Long-range electron transfer in porphyrin-containing [2]-rotaxanes: tuning the rate by metal cation coordination

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)).     

Long-range electron transfer over 4 nm governed by an inelastic hopping mechanism in self-assembled monolayers of helical peptides

Well-ordered self-assembled monolayers (SAMs) were prepared on gold from helical peptides carrying a ferrocene (Fc) moiety at the N- or C-terminal end, and long-range electron transfer (ET) from Fc to gold was investigated. Electrochemical studies revealed that an inelastic hopping mechanism dominated over the superexchange mechanism in the ET reactions in the present SAMs and the dipole moment of the helix accelerated the ET reactions probably due to the lowering of the barrier height between the gold surface and peptide layer.     

Intramolecular electron transfer initiated cation and radical formation through carbon-carbon bond activation

Radical cations can be formed in a spatially and temporally controlled manner by appending a sacrificial photooxidant to an easily oxidized substrate, leading to intramolecular electron transfer upon irradiation. The anthraquinone carboxyl group is an effective photooxidant that can promote single electron oxidation from an appended arene. The resulting intermediates undergo a cleavage reaction through carbon-carbon bond activation to provide either cations or radicals that react to form a range of products.     

Photoinduced electron transfer in Langmuir-Blodgett monolayers of porphyrin-fullerene dyads

A series of electron donor-acceptor (DA) dyads, composed of a porphyrin donor and a fullerene acceptor covalently linked with two molecular chains, were used to fabricate solid molecular films with the Langmuir-Blodgett (LB) technique. By means of the LB technique, the DA molecules can be oriented perpendicular to the plane of the substrate. In DHD6ee and its zinc derivative hydrophilic groups are attached to the phenyl moieties in the porphyrin end of the molecule; while in the other three dyads, TBD6a, TBD6hp, and TBD4hp, the hydrophilic groups are in the fullerene end of the molecule. This makes it possible to alternate the orientation of the molecules in two opposite directions with respect to the air-water interface and to fabricate molecular assemblies in which the direction of the primary photoinduced vectorial electron transfer can be controlled both by the deposition direction of the LB monolayer and by the selection of the used DA molecule. This was proved by the time-resolved Maxwell displacement charge measurements. The spectroscopic properties of the DA films were studied with the steady-state absorption and fluorescence methods. In addition, the time correlated single photon counting technique was used to determine the fluorescence properties of the dyad films.     

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.     

Naphthalimide appended rhodamine derivative: through bond energy transfer for sensing of Hg2+ ions

A naphthalimide appended rhodamine based fluorescent chemosensor '1' is synthesized which undergoes through bond energy transfer in the presence of Hg(2+) ions in mixed aqueous media.     

Impact of reductive cleavage of an intramolecular disulfide bond containing cationic gemini surfactant in monolayers and bilayers

The properties of a novel disulfide-bond-containing gemini surfactant bis[N,N-dimethyl-N-hexadecyl-N-(2-mercaptoethyl)ammonium bromide] disulfide (DSP) were studied using a Langmuir balance, supported monolayers, differential scanning calorimetry, giant vesicles, and LUVs. In 150 mM NaCl the cmc for DSP was 7.5 microM whereas that of the monomer N,N-dimethyl-N-hexadecyl-N-(2-mercaptoethyl)ammonium bromide (MSP) was 12.1 microM. Both surfactants exhibited single endotherms upon DSC, with peak temperatures Tm at 21.7 and 20.1 degrees C for DSP and MSP, respectively. The endotherm for MSP was significantly broader indicating less cooperative melting. Both in monolayers and in vesicles reductive cleavage of the disulfide bond of DSP could be obtained by glutathione (GSH). For Langmuir films of DSP the addition of GSH into the subphase led to a decrease in surface pressure pi as well as surface dipole potential psi. Although the cleavage by GSH was significantly slower in the presence of a charge saturating concentration of DNA, it did not prevent the reaction. The resulting monomers detached from supported monolayers, leading to loss of affinity of the surface for DNA. Disruption of giant vesicles containing DSP within approximately 30 s following a local injection of GSH was observed, revealing membrane destabilization.     

Crystal-chemical functions of [Hg2]2+ and [Hg3]4+ groups in inorganic structures

About 20 structures of natural and synthetic inorganic compounds containing a [H_g2] (dumbbell) pair (Hg-Hg = 2.47?2.56 å) or a [H_g3] triangle (Hg-Hg_av = 2.7 å) are analyzed. From crystal-chemical viewpoint, it is reasonable to consider these groups as a single cation with coordinates of its geometrical center. In all cases, the coordination polvhedra are convex polxhedra with 8–14 vertices and with distances from the center to the vertex of 3.00?3.91 å, which makes the [H_g2]²⁺ and [Hg3]⁴⁺ cations analogs of Cs. Examples are given to demonstrate the advantages of the suggested analysis of structures with a [Hg_n] group. This approach extends classical crystal chemistry to compounds with nonuniform chemical bonds, in particular, to compounds with pseudocluster and cluster groups inserted in the ionic structure; therefore, it is especially useful for comparison and classification purposes and for analysis of structure-foming factors.     

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.     

Scanning electrochemical microscopy. 59. Effect of defects and structure on electron transfer through self-assembled monolayers

Electron transfer (ET) rate kinetics through n-alkanethiol self-assembled monolayers (SAMs) of alkanethiols of different chain lengths [Me(CH2)nSH; n=8, 10, 11, 15] on Au and Hg surfaces and ferrocene (Fc)-terminated SAMs (poly-norbornylogous and HS(CH2)12CONHCH2Fc) on Au were studied using cyclic voltammetry and scanning electrochemical microscopy (SECM). The SECM results allow determination of the ET kinetics of solution-phase Ru(NH3)63+/2+ through the alkanethiol SAMs on Au and Hg. A model using the potential dependence of the measured rate constants is proposed to compensate for the pinhole contribution. Extrapolated values of koML for Ru(NH3)63+/2+ using the model follow the expected exponential decay (beta is 0.9) for different chain lengths. For a Fc-terminated poly-norbornyl SAM, the standard rate constant of direct tunneling (ko is 189+/-31 s(-1)) is in the same order as the ko value of HS(CH2)12CONHCH2Fc. In blocking and Fc SAMs, the rates of ET are demonstrated to follow Butler-Volmer kinetics with transfer coefficients alpha of 0.5. Lower values of alpha are treated as a result of the pinhole contribution. The normalized rates of ET are 3 orders of magnitude higher for Fc-terminated than for blocking monolayers. Scanning electron microscopy imaging of Pd nanoparticles electrochemically deposited in pinholes of blocking SAMs was used to confirm the presence of pinholes.     

Photoinduced electron transfer in self-assembled monolayers of porphyrin-fullerene dyads on ITO

Two porphyrin-fullerene dyads were synthesized to form self-assembled monolayers (SAMs) on indium-tin oxide (ITO) electrode, with either ITO-porphyrin-fullerene or ITO-fullerene-porphyrin orientations. The dyads contain two linkers for connecting the porphyrin and fullerene moieties and enforcing them essentially to similar geometries of the donor-acceptor pair, and two linkers to ensure the attachment of the dyads to the ITO surface with two desired opposite orientations. The transient photovoltage responses (Maxwell displacement charge) were measured for the dyad films covered by insulating LB films, thus ensuring that the dyads interact only with the ITO electrode. The direction of the electron transfer was from the photoexcited dyad to ITO independent of the dyad orientation. The response amplitude for the ITO-fullerene-porphyrin structure, where the primary intramolecular electron-transfer direction coincides with the direction of the final electron transfer from the dyad to ITO, was 25 times stronger than that for the opposite ITO-porphyrin-fullerene orientation of the dyad. Static photocurrent measurements in a liquid electrochemical cell, however, show only a minor orientation effect, indicating that the photocurrent generation is controlled by the processes at the SAM-liquid interface.     

Photochemical electron transfer through [corrected] the interface of hybrid films of titania nano-sheets and mono-dispersed spherical mesoporous silica particles

Photochemical Electron Transfer (ET) between an organic dye, the porphyrin derivative TMPyP, and an electron acceptor, methyl viologen MV2+, have been investigated at the interface of two different inorganic films, i.e., layered titania nano-sheets (TNS) and a monolayer film of spherical and mono-dispersed mesoporous silica (sMPS) particles (ca. 0.5 microm). TMPyP ions were intercalated within the sMPS nano-cavities to form (TMPyP-sMPS) while MV2+ ions were intercalated into the TNS interlayers to form (MV2+-TNS). The (TMPyP-sMPS) and (MV2+-TNS) films were then stacked on a silica substrate in this order to form a (MV2+-TNS)/(TMPyP-sMPS) film and, upon UV light irradiation, ET could be induced. However, when this film was stacked inversely, i.e., for the (TMPyP-sMPS)/(MV2+-TNS) films on a silica substrate, no photoinduced ET were observed. Interestingly, however, even for this photo-inactive inversely stacked film, ET could be generated by inserting a gold vapor-deposited layer between the (MV2+-TNS) and (TMPyP-sMPS) films. The conjugation conditions at the interface of the inversely stacked (TMPyP-sMPS)/(MV2+-TNS) hybrid film were, thus, confirmed to strongly affect the photoinduced electron transfers and their efficiencies.     

Mechanism of C[bond]H bond activation/C[bond]C bond formation reaction between diazo compound and alkane catalyzed by dirhodium tetracarboxylate

The B3LYP density functional studies on the dirhodium tetracarboxylate-catalyzed C-H bond activation/C-C bond formation reaction of a diazo compound with an alkane revealed the energetics and the geometry of important intermediates and transition states in the catalytic cycle. The reaction is initiated by complexation between the rhodium catalyst and the diazo compound. Driven by the back-donation from the Rh 4d(xz) orbital to the C[bond]N sigma*-orbital, nitrogen extrusion takes place to afford a rhodium[bond]carbene complex. The carbene carbon of the complex is strongly electrophilic because of its vacant 2p orbital. The C[bond]H activation/C[bond]C formation proceeds in a single step through a three-centered hydride transfer-like transition state with a small activation energy. Only one of the two rhodium atoms works as a carbene binding site throughout the reaction, and the other rhodium atom assists the C[bond]H insertion reaction. The second Rh atom acts as a mobile ligand for the first one to enhance the electrophilicity of the carbene moiety and to facilitate the cleavage of the rhodium[bond]carbon bond. The calculations reproduce experimental data including the activation enthalpy of the nitrogen extrusion, the kinetic isotope effect of the C[bond]H insertion, and the reactivity order of the C[bond]H bond.     

Electrochemical electron transfer and its relation to charge transport in single molecule junctions

Ability to control charge transport at nanometer scale lies in the heart of design of fast reliable electronic devices. Molecular electronics thrive to use functional molecules for such transport. If the molecule contains redox center(s), a diode-like or transistor-like behavior can be easily explored by controlling not only the voltage difference between two metallic contacts of the molecular junction but also the potential of one of the contacting electrodes with respect to some reference. Thus, one needs to understand the relationship between electrochemical electron transfer and charge transport in metal–molecule–metal junctions. This review presents latest theoretical approaches toward understanding of such relationship and discusses pivotal experimental works to validate them. Tunneling and hopping pathways may operate in parallel (two-channel model), but experimental conditions dictate the channel preference.     

The role of chalcogen-chalcogen interactions in the intrinsic basicity acidity of beta-chalcogenovinyl(thio)aldehydes HC([double bond]X)[bond]CH[double bond]CH[bond]CYH

The intrinsic acidity and basicity of a series of beta-chalcogenovinyl(thio)aldehydes HC([double bond]X)[bond]CH[double bond]CH[bond]CYH (X=O, S; Y=Se, Te) were investigated by B3LYP/6-311+G(3df,2p) density functional and G2(MP2) calculations on geometries optimized at the B3LYP/6-31G(d) level for neutral molecules and at the B3LYP/6-31+G(d) level for anions. The results showed that selenovinylaldehyde and selenovinylthioaldehyde should behave as Se bases in the gas phase, because the most stable neutral conformer is stabilized by an X[bond]H...Se (X=O, S) intramolecular hydrogen bond (IHB). In contrast the Te-containing analogues behave as oxygen or sulfur bases, because the most stable conformer is stabilized by typical X...Y[bond]H chalcogen-chalcogen interactions. These compounds have a lower basicity than expected because either chalcogen-chalcogen interactions or IHBs become weaker upon protonation. Similarly, they are also weaker acids than expected because deprotonation results in a significantly destabilized anion. Loss of the proton from the X[bond]H or Y[bond]H groups is a much more favorable than from the C[bond]H groups. Therefore, for Se compounds the deprotonation process results in loss of the X[bond]H...Se (X=O, S) IHBs present in the most stable neutral conformer, while for Te-containing compounds the stabilizing X...Y[bond]H chalcogen-chalcogen interaction present in the most stable neutral conformer becomes repulsive in the corresponding anion.     

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.