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[60]Fullerene-Stoppered Porphyrinorotaxanes: Pronounced Elongation of Charge-Separated-State Lifetimes
Abstract
A series of Sauvage-type porphyrinorotaxanes containing [60]fullerene stoppers have been synthesized by a convergent route. Photoinduced energy transfer and electron-transfer reactions in these rotaxanes yield long-distance charge-separated radical-pair states, whose lifetimes in solution at ambient temperatures are as high as 32 mus, depending on the distance between the fullerene and zinc porphyrin chromophores.
... Therefore, the former keeps the ZnP and C 60 moieties at longer and xed distances, while the latter brings them closer to each other, a process that is driven by secondary interactions between the chromophores and allowed by the unclosed ring of the rotaxane. [11][12][13][14][15][16][17][18][19] One of the well-established synthetic protocols to assemble rotaxanes and catenanes is Sauvage's Cu(I)-templated synthesis. [20][21][22][23][24] In this synthetic strategy, two 1,10-phenanthroline moieties (phen) become orthogonally arranged as a result of their tetrahedral coordination to the Cu(I) template ion, which creates the cross-over points needed for the formation of the mechanical bond. ...
... In line with previous reports, the latter is assigned to the one electron reduced form of C 60 , while the former correlates with the one electron oxidized form of ZnP. 11,14,15,18,33,35 From the corresponding extinction coefficients, that is, 0.82 Â 10 4 M À1 cm À1 for the ZnP triplet excited state at 840 nm 36 and 1.5 Â 10 4 M À1 cm À1 for the one electron reduced form of C 60 , 37 we conclude that the major product is the former. However, exact values for the yields of charge separation could not be determined. ...
... In comparison to previously studied rotaxanes incorporating either ZnPc, 61 Fc, 16 or ZnP, 14,15,18 signicantly longer charge separated state lifetimes have been achieved in the new rotaxanes 1, 2, and 3. For example, in ferrocene stoppered [Cu(phen) 2 ] + -C 60 rotaxanes only the (Fc) 2 -[Cu(phen) 2 ] 2+ -C 60 c À charge separated state with a lifetime of 15-16 ns (k CR ¼ 6.3 to 6.7 Â 10 7 s À1 ) was observed, without any appreciable evidence for a subsequent charge shi to the ferrocene units. ...
A new set of [Cu(phen)2]+ based rotaxanes, featuring [60]-fullerene as an electron acceptor and a variety of electron donating moieties, namely zinc porphyrin (ZnP), zinc phthalocyanine (ZnPc) and ferrocene (Fc), has been synthesized and fully characterized with respect to electrochemical and photophysical properties. The assembly of the rotaxanes has been achieved using a slight variation of our previously reported synthetic strategy that combines the Cu(I)-catalyzed azide–alkyne cycloaddition reaction (the “click” or CuAAC reaction) with Sauvage's metal-template protocol. To underline our results, complementary model rotaxanes and catenanes have been prepared using the same strategy and their electrochemistry and photo-induced processes have been investigated. Insights into excited state interactions have been afforded from steady state and time resolved emission spectroscopy as well as transient absorption spectroscopy. It has been found that photo-excitation of the present rotaxanes triggers a cascade of multi-step energy and electron transfer events that ultimately leads to remarkably long-lived charge separated states featuring one-electron reduced C60 radical anion (C60˙−) and either one-electron oxidized porphyrin (ZnP˙+) or one-electron oxidized ferrocene (Fc˙+) with lifetimes up to 61 microseconds. In addition, shorter-lived charge separated states involving one-electron oxidized copper complexes ([Cu(phen)2]2+ (τ < 100 ns)), one-electron oxidized zinc phthalocyanine (ZnPc˙+; τ = 380–560 ns), or ZnP˙+ (τ = 2.3–8.4 μs), and C60˙− have been identified as intermediates during the sequence. Detailed energy diagrams illustrate the sequence and rate constants of the photophysical events occurring with the mechanically-linked chromophores. This work pioneers the exploration of mechanically-linked systems as platforms to position three distinct chromophores, which are able to absorb light over a very wide range of the visible region, triggering a cascade of short-range energy and electron transfer processes to afford long-lived charge separated states.
... Rotaxanes containing C 60 as the electron-acceptor component and different types of electron-donors have been reported in the literature. Among them, porphyrins and phthalocyanines have been widely explored as the electron-donors, [59][60][61][62][63][64][65][66][67][68] in which charge separation (CS) process can occur through the singlet or triplet excited states, depending on the distance between the components. [58,65] Furthermore, changes in the relative distance between donors and acceptors seems to play a key role in the photoinduced electron transfer (PET) process. ...
... [4] Thus, changes in topology have shown to result in substantially longer lifetimes, greater than analogous covalently linked Zinc porphyrin-C 60 (ZnP-C 60 ) systems, showing lifetimes as long as 32 μs in the case of dyad 15 (Scheme 6), in which the bisphenanthroline template act as an energy or electron transfer mediator. [62] Scheme 6. Schematic representation of the rotaxane dyad combining C 60 and Zn-porphyrin. ...
Considerable research efforts have been devoted to the development of rotaxanes and the study of their unique dynamic properties. This minireview provides an overview of the main advances that have been realized in rotaxane architectures involving different types of carbon nanostructures. In particular, rotaxanes based on fullerenes and carbon nanotubes will be discussed.
... 20,27,30,32,34 In principle, such bulky substituents can increase the MLCT lifetime and affect the energetics of electron transfer. 20,27,32,34 Despite their similarities to [Ru(bpy) 3 19,32,[35][36][37][38][39][40][41][42][43][44][45][46]31,47 This situation likely arises for two reasons: First, the lability of Cu(I) generally means that an equilibrium mixture of copper complexes exists in solution if multiple similar-type ligands are present, thus complicating any data interpretation. Second, and probably more importantly, the [Cu(II)P 2 ] 2+ product of the initial MLCT oxidative quenching is such a weak oxidant that easily oxidizable donors must be employed. ...
... ; t, triplet; q, quadruplet, m, multiplet; br, broad), the value of the coupling constants in Hz if applicable, the number of protons implied and finally the assignment. In the 1 H NMR assignment (δ), H o and H m refer to the hydrogen atoms at the ortho and meta positions, respectively, of the phenyl ring attached to the phenanthroline ring system, whose hydrogen atoms are numbered H3,8 , H 4,7 , H 5,6 , respectively. Ar is used as an abbreviation for aromatic ring. ...
... Furthermore, to construct a D-C-A capable of multi-step electron transfer CS formation, a stable heteroleptic Cu(I)(L-D)(L-A) complex must be formed, having one ligand appended with the electron donor (L-D) and the other ligand appended with an electron acceptor (L-A). However, it has been repeatedly demonstrated that sterically crowding the Cu(I) coordination sphere hinders geometrical distortion upon photoexcitation which has two significant effects: (1) anodically shifting the Cu II/I redox potential, such that reductive quenching of the MLCT is possible and (2) Notably, other heteroleptic D-C-A systems were reported prior that contain Cu(I)(L) 2 complexes, but the Cu(I) unit is not the sole chromophore and a dynamic equilibrium exists between the multi-step and single-step CS states [73,74,[77][78][79]113]. The photo-induced electron transfer mechanism of this novel system is elucidated using transient absorption spectroscopy in conjunction with electrochemical, spectroelectrochemical, UV-Vis, and 1 H NMR studies. ...
... Nevertheless, these results are expected because it is well-known from the literature, [193,194] that the conjugated π-system of the fullerene core (and also that of the monoadducts) is a very good electron acceptor whereas porphyrins possess the ability to act as electron donors. As a direct consequence, a strong reduction of the fluorescence as well as of the singlet oxygen quantum yields was observed for 71 ...
... 15 Using a related structure, which included a zinc porphyrin as well, Li et al. have detected MLCT luminescence from the bis(1,10-phenanthroline)copper center, but the lifetime is very short due to oxidative quenching by C 60 acceptors. 16 For convenience, the emission studies described below utilize [2]pseudorotaxanes, which lack the bulky ''stopper'' groups that prevent unthreading in a true [2]rotaxane. Figure 1 shows the structure of the m-30 ring ligand, which all systems have in common. ...
This report describes photoluminescence studies of copper-containing [2]pseudorotaxanes that mimic elements of functioning molecular machines. Excitation with visible light induces a formal oxidation of the metal center and simulates an actuation process. In all four [2] pseudorotaxanes studied, the ring ligand is the same, but the thread ligand is variable, namely 2,9-di(anisol-4-yl)-1,10-phenanthroline (dap), 6,6'-di(anisol-4-yl)-2,2'-bipyridine (o-dabipy), 5,5'-di(anisol-4-yl)-2,2'-bipyridine (m-dabipy), or 8,8'-di(anisol-4-yl)-3,3'-bi-isoquinoline (dabiiq). The absorbance bandshapes suggest that aryl substituents extending from the core ligands engage in stacking interactions and induce a partially flattened structure in the ground state. More severe flattening occurs in the excited state and precludes the observation of emission if inter-ligand steric forces do not limit the distortion. Thus, the [2] pseudorotaxanes containing dap or o-dabipy as the thread ligand exhibit uncorrected emission maxima at around 720 nm in room-temperature dichloromethane, while the less constrained analogues, containing dabiiq or m-dabipy, are not emissive in fluid solution and barely exhibit a signal in rigid media. In dichloromethane, the luminescence quantum yields of the dap- and o-dabipy-containing systems are 6 x 10(-4) and 4 x 10(-4), and the excited-state lifetimes are 98 ns and 90 ns, respectively.
... The facile templated synthesis of Cu(I)-bisphenanthrolines ([Cu(NN) 2 ] + ) prompted the design and construction of sophisticated multicomponent architectures comprising one or more [Cu(NN) 2 ] + centers in tandem with other chromophores, 63 including C 60 fullerenes. 57,64 The first example of these hybrids was a rotaxane containing a [Cu(NN) 2 ] +type core and two methanofullerene moieties as stoppers (1), Figure 3. 50 In this system, upon photoexcitation, the electronic excited states typical of its individual components are strongly quenched, namely MLCT level of the [Cu(NN) 2 ] + core, fullerene singlet, and fullerene triplet. This molecule becomes spectroscopically silent because all of the excited states localized on the molecular components are deactivated by means of a sequence of energyand electron-transfer steps to a low energy charge-separated state which is made available at about 1.45 eV in the multicomponent rotaxane 1 thanks to the complementary electronic character of the Cu(I)-complexed core (electron donor) and of the C 60 terminals (electron acceptors). ...
The optical, electrochemical and excited state properties of C60 fullerene and its derivatives, combined with their peculiar structural features, have made them ideal modules for the construction of complex architectures which feature light induced processes. Here are described three selected classes of such systems investigated in our group by taking advantage of some intrinsic excited state properties of C60 fullerenes such as fluorescence, triplet lifetimes, or sensitized singlet oxygen luminescence. Fullerene hybrid assemblies with Cu(I), Ru(II), and Re(I) complexes undergo ultrafast photoinduced electron transfer (PET) upon excitation of the metal-to-ligand-charge-transfer (MLCT) excited states of the metal-complexed moiety. In the case of the Cu(I) system, occurrence of PET by fullerene excitation depends on the specific functionalization of the carbon sphere. Fullerodendrimers equipped with oligophenylenevinylene moieties and showing enhanced or reduced PET as a function of dendrimer structure and size are presented along with simpler monochromophoric fullerodendrimers which illustrate the capability of fullerene triplets to probe dendritic shielding effects. The peculiar ground and excited-state properties of rigid and conformationally flexible fullerene-porphyrin systems arranged in a face-to-face fashion are described, both with meso,meso-linked or triply fused porphyrin oligomers. Control of the direction and nature of photoinduced processes is achieved in such systems, which additionally show charge transfer or porphyrin-centered near-infrared luminescence. The whole work is discussed in a historic perspective and further development for photoactive fullerodendrimers and face-to-face arrays, particularly in solar energy conversion devices, is envisaged.
... Inspired by the work of Sauvage and colleagues, [41][42][43][44][45][46] we have designed and prepared analogous [Cu(phen) 2 ] + based rotaxanes and catenanes bearing ZnP as the electron donor, but with C 60 as the ultimate electron acceptor in place of AuP. [47][48][49][50] The choice of C 60 is grounded on its extraordinary redox and spectroscopic properties, 51,52 which have allowed determination of rates of formation and decay of electronically excited and charge separated states that were impossible to obtain in the previous purely porphyrinic interlocked systems. ...
The effect of molecular topology, and conformation on the dynamics of photoinduced electron transfer (ET) processes has been studied in interlocked electron donor-acceptor systems, specifically rotaxanes with zinc(II)-tetraphenylporphyrin (ZnP) electron donor and [60]fullerene (C(60)) as the electron acceptor. Formation or cleavage of coordinative bonds was used to induce major topological and conformational changes in the interlocked architecture. In the first approach, the tweezers-like structure created by the two ZnP stopper groups on the thread was used as a recognition site for complexation of 1,4-diazabicyclo[2.2.2]octane (DABCO), which creates a bridge between the two ZnP moieties on the rotaxane, generating a catenane structure. The photoinduced processes in the DABCO-complexed (ZnP)(2)-[2]catenate-C(60) system were compared with those of the (ZnP)(2)-rotaxane-C(60) precursor and the previously reported ZnP-[2]catenate-C(60). Steady-state emission and transient absorption studies showed that a similar multistep ET pathway emerged for rotaxanes and catenanes upon photoexcitation at various wavelengths, ultimately resulting in a long-lived ZnP(•+)/C(60) (•-) charge separated radical pair state. However, the decay kinetics of the latter states clearly reflect the topological differences between the rotaxane, the catenate, and DABCO-complexed-catenate architectures. The lifetime of the long-distance ZnP(•+)-[Cu(I)phen(2)](+)-C(60) (•-) charge separated state is more than four times longer in 3 (1.03 µs) than in 1 (0.24 µs) and approaches that in catenate 2 (1.1 µs). The results clearly showed that adoption of a catenane from a rotaxane topology inhibits the charge recombination process. In a second approach, the Cu(I) ion used as template to assemble the (ZnP)(2)-[Cu(I)phen(2)](+)-C(60) rotaxane was removed, and structural analysis suggested a major topographical change occurred, such that charge separation between the chromophores was no longer observed upon photoexcitation in nonpolar as well as polar solvents. Only ZnP and C(60) triplet excited states were observed upon laser excitation. These results highlighted the critical importance of the central Cu(I) ion for long range ET processes in these large interlocked electron donor-acceptor systems.
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.
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.
Double-bridged cofacial Ni porphyrin dimers 2 with 2,2'-bipyridyl pillars were effectively prepared by a one-step reductive homocoupling reaction of bis(chloropyridyl)-substituted Ni porphyrin derivatives followed by a specific separation of a cyanopropyl-modified silica gel column using pyridine eluent systems. The structural analyses of 2 and its Pd complex were carried out in their solid and solution states by means of X-ray single crystal analysis and NMR, respectively. The complexation of η3-allylpalladium chloride (Pd) with 2 on the spatially restricted 2,2-bipyridine moieties on 2 gave a 2:1 (Pd:2) complex, in which the 2,2'-bipyridine ligands only provided one of the N atoms on a 2,2'-bipyridine ligand to a Pd. Therefore, the 2,2-bipyridine moieties acted as a monodentate ligand.
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.
Generalized material on the synthesis of monosubstituted methanofullerenes C 60 , obtained by the addition–elimination mechanism.
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.
The structure and function of photosynthetic reaction centers (PRCs) have been modeled by designing and synthesizing electron donor-acceptor ensembles including electron mediators, which can mimic multi-step photoinduced charge separation occurring in PRCs to obtain long-lived charge-separated states. PRCs in photosystem I (PSI) or/ and photosystem II (PSII) have been utilized as components of solar cells to convert solar energy to electric energy. Biohybrid photoelectrochemical cells composed of PSII have also been developed for solar-driven water splitting into H2 and O2. Such a strategy to bridge natural photosynthesis with artificial photosynthesis is discussed in this minireview.
Transformation of a methylene group of the pillar[5]arene scaffold into a ketone has been achieved by treatment with N‐bromosuccinimide followed by hydrolysis of the bromide intermediate and oxidation of the resulting secondary benzylic alcohol with BaMnO4. Condensation of the resulting macrocycle including a ketone function with p‐toluenesulfonyl hydrazide followed by reaction of the corresponding tosylhydrazone with C60 under modified Bamford‐Stevens conditions gave a fulleropillar[5]arene derivative. This building block has been used to prepare a rotaxane. The resulting molecule combining the fullerene‐functionalized macrocycle with an axle bearing a porphyrin stopper is a photoactive molecular device in which the porphyrin emission is efficiently quenched by the fullerene moiety.
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In this review article, we highlight recent advances in the field of solar energy conversion at a molecular level. We focus mainly on investigations regarding fullerenes as well as endohedral metallofullerenes in energy and/or electron donor-acceptor conjugates, hybrids, and arrays, but will also discuss several more advanced systems. Hereby, the mimicry of the fundamental processes occurring in natural photosynthesis, namely light harvesting (LH), energy transfer (EnT), reductive/oxidative electron transfer (ET), and catalysis (CAT), which serve as a blue print for the rational design of artificial photosynthetic systems, stand at the focalpoint. Importantly, the key processes in photosynthesis, that is, LH, EnT, ET, and CAT, define the structure of this review with the only further differentiation in terms of covalent and non-covalent systems. Fullerenes as well as endohedral metallofullerenes are chosen by virtue of their small reorganization energies in electron transfer processes, on the one hand, and their exceptional redox behaviour, on the other hand.
AbbreviationsIntroductionBasic Photochemistry of PorphyrinsDonor–Acceptor Compounds for PetApplied PhotochemistryAcknowledgmentReferences
A study is conducted for harnessing the synergistic and complementary properties of fullerene (C60) and transition-metal compounds for nanomaterial applications. C60 and the rest of the fullerene family are electron-deficient polyolefins and can act as metal-binding ligands coordinating transition metals in different binding modes directly to the carbon cage. A number of complexes of this type have been synthesized and characterized incorporating a wide range of heavy, mainly second- and third-row transition metals attached to the fullerene cage.
Recent progress on the design, construction, and photoinduced energy- and electron-transfer applications of self-assembled donor- acceptor arrays has been reviewed, focusing mainly on the multiporphyrin-fullerene and multiporphyrin-SWCNT supramolecular arrays to construct photosynthetic antenna and reaction center models. In addition to the covalently bonded entities, self-assemblies through intermolecular interactions including π-π attraction, ion-pairing, cation-dipole, hydrogen bonding, utilized in the construction of multiporphyrin-fullerene or multiporphyrin- SWCNT are discussed. The control over the energy- and electrontransfer events upon excitation of the multiporphyrins in such supramolecular arrays is accomplished. These studies demonstrate that it is possible to develop self-assembled multiporphyrin conjugates capable of generating considerably high light converting and long-lived charge-separated species useful for light-energyharvesting applications.
In this thesis work the wide variety of chemical and physical properties exhibited by fullerenes have been used to generate and study novel interesting mechanical interlocked architectures. A fullerene rotaxane was designed to act as a macromolecular multitasking backbone, able to be exploited for different applications. In particular, such backbone has been modified with positive charges in order to build up a high efficiency driven electrochemically molecular shuttle and with different chromophores (metal porphyrins) to assemble a system that mimic the photoreaction center, an essential part of the whole photosynthesis process.
IntroductionBasic Principles of ElectrochemistryOverview of Electrochemical TechniquesElectrochemical Analysis of Supramolecular SystemsSelected ExamplesConcluding RemarksAcknowledgmentsReferences
IntroductionOptical Properties of Cu(I) ComplexesOld Systems for New ChallengesSummaryReferences
A new series of nanoscale electron donor-acceptor systems with [2]catenane architectures has been synthesized, incorporating magnesium porphyrin (MgP) or free base porphyrin (H2P) as electron donor and C60 as electron acceptor, surrounding a central tetrahedral Cu(i)-1,10-phenanthroline (phen) complex. Model catenated compounds incorporating only one or none of these photoactive moieties were also prepared. The synthesis involved the use of Sauvage's metal template protocol in combination with the 1,3-dipolar cycloaddition of azides and alkynes ("click chemistry"), as in other recent reports from our laboratories. Ground state electron interactions between the individual constituents was probed using electrochemistry and UV-vis absorption spectroscopy, while events occurring following photoexcitation in tetrahydrofuran (under both aerobic and anaerobic conditions) at various wavelengths were followed by means of time-resolved transient absorption and emission spectroscopies on the femtosecond and nanosecond time scales, respectively, complemented by measurements of quantum yields for generation of singlet oxygen. From similar studies with model catenates containing one or neither of the chromophores, the events following photoexcitation could be elucidated. The results were compared with those previously reported for analogous catenates based on zinc porphyrin (ZnP). It was determined that a series of energy transfer (EnT) and electron transfer (ET) processes take place in the present catenates, ultimately generating long-distance charge separated (CS) states involving oxidized porphyrin and reduced C60 moieties, with lifetimes ranging from 400 to 1060 nanoseconds. Shorter lived short-distance CS states possessing oxidized copper complexes and reduced C60, with lifetimes ranging from 15 to 60 ns, were formed en route to the long-distance CS states. The dynamics of the ET processes were analyzed in terms of their thermodynamic driving forces. It was clear that intramolecular back ET was occurring in the inverted region of the Marcus parabola correlating rates and driving forces for electron transfer processes. In addition, evidence for triplet excited states as a product of either incomplete ET or back ET was found. The differences in behavior of the three catenates upon photoexcitation are analyzed in terms of the energy levels of the various intermediate states and the driving forces for EnT and ET processes.
Platinum(II) bis(N-(4-ethynylphenyl)carbazole)bipyridine fullerene complexes, (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60, were synthesized. Their photophysical properties were studied by electronic absorption and emission spectroscopy and the origin of the transitions was supported by computational studies. The electrochemical properties were also studied and the free energies for charge-separation and charge-recombination processes were evaluated. The photoinduced electron transfer reactions in the triads were investigated by femtosecond and nanosecond transient absorption spectroscopy. In dichloromethane, both triads undergo ultrafast charge separation from the 3MLCT/LLCT excited state within 300 fs to yield their respective triplet charge-separated (CS) states, namely (Cbz)2˙+–Pt(bpy)–C60˙− and (tBuCbz)2˙+–Pt(bpy)–C60˙−, and the CS states would undergo charge recombination to give the 3C60* state, which subsequently decays to the ground state in 22–28 μs.
Porphyrin-containing [2]catenanes were obtained from relatively simple organic precursors thanks to "coordination chemistry-only". When the central porphyrin metal is Zn2+ the [2]catenanes were assembled in two steps by using Cu(I)–N interactions to assemble acyclic complexes and Zn(II)–N interactions to generate rings. These sophisticated architectures were obtained in excellent yield since the formation of the thermodynamic products is favored over that of kinetic products. This also explains why a perfect fit in terms of distances and angles between the components to assemble leads to highly stable coordination chemistry-assembled species. When a second row metal with an inert coordination sphere, like Rh(III) was chosen as metal centre of the porphyrins, direct metalation of porphyrinic compounds with [Rh(CO)2Cl]2 was not met with success when the bidentate chelate used as connector was metal free. A three-step strategy was used for the synthesis of a tetra-rhodiumporphyrin[2]-catenane: (i) copper(I)-driven entwining of two free-base porphyrin-bearing bidentate chelates followed (ii) by insertion of rhodium in the porphyrin nuclei and finally (iii) cyclisation via formation of N-Rh coordination bonds.
Donor-chromophore-acceptor triads, (PTZ)2-Pt(bpy)-C60 and ((t)BuPTZ)2-Pt(bpy)-C60, along with their model compound, (Ph)2-Pt(bpy)-C60, have been synthesized and characterized; their photophysical and electrochemical properties have been studied, and the origin of the absorption and emission properties has been supported by computational studies. The photoinduced electron transfer reactions have been investigated using the femtosecond and nanosecond transient absorption spectroscopy. In dichloromethane, (Ph)2-Pt(bpy)-C60 shows ultrafast triplet-triplet energy transfer from the (3)MLCT/LLCT excited state within 4 ps to give the (3)C60* state, while in (PTZ)2-Pt(bpy)-C60 and ((t)BuPTZ)2-Pt(bpy)-C60, charge-separated state forms within 400 fs from the (3)MLCT/LLCT excited state with efficiency of over 0.90, and the total efficiency with the contribution of (3)C60* is estimated to be 0.99. Although the forward electron transfer reactions are very rapid, the charge-separated state recombines to the singlet ground state at a time of hundreds of nanoseconds because of the difference in spin multiplicity between the charge-separated state and the ground state.
The electron and energy accepting properties of fullerene C60 can be readily exploited upon the selection of suitable donating species to construct supramolecular assemblies which display photoinduced electron and/or energy transfer processes. In this account we focus on the use of transition metal complexes in the role of the electron/energy donor, in particular we discuss the photophysical properties of some fullerene hybrid systems containing tris(2,2′-bipyridine)ruthenium(II), (2,2′-bipyridine)rhenium(I) or bis(1,10-phenanthroline)copper(I) subunits that we have investigated over the years. In fullerene hybrid dyads with Ru(II) and Re(I) complexed moieties, electron transfer is observed at room temperature in both cases, while it is blocked at 77 K in the former case. For multicomponent arrays (rotaxane, dendrimers, helicates) with Cu(I)-bisphenanthroline moieties, quenching by electron/energy transfer of the MLCT excited states of the metal complexed moiety is always found. When excitation is addressed to the carbon cage, photoinduced processes occurs when methanofullerenes are involved, but not with bismethanofullerenes. To cite this article: J.N. Clifford et al., C. R. Chimie (9) 2006.
Highlights of this year's literature include the characterisation of [CuMe], the first diazene-bridged dicopper(I) complex, and illumination of dioxygen binding by dicopper(I) complexes. The observation of {Cu(η2-H2)}+ species has been a major development in studies of copper-exchanged zeolites. Among polynuclear copper(II) species, examples of square planar μ4-bridging hydroxide and trigonal planar μ3-chloride are remarkable.
This review complements previous reports and is a collection of the highlights revealed by database searches for the year 2004. Highlights of the year include the isolation of a small [50]fullerene derivative, C50Cl10, use of a cyclodextrin and [60]fullerene complex to achieve nitrogen fixation and the continued development of fullerene- and nanotube-based electronic devices.
This review explores recent developments of photoinduced electron transfer systems based on interlocked molecules, especially [60]fullerene (C 60) and porphyrin-containing [2]rotaxanes. A number of synthetic methodologies for the construction of C60-containing interlocked molecules with various photo- and electro-active moieties, such as metalloporphyrins, metallophthalocyanines, triarylamines, and ferrocenes, in addition to the photoinduced electron transfer behaviors of these interlocked molecules, are examined. A synthetic strategy for a multi-step photoinduced electron transfer system based on interlocked molecules is also discussed.
This review article highlights the recent progress made in the design and study of self-assembled supramolecular architectures based on tetrapyrrolemacrocycles as donors, and fullerene and carbon nanotubes as electron acceptors for electron and energy transfer applications in solution. The remarkable features of the utilized biomimetic organization principles viz., hydrogen bonding, π–π stacking, metal-mediated complexation, and electrostatic attraction in governing the stability and geometry of the nanohybrids, and their significance in controlling the structure and electron transfer properties are discussed.
The recent literature on photoactive interlocked structures containing porphyrins is reviewed. Catenanes and rotaxanes studied both in the author's laboratory and by other groups, displaying either photoinduced energy or electron transfer processes are reported. In addition to porphyrins, the examined structures contain photo or electroactive components as C60, paraquat, ferrocene, aromatic amines. Both metal catenanes/rotaxanes and free catenanes/rotaxanes are discussed and the differences in their behavior is outlined with respect to structural rigidity and electronic coupling properties. The role of different conformations and their effect on photophysical properties is examined. In spite of their uncommon topology, these arrays behave similarly to covalently or self-assembled photoactive multi-component architectures and display fast energy/electron transfer rates and high charge separation efficiency. A rationale for this behavior is provided.
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).
This Perspective article traces the career of the author in the area of mechanistic organic photochemistry from the primitive state of this field in the late 1950s and early 1960s until its maturity into a highly sophisticated discipline at the present time. It focuses on studies carried out by the author and his associates to delineate mechanisms of basic types of photochemical reactions of small organic molecules to photophysical studies in recent years involving electron transfer processes occurring in nanoscale artificial photosynthetic systems in ultrashort time domains. The important role that serendipitous events played in directing key career decisions and events is emphasized. The important ways that developments in instrumentation influenced the choices, possibilities and accomplishments of doing research in photochemistry between 1960 and the present time are emphasized. Acknowledgment is given by the author to the many people who contributed directly and indirectly to the course that his career has taken over more than half a century.
Synthesis of [60]fullerene (C(60))-functionalized rotaxanes via Diels-Alder reactions with C(60) is described. Diels-Alder reaction of C(60) and sulfolene moiety as masked diene attached on the wheels of rotaxanes results in high yields of C(60) incorporation. Rotaxanes are prepared by tin-catalyzed urethane-forming end-capping reaction with isocyanate of pseudorotaxane having the wheel carrying C(60) functionality as introduced by the Diels-Alder reaction. The Diels-Alder reaction was accomplished as end-capping reaction between C(60) and pseudorotaxane bearing sultine moiety as masked diene on the axle terminal. A variety of C(60)-containing [2]rotaxanes was prepared in moderate to good yields by these Diels-Alder protocols. (c) 2005 Elsevier Ltd. All rights reserved.
Electron transfer processes occurring through-space and through-bond were studied for novel mechanically linked triad [2]rotaxanes, which contain a porphyrin (MP) unit as a pendant and [60]fullerene (C(60)) and triphenylamine (TPA) moieties as stopper groups at the axle ends (abbreviated as (MP;C(60)-TPA)(rol+) with NIP = H(2)P or ZnP. The photophysical properties were investigated by means of time-resolved fluorescence and transient absorption measurements with changing solvent polarity. The charge separation took place mainly via ((1)MP*;C(60)-TPA)(rol+) and (MP;(1)C(60)*-TPA)(rol+) in polar solvents. Within the charge-separated states of triads (MP;C(60)-TPA)(rol+) hole-shift and/or back electron transfer took place competitively. The lifetime of the charge separation state of (ZnP;(1)C(60)-TPA)(rol+) was similar to that of dyad (C(60)-TPA)(rol+) (140 ns in dimethylformamide), whereas that of (H(2)P;C(60)-TPA)(rol+) was as long as 230 ns, suggesting two final charge separation states such as (MP;C(60)(center dot-)-TPA(center dot+))(rol+) and (MP(center dot+);C(60)(center dot-)PA)(rol+) depending on the kind of porphyrin. Copyright (c) 2006 Society of Porphyrins & Phthalocyanines.
Photoinduced electron-transfer processes of a newly synthesized rotaxane containing porphyrinatozinc (ZnP), [60]fullerene (C(60)), and ferrocene (Fc) have been studied in terms of the time-resolved fluorescence and transient absorption measurements in polar and nonpolar solvents. In this rotaxane, (ZnP;C(60)-Fc)(rotax), ZnP was chosen as a pendant of a crown-ether necklace, through which an axle with C(60) and Fc at both termini was penetrated. By the selective excitation of the ZnP moiety in a nonpolar solvent, energy transfer predominantly takes place to the C(60) moiety of (ZnP;C(60)-Fc)(rotax). In polar solvents, charge-separation process takes place via the excited singlet state of the ZnP moiety in addition to the energy-transfer process. From the nanosecond transient absorption spectra, a clear absorption band of the C(60)(center dot) moiety was observed at 1000 mn as well as a broad absorption in the 600-800 nm region due to ZnP(center dot+), suggesting the generation of (ZnP(center dot+);C(60)(center dot-)-Fc)(rotax) in the first step. Afterward, the hole-transferring process from ZnP(center dot+) to Fc is thermodynamically possible, although this process is not fast because of its through-space process character. The final lifetimes of the C(60)(center dot-) moiety were evaluated to be 290 and 370 ns in benzonitrile and DMF, respectively. The ratios of the charge-separation rates to charge-recombination rates were ca. 1000, indicating that (ZnP;C(60)-Fc)(rotax) affords an efficient photosynthetic model. Copyright (C) 2005 Society of Porphyrins & Phthalocyanines.
A molecular triad assembly consisting of an electron donor, a bis(phenanthroline)copper(I) chromophore, and an electron acceptor has been prepared. Under visible-light excitation, this assembly undergoes efficient (ca. 50%) photoinduced, multistep formation of a diradical cation charge-separated state that has a lifetime of >100 ns and stores >1.0 eV of energy. This system constitutes an earth-abundant functional analogue of related Ru(bpy)(3) triad systems.
We have prepared a family of six new bimetallic complexes containing electron-donating trans-{RuIICl(pdma)2}+ [pdma = 1,2-phenylenebis(dimethylarsine)] centers linked to trans-[RuIICl(pdma)(2,2‘-bpy)(NO)] (bpy = bipyridyl) or fac-{ReI(biq)(CO)3}+-based (biq = 2,2‘-biquinolinyl) electron acceptors. The extended bridging units are based on 4,4‘-bpy or bpe [E-1,2-bis(4-pyridyl)ethylene] ligands, and these assemblies are designed to display long-lived photoinduced charge-separated states. A number of new monometallic precursor complexes have also been synthesized and fully characterized. The electronic absorption spectra of the bimetallic species are dominated by intense, visible d(RuII) → π*(4,4‘-bpy/bpe) metal-to-ligand charge-transfer (MLCT) bands and d(ReI) → π*(biq) absorptions in the near-UV region. Cyclic voltammetric studies show both metal-based oxidation and ligand-based reduction processes and provide evidence that these assemblies contain thermodynamic driving forces for charge separation. Single-crystal X-ray structures have been determined for the monometallic complex salts fac-[ReI(biq)(CO)3(MeCN)]CF3SO3·CHCl3, fac-[ReI(biq)(CO)3(4-BrCH2C5H4N)]PF6, fac-[ReI(biq)(CO)3(4-HCO2CH2C5H4N)]PF6·Me2CO, fac-[ReI(biq)(CO)3(ISNE)]CF3SO3 (ISNE = ethylisonicotinate), and fac-[ReI(biq)(CO)3(NC5H4CO2)]2·HPF6·3MeCN. Ultrafast time-resolved infrared and Raman spectroscopic measurements will be employed to investigate the photoexcitation properties of the bimetallic species and of monometallic reference complexes.
Hydrogen-bonding effects on film structures and photophysical, photoelectrochemical, and photovoltaic properties have been examined in mixed films of porphyrin and fullerene composites with and without hydrogen bonding on nanostructured TiO2 electrodes. The nanostructured TiO2 electrodes modified with the mixed films of porphyrin and fullerene composites with hydrogen bonding exhibited efficient photocurrent generation compared to the reference systems without hydrogen bonding. Atomic force microscopy, infrared reflection absorption and ultraviolet−visible absorption spectroscopies, and time-resolved fluorescence lifetime and transient absorption spectroscopic measurements disclosed the relationship between the film structures and optical and photoelectrochemical properties relating to the formation of hydrogen bonding between the porphyrins and/or the C60 moieties in the films on the electrode surface. These results show that hydrogen bonding is a potential methodology for the fabrication of donor and acceptor composites on a nanostructured TiO2 electrode, which exhibits high open circuit potential relative to that of the corresponding SnO2 electrode.
A new macrocycle and the corresponding [2]catenane were prepared quantitatively from relatively large organic fragments using coordination chemistry bonds only (Zn–N bonds for the ring and Cu–N bonds for the catenane). The two constitutive macrorings of the [2]catenane are 43-membered rings. The organic components consist of a bis-Zn porphyrin and a 1,10-phenanthroline derivative, both fragments being significantly larger than those previously used for making related compounds. The catenane was assembled in two steps from a bis-Zn porphyrin and a 2,9-di(4′′-pyridyl-4′-phenyl)-1,10-phenanthroline. In a first step the entwined copper(I) complex of the phenanthroline derivative was formed via Cu(I)–N interactions. Subsequently, a double ring closing reaction was performed by coordination of two equivalents of bis-Zn porphyrin to the entwined copper(I) complex. The tetraporphyrinic [2]catenane was thus obtained thanks to the formation of four Zn(II)–pyridine bonds. It has a high association constant, 4 × 1010 M−2, due to the good geometrical fit between the constituents.
Cyclopropanations between C60 and readily available malonates bearing different steroid moieties, I [R = CH2CH2CHMe2, CH2CHEtCHMe2-(R)] and II, by the Bingel-Hirsch protocol has allowed the synthesis of a new series of hybrid functionalized chimeras [III (X1-X2 = C60-fullerene) and IV]. Whereas cycloadducts III showed the expected chem. structures, the presence of the diene moiety in the ergosterol unit of malonate II resulted in the prodn. of the corresponding cycloadduct with an addnl. oxygen mol. A thorough spectroscopical study (1H and 13C NMR, COSY, DEPT, HMQC, and HMBC) allowed the structure of monocycloadduct IV to be unambiguously unraveled as that of an endoperoxide, as a result of the sensitizing effect of the C60 unit, which efficiently promotes the addn. of excited singlet oxygen to the diene moiety of the steroid. Cyclic voltammetry of hybrids III and IV, as well as their electronic spectra, support the exclusive formation of the corresponding monoadducts.
Photophysical investigations on a series of (2,4,6)-tris-substituted metalloporphyrin-fullerene conjugates revealed the effects of an electron-rich microenvironment surrounding the electron-donating porphyrin as a function of the metal center. On one hand, for all conjugates-water-soluble and non-water-soluble-ultrafast charge separation was observed upon photoexcitation. On the other hand, when examining the charge recombination dynamics for the non-water-soluble conjugates it becomes obvious that the (2,4,6)-tris-substitution stabilizes the radical-ion-pair state relative to the mono-substitution in the ortho-, meta-, and para-position. The more efficient protection of the electron-donating porphyrin from solvation is thought to be the major cause for this impact. Nevertheless, the situation is slightly different for the water-soluble conjugates. At first glance, the radical-ion-pair state lifetimes are, also in the case of the (2,4,6)-tris-substitution, longer than for the mono-substituted ortho-, meta- and para-conjugates. Upon closer inspection, they fail, however, to exhibit any metal dependence. Competing with the protection from solvation of the dendrons, dipole-charge interactions impact the stabilization in the polar aqueous environment and, in turn, become the dominant force governing the electron-transfer dynamics.
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.
Applications of voltammetry and electrosynthesis in the chemistry of fullerenes and their derivatives are considered. Effect of structural factors and environment on the reduction potentials of fullerenes and stability of their anion intermediates are analyzed. Emphasis is laid on the reduction of phosphorylated methanofullerenes.
We propose a novel strategy using fullerenes for the construction of solar energy conversion systems that mimic the primary electron transfer events in photosynthesis. Redox-active fullerenes such as C60 and C70 were covalently bound to a porphyrin and the photophysical properties of the resulting compounds were investigated. Regardless of solvent and linkage, the charge-separated state is produced efficiently in zincporphyrin–fullerene dyads, showing that fullerenes are good electron acceptors. The most intriguing characteristic of fullerenes in electron transfer is that they accelerate photoinduced charge separation as well as charge shift and slow down charge recombination, properties that are in sharp contrast with those of conventional two-dimensional aromatic acceptors such as quinones and imides. The peculiar electron transfer properties of fullerenes can be explained by the small reorganization energies, which make it possible to optimize artificial photosynthetic multistep charge separation. A combination of the two strategies, multistep electron transfer and small reorganization energy of fullerenes, has been applied to light energy conversion systems as well as the more complex molecular systems such as triads. Highly efficient photosynthetic multistep electron transfer has been realized at gold electrodes modified with self-assembled monolayers of fullerene-containing molecules. These results will provide new principles and concepts to develop artificial photosynthetic materials as well as molecular devices.
The fluorescence quenching of a Zn(II) porphyrin linked to Cu(I)catenates relative to a model compound without Cu(I) was attributed to energy transfer from the Zn(II) porphyrin to the metal-to-ligand charge transfer (MLCT) state of the Cu(I)(phenanthroline)2 center at the core of the molecules. The similarity of the fluorescence spectra and fluorescence decays of a Zn(II) porphyrin linked to an Au(III) porphyrin, a Zn(II) porphyrin or a benzoate moiety through the catenate framework suggested that no fluorescence quenching by electron transfer to the Au(III) porphyrin occurred and that the copper(I) (phenanthroline)2 center acts as an energy sink. The value of the critical distance for Förster type energy transfer, determined from spectral data is compatible with the observed rate constants for energy transfer and dimensions of the macrocycle. The multi-exponential nature of the fluorescence decay is attributed to the presence of different slowly interconverting conformations of the macrocycle to which the porphyrins are attached. Copyright © 2001 John Wiley & Sons, Ltd.
A rotaxane made from a bis-phenanthroline Cu1 complex and two C60 units acting as stoppers and two C60 units acting as stoppers has been synthesized. Electrochemical, spectroscopic and photophysical properties of the individual components, a methanofullerene and a Cu1 catenate, were determined. The properties of the methanofullerene were also compared with those of plain C60 and rationalized with the aid of semiempirical calculations. The changes in the photophysical properties detected in the rotaxane with respect to the models were assigned to the occurrence of intramolecular processes. The excited single state localized on the fullerene and the MLCT excited state centred on the Cu1 complex are both quenched. Deactivation of the fullerene excited singlet state occurs by energy transfer to the Cu1-complex moiety, which competes with intersystem crossing to triplet fullerene, whereas the Cu1-complex excited state is mainly quenched by electron transfer to form the charge-separated state consisting of the oxidized metal centre [Cu(phen)2]2+ and the fullerene radical anion. The fullerene triplet, formed in reduced yield with respect to the model, is also quenched by electron transfer to the same charge-separated state. The ability of both model components to sensitize single oxygen is completely suppressed in the rotaxane. The occurrence of a fast back-electron-transfer reaction is postulated, as spectroscopic detection of the charge-separated state has not been achieved.
The synthesis of (E)-hex-3-ene-l, 5-diynes and 3-methylidenepenta-1, 4-diynes with pendant methano[60]-fullerene moieties as precursors to C60-substituted poly(triacetylenes) (PTAs, Fig. 1) and expanded radialenes (Fig. 2) is described. The Bingel reaction of diethyl (E)-2, 3-dialkynylbut-2-ene-1, 4-diyl bis(2-bromopropane-dioates) 5 and 6 with two C60 molecules (Scheme 2) afforded the monomeric, silyl-protected PTA precursors 9 and 10 which, however, could not be effectively desilylated (Scheme 4). Also formed during the synthesis of 9 and 10, as well as during the reaction of C60 with thedesilylated analogue 16 (Scheme5), were the macrocyclic products 11, 12, and 17, respectively, resulting from double Bingel addition to one C-sphere. Rigorous analysis revealed that this novel macrocyclization reaction proceeds with complete regio- and diastereoselectivity. The second approach to a suitable PTA monomer attempted N, N′-dicyclohexylcarbodiimide(DCC)-mediated esterification of (E)-2, 3-diethynylbut-2-ene-l, 4-diol (18, Scheme 6) with mono-esterified methanofullerene-dicarboxylic acid 23; however, this synthesis yielded only the corresponding decarboxylated methanofullerene-carboxylic ester 27 (Scheme 7). To prevent decarboxylation, a spacer was inserted between the reacting carboxylic-acid moiety and the methane C-atom in carboxymethyl ethyl 1, 2-methano[60]fullerene-61, 61-dicarboxylate (28, Scheme 8), and DCC-mediated esterification with diol 18 afforded PTA monomer 32 in good yield. The formation of a suitable monomeric precursor 38 to C60-substituted expanded radialenes was achieved in 5 steps starting from dihydroxyacetone (Schemes 9 and 10), with the final step consisting of the DCC-mediated esterification of 28 with 2-[1-ethynyl(prop-2-ynylidene)]propane-1, 3-diol (33). The first mixed C60-C70 fullerene derivative 49, consisting of two methano[60]fullerenes attached to a methano[70]fullerene, was also prepared and fully characterized (Scheme 13). The Cs-symmetrical hybrid compound was obtained by DCC-mediated esterification of bis[2-(2-hydroxy-ethoxy)ethyl] 1, 2-methano[70]fullerene-71, 71-dicarboxylate (46) with an excess of the C60-carboxylic acid 28. The presence of two different fullerenes in the same molecule was reflected by its UV/VIS spectrum, which displayed the characteristic absorption bands of both the C70 and C60 mono-adducts, but at the same time indicated no electronic interaction between the different fullerene moieties. Cyclic voltammetry showed two reversible reduction steps for 49, and comparison with the corresponding C70 and C60 mono-adducts 46 and 30 indicated that the three fullerenes in the composite fullerene compound behave as independent redox centers.
This Account reviews our main achievements in the field of excited-state properties of fullerene derivatives. The photosensitizing and electron-acceptor features of some relevant classes of functionalized fullerene materials are highlighted, considering the impact of functionalization on fullerene characteristics. In addition, the unique optimization in terms of redox potentials, water-solubility, and singlet oxygen generation is presented for several novel fullerene-based materials.
Photoinduced charge separation (CS) and charge recombination (CR) processes have been examined in various porphyrin-fullerene linked systems (i.e., dyads and triads) by means of time-resolved transient absorption spectroscopy and fluorescence lifetime measurements. The investigated compounds comprise a homologous series of rigidly linked, linear donor-acceptor arrays with different donor-acceptor separations and diversified donor strength: freebase porphyrin-C60 dyad (H2P-C60), zincporphyrin-C60 dyad (ZnP-C60), ferrocene-zincporphyrin-C60 triad (Fc-ZnP-C60), ferrocene-freebase porphyrin-C60 triad (Fc-H2P-C60), and zincporphyrin-freebase porphyrin-C60 triad (ZnP-H2P-C60). Most importantly, the lowest lying charge-separated state of all the investigated systems, namely, that of ferrocenium ion (Fc+) and the C60 radical anion (C60.-) pair in the Fc-ZnP-C60 triad, has been generated with the highest quantum yields (close to unity) and reveals a lifetime as long as 16 micros. Determination of CS and CR rate constants, together with the one-electron redox potentials of the donor and acceptor moieties in different solvents, has allowed us to examine the driving force dependence (-DeltaG0ET) of the electron-transfer rate constants (kET). Hereby, the semilogarithmic plots (i.e., log kET versus -DeltaG0ET) lead to the evaluation of the reorganization energy (lambda) and the electronic coupling matrix element (V) in light of the Marcus theory of electron-transfer reactions: lambda = 0.66 eV and V = 3.9 cm(-1) for ZnP-C60 dyad and lambda = 1.09 eV and V = 0.019 cm(-1) for Fc-ZnP-C60, Fc-H2P-C60, and ZnP-H2P-C60 triads. Interestingly, the Marcus plot in Fc-ZnP-C60, Fc-H2P-C60, and ZnP-H2P-C60 has provided clear evidence for intramolecular CR located in both the normal and inverted regions of the Marcus parabola. The coefficient for the distance dependence of V (damping factor: betaCR = 0.58 A(-1) is deduced which depends primarily on the nature of the bridging molecule.
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)).
Fullerenes and porphyrins are molecular architectures ideally suited for devising integrated, multicomponent model systems to transmit and process solar energy. Implementation of C60 as a 3-dimensional electron acceptor holds great expectations on account of their small reorganization energy in electron transfer reactions and has exerted a noteworthy impact on the improvement of light-induced charge-separation. This article describes how the specific compositions of porphyrin chromophores linked to C60--yielding artificial light harvesting antenna and reaction center mimics--have been elegantly utilized to tune the electronic couplings between donor and acceptor sites and the total reorganization energy. Specifically, the effects that these parameters have on the rate, yield and lifetime of the energetic charge-separated states are considered.
A series of Sauvage-type rotaxanes containing [60]fullerene and tetraarylporphyrin moieties has been synthesized by a convergent route. Photoinduced energy-transfer and electron-transfer reactions in these rotaxanes yield long-lived change-separated states, in agreement with the large distance between the fullerene and porphyrin chromophores.