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Long-Range Electron Transfer in Porphyrin-Containing [2]-Rotaxanes: Tuning the Rate by Metal Cation Coordination
Abstract
A series of [2]-rotaxanes has been synthesized in which two Zn(II)-porphyrins (ZnP) electron donors were attached as stoppers on the rod. A macrocycle attached to a Au(III)-porphyrin (AuP+) acceptor was threaded on the rod. By selective excitation of either porphyrin, we could induce an electron transfer from the ZnP to the AuP+ unit that generated the same ZnP*+-AuP* charge-transfer state irrespective of which porphyrin was excited. Although the reactants were linked only by mechanical or coordination bonds, electron-transfer rate constants up to 1.2x10(10) x s(-1) were obtained over a 15-17 A edge-to-edge distance between the porphyrins. The resulting charge-transfer state had a relatively long lifetime of 10-40 ns and was formed in high yield (>80%) in most cases. By a simple variation of the link between the reactants, viz. a coordination of the phenanthroline units on the rotaxane rod and ring by either Ag+ or Cu+, we could enhance the electron-transfer rate from the ZnP to the excited 3AuP+. We interpret our data in terms of an enhanced superexchange mechanism with Ag+ and a change to a stepwise hopping mechanism with Cu+, involving the oxidized Cu(phen)22+ unit as a real intermediate. When the ZnP unit was excited instead, electron transfer from the excited 1ZnP to AuP+ was not affected, or even slowed, by Ag+ or Cu+. We discuss this asymmetry in terms of the different orbitals involved in mediating the reaction in an electron- and a hole-transfer mechanism. Our results show the possibility to tune the rates of electron transfer between noncovalently linked reactants by a convenient modification of the link. The different effect of Ag+ and Cu+ on the rate with ZnP and AuP+ excitation shows an additional possibility to control the electron-transfer reactions by selective excitation. We also found that coordination of the Cu+ introduced an energy-transfer reaction from 1ZnP to Cu(phen)2+ (k = 5.1x10(9) x s(-1)) that proceeded in competition with electron transfer to AuP+ and was followed by a quantitative energy transfer to give the 3ZnP state (k = 1.5x10(9) x s(-1)).
... 49,50 Since the pioneering work by Sauvage et al., 51 Au(III) porphyrins have often been used as electron-accepting chromophores in donor−acceptor complexes for the study of photoinduced electron transfer due to their good stability and suitable redox properties. 52,53 The triplet state of Au(III) porphyrin is directly formed by irradiation with a yield of unity 52,54 and a lifetime of approximately 50 ns at room temperature. 55 These results inspired us to examine the effects of a heavy metal atom on the photovoltaic properties of Au(III) porphyrins acting as an acceptor unit in BHJ OSCs. ...
The widespread use of nonfullerene-based electron-accepting materials has triggered a rapid increase in the performance of organic photovoltaic devices. However, the number of efficient acceptor compounds available is rather limited, which hinders the discovery of new, high-performing donor:acceptor combinations. Here, we present a new, efficient electron-accepting compound based on a hitherto unexplored family of well-known molecules: gold porphyrins. The electronic properties of our electron-accepting gold porphyrin, named VC10, were studied by UV-Vis spectroscopy and by cyclic voltammetry (CV) , revealing two intense optical absorption bands at 500-600 and 700-920 nm and an optical bandgap of 1.39 eV. Blending VC10 with PTB7-Th, a donor polymer, which gives rise to an absorption band at 550-780 nm complementary to that of VC10, enables the fabrication of organic solar cells (OSCs) featuring a power conversion efficiency of 9.24% and an energy loss of 0.52 eV. Hence, this work establishes a new approach in the search for efficient acceptor molecules for solar cells and new guidelines for future photovoltaic material design.
... Numerous reports are available on the reaction between Fe(CN) 6 4− and metal complexes. [8][9][10][11][12] Gaswick et al. have reported that the hexacyanoferrate(II) anion can reduce pentamminecobalt(III) complexes to cobalt(II) via an outer-sphere electron transfer step [13] and they have also reported that the substituted pentamminecobalt(III) complexes could be reduced by hexacyanoferrate(II) with the formation of an ion pair. [14] A. R. Mustafina et al. [15] have studied the outersphere association of p-sulfanotothiacalix [4]arene (STCA) with some cobalt(III) complexes. ...
The kinetic study of reduction of single chain surfactant octahedral cobalt(III) complexes, cis-[Co(en)(2) (4AMP)(DA)](ClO4)(3) and cis-[Co(trien)(4CNP)(DA)](ClO4)(3) (where en = ethylenediamine; trien= triethylenetetramine; 4AMP = 4-aminopyridine; 4CNP= 4-cyanopyridine; DA= dodecylamine) with [Fe(CN)(6)](4-) has been carried out by spectrophotometric method under pseudo first order conditions using an excess of a reductant in different media namely, micelles, beta-cyclodextrim (beta-CD), liposome vesicles (DPPC) and ionic liquids ((BMIM) Br) at different temperatures in order to have deep insight into the molecular behavior during the electron transfer reaction. The analysis of kinetic results strongly suggests that the reduction reaction between the surfactant complex and [Fe(CN)(6)](4-) occurs via a second order outer sphere electron transfer mechanism. The remarkable increase in the rate of electron transfer from the oxidant to reductant is observed in ionic liquid medium as compared to the rest of all other media studied in the present investigation. This can be ascribed due to the fact that the formation of aggregated and constrained geometry of the surfactant cobalt(III) complexes (oxidants) in ionic liquid medium renders the rate of one electron transfer from the oxidants to the reductant, [Fe(CN)(6)](4-) to be higher. Furthermore, the rate constant value increases with increase in the concentration of micelles, however in the presence of beta-cyclodextrin medium the rate of electron transfer substantially decreases due to the inclusion of long aliphatic chain of the surfactant cobalt(III) complexes into beta-cyclodextrin. In liposome media, second order rate constant for this electron transfer reaction from both the oxidants was found to decrease with increase in the concentration of the liposome below the phase transfer temperature of DPPC. However, above the phase transition temperature the reaction was found to increase with increase in the concentration of the DPPC. The main driving force for this phenomenon is considered to be the intervesicular hydrophobic interaction between the surfactant complexes and vesicular surface. The activation parameters (Delta S-double dagger and Delta H-double dagger) of this outer sphere electron transfer reaction have been calculated by varying temperatures and the analysis of the data corroborates the kinetics of the reaction.
... Outer-sphere electron transfer between transition metal complexes plays an essential role both in vivo [1] and in the operation of molecular scale devices, such as molecular wires and logic gates [2][3][4]. Kinetic and stoichiometric studies of the reduction of cobalt(III) ammine complexes have yielded important information regarding the mechanism of electron transfer reactions. Bensen et al. have reported that Fe 2? can reduce cobalt(III) complexes via an outer-sphere electron transfer step. ...
The effect of unilamellar vesicles of dipalmitoylphosphotidylcholine (DPPC), both below and above the phase transfer region, on the second-order rate constants for outer-sphere electron transfer between Fe2+ and the surfactant–cobalt(III) complexes, cis-[Co(en)2(C12H25NH2)2]3+ and cis-[Co(trien)(C12H25NH2)2]3+ (en = ethylenediamine, trien = triethylenetetramine, C12H25NH2 = dodecylamine) was studied by UV–Vis absorption spectroscopy. Below the phase transition temperature of DPPC, the rate decreased with increasing concentration of DPPC, while above the phase transition temperature the rate increased with increasing concentration of DPPC. It is concluded that below the phase transition temperature, there is an accumulation of surfactant–cobalt(III) complexes at the interior of the vesicle membrane through hydrophobic effects, and above the phase transition temperature the surfactant–cobalt(III) complex is released from the interior to the exterior surface of the vesicle. Through isokinetic plots, we have established that the mechanism of the reaction does not alter during the phase transition of DPPC.
... Ruthenium polypyridyl complexes are of particular importance from the viewpoint of their unique photophysical and photochemical activity [1], which underlay the development of molecular machines and switches [2][3][4][5][6] or mimicking reactions of photosynthesis [7]. Another promising application of ruthenium polypyridyls is caused by their use as building blocks of heterometallic complexes, thus initiating the energy and electron transfer [8][9][10][11][12][13][14]. ...
The pH-dependent heterometallic complex formation with p-sulfonatothiacalix[4]arene (TCAS) as bridging ligand in aqueous solutions was revealed by the use of spectrophotometry, nuclear magnetic relaxation and fluorimetry methods. The novelty of the structural motif presented is that the appendance of emission metal center ([Ru(bpy)3]2+) is achieved through the cooperative non-covalent interactions with the upper rim of TCAS. The second metal block (Fe(III), Fe(II) and Mn(II)), bound with the lower rim of TCAS in the inner sphere coordination mode is serving as quencher of [Ru(bpy)3]2+ emission. The difference between the complex ability of Fe(III) and Fe(II) ions provides pH conditions for redox-dependent emission of [Ru(bpy)3]2+.
Ion pairs of charged π-electronic systems form assemblies with ordered arrangement through ⁱπ–ⁱπ interactions. The assemblies show various functionalities derived from the unique arrangement of the charged π-electronic systems with fascinating structures and electronic states. We summarize recent advances and achievements of the ion-pairing assemblies especially focusing on their assembly styles.
This review provides an overview of the π‐electronic ion pairs and assemblies that demonstrate photoresponsive behaviors through photoisomerization and photoinduced electron transfer. Various ion pairs arise from the incorporation of ionic substituents to photoisomerizable π‐electronic systems, such as azobenzene and diarylethene moieties, and their association with counterions. Conversely, numerous ion pairs, made up of positively and negatively charged π‐electronic systems serving as electron acceptor and donor units, respectively, are prepared through ion‐pair metathesis. In both scenarios, the design and synthesis of charged π‐electronic systems play a crucial role in ion‐pairing assemblies. Moreover, the structures and properties of these assembled entities are influenced by the inclusion of accompanying counterions. These ion pairs display photoresponsive actions, including phase transitions through photoisomerization and the emergence of excited‐state radical pairs due to photoinduced electron transfer. Notably, π‐stacked ion pairs (π‐sips) composed of charged porphyrins transform into π‐stacked radical pairs (π‐srps) upon photoexcitation. Multiple components, as oppositely charged π‐electronic systems associated by noncovalent interactions, possessing varying electronic states and traits are suitable for switching behaviors in response to external stimuli.
Various charged π-electronic systems have been synthesized for application in ion-pairing assemblies. Anion-responsive π-electronic systems, which form anion complexes as pseudo-π-electronic anions, have been functionalized by peripheral and core modifications. π-Electronic anions have also been constructed by the introduction of a negative charge to an aromatic system and the attachment of hydrogen-bonding-stabilized anionic units to the π-electronic systems via covalent linkages. Porphyrins have been found extended π-electronic systems that stabilize positively and negatively charged states. The combination of charged π-electronic systems enabled the formation of diverse ion pairs and their assemblies, exhibiting distinct assembling behaviors and properties depending on the constituent charged species.
More than three decades of research efforts have yielded powerful methodologies based on transition metal template-directed syntheses for the assembly of a huge number of interlocked systems, molecular knots, machines and synthesizers. Such template techniques have been applied in the preparation of mechanically linked electron donor–acceptor artificial photosynthetic models. Consequently, synthetic challenging photoactive rotaxanes and catenanes have been reported, in which the chromophores are not covalently linked but are still associated with undergoing sequential energy (EnT) and electron transfer (ET) processes upon photoexcitation. Many interlocked photosynthetic models produce highly energetic, but still long-living charge separated states (CSS). The present work describes in a historical perspective some key advances in the field of photoactive interlocked systems assembled by transition metal template techniques, which illustrate the usefulness of rotaxanes and catenanes as molecular scaffolds to organize electron donor–acceptor groups. The effects of molecular dynamics, molecular topology, as well as the role of the transition metal ion used as template species, on the thermodynamic and kinetic parameters of the photoinduced energy and electron transfer processes in the interlocked systems are also discussed.
Stable pillar[5]arene‐containing [2]rotaxane building blocks with pentafluorophenyl ester stoppers have been efficiently prepared on a multi‐gram scale. Reaction of these building blocks with various nucleophiles gave access to a wide range of [2]rotaxanes with amide, ester or thioester stoppers in good to excellent yields. The rotaxane structure is fully preserved during these chemical transformations. Actually, the addition‐elimination mechanism at work during these transformations totally prevents the unthreading of the axle moiety of the mechanically interlocked system. The stopper exchange reactions were optimized both in solution and under mechanochemical solvent‐free conditions. While amide formation is more efficient in solution, the solvent‐free conditions are more powerful for the transesterification reactions. Starting from a fullerene‐functionalized pillar[5]arene derivative, this new strategy gave easy access to a photoactive [2]rotaxane incorporating a C60 moiety and two Bodipy stoppers. Despite the absence of covalent connectivity between the Bodipy and the fullerene moieties in this photoactive molecular device, efficient through‐space excited state interactions have been evidenced in this rotaxane.
The complexation processes of N,N’‐dibutyl‐1,4,5,8‐naphthalene diimide (NDI) into two types of π‐electron‐rich molecular containers consisting of two Zn(II)‐porphyrins connected by four flexible linkers of two different lengths, were characterized by means of absorption and emission spectroscopies and molecular dynamics simulation. Notably, the addition of NDI leads to a strong quenching of the fluorescence of both cages only when they are in an open conformation suitable for guest encapsulation, a situation triggered by silver(I) ions binding to the lateral triazoles. Molecular dynamics simulations confirm the fast binding of NDI, likely assisted by NDI‐silver(I) interactions. Upon NDI complexation, the two porphyrin macrocycles get closer, with an optimized face to face orientation, suggesting an induced‐fit mechanism through π–π interactions with the NDI aromatic cycle. Ultrafast transient absorption experiments allowed to identify the process of quenching of the Zn‐porphyrin fluorescence as an efficient photoinduced electron transfer reaction between the cage porphyrin and the included NDI guest. The process occurs on fast and ultrafast time scales in the two complexes (1.5 ps and ≤300 fs) leading to a short‐lived charge separated state (charge recombination lifetimes in the order of 30–40 ps). The combined computational and experimental approach used here is able to furnish a reliable model of the NDI‐cage complexation mechanism and of the corresponding electron transfer reaction, attesting the allosteric control of both processes by the silver(I) ions.
Activatable triplet photosensitization refers to a photosentization process which can be turn on/off easily by external stimulus. Activatable triplet photosensitizations are normally achieved by interfere with the singlet excited state before intersystem cross process (ISC), i.e. the formation process of triplet states of sensitizer. To achieve novel activatable triplet photosensitization, a disulfide-bridged porphyrin zinc (II) dyad (ZnPor-S-S-ZnPor) is prepared. Although fast ISC can be conducted in this dyad, an extremely low efficiency is obtained when employing this dyad as triplet donor in triplet-triplet annihilation upconversion (TTA-UC) for sensitizing perylene. This is because of the presence of electron transfer from the triplet state of porphyrin zinc (II) unit to the disulfide bond, which quickly quenches the triplet state of the porphyrin zinc (II) unit. This electron transfer process can be stopped by the cleavage of the disulfide bond at the presence of thiol and TTA-UC efficiency can be enhanced significantly. Our result demonstrates for the first time that the disulfide bond can act as not only an easy cleavage linker, but also a triplet electron acceptor. Furthermore, quenching the triplet states of sensitizer by triplet electron transfer provides an alternative protocol for designing activatable triplet sensitizers except controlling the singlet excited state before ISC process.
A one-pot reaction is used to make a series of [5]rotaxanes. The protocol involves simultaneous threading-followed-by-stoppering to trap a macrocycle (dibenzo[24]crown 8, DB24C8) on an axle to form a mechanically interlocked molecule (MIM) – in this case a rotaxane – and the condensation of an aldehyde with a pyrrole to form a porphyrin precursor. For each [5]rotaxane, a different combination of recognition site and stoppering group was used; the protonation state of the [5]rotaxane can be used to generate different co-conformational states for each [5]rotaxane making these systems potential multi-state switches for further study in solution or the solid-state.
Ion-pairing assemblies consisting of appropriately designed charged π-electronic species afford various functional supramolecular assemblies, including crystals and soft materials; they are formed by the anisotropic arrangement of charged π-electronic species through electrostatic and other weak noncovalent interactions. The design, synthesis and combination of charged π-electronic species (ion-pair formation) are crucial for the preparation of functional dimension-controlled assemblies. This article includes the prologue and progress of ion-pairing assemblies comprising π-electronic ions. Synthesis of π-electronic ions, preparation of ion pairs, fabrication of assemblies as crystals, gels and liquid crystals and their characteristic properties are summarized.
Subphthalocyanine (SubPc)-stoppered [2]rotaxanes were synthesized for the first time. The rotaxane bearing unsubstituted SubPc as a stopper exhibited an equilibrium of slipping-on and slipping-off, whereas a perfluorinated SubPc stopper completely blocked slippage of the ring due to its slightly larger size. Kinetic studies revealed the Gibbs free energy of activation for the slipping-on and slipping-off processes. The optical properties of the rotaxanes, including photoinduced electron transfer, were also revealed.
Porphyrin–AuIII complexes were found to act as π-electronic cations, which can combine with various counteranions, including π-electronic anions. Single-crystal X-ray analyses revealed the formation of assemblies with contributions of charge-by-charge and charge-segregated assemblies, depending on the geometry and electronic state of the counteranions. Porphyrin–AuIII complexes possessing aliphatic alkyl chains formed dimension-controlled ion-pairing assemblies as thermotropic liquid crystals, whose ionic components were highly organized by π–π stacking and electrostatic interactions.
Porphyrins have been widely recognized for their unique photo‐ and electrochemical properties as well as their crucial roles in a wide variety of biological processes including catalysis, oxygen transport, and photosynthesis. In addition, their size and predictable coordination geometries makes them attractive components in the assembly of functional supramolecular assemblies. Advances in porphyrin synthesis have enabled their incorporation into rotaxanes not only as stoppers but also as components of the macrocycle, or even higher order rotaxane architectures, each design imparting unique functionality either through redox, fluorescence, or coordination environment. Here we describe advances in the synthesis of increasingly complex porphyrin‐containing rotaxane assemblies and their dynamic properties that arise from the porphyrin inclusion.
A sophisticated photoactive molecular device has been prepared by combining recent concepts for the preparation of multifunctional nanomolecules (click chemistry on multifunctional scaffolds) with supramolecular chemistry (self-assembly to prepare rotaxanes). Specifically, a clickable [2]rotaxane scaffold incorporating a free-base porphyrin stopper has been prepared and functionalized with ten peripheral Zn(II)-porphyrin moieties. Electrochemical investigations of the final compound revealed a peculiar behavior resulting from the intramolecular coordination of the Zn(II) porphyrin moieties to 1,2,3-triazole units. Finally, steady state investigations of the compound combining Zn(II) and free-base porphyrin moieties have shown that this compound is a light-harvesting device capable of channeling the light energy from the peripheral Zn(II)-porphyrin subunits to the core by singlet-singlet energy transfer.
Mechanically interlocked molecules (MIMs) which contain two or more different recognition sites in the shape of noncovalent bonds between their component parts provide the perfect prototypes for the design and synthesis of molecular switches and machines. In example, namely bistable catenanes and rotaxanes, it is convenient to identify a ground state co-conformation (GSCC) and a higher energy metastable state co-conformation (MSCC) where the GSCC:MSCC ratio at room temperature in solution is ideally for most purposes in the range of at least 10:1. The chapter discusses the most of the parameters that influence ring translation in n-donor/n-acceptor mechanomolecules and discusses competitive binding interactions that lead to mechanical switching motions as a result of molecular recognition. It also describes the single-molecule experiments and many of the molecular machines that involve molecular motion in condensed phases and at interfaces, motivating the pursuit of a more fundamental understanding of how these phases influence the thermodynamics and kinetics.
Supramolecular chemistry and self-assembly provide a valuable chance to understand the complicated topological structures on a molecular level. Two types of classical mechanically interlocked molecules, rotaxanes and catenanes, possess non-covalent mechanical bonds and have attracted more attention not only in supramolecular chemistry but also in the fields of materials science, nanotechnology and bioscience. In the past decades, the template-directed clipping reaction based on imine chemistry has become one of the most efficient methods for the construction of functionalized rotaxanes and catenanes. In this review, we outlined the main progress of rotaxanes and catenanes using the template-directed clipping approach of imine chemistry. The review contains the novel topological structures of rotaxanes and catenanes, functions and applications.
Metalloporphyrins are prominent building blocks in the synthetic toolbox of advanced photodriven molecular devices. When the central ion is paramagnetic, the relaxation pathways within the manifold of excited states are highly intricate so that unravelling the intramolecular energy and electron transfer processes is usually a very complex task. This fact is critically hampering the development of applications based on the enhanced coupling offered by the electronic exchange interaction. In this work, the dynamics of charge separation in a copper porphyrin-fullerene are studied with several complementary spectroscopic tools across the electromagnetic spectrum (from near-infrared to X-ray wavelengths), each of them providing specific diagnostics. Correlating the various rates clearly demonstrates that the lifetime of the photoinduced charge-separated state exceeds by about 10-fold that of the isolated photoexcited CuII porphyrin. As revealed by the spectral modifications in the XANES region, this stabilization is accompanied by a transient change in covalency around the CuII center, which is induced by an enhanced interaction with the C60 moiety. This experimental finding is further confirmed by state-of-the art calculations using DFT and TD-DFT including dispersion effects that explain the electrostatic and structural origins of this interaction, as the CuIIP cation becomes ruffled and approaches closer to the fullerene in the charge-separated state. From a methodological point of view, these results exemplify the potential of multielectron excitation features in transient X-ray spectra as future diagnostics of subfemtosecond electronic dynamics. From a practical point of view, this work is paving the way for elucidating out-of-equilibrium electron transfer events coupled to magnetic interaction processes on their intrinsic time-scales.
Porphyrin dyes were synthesized for use in p-type (NiO) dye sensitized solar cells based on different design principles. One porphyrin was designed with a significant charge transfer character in the excited state because of push-pull effects of the substituents. Another porphyrin had instead an appended NDI acceptor group (NDI = naphthalene diimide). The dyes were characterized by spectroscopic, electrochemical and DFT methods. Solar cells based on sensitized, meso-porous NiO showed rather poor performance compared to other organic dyes, but with a clear improvement for the dye with the NDI acceptor. Ultrafast transient absorption spectroscopy and nanosecond laser photolysis showed that hole injection into NiO was followed by unusually rapid charge recombination, predominantly on a 50-100 ps time scale, which is likely the main reason for the poor photovoltaic performance. Again the porphyrin with the NDI group showed a more long-lived charge separation that should lead to better dye regeneration in a solar cell, which can explain its better photovoltaic performance.
IntroductionSwitching of Electron-Transfer Processes Photon Inputs Long-Lived SwitchingFast and Ultrafast SwitchingRedox InputsAcid–Base InputsOther FactorsSwitching of Energy-Transfer Processes Photon InputsRedox InputsAcid–Base InputsOther Factors Photon Inputs Long-Lived SwitchingFast and Ultrafast SwitchingRedox InputsAcid–Base InputsOther Factors Long-Lived SwitchingFast and Ultrafast Switching Photon InputsRedox InputsAcid–Base InputsOther Factors
Reaction of 5-iodo-10,15,20-tris(3,5-di-tert-butylphenyl)porphyrin with excess trimethylsilylacetylene under Sonogashira conditions gives an unexpected porphyrinic enyne suitable for further transformations. The NMR spectrum and structural analysis of the porphyrin enynes show that the substituent on the acetylene group is positioned above the porphyrin ring. This structural characteristic makes the microenvironment of the porphyrin center in a controllable fashion.
A donor-acceptor [2]catenane, in which an extended tetracationic cyclophane is mechanically interlocked by a porphyrin-containing macrocycle, was synthesised using a template-directed protocol and alkene metathesis as the ring-closing step. In the ground state of this [2]catenane, the porphyrin ring resides inside the cavity of the cyclophane on account of favourable charge-transfer interactions between the electron-rich porphyrin and the electron-deficient cyclophane. The [2]catenane can act as a push-button molecular switch where the co-conformations of the [2]catenane can be controlled either chemically or electrochemically. Addition of acid protonates the porphyrin ring and a relative circumrotational motion of the macrocycle positions the charged porphyrin ring outside the cavity of the cyclophane. The switch can be reset to its ground-state co-conformation by the addition of base. Electrochemical reduction of the extended bipyridinium units of the cyclophane decreases the strength of the donor-acceptor interactions in the [2]catenane, leading to a loss of recognition between the mechanically interlocked rings. The chemical and electrochemical switching mechanisms are both reversible.
A bistable bis-branched [3]rotaxane with a perylene bisimide (PBI) chromophore that separates two mechanically interlocked [2]rotaxane arms including a diferrocene-functionalized dibenzo-24-crown-8 ring, was designed, prepared, and characterized by 1H NMR and 13C NMR spectroscopy and HR-ESI mass spectrometry. The uniform relative shuttling movement of target [3]rotaxane in response to external acid-base stimuli was confirmed by 1H NMR spectroscopy. Furthermore, the shuttling motion of the macrocycles in the system was accompanied by a remarkably visual fluorescence change due to the distance-dependent photoinduced electron transfer process from the ferrocene units to the PBI fluorophore. This kind of rotaxane has important potential for designing more sophisticated mechanically interlocked molecules with controllable functions.
High yields syntheses of Cu (II) and Ni(II) templated [2]pseudorotaxane precursors, CuPRT and NiPRT are achieved by threading bis-azide-bis-amide-2,2´-bipyridine axle into bis-amide-tris-amine macrocycle. Single crystal X-ray structural analysis of CuPRT reveals complete threading of the axle fragment into the wheel cavity where strong aromatic pi-pi stacking interactions (ranges from 3.474 to 3.494 Angstrom) between two parallel arene moieties of the wheel and the pyridyl unit of axle are operative besides metal ion templation. These azide terminated [2]pseudorotaxane precursors are further explored towards synthesis of metal free [2]rotaxane upon attaching a newly developed bulky stopper molecule with alkyne terminal via Cu(I)-catalyzed azide-alkyne cycloaddition reaction. Attachment of the stopper to CuPRT precursor via click chemistry, fails due to de-threading of azide terminated axle in the reaction conditions. However, synthesis of metal free [2]rotaxane containing triazole with other functionalities in the axle is achieved upon coupling between azide terminated NiPRT and the alkyne terminated stopper with ~45% yield. The [2]rotaxane is characterized by mass spectrometry, 1D and 2D NMR (COSY, DOSY, ROESY) experiments. Comparative solution state NMR studies of [2]rotaxane and in its protonated state are carried out to locate the position of the wheel on the axle of metal free [2]rotaxane. Furthermore, variable temperature 1H- NMR study in DMSO-d6 of [2]rotaxane supports the kinetic-inertness of the interlocked structure, where newly developed stopper prevents dethreading of 30-membered wheel from the axle.
Rotaxanes are currently seen as one of the most promising components in artificial molecular machines. In the last decades, a lot of effort has been dedicated to the developing of such interlocked architectures, which can be subjected to submolecular motion. Rotaxanes field has evolved to stimuli-responsive molecular shuttle, capable of responding to several different external stimuli. The interest on molecular shuttles arises from the possibility of achieving controlled submolecular motion of their components with respect to one another.
Here, we present an overview on benzylic amide rotaxanes, smart molecules able to function as electrochemically switchable molecular shuttles, both in solution and on surface.
Transposing working systems from solution phase onto surfaces and into the solid state is fundamental for their applicability in different technological fields including electromechanical switches and molecular electronic devices.
The synthesis of benzylic amide rotaxanes is based on template effects mediated by hydrogen bonds, which play a key role also in the shuttling process. Given the electrostatic nature of hydrogen bonds, their interactions can be modulated through electrons injection. Electrochemistry results a reagent-free powerful tool to operate stimulus and characterize the molecular level motion across different environments, necessary from the point of view of future integration with current technologies.
The conditions for the formation of heteroleptic complexes of a lanthanide(iii) ion (Ln = La, Gd, and Tb) with p-sulfonatothiacalix[4]arene and a heteroaromatic chromophore in water were found using X-ray diffraction analysis, pH-metry,
1H NMR and UV—Vis spectroscopy, and nuclear magnetic relaxation. In the resulting complexes, the heteroaromatic chromophore
is in the calix[4]arene cavity and the lanthanide ion is coordinated by the electron-donating groups of the upper or lower
calix[4]arene rim. Emission spectroscopic studies revealed changed emission properties of TbIII ions in the terbium(iii)—p-sulfonatothiacalix[4]arene—bipy complex.
Recent advances in electron transfer chemistry of electron donor-acceptor ensembles reported containing porphyrins and metalloporphyrins reported in ICPP-2 are reviewed briefly by focusing on the fundamental aspects of electron transfer reactions.
An overview of the synthesis and applications of molecular containers containing more than one porphyrin in their framework and enclosing a three-dimensional (3D) cavity is presented. Multiporphyrinic molecular cages held together by more than two linkers are considered in the review, as these multiple linkers restrict or prevent free rotation of the porphyrin derivatives and in such molecular architectures an internal 3D void space exists. The synthesis of covalent multiporphyrinic cages requires an important effort in the case of a stepwise synthesis which starts with the preparation of a functionalized porphyrin ready to cyclize.
Recent advances in electron transfer chemistry of electron donor-acceptor ensembles reported containing porphyrins and metalloporphyrins reported in ICPP-2 are reviewed briefly by focusing on the fundamental aspects of electron transfer reactions.
In this paper, there is an attempt to review the developments in the design and synthesis of pyridine containing [1+1] and [2+2] macrocyclic Schiff-base ligands, formed by condensations of 2,6-diacylpyridine or 2,6-diformylpyridine and appropriate polyamines, which utilize the templating capability of different metal ions to direct the synthetic pathway. The reduction of the cyclic Schiff-bases to their related amine derivatives is also considered since this leads to more flexible ligands capable of structural elaboration through donor groups. Attention is mainly paid to the synthetic and structural aspects of the resulting metal complexes, particularly to the role of the coordination preferences of the different metal ions in directing the synthesis totally or preferentially towards mono-, di- or poly-nuclear entities. The preparation of functionalized ligands, containing pendant arms, capable of promoting rapid complexation and decomplexation and their use in selective metal ion transportation and separation are also paid attention to. Furthermore, a summary of the new approach of these compounds such as mechanically interlocked molecules, catalytic properties and cofactors and artificial metalloenzymes is reviewed.
Within the fields of coordination and organometallic chemistry there has recently been much interest in the design, synthesis and study of novel complexes having potential for applications in the nascent technologies of molecular electronics and photonics. Here we provide an overview of the current status of research involving organotransition metal complexes in the following important areas: liquid crystalline materials, nonlinear optical materials, molecular wires and switches, chromophore-quencher complexes, dye-sensitized photovoltaic cells and organic light-emitting diodes. Cover-age is selective, generally focusing on highlights and the most recent developments, with the broad aim of conveying the essence and excitement of a group of related research topics at the forefront of modern inorganic chemistry.
A supramolecular assembly is formed upon mixing millimolar concentrations of a tetrakis-tetrathiafulvalene calix[4]pyrrole (TTF-C4P) and a porphyrin tetraethylammonium carboxylate salt in benzonitrile (PhCN). The TTF-C4P binds to the carboxylate moiety of the porphyrin with a 1:1 stoichiometry and a binding constant of 6.3 × 104 M–1 in this solvent at 298 K. Laser photoexcitation of the supramolecular complex results in formation of the triplet charge-separated (CS) state composed of a radical cation of the TTF-C4P receptor and the radical anion of the porphyrin carboxylate. These processes and the resulting states were characterized by means of transient absorption and electron spin resonance (ESR) spectroscopies. The rate constants corresponding to the forward and backward intramolecular electron-transfer (ET) processes were determined to be 2.1 × 104 and 3.6 × 102 s–1, respectively. The rate constants of intermolecular forward and backward electron transfer were also determined to be 4.4 × 108 and 9.8 × 108 M–1 s–1, respectively. The electronic coupling constant (V), 1.2 × 10–2 cm–1, and the reorganization energy (λ), 0.76 eV, for back electron transfer were evaluated from the temperature dependence of the rate constants of intramolecular electron transfer. The small V value indicates little spin-forbidden interaction between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and substantiates the long-lived CS lifetime. These results were corroborated by density function theory (DFT) calculations, which provided support for the conclusion that the HOMO and LUMO, located on a TTF moiety of the TTF-C4P and the porphyrin core, respectively, have little interaction though space.
An ortho-dimethyl substituted meso-tetrakisarylporphyrin prefunctionalized with triflate groups was prepared in good yield from an accessible 2,6-dimethyl-4-(triflyloxy)benzaldehyde. This porphyrin is an interesting building block, which could directly be engaged in Suzuki cross-coupling reactions, to be further tetra functionalized with 3-pyridyl ligands in yields equal or above 85%. The porphyrin core of the various compounds bearing four remote coordination sites was metalated with zinc(II). The molecular structures of the starting triflate porphyrin derivative and of the zinc(II) porphyrin substituted with four 3-pyridyl groups bearing a protected alcohol were determined using X-ray crystallography.
The ability of a dibenzylammonium dumbbell to form [2]rotaxanes with less than [24]crownethers has been investigated in order to correlate the spatial requirement of the ammonium moiety in the dibenzylammonium ion with the size of the encircling crown ether. The investigation hinges on template-directed ring-closing olefinmetathesis using 2nd generation Grubbs’ catalyst, known as the clipping approach. A series of acyclic diolefinpolyethers were independently subjected to the ring-closing metathesis in the presence of dibenzylammonium hexafluorophosphate. In this study, we obtained nine threaded molecules involving dibenzylammonium ion and crown ethers having less than 24 atoms. Three of the nine threaded molecules exhibited pseudorotaxane character; all the three pseudorotaxanes generated were 1:1 complexes between dibenzylammonium ion and three different [23]crownethers, with reasonably high association constant values. The remaining six threaded molecules incorporating unsaturated and saturated [20], [21] and [22]crownethers onto the ammonium moiety of the dibenzylammonium ion were [2]rotaxanes, where the benzene rings acted as stoppers. The smallest crown ether encompassing the dibenzylammonium dumbbell was found to be an unsaturated [20]crown ether. These threaded and interlocked species are well defined and well characterized by 1H NMR, 13C NMR and HRMS. In addition, single crystals suitable for crystallographic analysis were obtained and characterized, not only confirming the threaded structures but also substantiating our claim.
The design and synthesis of two recently reported examples of a new class of mechanically interlocked molecules - suitanes - is described. Suitanes consist of a rigid "body" template that is wrapped up in an all-in-one "suit" that fits the body to a T such that the two cannot be separated. These architecturally distinct interlocked molecular compounds are synthesized from a pool of components through a template-directed self-assembly process by taking advantage of dynamic covalent bond formation.
Brücken bauen: Die Reife eines [3]Rotaxans wurden intramolekular verbrückt, wobei cyclochirale, verzahnte Verbindungen entstanden, die als Bonnane (nach der Stadt Bonn) bezeichnet wurden (siehe Bild). Die Enantiomere und meso-Formen der Bonnane wurden an immobilisierten chiralen Phasen getrennt und charakterisiert.
The kinetics and mechanism of reduction of the surfactant-cobalt(III) complex ions, cis-[Co(bpy)2(C12H25NH2)2]3+ and cis-[Co(phen)2(C12H25NH2)2]3+ (bpy = bipyridyl, phen = 1,10-phenan-throline, C12H25NH2 = dodecylamine) by Fe(CN6)4− in self-micelles were studied at different temperatures. Experimentally the reaction was found to be second order and the
electron transfer postulated as outersphere. The rate constant for the electron transfer reaction for both the complexes was
found to increase with increase in the initial concentration of the surfactant-cobalt(III) complex. This peculiar behaviour
of dependence of second-order rate constant on the initial concentration of one of the reactants has been attributed to the
presence of various concentration of micelles under different initial concentration of the surfactantcobalt(III) complexes
in the reaction medium. The effect of inclusion of the long aliphatic chain of the surfactant complex ions into β-cyclodextrin
on these reactions has also been studied.
A series of porphyrins, strapped with aryloxy-substituted polyether chains of various lengths and different substitution patterns, has been photophysically characterised. These molecules act as receptors for bipyridinium dications and their association constants with paraquat in acetonitrile solutions have been determined by absorption and emission spectroscopy; they have been found to be in the range 102–105. The photophysical characterisation of the complexes has been made in solutions containing more than 95% of associated porphyrin component. A photoinduced electron transfer from the porphyrin excited state to the paraquat guest, responsible for the porphyrin luminescence quenching, takes place with τ≪20 ps and is followed by a very fast recombination of the resulting charge-separated state. In only a few cases could the charge-separated state be detected and a lifetime of 20 ps was measured, whereas in most complexes the recombination is faster than 20 ps. In none of the examined complexes was escape from the geminate recombination of the charge-separated state observed.
The synthesis of a pseudo-rotaxane containing two metal centres has been achieved using the coordination of a ditopic organic ligand on an iridium(III) complex followed by the coordination of a copper(I) centre. Preliminary photophysical studies indicate that the excited state of the iridium is quenched by the copper unit.
The syntheses of two novel CuI-bis(benzonitrile) complexes of the type [Cu(dCNn)2][PF6] {where dCNn = NCPhO(CH2)nOPhCN, n = 3, 4} are described. Both compounds have been characterized by X-ray crystallography. To our surprise, and although the two complexes were quite similar, two very distinct structures were obtained. For the [Cu(dCN4)2][PF6] complex, a tetrahedral geometry was observed whereas for [Cu(dCN3)2][PF6], a polymeric assembly was formed. A further extension of this work involved the syntheses of two threaded species bearing the M30 macrocycle, Cu metal and a dCNn ligand. The formation of the desired complexes was confirmed by means of mass spectrometry as well as 1-D and 2-D 1H NMR spectroscopy. Finally, the cyclic voltammograms of all 4 new species were recorded giving rise to redox potentials ranging from +0.88 to +1.25 V. These new threaded complexes are of particular interest since they can form the basis of novel rotaxane structures. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)
Routes for the synthesis of a series of [2]rotaxanes and [2]catenanes incorporating porphyrin, ferrocene and fullerene moieties by the Sauvage metal template technique are described. Photophysical studies on these materials show that excitation of the porphyrin component causes a series of events along an energy gradient leading to long-lived long distance charge-separated radical pair states with lifetimes as long as 32 μs in tetrahydrofuran solution at ambient temperatures. To cite this article: D.I. Schuster et al., C. R. Chimie 9 (2006).
In the past few decades, bi-stable [2]rotaxanes have been extensively studied because of their applications in the fields of functional molecules and molecular machines. In this paper, a di-ferrocene-functionalized [2]rotaxane with two fluorophores as stoppers was designed, perpared and studied. In this bi-stable [2]rotaxane, a dibenzo-24-crown-8 macrocycle functionalized with two ferrocene moieties as electron donors can reversibly shuttle between two distinct stations, namely, a dialkylammonium recognition site and a N-methyltriazolium recognition site, by external acid-base stimuli, which has been demonstrated using 1H NMR spectroscopy. It has been shown that, by introducing two ferrocene units into the macrocycle component, the fluorescence of two fluorescent stoppers, such as the anthracene fluorophore and the 4-morpholin-naphthalimide fluorophore, respectively, can be changed in an alternate mode by an adjustable, distance-dependent photo-induced electron transfer process that occurs between the ferrocene electron donors and each of the two fluorophores.
The photosensitized electron-transfer processes in the rotaxane hybrids composed with electron-accepting fullerenes and various electron-donors placed in the rotaxanes are revealed with time-resolved fluorescence and absorption spectral methods. Porphyrins are most useful as light-harvesting donors and photosensitizing donors. In addition, aromatic amines and ferrocene act as electron-donor and also hole-shifting reagents in multi-component rotaxanes. In the rotaxanes with spatially placed donor-acceptor molecules, the role of triplet states becomes important compared with the covalently connected donor-acceptor molecular systems, which may be related to the "through-space" and "through-bond" electron transfer, respectively. In the designed multi-component rotaxanes which maintain mechanically or topologically the electron-acceptor, electron-donor, and hole-shifter, the photoinduced electron transfer, hole-shift, electron-hole recombination are established. As a whole, contribution of the triplet excited states is prominent compared with the covalently bonded molecules and supramolecular systems constructed with coordination bonds. (C) 2010 Elsevier B.V. All rights reserved.
In the context of long-range electron transfer for solar energy conversion, we present the synthesis, photophysical, and computational characterization of two new zinc(II) phthalocyanine oligophenylene-ethynylene based donor-bride-acceptor dyads: ZnPc-OPE-AuP(+) and ZnPc-OPE-C(60). A gold(III) porphyrin and a fullerene has been used as electron accepting moieties, and the results have been compared to a previously reported dyad with a tin(IV) dichloride porphyrin as the electron acceptor (Fortage et al. Chem. Commun.2007, 4629). The results for ZnPc-OPE-AuP(+) indicate a remarkably strong electronic coupling over a distance of more than 3 nm. The electronic coupling is manifested in both the absorption spectrum and an ultrafast rate for photoinduced electron transfer (k(PET) = 1.0 × 10(12) s(-1)). The charge-shifted state in ZnPc-OPE-AuP(+) recombines with a relatively low rate (k(BET) = 1.0 × 10(9) s(-1)). In contrast, the rate for charge transfer in the other dyad, ZnPc-OPE-C(60), is relatively slow (k(PET) = 1.1 × 10(9) s(-1)), while the recombination is very fast (k(BET) ≈ 5 × 10(10) s(-1)). TD-DFT calculations support the hypothesis that the long-lived charge-shifted state of ZnPc-OPE-AuP(+) is due to relaxation of the reduced gold porphyrin from a porphyrin ring based reduction to a gold centered reduction. This is in contrast to the faster recombination in the tin(IV) porphyrin based system (k(BET) = 1.2 × 10(10) s(-1)), where the excess electron is instead delocalized over the porphyrin ring.
A unique poly(pseudo)rotaxane-like network constituted by C−H···N hydrogen bonds in the solid-state structure of 1,7-phenanthroline and a self-inclusion host−guest type assembly in its salt form have been discussed. The assemblies are characterized by single-crystal X-ray diffraction methods.
Two series monotailed porphyrins, Cobalt-5-{4-[ω-(1-adamantaneamino) alkyloxy]phenyl}-10,15,20-triphenyl porphyrinate (CoPCnA, n=4,5,6) and Nickel-5-{4-[ω-(1-adamantaneamino)alkyloxy]phenyl}-10,15,20-triphenyl porphyrinate (NiPCnA, n=4,5,6), were synthesized, in which the porphyrin moiety was connected to 1-adamantanamine via a flexible hydrocarbon chain. The fluorescence quenching between these donor substrates and mono-6-p-nitrobenzoyl-β-cyclodextrin (NBCD) was studied in detail. Distinct fluorescence quenching occured in these supramolecular systems. This quenching was attributed to the photoinduced electron transfer (PET) inside the supramolecular assembly between the porphyrin donors and cyclodextrin acceptors. Detailed Stern-Volmer constants were measured and they were partitioned into dynamic Stern-Volmer quenching constants and static binding constants. It was demonstrated that the PET interaction between the porphyrin subunits and NBCD is indeed effective.
Gold(III) porphyrins are easily reduced to the corresponding radical anions, which are stable in water over a wide pH range. Further reduction results in formation of the phlorins with relatively little tendency to form chlorins. Both radical anion and phlorin will reduce water to H2 on the surface of a colloidal Pt catalyst. From kinetic studies the radical anion is by far the better reducing species. Using NADH as reducing agent, a photosystem has been devised which results in the overall storage of visible light. Oxidation of the gold(III) porphyrins results in destruction of the porphyrin ring.
A theoretical analysis is made of the rate of intramolecular transfer of the odd electron between the two phenyl groups in the mononegative ions of the α,ω‐diphenyalkanes, ϕ—(CH2)n—ϕ. The essential features of the calculations are: (a) It is shown that the polymethylene chain can be replaced by a pseudopotential corresponding to an effective direct transfer between the rings. (b) There is a strong tendency for self‐trapping of the odd electron on one phenyl ring, or the other, due to solvent polarization and bond distortions in the rings. This self‐trapping greatly reduces the rate of intramolecular charge transfer. (c) The intramolecular charge transfer occurs as an electronic resonance effect when a short‐lived thermally activated molecular state is formed in which the two rings appear to the odd electron to be equivalent to one another. The activation energy is estimated to be of the order of 1000 cm—1. (d) It is found that the rate of intramolecular charge transfer decreases exponentially with the length of the polymethylene chain, the decrease being as much or more than a factor of ten for each added methylene group.
Condensation of dipyrrylmethane units onto a copper(I) complex, in which a diphenylphenanthroline ligand bearing aldehyde functions is threaded into a phenanthroline-containing macrocycle, afforded a copper(I)-complexed bisporphyrin-stoppered [2]-rotaxane or a bis-porphyrin-bridged [3]-catenate, depending on whether a second, aromatic aldehyde was added to the reaction mixture or not. The isolated yields were 18 and 5% respectively. In a separate experiment, a macrocyclic compound identical to the central ring of the [3]-catenate (46-membered ring) has also been synthesized in 28% yield.
Reaction of pyrrole and an aromatic aldehyde at room temperature affords the corresponding tetraarylporphyrinogen in high yield at thermodynamic equilibrium. Oxidation yields the porphyrin. The reaction conditions are of broad scope.
The synthesis of bis-porphyrins disposed in an oblique fashion is described. The tetrapyrrole rings are linked via a 2,9-diphenyl-1,10-phenanthroline spacer. Two routes have been investigated: a stepwise procedure and a direct strategy involving a double cyclisation step. From the bis-free base porphyrin, synthesized with an overall yield of ∼1% from 2,9-di(p-tolyl)-1,10-phenanthroline, an unsymmetrical zinc (II) porphyrin free base system was prepared and isolated in view of excited state energy and electron transfer.
We describe here the detailed template synthesis of a copper(I) catenate. Demetallation leads to the corresponding free ligand, a catenand. Both molecules are made of two interlocked 30 membered rings.
Absorption spectra are reported for a series of metalloporphyrin π-radical cations produced by chemical oxidation in CH2Cl2. Where the parent metalloporphyrin is diamagnetic, the absorption spectral features of the π-radical cations are similar and there is a good correlation between the energies of the first spin-allowed transitions for the π-radical cation and for the parent porphyrin. In general, this is not the case for π-radical cations derived from paramagnetic metalloporphyrins where the lowest energy spin-allowed transition occurs at higher wavelength than anticipated from the studies with diamagnetic metalloporphyrins.
Photoredox and electrochemical reactions of water-soluble gold porphyrins (AuTMPyP, AuTPyP and AuTSPP) have been investigated in aqueous solution. AuTMPyP is the most durable photoredox catalyst for these porphyrins operating through the reductive electron transfer process. Photogenerated AuTMPyP (π-radical anion) is able to reduce water at pH < 4, although it undergoes disproportionation to form a AuTMPyP(phlorin). The AuTMPyP(phlorin) is completely oxidized back to the parent AuTMPyP in the dark with the appropriate oxidant, but does not have sufficient potential for the reduction of water.
Using two coordination bonds, a porphyrin–quinone supramolecule with a large association constant is assembled; intramolecular photoinduced electron transfer from the excited singlet state of the porphyrin to the quinone is observed by steady-state fluorescence quenching and time-resolved fluorescence studies.
A dicopper(I) trefoil knot is synthesized in 30% yield by the use of 1,3-phenylene spacers, the double stranded helical precursor complex being formed quantitatively.
A rotaxane with two rigidly held porphyrins as stoppers is synthesized by a copper(I)-based template strategy.
A catenane-type triad consisting of a donor (phenothiazine), a sensitizer (Ru(2,2'-bipyridine)(3)) and an acceptor (cyclobis(paraquat-p-phenylene)) was prepared by stepwise coordination. Multistep noncovalent and covalent electron transfer was successfully carried out in this triad.
Transition metal complexes formed from oligopyridino ligands covalently attached to porphyrin modules have been used to assemble well-defined, photoactive multicomponent arrays. Apart from gathering together porphyrinic components, the metal complex plays an essential role by modulating the degree of electronic coupling between the porphyrins or by direct participation in sequential electron-transfer steps. Several such-systems have been built and investigated by ultrafast transient spectroscopy, allowing critical comparison with the bacterial photosynthetic reaction centre complex.
Electron-transfer reactions in an H-bonded assembly composed of a diacylaminopyridine bearing zinc porphyrin and 1,8:4,5-naphthalenetetracarboxylic diimide (Ka = 2.8 × 102 dm3 mol–1) in benzene occur with kCS = 4.1 × 1010 s–1 and kCR = 3.7 × 109 s–1, while the corresponding covalently linked model with a comparable distance and energy gap undergoes electron transfer with kCS = 9.9 × 1010 s–1 and kCR = 6.7 × 108 s–1.
A new bis(porphyrin) has been synthezised in which a catenane is used as a spacer between the two chromophores, the donor and acceptor porphyrins being located as pendant groups, arranged linearly on each side of the catenane core.
A synthetic strategy is described that provides access to oligopyridine-based ditopic ligands bridged by an alkyne spacer comprising one to four ethynyl groups. The spacer serves as both a rigid girder to maintain structural inetgrity and a conduit for electron flow. These ditopic ligands, bearing 2,2′-bipyridine, 1,10-phenanthroline or 2,2′:6′,2″-terpyridine coordination sites, are used to construct a dichotomous series of polynuclear metal complexes. Judicious selection of ligand and metal cation permits the assembly of novel molecular architectures incorporating a logical gradient of redox units along the molecular axis. Different molecular shapes become possible by changing the position at which the alkyne bridge is connected to the terminal ligand and varying the nature of the complexing cation. The elecrochemical and photochemical properties of many such molecular arrays are enumerated with a view to the future design of molecular electronic devices. The alkyne bridge actively promotes long-range electronic coupling between remote cationic units, especially under illumination with visible light. Intramolecular triplet energy transfer, photon migration and light-induced electron transfer occur by way of extremely rapid superexchange involving LUMO and HOMO states localized on the bridge. It is also shown that facile electron delocalization over an extended π* orbital takes place in the triplet excited states of symmetrical binuclear complexes. This process extends the triplet lifetime and thereby facilitates secondary reactions that are otherwise unattainable. The level of electronic communication along the molecular wire can be controlled by insertion of suitable insulating groups into the bridge, these groups also providing further means by which to manipulate the stereochemistry. In certain cases, the alkyne bridge undergoes reductive electropolymerization to generate molecular films having metallo centres dispersed along a conjugated backbone.
Using sterically hindered 2,9-diarylphenanthrolines (I) as well as unsubstituted or 4,7-disubstituted phenanthrolines (II) the first preparation of open chain heteroleptic bis(phenanthroline) complexes [M(I)(II)]n+ with M = Ag+ or Zn2+ has been realised. Their structure was confirmed by electrospray MS, NMR and in one case X-ray data. With two sets of phenanthroline ligands, three stereoisomers of [M(I)(II)]+ were detected in the silver(I) and for comparative reasons in the corresponding copper(I) complexes. On the basis of NMR experiments these were assigned as in-in, in-out and out-out isomers.
Energy and electron transfer processes taking place in [3]-rotaxanes containing various photo- and electro-active metal-complex fragments, such as zinc(II)- and gold(III)-porphyrin stoppers, copper(I)- and silver(I)-bis-phenanthroline complex moieties, were studied by spectroscopic steady state and time resolved techniques. Energy transfer from the zinc porphyrin excited singlet state to the MLCT excited state of the Cu(I) complex fragments was observed in the [3]-rotaxane [PZnCuCuPZn]2+, with a lifetime of 180 ps. In the rotaxane [PZnCu-PZn]+energy transfer occurred only from the porphyrin (τ=490 ps) close to the Cu(I) complex, while the lifetime of the distant zinc porphyrin was unaffected. Quenching of the distant porphyrin was partly restored in [PZnCuAgPZn]2+, while the one close to the Cu(I) complex displayed a luminescence lifetime of 300 ps. In the case of the free [3]-rotaxane [PZn--PAu]+, the data suggest that a very slow electron transfer process takes place between the porphyrin stoppers probably through the extended “necklace’' spacer.
The excited triplet state deactivation of zinc(II) meso-diaryloctaalkylporphyrins (ZnDAOAP) has been studied over a wide temperature range using transient triplet-triplet absorption spectroscopy together with steady-state and time-resolved phosphorescence techniques, The results from transient absorption measurements show that the depopulation of the initially formed triplet state (T-1A state) is unusually fast at temperatures above 150 K. The efficiency of the deactivation originates from a spin allowed transition to a second tripler state (T-1B state), The transformation process T-1A-->T-1B is therefore the dominating deactivation channel of the T-1A state in this temperature range, and direct intersystem crossing T-1A-->S-0 makes negligible contribution. The subsequent ground-state recovery T-1B-->S-0 is also very efficient in comparison to many other porphyrins. Due to the substantial activation energy found for the transformation process, it most likely involves a conformational distortion of the porphyrin macrocycle. At low temperature, however, the relaxation of the T-1A State occurs by direct intersystem crossing to the ground state.
Zinc-meso-tetrasulfonatophenyl-porphyrin (ZnTPPS4-) spontaneously forms a complex with methyl viologen (MV2+) in aqueous solutions. Both the free porphyrin and its complex with viologen were studied by pump−probe spectroscopy. Excitation in the porphyrin Soret band with a short laser pulse (fwhm = 200 fs) populated the upper excited-state S2, and the subsequent processes were followed by the transient absorbance changes involved. For the free ZnTPPS,4- the transient absorption spectrum of the upper excited-state S2 was different from that of the lowest excited-state S1. The S2 to S1 internal conversion could therefore be followed, and a S2 lifetime of 1.3 ± 0.2 ps was determined. In the ZnTPPS4-−MV2+ complex, excitation in the Soret band produced spectral changes attributed to the charge separated state ZnTPPS3-−MV+, with a rate limited by the laser pulse (rate constant >5 × 1012 s-1). This suggests that the photoinduced electron transfer from the excited porphyrin to the methyl viologen occurred directly from the S2 state of the porphyrin, before internal conversion to S1. The reverse electron transfer that followed regenerated the ground state reactants with a rate constant (1.3 ± 0.2) × 1012 s-1.
It has been found that for mono- and di-meso-phenyl-substituted octaethylporphyrin (OEPs), etioporphyrins, corresponding Zn complexes and OEP chemical dimers (with a meso-phenyl spacer) a drastic shortening of triplet lifetimes from a ms to μs timescale in deaerated toluene solutions at 295K takes place without any changes of the spectral-kinetic parameters of the S0 and S1 states. The observed effects are connected with torsion librations of the phenyl ring around a single C–C bond in sterically encumbered porphyrins leading to non-planar dynamic distorted conformations in excited T1 states accompanied by an enhancement of the T1-state nonradiative deactivation pathways.
Molecular knots have long been the subject of speculation. Now, for the first time, it has been possible to synthesize a “knotted” molecule having the topological chirality of a trefoil knot. Compound 1 was synthesized via a double‐helix complex by exploiting the template effect of CuI ions. The chirality of 1 could be demonstrated by ¹H NMR spectroscopy. The “unraveled” topological isomer of 1 was also synthesized. (Figure Presented.)
In our lecture we first describe the history and methods of membrane protein crystallization, before we show how the structure of the photosynthetic reaction center from the purple bacterium Rhodopseudomonas viridis was solved. The structure of this membrane protein complex is then correlated with its function as a light-driven electron pump across the photosynthetic membrane. Finally, we draw conclusions on the structure of the photosystem II reaciton center of plants and discuss the aspects of membrane protein structure. Sections 1 (crystallization), 4 (conclusions on the structure of photosystem II reaction center and evolutionary aspects) and 5 (aspects of membrane protein structure) were presented and written by H. M., sections 2 (determination of the structure) and 3 (structure and function) by J. D. We have arranged the manuscript in this way in order to facilitate continuous reading.
The electrochemical properties of
bis(2,9-diaryl-1,10-phenanthroline)silver complexes are reported. The
redox potentials of the coordinated metal centre are strongly dependent on
the topology of the organic backbone surrounding the metal centre and its
set of ligands. This particular property can be used for the evaluation of
the topological characteristics of different polypyrrole matrices built
around entwined 2,9-diaryl-1,10-phenanthroline moieties.
The copper(I)-templated synthesis of the first [2]catenate incorporating electron-donor and -acceptor porphyrins linked mechanically, though not covalently, has been achieved, along with that of related model. compounds incorporating only electron donating zinc(Ir) porphyrins. The preparations were approached by first constructing macrocycles incorporating 2,9-diphenyl-1,10-phenanthroline residues and pendant porphyrins. An unusual aggregation of one of these macrocycles, incorporating a gold(III) porphyrin, has been observed and appears to be driven by pi-pi stacking interactions, but proves to be no impediment to the preparation of the catenates. The gross conformation of the catenates in solution seems to be approximately that of the envisaged design where the porphyrins are directed to opposite sides and laterally away from the heart of the catenate, since no inter-porphyrin interactions could be detected by NMR or UV-visible spectroscopy. The removal of the templating copper(I) ion from the catenates is inhibited by the presence of appended zinc(II) porphyrins, while the free catenands may be obtained when the zinc(II) ion is absent.
A Ru(II) complex (Ru) containing as ligands a tridentate 4‘-p-tolyl-2,2‘:6‘,2‘‘-terpyridine (ttpy) and a tridentate 2,6-bis(4‘-phenyl-2‘-quinolyl)pyridine (bpqpy) has been covalently linked to a porphyrin module (PH2) to obtain a PH2−Ru dyad. The corresponding PZn−Ru dyad has then been obtained by metalation of the free base porphyrin with Zn(II) acetate. The photoinduced processes which occur on excitation of the PH2−Ru and PZn−Ru dyads, as well as of the PH2 and PZn porphyrin units and the [Ru(ttpy)(bpqpy)]2+ model compound Ru, have been investigated in butyronitrile rigid matrix at 77 K and fluid solution at 295 K. In both dyads at low temperature, the lowest singlet excited state of the porphyrin moiety (S1) is quenched by energy transfer to give the triplet metal-to-ligand charge-transfer excited state of the Ru complex (3MLCT) which, in its turn, is quenched by energy transfer to yield the triplet excited state of the porphyrin moiety (T1). At room temperature, a charge-transfer (CT) excited state corresponding to the transfer of an electron from the porphyrin moiety to the Ru-based moiety comes into play. For the PZn−Ru dyad, where the CT state lies below the S1 excited state of the porphyrin moiety, the deactivation of S1 (k ≥ 5 × 1010 s-1) occurs mainly by electron transfer to give the CT level that then deactivates to the T1 excited state of the porphyrin moiety (100% efficiency; k = 9.3 × 109 s-1). Since the T1 level is intrinsically long lived (τ 210 μs), its deactivation occurs essentially via an activated process through the upper lying CT level (k = 5.7 × 106 s-1). The 3MLCT excited state of the Ru-based moiety directly formed by light absorption appears to decay unperturbed with its intrinsic lifetime (k = 1.1 × 1010 s-1). In the case of the PH2−Ru dyad, the CT level lies slightly above S1. As a consequence, only a fraction (ca. 30%) of the S1 excited states are quenched by electron transfer, the remaining part being quenched by energy transfer to give the 3MLCT excited state of the Ru-based moiety. Deactivation of the CT state leads to the formation of T1 (k = 8.7 × 109 s-1), whereas the 3MLCT excited state undergoes unperturbed deactivation (k = 1.2 × 1010 s-1) directly to the ground state. For the latter dyad, the T1 excited state is very long lived (280 μs) since deactivation via the upper lying CT level is precluded for energetic reasons.
Ortho-disubstituted tetraphenylporphyrins such as tetramesitylporphyrin have been widely used in model systems, but these "sterically hindered" porphyrins have been difficult to synthesize under mild as well as forcing conditions. Mesitaldehyde is highly discriminating in its exacting requirements for catalysis, but little steric hindrance is observed when these catalytic requirements are satisfied. A key feature of these catalytic conditions involves BF3-ethanol cocatalysis. Application of these conditions to 14 ortho-substituted benzaldehydes resulted in a clear reactivity pattern: cocatalysis gave improved yields with 2-alkyl-, 2-alkoxy-, and 2,6-dialkoxybenzaldehydes, but six o-halogen-substituted benzaldehydes showed little or no increase. Four ortho-disubstituted aldehydes failed to react under any conditions. The structural effects of substituents can be partly understood by examining the packing of the aldehyde ortho substituents about the tetrahedral meso carbon in the porphyrinogen, the precursor to the porphyrin.
A guanine radical cation (G+•) was site-selectively generated in double stranded DNA and the charge transfer in different oligonucleotide sequences was investigated. The method is based on the competition between a charge transfer from G+• through the DNA and its trapping reaction with H2O. We analyzed the hole transfer from this G+• to a GGG unit through one, two, three, and four AT base pairs and found that the rate decreases by about 1 order of magnitude with each intervening AT base pair. This strong distance dependence led to a β-value of 0.7 ± 0.1 Å-1. Within the time scale of this assay the charge transfer nearly vanished when the G+• was separated by four AT base pairs from the GGG unit. However, if the second or the third of the four intervening AT base pairs was exchanged by a GC base pair, the rate of the hole transfer from the G+• to the GGG unit increased by 2 orders of magnitude. In addition, a long-range charge transfer over 15 base pairs could be observed in a mixed strand that contained AT as well as GC base pairs. Because G+• can oxidize G but not A bases, the long-range charge transport can be explained by a hopping of the positive charge between the intervening G bases. Thus, the overall charge transport in a mixed strand is a multistep hopping process between G bases where the individual steps contribute to the overall rate. The distance dependence is no longer described by the β value of the superexchange mechanism.
Photoinduced electron transfer within donor−(salt bridge)−acceptor complexes has been investigated. We now report the first comparative study of electron transfer through an asymmetric salt bridge interface formed from the 1:1 association of an amidinium to a carboxylate via two hydrogen bonds. Laser flash excitation prompts an electron to transfer from a highly reducing excited state of a derivatized Ru(II) bipyridine donor complex to a dinitrobenzene acceptor juxtaposed by the salt bridge interface. The rate of electron transfer through the D−(amidinium−carboxylate)−A salt bridge is 102 times slower than that for the pair when the interface is switched, D−(carboxylate−amidinium)−A. This large difference shows that a salt bridge can significantly influence the kinetics of electron transfer and, accordingly, bears considerably on electron transport within the biological milieu of proteins and enzymes.
Spectroscopic and electrochemical investigations have been carried out on a collection of hydrogen-bonded mixed-valence adducts of ruthenium complexes. The electron donors (H-bond acceptors) are Ru(II) cyano species and the electron acceptors (H-bond donors) are Ru(III) ethylenediamine species, and NIR spectroscopic transitions in the adducts are assigned to intervalence transfer through the hydrogen bonds holding the adducts together (HBIT). Spectroscopic studies using Job's method indicate that the adducts are 2:1 ternary aggregates of formulations such as [((trpy)(bpy)RuII(CN))2,(en)2RuIII(bpy)]5+ and [((bpy)2RuII(CN)2)2,(en)2RuIII(bpy)]3+. Voltammetric investigations show substantial repulsion of the redox waves of the parent complexes in mixtures containing both donor and acceptor. Comparison with known electronic coupling data for mixed-valence ruthenium dimers covalently bound through dithiaspiroalkane bridging ligands indicates that the electronic coupling through H bonds of this type is 65−75% as strong as through σ-covalent bonds.
Molecular composite knots have been prepared from transition metal-assembled precursors via a Glaser acetylenic coupling reaction. The templating metal is copper(I), and the coordinating fragments incorporated into the final structure are 1,10-phenanthroline-type chelates. The compounds are composite knots as opposed to prime knots such as the classical trefoil knot. By combining two tied open-chain fragments in a cyclodimerization reaction, the simplest composite knots are obtained as a mixture of two topological diastereomers. The minimum number of crossing points used to represent the molecules in a plane is six. Due to the complexity of the entangled precursors and to the several cyclization possibilities, the formation yield of composite knots is only modest (3%). On the other hand, the compound has been fully characterized by ES-MS (molecular weight, 4037.8) and by 1H NMR spectroscopy, including 2D NMR (NOESY).
While Cu II porphyrins are known to luminesce, Ag II complexes do not. It is shown here that silver(III) octaethylporphyrin has no emission while gold(III) tetraphenylporphyrin has a moderately intense phosphorescence with a nonexponential decay fit with two decay times of 63 and 184 μs. I n contrast to Cu II and Au III porphyrins, the Ag complexes have a metal redox potential, II ⇄ III, between that of ring oxidation and ring reduction suggesting that luminescence is quenched by low-energy charge transfer transitions Ag II → ring or ring → Ag III. Near-infrared (1100-700 nm) absorption spectra confirm the presence of weak absorption bands in Ag II and Ag III complexes that are not observed in complexes of Cu II and Au III. The near-IR absorption of Cu II(TPP) and the quenching of its unusually broad emission by pyridine suggest that a charge transfer state is close to the emitting level. Iterative extended Hückel calculations explain these facts by the energy of orbital b 1g(d x2-y2), which rises along the series Cu < Ag < Au.
Synthese d'un catenane comprenant 2 macrocycles imbriques (ce macrocycle etant un hexaoxa [16] paracyclo [0] phenanthrolino-1,10 [0] paracyclophane) a partir de diiodo-1,14 tetraoxa-3,6,9,12tetradecane, de bis-[hydroxy-4 phenyl]-2,9 phenanthroline-1,10 et de Cu(CH 3 CN) 4 + •BF 4 − suivie de demetallation du cuprocatenane obtenu. Proprietes du cuprocatenane
Copper(I) complexes of chelates and cyclic ligands containing 1,10-phenanthroline coordinating units have been prepared. Kinetic studies have been carried out in order to gain insight into the particular properties of the recently synthesized catenates. In the presence of the decomplexing agent CN-, two dissociation mechanisms are involved: a bimolecular attack of CN- on the copper(I) center and a monomolecular demetallation pathway. The rate law has been obtained and is in good agreement with the two dissociation pathways for each complex studied. The decomplexation rate depends strongly on both the topography and the topology of the molecules. For bis(2,9-diphenyl-1,10-phenanthroline)copper(I), Cu(dpp)2+, accessibility of the metal by CN- is hindered as compared to less substituted complexes, making the bimolecular dissociation process much slower. Such an effect is due to the particular shape of the molecule, two dpp's, fitting in together while encaging the copper atom. A pronounced cyclic effect has also been demonstrated, even for mixed complexes containing only one ring surrounding the chelated metal. A marked catenand effect, of topological origin, has been found: when the ligand is composed of two interlocked rings, the unravelling of the two cycles (necessary to demetallation) renders the latter reaction several orders of magnitude slower than for the acyclic analogue of the catenate.
A series of molecules 1 was synthesized containing a 1,4-dimethoxynaphthalene donor (D) an a 1,1-dicyanoethylene acceptor (a) interconnected by five different, rigid, nonconjugated bridges. The length of the bridges varies with increments of two sigma-bonds from four in 1(4) to 12 sigma-bonds in 1(12), to provide donor-acceptor center-to-center separations (R/sub c/) ranging from 7.0-14.9 A. In solvents of medium and high polarity, excitation of the donor D is followed by rapid intramolecular electron transfer. The rate constant (k/sub et/) shows only small dependence upon the solvent polarity (a factor of 2-3 between benzene and acetonitrile, for example) but decreases with increasing separation ranging from >10¹¹ sâ»Â¹ for a four-bond separation to approx. =4 x 10⸠sâ»Â¹ for a 12-bond separation. In saturated hydrocarbon solvents photoinduced electron transfer is not observed for 10 and 12-bond separations, while it is not significantly decreased for the shorter homologues. Therefore the absence of electron transfer at 10- and 12-bond separations in saturated hydrocarbon solvents is attributed to a thermodynamic rather than to a kinetic effect. In solvents where electron transfer is thermodynamically feasible, its rate is considerably greater than that found from various other experimental studies where either different bridges were used or intermolecular electron transfer was studied. Through-bond interaction involving sigma/..pi.. interaction between the bridge and the donor-acceptor pair is proposed to explain the very high electron transfer rates observed in 1; this is qualitatively correlated with independent information about this coupling derived from both theory and experiment (photoelectron spectroscopy).
The coordinating properties of a catenand, consisting of two interlocked 30-membered rings, have been studied. Several complexes, the catenates, have been prepared and fully characterized. The electron spectra of catenates have been measured, showing intense absorption bands in the visible for the CuI and NiI complexes. The strong color of copper(I) and nickel(I) catenates corresponds to a metal-to-ligand charge-transfer (MLCT) transition. Many of the catenates studied are strong photoemitters, the excitation light being in the near-UV or visible region. Both ligand-localized or MLCT excited states are responsible for the emission properties observed, depending on the metallic species complexed. The two 2,9-diphenyl-1,10-phenanthroline (dpp) subunits which form the complexing species of the catenand, adopt an "entwined" geometry in all the catenates isolated. This special shape was clearly demonstrated by 1H NMR studies for copper(I), silver(I), zinc(II), and cadmium(II) catenates and for their corresponding acyclic analogues containing two 2,9-di-p-anisyl-1,10-phenanthroline (dap) chelates. The molecular topography of the system in solution is thus in perfect agreement with the solid-state structure of copper(I) catenate, as earlier determined by X-ray crystallography. A detailed electrochemical study of the various catenates prepared has been carried out. The very general trend is that low oxidation states of transition-metal catenates are strongly stabilized. Some one-electron reductive processes have clearly been shown to occur on the ligand without decomposition of the complex. This is the case for lithium(I), copper(I), and zinc(II) catenates. It is even possible to generate stable solutions of the anionic copper complex by two-electron reduction of copper(I) catenate. In other instances, electron transfer takes place on the metal. The most straightforward situation is that of NiII, which is very readily reduced to NiI (d9), this monovalent nickel catenate being surprisingly stable toward reoxidation. The nature of the orbitals involved in the reduction of FeII, CoII, AgI, and CdII (ligand or metal centered) is not certain as yet. In any case, the destabilizing effect toward high oxidation states was so pronounced that it turned out to be impossible to generate trivalent states like FeIII or CoIII. Rather, oxidation of the ligand part (E > 1.4 V versus SCE) was observed.
A [2]-rotaxane Zn2−Au+ made from a dumbbell component ended by Zn(II) porphyrin stoppers and a ring component incorporating a Au(III) porphyrin has been assembled in 13% yield using the transition metal templating route. 1H NMR studies show that its conformation in solution is very different from those of its complexes with Cu+, Ag+, and Li+. In particular, removal of the templating metal resulted in a changeover of the molecule, the threaded macrocycle undergoing a pirouetting motion placing the Au(III) porphyrin in the cleft formed by the two Zn(II) porphyrin stoppers. At room temperature, the changeover could be either complete or partial, depending on the solvent used. Photoinduced electron transfer from one of the Zn(II) porphyrins to the Au(III) porphyrin of the macrocycle was evidenced in the case of the free rotaxane and its Cu(I) complex, Zn2Cu+Au+. In the former case, the photoinduced electron transfer process could be clearly resolved for an extended conformation that is characterized by the Zn(II) porphyrins pointing far from the Au(III) porphyrin electron acceptor, and accounting for 30% of the total in acetonitrile at room temperature. In both Zn2Cu+Au+ and Zn2−Au+ rotaxanes, the charge-separated state, in which the Zn(II) porphyrin is a cation radical and the Au(III) porphyrin a neutral radical, was generated at a rate of 5 × 109 s-1 and disappeared at a rate of 2 × 108 s-1. In the case of Zn2Cu+Au+, the primary step is very likely energy transfer from the Zn(II) porphyrin singlet excited state to the MLCT state of the central Cu(I) complex, followed by an electron transfer from the excited Cu(I) unit to the Au(III) porphyrin and a successive charge shift from the Zn(II) porphyrin to the oxidized Cu(II) complex. [2]-rotaxane Zn2−Au+, in which no bond pathway can be identified between the donor and the acceptor, is a typical case of electron transfer involving molecular fragments connected by mechanical bonds.
The absorption features of 1(π,π*), 3(π,π*), 3(d,π*), and (d,d) excited states of metalloporphyrins have been closely examined between 420 and 900 nm with subpicosecond/picosecond transient absorption spectroscopy. The spectra of all of these excited states exhibit strong but not readily distinguishable absorption between the Soret- and Q-band bleachings, a region used extensively and often exclusively in transient absorption studies on porphyrins. The absorption features between 600 and 900 nm are smaller than those observed in the Soret region but have distinctive characteristics that aid in assessing the presence or absence of a particular type of transient state. We discuss the electronic origins of the prominent bands in the excited-state spectra. Our results and discussion provide fundamental information on the optical properties of metalloporphyrin excited states and a much needed framework for better interpreting the results of in vivo and in vitro transient absorption studies on these complexes.
Recent optical data suggest that porphyrin-guanine (PG) interacts in aprotic solvents with quinone-cytosine (QC) to form a 1 : 1 donor-acceptor preorganized base-paired supramolecule (PG-QC). To confirm such significant findings, for this never class of photosynthetic model system, we report here on an independent study by time-resolved electron paramagnetic resonance spectroscopy, combined with selective light excitation. The assemblies (BG-QC) of different porphyrins (zinc-porphyrin, Zn, and free base-porphyrin, H-2) were oriented in liquid crystals of different properties, and the results confirm unambiguously the formation of photoinduced charge-separated states, i.e., ZnG(.+)...QC(.-) and H(2)G(.+)...QC(.-), With lifetimes of a few microseconds. The unique spin-polarized EPR spectra, which depend on the temperature and solvent, allow the determination of whether the genesis of the electron transfer route is the photoexcited singlet or triplet states of the porphyrin-guanine subunits.
The transition metal-directed threading of a molecular fragment containing a central chelate and an end-attached gold(III) porphyrin into a presynthesized coordinating ring affords a general precursor to rotaxane-type structures. After threading, construction of the second porphyrin acting as an efficient stopper affords a copper(I)-complexed [2]-rotaxane as well as a novel compartmental [3]-rotaxane. The latter compound consists of a three-porphyrin-containing molecular fragment threaded through two 30-membered rings. Quantitative demetalation of the copper(I)-complexed [2]-rotaxane affords a free-ligand [2]-rotaxane. The two porphyrins are bulky enough to inhibit any process leading to separate fragments, the cycle and the thread. H-1 NMR spectroscopy demonstrates that upon decomplexation the [2]-rotaxane undergoes profound conformation changes. Recomplexation using zinc(II) regenerates an entwined and compact structure similar to that of the copper(I)-complexed [2]-rotaxane obtained in the template reaction.
Five new dicopper(I) knots have been synthesized as well as their face-to-face isomers. The knots range from 80- to 90-membered rings, and their preparation yields depend crucially on structural parameters such as number of methylene fragments linking the two chelating units and length of the poly(ethyleneoxy) unit used in the cyclization reaction. The best yield was obtained for an 84-membered knotted ring with a -(CH2)6- connector: this relatively long fragment allows pronounced winding of the double-helix precursor and is thus favorable to the knotting reaction. The face-to-face complexes were in some instances the major products, being obtained in yields amounting to 24% in the case of the dicopper(I) bis(43-membered-ring) system. The electrochemical properties of the copper complexes also depend on their structure. The redox potential values of the Cu(II)/Cu(I) couple span over a wide range (approximately 0.5-0.75 V vs SCE), the most electrochemically stable copper(I) complex being the 84-membered knotted compound. In CH2Cl2 solution, both the Cu2 knots and their face-to-face isomers exhibit metal-to-ligand charge-transfer absorption bands in the visible region and emission bands in the red spectral region. The profile of the absorption spectra and the luminescence properties (lambda(max) quantum yield, lifetime, and rate of excited-state quenching by acetone) depend on the length of the connectors. In agreement with the electrochemical results, the -(CH2)6- linker has a pronounced shielding effect on the metal center as well as a special ability to impose geometrical constraint.
A set of rotaxanes has been constructed consisting of a 30-membered macrocyclic ring, incorporating a 2,9-diphenyl-1,10-phenanthroline residue, threaded onto a second 2,9-diphenyl-1,10-phenanthroline residue with gold-(III) and zinc(II) porphyrins acting as terminal stoppers. The two chelating groups may be coordinated to copper(I) or zinc(II) cations or left free. Upon selective excitation of either porphyrin, rapid electron transfer occurs from the zinc porphyrin to the appended gold porphyrin and the ground state is restored by relatively slow reverse electron transfer. The rates of the various electron-transfer steps show a marked dependence on the molecular architecture and may be understood in terms of a frontier molecular orbital energy diagram involving through-bond electron or hole transfer. The coordinating cation modulates the energy of orbitals on the spacer and, thereby, affects the rate of electron transfer. The central metal complex may also be involved as a [open quotes]real[close quotes] intermediate in the electron-transfer pathway. Compared to the corresponding bis-porphyrin, rates of electron transfer at zero activation free energy change and the magnitude of electronic coupling between the porphyrins are significantly lower in the rotaxanes. This may be a consequence of subtle changes in the stereochemistry. 41 refs., 6 figs., 4 tabs.
A rotaxane has been built around a central copper(I) bis(1,10-phenanthroline) complex with gold(III) and zinc(II) porphyrins acting as terminal stoppers. Upon selective excitation of either porphyrin, rapid electron transfer occurs from the zinc porphyrin to the appended gold porphyrin. The copper(I) complex donates an electron to the resultant zinc porphyrin [pi]-radical cation, and the ground-state system is restored by relatively slow electron transfer from the gold porphyrin neutral radical to the copper(II) complex. The rates of the various electron-transfer steps observed with the rotaxane are compared to those occurring in closely related systems, and it is concluded that the copper(I) complex mediates photoinduced electron transfer between the terminal porphyrinic subunits but not the reverse reaction. A qualitative understanding of the electron-transfer rate is presented in terms of a simple orbital energy diagram involving through-bond electron or hole transfer. 39 refs., 3 figs., 2 tabs.
Intramolecular photoinduced charge separation (CS) and charge recombination (CR) of the product ion pair (IP) state of a series of fixed-distance dyads consisting of free-base porphyrin or zinc porphyrin and quinones have been investigated by means of picosecond-femtosecond laser spectroscopies in order to examine the energy gap and temperature dependences of CS and CR reactions in nonpolar media. Obtained CS rates were in the normal region, up to the neighborhood of the top region, and CR rates were in the inverted region; their energy gap dependences at room temperature were approximately reproduced by a semiclassical formula taking into consideration the high-frequency quantum mode of nuclear vibrations, although the CS rates near the top region did not show indication of the shift to the inverted region, contrary to the calculation. We have confirmed that the activation barrier for the CS reaction increases with a decrease of the energy gap, while the CR process is activationless, indicating the dominant effect of the high-frequency quantum mode in the inverted region. However, we could hardly find optimum parameter values for reorganization energies, etc., in the theoretical equation which could reproduce quantitatively both the energy gap dependence and the temperature dependence of the CS and CR rates at the same time. We have examined also the solvent polarity effect upon the energy gap (-DELTAG(CS)) dependence of the CS rate constant (k(CS)) which showed a rather large systematic change corresponding to the increase of the solvent reorganization energy with the increase of the solvent polarity, while the energy gap (-DELTAG(CR)) dependence of the CR rate constant (k(CR)) showed little solvent polarity dependence, leading to the crossing between the k(CS) vs -DELTAG(CS) curve in the normal to near the top region and the k(CR) vs -DELTAG(CR) in the inverted to near the top region. Implications of these results, which seem difficult to interpret on the basis of the conventional electron-transfer theories, are discussed on the basis of the dominant effect of the quantum mechanical tunneling in the inverted region and/or the existence of nonlinear or some specific interactions between the IP state and the surrounding polar solvent.
Bisgold(III) bisporphyrins, in which a 1,10-phenanthroline spacer imposes a constrained geometry, have been synthesized. Flash photolysis studies indicate that, at high laser intensity, triplet exciton annihilation occurs with a diffusion coefficient of 4 x 10(-5) cm2 s-1. Two such molecules coordinate to a copper(I) ion, via the 1, 10-phenanthroline spacers, to form a tetrameric porphyrin ensemble in which triplet exciton annihilation competes with electron transfer from the copper(I) complex to a gold porphyrin triplet. A mixed multicomponent array, comprising gold(III) and zinc(II) bisporphyrins covalently-linked to a copper(I) bis(1,10-phenanthroline) complex, undergoes a variety of electron-transfer reactions according to which porphyrin absorbs the photon energy. The copper(I) complex participates in these electron-transfer processes via both direct (redox) and indirect (superexchange) mechanisms.
