation o f copper(1) either in a hornoleptic, entwined complex, or i n a rotaxane complex whose threaded ring is a dpp-containing m acrocycle. Photoinduced electron transfer between the zinc porphyrin excited state and the gold porphyrin cation has been studied by laser f lash spectroscopy and its rate has been shown to depend on the nature of the bridging s pecies lying in between the metal porphyrins : 55 ps in the dpp- bridged dimer, a nd 1 ps only in the rotaxane copper(1) complex.

A Cyclic (4)rotaxane that Behaves as a Switchable Molecular Receptor: Formation of a Rigid Scaffold from a Collapsed Structure by Complexation with Copper(I) Ions**

The most efficient molecular receptors are usually rigid edifices with a hollow part that is able to accommodate the complexed species through an electronic and geometrical complementarity between the substrate and the complexing parts of the host. By analogy with biological processes related to induced fit, other host–guest processes are based on flexible hosts that are able to adapt their geometry to that of the species to be recognized. In the very active field of catenanes, rotaxanes, and molecular machines, very few systems have been considered as interesting receptors for molecular guests. One of the main contributions to this subfield of research is that of anion recognition by various interlocking compounds. Our research group has also recently described a [3]rotaxane that is able to act as an adjustable receptor. The system consists of two rings threaded by an axis on which the rings can move freely. The complexing units are zinc porphyrins that are firmly attached to the rotaxane rings and are able to interact with given substrates that consist of doubly end functionalized compounds bearing 4-pyridyl groups. It was shown that the marked geometrical adaptability of the host in its metal-free form allows interaction with guests of very different sizes. Herein we report the properties of a related compound, namely a cyclic bisporphyrin [4]rotaxane, the behavior of which is totally different from that of a previously studied linear [3]rotaxane. Contrary to the latter compound, the metal-free [4]rotaxane collapses completely and does not show any complexation properties, whereas the copper(I)complexed compound is a good and selective receptor for diamine and dipyridyl substrates because of the scaffolding effect of the four metal centers (Figure 1). The recognition process can thus be switched on and off by complexing the free ligand to four Cu ions or demetalating the metalcomplexed species, respectively. The synthesis of rotaxane 1 [7] as well as that of its related [4]pseudorotaxane have already been reported. 1 was demetalated using a large excess of KCN (ca. 50 equivalents) at room temperature. The reaction mixture was stirred for 2.5 hours, and the crude product was then purified by chromatography on silica gel to afford the demetalated rotaxane 2 in 88% yield (Scheme 1). Rotaxane 2 was characterized by H NMR spectroscopy (COSY, ROESY, DOSY), electrospray mass spectrometry and UV/Vis spectroscopy. Very surprisingly, the H NMR spectrum of rotaxane 2 shows considerable loss of symmetry compared to the metalated system 1, in which all the Cu centers as well as the four stoppers were chemically equivalent. A quarter of the rotaxane 1 only had thus to be considered for complete NMR assignment. By contrast, the H NMR signals of 2 were doubled compared to those of rotaxane 1. Two distinct stoppers with significantly different H NMR signals and thus markedly different environments can be identified. NOE interactions between some parts of 2 were clearly detected, whereas these fragments were too far away from one another in 1 to show any interaction. For instance, some protons of the tBu groups of one type of stopper (H-re’; r= “rod”) correlate to protons H-b4’ and Hb7’ (b=bismacrocycle) of the 1,10-phenanthroline unit in 2, thus indicating that this stopper and a given 1,10-phenanthroline unit are located very close to one another. As the bismacrocycles and the axles are both rigid, the proximity between two such fragments tends to indicate that the topography of 2 is markedly different from that of 1, and that 2 has a totally collapsed structure. This hypothesis was confirmed by additional observations. For instance, one proton of the (CH2)3 linker (H-r9) is strongly shielded upon decomplexation; the corresponding signal moves from Figure 1. Principle of the recognition process that is switched on and off by metalation or demetalation. Small gray dots: Cu ions; gray squares: porphyrins; black double arrow: guest compound. The chemical structures of 1 and 2 are shown in Scheme 1.

Ion‐Pair‐Mediated Asymmetric Synthesis of a Configurationally Stable Mononuclear Tris(diimine)–Iron(II) Complex

Configurational stability is conferred on the complex 1 (R=4-MeOC6H4) by the carefully designed tetradentate bis(1,10-phenanthroline) ligand. The resolution and asymmetric synthesis of 1 were readily achieved by using tris(tetrachlorobenzenediolato)phosphate(V) anions (2) as chiral auxiliaries. The picture shows the separation of the enantiomers of 1 by preparative ion-pair thin-layer chromatography.

Transition metal-assembled multiporphyrinic systems as models of photosynthetic reaction centre

Abstract A multicomponent system consisting of a zinc(II) porphyrin and a gold(III) porphyrinic module assembled via a central bis(phenyl-terpyridine) ruthenium(II) complex has been prepared following a new gathering strategy. Excitation into the zinc porphyrin moiety is followed by rapid stepwise intramolecular electron transfer leading to a relatively long-lived (33 ns), charge-separated state consisting of a zinc porphyrin radical cation and a neutral radical localized on the gold(III) porphyrin ring. The edge-to-edge separation beween the porphyrinic units is ca. 20 A.

ROTAXANES AND OTHER TRANSITION METAL-ASSEMBLED PORPHYRIN ARRAYS FOR LONG-RANGE PHOTOINDUCED CHARGE SEPARATION

Abstract Bis-porphyrin conjugates have been synthesized for performing long-range photoinduced charge separation. The electron donor is a zinc(II) porphyrin in the excited state and the electron acceptor is a gold(III) porphyrin. These elements or their free-base analogs have been assembled following two different strategies. In a first approach, the porphyrinic subunits, separated by one or more 2,9-diphenyl-1,10-phenanthroline spacers, have been incorporated into rotaxane structures containing up to four threaded macrocycles by copper(I) templated synthesis. In a second approach, a triad having strictly controlled geometry has been prepared following a new gathering strategy. This triad, consisting of two porphyrinic moieties assembled via a central bis(phenyl-terpyridine)ruthenium(II) complex, uses coordination rather than covalent chemistry to interconnect the components.