Conformation effect of oligosilane linker on photoinduced electron transfer of tetrasilane-linked zinc porphyrin–[60]fullerene dyads

A series of zinc porphyrin–[60]fullerene dyads linked by conformation-constrained tetrasilanes and permethylated tetrasilane have been synthesized for the evaluation of the conformation effect of the tetrasilane linkers on the photoinduced electron transfer. The excited-state dynamics of these dyads have been studied using the time-resolved fluorescence and absorption measurements. The fluorescence of the zinc porphyrin moiety in each dyad was quenched by the electron transfer to the fullerene moiety. The transient absorption measurements revealed that the final state of the excited-state process was a radical ion pair with a radical cation on the zinc porphyrin moiety and a radical anion on the fullerene moiety as a result of the charge separation. The charge separation and charge recombination rates were found to show only slight conformation dependence of the tetrasilane linkers, which is characteristic for the Si-linkages.     

Synthesis of [60]fullerene-containing [2]rotaxanes using axle molecules bearing donor moiety

[2]Rotaxanes, consisting of a fullerene derivative bearing an electron-donating 1,5-dialkoxynaphthalene moiety and a macrocycle containing electron-deficient naphthalenetetracarboxylic diimide moieties, were first successfully synthesized and characterized.     

Exploring the photoinduced electron transfer reactivity of aza[60]fullerene iminium cation

Photolysis of (C₅₉N)₂ solutions in the presence of neutral π-donors, such as arenes and electron-rich alkenes leads to a series of novel aza[60]fullerene monoadducts. The key step of the reaction involves a photoinduced electron transfer from the donor molecule to the iminium cation of aza[60]fullerene, followed by radical coupling of the resulting aza[60]fullerenyl radical with an intermediate stabilized radical derived from the substrate. This type of reactivity has been proven efficient with arenes having oxidation potential higher than about 1.5 V. Simple olefins, such as tri- and tetra-methylethylene, as well as cyclohexene, can also participate in this kind of photoinduced electron transfer-initiated reaction with C₅₉N⁺, affording the corresponding aza[60]fullerene derivatives. In the case of 2-methoxyprop-1-ene, 2,4-hexadiene, and β,β-dimethylstyrene, [2+2] cycloaddition reactions with the aza[60]fullerene carbon shell dominate, leading to a mixture of unidentified multiadducts.     

Photoinduced electron transfer processes of fullerene rotaxanes containing various electron-donors

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.     

Synthesis and photoinduced electron-transfer properties of phthalocyanine-[60]fullerene conjugates

A series of three novel ZnPc-C60 conjugates (Pc=phthalocyanine) 1 a-c bearing different spacers (single, double, and triple bond) between the two electroactive moieties was synthesized and compared to that of ZnPc-C60 conjugate 2, in which the two electroactive moieties are linked directly. The synthetic strategy- towards the preparation of 1 a-c- involved palladium-catalyzed cross-coupling reactions over a monoiodophthalocyanine precursor 4 to introduce the corresponding spacer, and subsequent dipolar cycloaddition reaction to C60. Detailed photophysical investigations of 1 a-c and 2 prompted an intramolecular electron transfer that evolves from the photoexcited ZnPc to the electron-accepting C60. In particular, with the help of femtosecond laser photolysis charge separation was indeed confirmed as the major deactivation channel. Complementary time-dependent density functional calculations supported the spectral assignment, namely, the spectral identity of the ZnPc(*+) radical cation and the C60 (*-) radical anion as seen in the differential absorption spectra. The lifetimes of the correspondingly formed radical ion-pair states depend markedly on the solvent polarity: they increase as polarity decreases. Similarly, although to a lesser extent, the nature of the linker impacts the lifetime of the radical ion-pair states. In general, the lifetimes of these states tend to be shortest in the system that lacks any spacer at all (2), whereas the longest lifetimes were found in the system that carries the triple-bond spacer (1 a).     

Study of photoinduced electron transfer between [60]fullerene and proton-sponge by laser flash photolysis: addition effects of organic acid

Photoinduced electron-transfer processes between fullerene (C60) and 1,8-bis(dimethylamino)naphthalene, which is called a proton-sponge (PS), have been investigated by means of laser flash photolysis in the presence and absence of CF3CO2H. For a mixture of C60 and PS, the transient absorption spectra showed the rise of the C60 radical anion with concomitant decay of the C60 triplet (3C60), suggesting that photoinduced intermolecular electron transfer occurs via 3C60 in high efficiency in polar solvent. For a covalently bonded C60-PS dyad, photoinduced intramolecular charge-separation process takes place via the excited singlet state of the C60 moiety, although charge recombination occurs within 10 ns. For both systems, electron-transfer rates were largely decelerated by addition of a small amount of CF3CO2H, leaving the long-lived 3C60. These observations indicate that the energy levels for charge-separated states of the protonated PS and C60 become higher than the energy level of the 3C60 moiety, showing low donor ability of the protonated PS. Thus, intermolecular electron-transfer process via 3C60 for C60-PS mixture and intramolecular charge-separation process via 1C60-PS for C60-PS dyad were successfully controlled by the combination of the light irradiation with a small amount of acid.     

Synthesis of [60]fullerene-functionalized rotaxanes

Synthesis of [60]fullerene (C₆₀)-functionalized rotaxanes via Diels-Alder reactions with C₆₀ is described. Diels-Alder reaction of C₆₀ and sulfolene moiety as masked diene attached on the wheels of rotaxanes results in high yields of C₆₀ incorporation. Rotaxanes are prepared by tin-catalyzed urethane-forming end-capping reaction with isocyanate of pseudorotaxane having the wheel carrying C₆₀ functionality as introduced by the Diels-Alder reaction. The Diels-Alder reaction was accomplished as end-capping reaction between C₆₀ and pseudorotaxane bearing sultine moiety as masked diene on the axle terminal. A variety of C₆₀-containing [2]rotaxanes was prepared in moderate to good yields by these Diels-Alder protocols.     

Convergent synthesis and photophysics of [60]fullerene/porphyrin-based rotaxanes

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.     

Energy and electron transfer in beta-alkynyl-linked porphyrin-[60]fullerene dyads

Three porphyrin-fullerene dyads, in which a diyne bridge links C(60) with a beta-position on a tetraarylporphyrin, have been synthesized. The free-base dyad was prepared, as well as the corresponding Zn(II) and Ni(II) materials. These represent the first examples of a new class of conjugatively linked electron donor-acceptor systems in which pi-conjugation extends from the porphyrin ring system directly to the fullerene surface. The processes that occur following photoexcitation of these dyads were examined using fluorescence and transient absorption techniques on the femtosecond, picosecond, and nanosecond time scales. In sharp contrast to the photodynamics associated with singlet excited-state decay of reference tetraphenylporphyrins (ZnTPP, NiTPP, and H(2)TPP), the diyne-linked dyads undergo ultrafast (<10 ps) singlet excited-state deactivation in toluene, tetrahydrofuran (THF), and benzonitrile (PhCN). Transient absorption techniques with the ZnP-C(60) dyad clearly show that in toluene intramolecular energy transfer (EnT) to ultimately generate C(60) triplet excited states is the dominant singlet decay mechanism, while intramolecular electron transfer (ET) dominates in THF and PhCN to give the ZnP(*+)/C(60)(*-) charge-separated radical ion pair (CSRP). Electrochemical studies indicate that there is no significant charge transfer in the ground states of these systems. The lifetime of ZnP(*+)/C(60)(*-) in PhCN was approximately 40 ps, determined by two different types of transient absorption measurement in two different laboratories. Thus, in this system, the ratio of the rates for charge separation (k(CS)) to rates for charge recombination (k(CR)), k(CS)/k(CR), is quite small, approximately 7. The fact that charge separation (CS) rates increase with increasing solvent polarity is consistent with this process occurring in the normal region of the Marcus curve, while the slower charge recombination (CR) rates in less polar solvents indicate that the CR process occurs in the Marcus inverted region. While photoinduced ET occurs on a similar time scale in a related dyad 15 in which a diethynyl bridge connects C(60) to the para position of a meso phenyl moiety of a tetrarylporphyrin, CR occurs much more slowly; i.e., k(CS)/k(CR) approximately equal to 7400. Thus, the position at which the conjugative linker is attached to the porphyrin moiety has a dramatic influence on k(CR) but not on k(CS). On the basis of electron density calculations, we tentatively conclude that unfavorable orbital symmetries inhibit charge recombination in 15 vis a vis the beta-linked dyads.     

Competitive photoinduced electron transfer by the complex formation of porphyrin with cyclodextrin bearing viologen

Photoinduced electron transfer between a porphyrin and a new guest cyclodextrin bearing viologen occurs by a supramolecular formation with conformational change of a guest molecule.     

Energy and electron transfer in polyacetylene-linked zinc-porphyrin-[60]fullerene molecular wires

The synthesis and electrochemical and photophysical studies of a series of alkyne-linked zinc-porphyrin-[60]fullerene dyads are described. These dyads represent a new class of fully conjugated donor-acceptor systems. An alkynyl-fullerene synthon was synthesized by a nucleophilic addition reaction, and was then oxidatively coupled with a series of alkynyl tetra-aryl zinc-porphyrins with 1-3 alkyne units. Cyclic and differential pulse voltammetry studies confirmed that the porphyrin and fullerene are electronically coupled and that the degree of electronic interaction decreases with increasing length of the alkyne bridge. In toluene, energy transfer from the excited zinc-porphyrin singlet to the fullerene moiety occurs, affording fullerene triplet quantum yields of greater than 90 %. These dyads exhibit very rapid photoinduced electron transfer in tetrahydrofuran (THF) and benzonitrile (PhCN), which is consistent with normal Marcus behavior. Slower rates for charge recombination in THF versus PhCN clearly indicate that charge-recombination events are occurring in the Marcus inverted region. Exceptionally small attenuation factors (beta) of 0.06+/-0.005 A(-1) demonstrate that the triple bond is an effective mediator of electronic interaction in zinc-porphyrin-alkyne-fullerene molecular wires.     

Synthesis of [60]fullerene acetals and ketals: reaction of [60]fullerene with aldehydes/ketones and alkoxides

The reaction of C(60) with propionaldehyde (butyraldehyde or phenylacetaldehyde) and MeONa-MeOH or EtONa-EtOH in anhydrous chlorobenzene in the presence of air at room temperature unexpectedly gave rare fullerene acetals 2aa-cb, while the reaction of C(60) with acetone (acetophenone, cyclohexanone, or cyclopentanone) and MeONa-MeOH or EtONa-EtOH under the same conditions afforded the uncommon fullerene ketals 4aa-db. A possible reaction mechanism for the formation of the fullerene acetals and ketals is proposed based on further experimental results.     

Carbon nanotubes with covalently linked porphyrin antennae: photoinduced electron transfer

Single- and multiwalled carbon nanotubes have been covalently functionalized with free-base porphyrin. The quantity of porphyrin linked to the surface was determined from thermogravimetric and UV-vis analysis. A reversible protonation equilibrium between the attached porphyrin and the residual acid groups of the carbon nanotubes has been identified. Steady-state fluorescence emission spectrum of the solutions of porphyrin-linked carbon nanotubes shows that the porphyrins act as energy absorbing and electron transferring antennae, and the carbon nanotubes act as efficient electron acceptors. The porphyrin-linked carbon nanotubes show 95-100% emission quenching, indicating a fast photoinduced electron transfer.     

Synthesis and photophysical investigation of new porphyrin derivatives with beta-pyrrole ethynyl linkage and corresponding dyad with [60] fullerene

Two new beta-substituted arylethynyl meso-tetraphenylporphyrins, 2-[(4'-formyl)phenyl]ethynyl-5,10,15,20-tetraphenylporphyrin (system A) and 2-[(4'-methyl)phenyl]ethynyl-5,10,15,20-tetraphenylporphyrin (system B) and their zinc derivatives were synthesized by palladium catalysis, using a synthetic approach that affords high yields of the target systems. Comparative ultraviolet-visible (UV-vis), NMR, and cyclic voltammetry studies of such macrocycles reveal the presence of an extensive conjugation between the tetrapyrrolic ring and the linker, through pi-pi orbital interaction. This interaction was observed in the form of a "push-pull" effect that moves the electronic charge between the porphyrin and the aldehyde group of system A. System B, bearing a methyl group instead of the formyl group, was synthesized in order to evaluate the effect of the substitution on the charge delocalization, which is necessary to corroborate the push-pull mechanism hypothesis. The new porphyrin, system A, was also used as a starting material for the synthesis of new porphyrin-fullerene dyads in which the [60]fullerene is directly linked to the tetrapyrrolic rings by ethynylenephenylene subunits. Fluorescence and transient absorption measurements of the new dyads reveal that ultrafast energy and electron transfer occur, respectively, in nonpolar and polar solvents, with high values of the rate constant. The UV-vis, NMR, and cyclic voltammetry results show that it is possible for both energy and electron transfer between porphyrin and fullerene to take place through the pi-bond interaction. Such results evidence that the coupling between the donor and acceptor moieties is strong enough for possible photovoltaic applications.     

Electrostatic complexation and photoinduced electron transfer between Zn-cytochrome c and [olyanionic fullerene dendrimers

Two dendritic fullerene (DF) monoadducts, 2 and 3, which can carry up to 9 and 18 negative charges, respectively, were examined with respect to electrostatic complexation with Cytochrome c (Cytc). To facilitate comprehensive photophysical investigations, the zinc analogue of Cytc (ZnCytc) was prepared according to a novel, modified procedure. The association of ZnCytc and DF, and consequential photoinduced electron transfer within ZnCytc-DF from the photoexcited protein to the fullerene, was proven by fluorescence spectroscopy and transient absorption spectroscopy. These findings were also supported by circular dichroism as well as by extensive molecular dynamics (MD) simulations.     

Taming C60 fullerene: tuning intramolecular photoinduced electron transfer process with subphthalocyanines

Two subphthalocyanine–C₆₀ conjugates have been prepared by means of the 1,3-dipolar cycloaddition reaction of (perfluoro) or hexa(pentylsulfonyl) electron deficient subphthalocyanines to C₆₀. Comprehensive assays regarding the electronic features – in the ground and excited state – of the resulting conjugates revealed energy and electron transfer processes upon photoexcitation. Most important is the unambiguous evidence – in terms of time-resolved spectroscopy – of an ultrafast oxidative electron transfer evolving from C₆₀ to the photoexcited subphthalocyanines. This is, to the best of our knowledge, the first case of an intramolecular oxidation of C₆₀ within electron donor–acceptor conjugates by means of only photoexcitation.     

Synthesis of [60]fullerene adducts bearing carbazole moieties by Bingel reaction and their properties

Carbazole-linked [60]fullerene adducts were successfully prepared by the Bingel reactions using carbazole derivatives bearing one or two ethyl malonate moieties. In the latter cases, specific bisadduct regioisomers were obtained, depending on the spacer unit between two ethyl malonate moieties. [reaction: see text]     

Photoinduced electron transfer reactivity of aza[60]fullerene: three discrete functionalization pathways with a single substrate

Azafullerenium carbocation, C59N+, shows photoinduced electron transfer (PET) reactivity toward benzyltrimethylsilane. The reaction between (C59N)2 and benzyltrimethylsilane gives three different aza[60]fullerene monoadducts depending on the reaction conditions used.     

Photoinduced energy and electron transfer processes in hexapyropheophorbide a- fullerene [C(60)] molecular systems

The photophysical properties of the novel hexapyropheophorbide a (P6), and hexakis (pyropheophorbide a)-C60 (FP6) were studied and compared with those of hexakis (pyropheophorbide a)-fullerene [5:1] hexaadduct (FHP6). It was found that after light absorption the pyropheophorbide a molecules in all three compounds undergo very efficient energy transfer as well as partly excitonic interactions. The last process results in the formation of energy traps, which could be resolved experimentally. For P6, due to shorter distances between neighboring dye molecules, stronger interactions between pyropheophorbide a units than for FHP6 were observed. As a consequence, the excitation energy is delivered rapidly to traps formed by stacked pyropheophorbide a molecules resulting in the reduction of fluorescence, intersystem crossing, and singlet oxygen quantum yields compared to the values of FHP6. For FP6 the reduction of these values is much stronger due to an additional fast and efficient deactivation process, namely photoinduced electron transfer from pyropheophorbide a to the fullerene moiety. Consequently, FP6 can be considered as a combination of a light-harvesting system consisting of several separate pyropheophorbide a molecules and a charge-separating center.