Dendrimer-functionalized single-wall carbon nanotubes: synthesis, characterization, and photoinduced electron transfer

We describe the synthesis and characterization of a series of single-wall carbon nanotubes (SWNTs) functionalized with polyamidoamine dendrimers. Importantly, the dendrimers are linked directly to the SWNT surface using a divergent methodology. This approach allows the number of functional groups on the nanotubes to be increased without provoking significant damage to the conjugated pi-system of the SWNTs. Several tetraphenylporphyrin moieties can be linked to the periphery of the dendrimers, and the photophysical properties of the resulting nanoconjugates have been investigated with a series of steady-state and time-resolved spectroscopy. The fluorescence kinetics provide evidence for two transient decays, one very short-lived (i.e., 0.04 +/- 0.01 ns) and one long-lived (i.e., 8.6 +/- 1.2 ns). A possible explanation is that some porphyrin units do not interact with the nanotubes, thus exhibiting a fluorescence lifetime similar to that of the free porphyrin. Complementary transient absorption measurements not only corroborate the fast decay of the photoexcited tetraphenylporphyrin but also confirm that intraconjugate charge separation evolves from the excited porphyrin to the SWNTs.     

Electron donor-acceptor dyads and triads based on tris(bipyridine)ruthenium(II) and benzoquinone: synthesis,characterization, and photoinduced electron transfer reactions

Two electron donor-acceptor triads based on a benzoquinone acceptor linked to a light absorbing [Ru(bpy)(3)](2+) complex have been synthesized. In triad 6 (denoted Ru(II)-BQ-Co(III)), a [Co(bpy)(3)](3+) complex, a potential secondary acceptor, was linked to the quinone. In the other triad, 8 (denoted PTZ-Ru(II)-BQ), a phenothiazine donor was linked to the ruthenium moiety. The corresponding dyads Ru(II)-BQ (4) and PTZ-Ru(II) (9) were prepared for comparison. Upon light excitation in the visible band of the ruthenium moiety, electron transfer to the quinone occurred with a rate constant k(f) = 5 x 10(9) s(-)(1) (tau(f) = 200 ps) in all the quinone containing complexes. Recombination to the ground state followed, with a rate constant k(b) approximately 4.5 x 10(8) s(-)(1) (tau(b) approximately 2.2 ns), for both Ru(II)-BQ and Ru(II)-BQ-Co(III) with no indication of a charge shift to generate the reduced Co(II) moiety. In the PTZ-Ru(II)-BQ triad, however, the initial charge separation was followed by a rapid (k > 5 x 10(9) s(-)(1)) electron transfer from the phenothiazine moiety to give the fairly long-lived PTZ(*)(+)-Ru(II)-BQ(*)(-) state (tau = 80 ns) in unusually high yield for a [Ru(bpy)(3)](2+)-based triad (> 90%), that lies at DeltaG degrees = 1.32 eV relative to the ground state. Unfortunately, this triad turned out to be rather photolabile. Interestingly, coupling between the oxidized PTZ(*)(+) and the BQ(*)(-) moieties seemed to occur. This discouraged further extension to incorporate more redox active units. Finally, in the dyad PTZ-Ru(II) a reversible, near isoergonic electron transfer was observed on excitation. Thus, a quasiequilibrium was established with an observed time constant of 7 ns, with ca. 82% of the population in the PTZ-Ru(II) state and 18% in the PTZ(*)(+)-Ru(II)(bpy(*)(-)) state. These states decayed in parallel with an observed lifetime of 90 ns. The initial electron transfer to form the PTZ(*)(+)-Ru(II)(bpy(*)(-)) state was thus faster than what would have been inferred from the Ru(II) emission decay (tau = 90 ns). This result suggests that reports for related PTZ-Ru(II) and PTZ-Ru(II)-acceptor complexes in the literature might need to be reconsidered.     

Benzophenone and zinc(II) phthalocyanine dichromophores-labeled poly (aryl ether) dendrimer: synthesis,characterization and photoinduced energy transfer

A series of benzophenone chromospheres and zinc(II) phthalocyanine dichromophores labeled poly (aryl benzyl ether) dendrimer (G_n-DZnPc(BP)_8n, n = 1?2) were synthesized. Their structures were characterized by elemental analysis, ¹H NMR, IR, UV–vis and matrix-assisted laser desorption/ionization time-of-flight spectrometry (MALDI-TOF MS). Their photophysical properties were examined by steady-state and time-resolved fluorescence methods. Both the poly (aryl benzyl ether) dendrimer and BP terminal chromophores had a significant effect on photophysical properties of the zinc(II) phthalocyanine core. Time-resolved spectroscopic measurements indicated that the lifetime of benzophenone (donor) chromophore was longer than that of the zinc(II) phthalocyanine (acceptor). The fluorescence of the peripheral benzophenone chromophores was quenched by the phthalocyanine group attached to the focal point. All of these observations suggest that an intramolecular singlet energy transfer occurs in G_n-DZnPc(BP)_8n molecules. The light-harvesting abilities of these molecules increased with generations due to an increase in the number of benzophenone chromophores. The energy transfer efficiencies were ca. 0.49 and 0.68 for generations 1 and 2, respectively, and the rate constants of the singlet-singlet energy transfer were ca. 10⁸ s^?1. The rate constants changed inconspicuously with increase of dendron generations. The intramolecular singlet-singlet energy transfer is proposed to proceed mainly via a Förster-type interaction mechanism involving the dendrimer backbone as a scaffold to hold the peripheral benzophenone chromophores and the phthalocyanine core together. This dendrimer was an effective new energy transmission complex with high efficiency and could be used as a potential light-harvesting system.     

Synthesis, characterization, and photoinduced energy and electron transfer in a supramolecular tetrakis (ruthenium(II) phthalocyanine) perylenediimide pentad

Metal coordination was probed as a versatile approach for designing a novel electron donor/acceptor hybrid [PDIpy(4){Ru(CO)Pc}(4)] (1), in which four pyridines placed at the bay region of a perylenediimides (PDIpy(4)) coordinate with four ruthenium phthalocyanine units [Ru(CO)Pc]. This structural motif was expected to promote strong electronic coupling between the electron donors and the electron acceptor, a hypothesis that was confirmed in a full-fledged physicochemical investigation focusing on the ground and excited state reactivities. As far as the ground state is concerned, absorption and electrochemical assays indeed reveal a notable redistribution of electron density, that is, from the electron-donating [Ru(CO)Pc] to the electron-accepting PDIpy(4). The most important thing to note in this context is that both the [Ru(CO)Pc] oxidation and the PDIpy(4) reduction are rendered more difficult in 1 than in the individual building blocks. Likewise, in the excited state, strong electronic communication is the inception for a rapid charge-transfer process in photoexcited 1. Regardless of exciting [Ru(CO)Pc] or PDIpy(4), spectral characteristics of the [RuPc] radical cation (broad absorptive features from 425 to 600 nm with a maximum at 575 nm, as well as a band centered at 725 nm) and of the PDI radical anion (780 nm maximum) emerge. The correspondingly formed radical ion pair state lasts for up to several hundred picoseconds in toluene, for example. On the other hand, employing more polar solvents, such as dichloromethane, destabilizes the radical ion pair state.     

A highly selective and sensitive fluorescent PET (photoinduced electron transfer) chemosensor for Zn(II)

The naphthalimide derivative 1 was designed as a fluorescence PET sensor for Zn(II); 1 showed excellent selectivity for Zn(II) at pH 7.4, even in the presence of other competitive cations, the emission, being pH independent above pH 3.5, was switched on upon Zn(II) recognition.     

Visible-light-induced synthesis of phosphorylated N-heterocycles through proton-coupled electron transfer

正Phosphorus-containing organic compounds are important feedstock for the synthesis of value-added bioactive molecules. Therefore, the development of highly efficient synthetic methods for the construction of phosphorus-element bonds has drawn huge attention in the past decades [1].Particularly, the formation of P–C bonds from phosphoruscentered radicals has been demonstrated to be one of the most efficient and convenient strategies, which has been widely applied for the synthesis of organic phosphorus compounds in recent years.     

The possible role of proton-coupled electron transfer (PCET) in water oxidation by photosystem II

All higher life forms use oxygen and respiration as their primary energy source. The oxygen comes from water by solar-energy conversion in photosynthetic membranes. In green plants, light absorption in photosystem II (PSII) drives electron-transfer activation of the oxygen-evolving complex (OEC). The mechanism of water oxidation by the OEC has long been a subject of great interest to biologists and chemists. With the availability of new molecular-level protein structures from X-ray crystallography and EXAFS, as well as the accumulated results from numerous experiments and theoretical studies, it is possible to suggest how water may be oxidized at the OEC. An integrated sequence of light-driven reactions that exploit coupled electron-proton transfer (EPT) could be the key to water oxidation. When these reactions are combined with long-range proton transfer (by sequential local proton transfers), it may be possible to view the OEC as an intricate structure that is "wired for protons".     

A mixed fullerene-ferrocene thermotropic liquid crystal: synthesis, liquid-crystalline properties, supramolecular organization and photoinduced electron transfer

Grafting of a ferrocene-containing liquid-crystalline malonate derivative to C60 led to the mixed fullerene-ferrocene material 1 which gave rise to a smectic A phase. Cholesterol was used as liquid-crystalline promoter. X-ray diffraction experiments and volumetric measurements indicated that 1 is organized in double layered structures. The corresponding supramolecular organization within the mesomorphic lamellar phase is characterized by a microsegregation of the different units (ferrocene, fullerene, and cholesterol) in distinct sublayers. In such a smectic A phase, C60 imposes the arrangement of the other molecular moieties. Photophysical studies revealed that electron transfer occurs from the donor ferrocene to the electron accepting fullerene. The formation of a long-lived radical pair, with lifetimes of the order of several hundred nanoseconds, was confirmed by time-resolved spectrometry, especially in the near infrared region, in which the radical anion of the fullerene moiety displays its characteristic fingerprint absorption.     

Synthesis, characterization, and photoinduced electron transfer in functionalized single wall carbon nanohorns

Single-wall carbon nanohorns (SWNHs) are a new class of material that is closely related to single-wall carbon nanotubes. Here, we describe the synthesis and characterization of a series of SWNHs functionalized with ethylene glycol chains and porphyrins. Functionalization of carbon nanohorns has been achieved using two different synthetic protocols: (1) direct attack of a free amino group on the nanohorn sidewalls (nucleophilic addition) and (2) amidation reaction of the carboxylic functions in oxidized nanohorns. The nanohorn derivatives have been characterized by a combination of several techniques, and the electronic properties of the porphyrin/nanohorn assemblies (SWNH/H2P) have been investigated by electrochemistry, spectroelectrochemistry, and a series of steady-state and time-resolved spectroscopy. The cyclic voltammetry curve of nanohorn/porphyrin conjugate 6 showed a continuum of faradic and pseudocapacitive behavior, which is associated with multiple-electron transfers to and from the SWNHs. Superimposed on such a pseudocapacitive current, the curve also displays three discrete reduction peaks at -2.26, -2.57, and -2.84 V and an oxidation peak at 1.12 V (all attributed to the porphyrin moiety). Steady-state and time-resolved fluorescence demonstrated a quenching of the fluorescence of the porphyrin in SWNH/H2P conjugates 5 and 6 compared to the reference free base porphyrin. Transient absorption spectra permitted the electron-transfer process between the porphyrins and the carbon nanostructures to be highlighted.     

Effect of the addition of a fused donor-acceptor ligand on a Ru(II) complex: synthesis, characterization, and photoinduced electron transfer reactions of [Ru(TTF-dppz)2(Aqphen)]2+

The synthesis and the photophysical properties of the complex [Ru(TTF-dppz)(2)(Aqphen)](2+) (TTF = tetrathiafulvalene, dppz = dipyrido-[3,2-a:2',3'-c]phenazine, Aqphen = anthraquinone fused to phenanthroline via a pyrazine bridge) are described. In this molecular triad excitation into the metal-ligand charge transfer bands results in the creation of a long-lived charge separated state with TTF acting as electron donor and anthraquinone as terminal acceptor. The lifetime of the charge-separated state is 400 ns in dichloromethane at room temperature. A mechanism for the charge separation involving an intermediate charge-separated state is proposed based on transient absorption spectroscopy.     

First fluorescent photoinduced electron transfer (PET) reagent for hydroperoxides

A novel fluorescent reagent for hydroperoxides, 4-(2-diphenylphosphinoethylamino)-7-nitro-2,1,3-benzoxadiazole (1), was developed on the basis of the method for designing photoinduced electron transfer (PET) reagents having a benzofurazan skeleton. Compound 1 was quantitatively reacted with hydroperoxides to give its fluorescent derivative, 2. In acetonitrile, the Phi value (0.44) of 2 was 31 times greater than that of 1. The long excitation (458 nm) and emission (520 nm) wavelengths of 2 are suitable for the determination of hydroperoxides, especially in biosamples. [structure: see text]     

Directional photoinduced electron transfer in paraquat silicate thin films containing entrapped ruthenium(II)-tris(bathophenanthroline-disulfonate)

We have demonstrated directional photoinduced electron transfer in paraquat silicate thin films containing entrapped ruthenium(II)-tris(bathophenanthroline-disulfonate (RuBPS). The films were made by electrochemically-induced hydrolysis of a silane analogue of paraquat with ruthenium(II)-tris(bathophenanthroline-disulfonate as its ion pair.     

Energetics of direct and water-mediated proton-coupled electron transfer

Proton-coupled electron transfer (PCET) is an elementary chemical reaction crucial for biological oxidoreduction. We perform quantum chemical calculations to study the direct and water-mediated PCET between two stacked tyrosines, TyrO(?) + TyrOH → TyrOH + TyrO(?), to mimic a key step in the catalytic reaction of class Ia ribonucleotide reductase (RNR). The energy surfaces of electronic ground and excited states are separated by a large gap of ~20 kcal mol(-1), indicative of an electronically adiabatic transfer mechanism. In response to chemical substitutions of the proton donor, the energy of the transition state for direct PCET shifts by exactly half of the change in energetic driving force, resulting in a linear free energy relation with a Br?nsted slope of ?. In contrast, for water-mediated PCET, we observe integer Br?nsted slopes of 1 and 0 for proton acceptor and donor modifications, respectively. Our calculations suggest that the π-stacking of the tyrosine dimer in RNR results in strong electronic coupling and adiabatic PCET. Water participation in the PCET can be identified perturbatively in a Br?nsted analysis.     

Fluorescent photoinduced electron transfer (PET) sensing molecules with p-phenylenediamine as electron donor

Fluorescent photoinduced electron-transfer sensors were made from p-phenylenediamine-substituted azacrown ethers attached with a dansyl group, in which the p-phenylenediamine moiety serves as electron donor and the dansyl group acts as the acceptor. Chelation-enhanced fluorescence was observed upon addition of metal salts. Selective fluorescence response was observed for Mg(2+) and/or Ca(2+) versus Na(+) and K(+) due to size match and charge density sensitivity of the p-phenylenediamine moiety.     

Efficient complexation of pyrrole-bridged dizinc(II) bisporphyrin with fluorescent probe pyrene: synthesis, structure, and photoinduced singlet-singlet energy transfer

A diethylpyrrole‐bridged dizinc(II) bisporphyrin (Zn₂DEP) is reported that encapsulates fluorescent probe pyrene molecules through strong π–π interactions, which can relay information about the chemical environment in the interior of the host–guest supramolecular assembly. X‐ray structures of both Zn₂DEP and the encapsulated pyrene complex are reported, which provides a rare opportunity to investigate the structural changes upon guest binding. A comparative structural analysis demonstrated the exceptional ability of this bisporphyrin platform to open its binding pocket for pyrene encapsulation by a vertical displacement of more than 2.45 Å, although both Zn₂DEP and the pyrene complex have nearly parallel porphyrin ring orientations. The ¹H NMR spectrum of the encapsulated pyrene complex in solution shows the upfield shifts of the pyrene protons due to a strong ring current effect, which demonstrates the retention of the solid‐state structure in solution. To further assess the extent to which pyrene guests remain encapsulated in solution, a known fluorescence quencher, dimethylaniline, was added to the host–guest assembly, which shows no exciplex formation for days in nonpolar solvents. Thus, the assembly also retained the structural integrity in solution for a long time. The association constant (K_asso) for such a complexation process in solution was observed to be 1.78×10⁵ M ^?2 for 1:2 binding. Steady‐state fluorescence and lifetime studies indicate significant photoinduced singlet–singlet energy transformation from the excited state of pyrene to zinc bisporphyrin.     

Heteroleptic Pd(II) dithiocarbamates: synthesis,characterization, packing and in vitro anticancer activity against HeLa cell line

Two new heteroleptic Pd(II) dithiocarbamates (1 and 2) have been synthesized by reaction of equimolar quantities of palladium(II) chloride, sodium 4-(3-methoxyphenyl)piperazine-1-carbodithioate, and appropriate substituted triphenylphosphines. The synthesized complexes have been characterized by their physical, spectral (IR, ¹H, ¹³C, and ³¹P NMR), and X-ray crystallographic data. Complexes 1 and 2 showed square-planar geometry both in solution and solid states. The crystal packing of both complexes revealed similar 3-D-supramolecular networks comprised of 1-D chains. However, the nature and strength of various non-covalent interactions of these networks were slightly different. The DNA interaction studies of the complexes have been carried out by UV–visible spectroscopy to evaluate their anticancer potential. The study suggested intercalative interaction with 2.402 × 10⁴ and 2.713 × 10³ M^?1 binding constants, respectively. The complexes have also been evaluated for their anticancer activity against HeLa cell line. Both complexes showed higher activity with IC₅₀ values much lower (22.176 and 26.166 μM for 1 and 2, respectively) than the standard cisplatin (78.075 μM). Furthermore, the complexes induced stronger DNA fragmentation as investigated by DNA ladder assay for apoptosis. Our findings suggested that the anticancer action of these compounds stems from their interaction with DNA leading to DNA damage and apoptosis. The excellent activity of 1 and 2 deserves to be a focus for further research and in vivo studies.     

Proton-promoted oxygen atom transfer vs proton-coupled electron transfer of a non-heme iron(IV)-oxo complex

Sulfoxidation of thioanisoles by a non-heme iron(IV)-oxo complex, [(N4Py)Fe(IV)(O)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), was remarkably enhanced by perchloric acid (70% HClO(4)). The observed second-order rate constant (k(obs)) of sulfoxidation of thioaniosoles by [(N4Py)Fe(IV)(O)](2+) increases linearly with increasing concentration of HClO(4) (70%) in acetonitrile (MeCN)at 298 K. In contrast to sulfoxidation of thioanisoles by [(N4Py)Fe(IV)(O)](2+), the observed second-order rate constant (k(et)) of electron transfer from one-electron reductants such as [Fe(II)(Me(2)bpy)(3)](2+) (Me(2)bpy = 4,4-dimehtyl-2,2'-bipyridine) to [(N4Py)Fe(IV)(O)](2+) increases with increasing concentration of HClO(4), exhibiting second-order dependence on HClO(4) concentration. This indicates that the proton-coupled electron transfer (PCET) involves two protons associated with electron transfer from [Fe(II)(Me(2)bpy)(3)](2+) to [(N4Py)Fe(IV)(O)](2+) to yield [Fe(III)(Me(2)bpy)(3)](3+) and [(N4Py)Fe(III)(OH(2))](3+). The one-electron reduction potential (E(red)) of [(N4Py)Fe(IV)(O)](2+) in the presence of 10 mM HClO(4) (70%) in MeCN is determined to be 1.43 V vs SCE. A plot of E(red) vs log[HClO(4)] also indicates involvement of two protons in the PCET reduction of [(N4Py)Fe(IV)(O)](2+). The PCET driving force dependence of log k(et) is fitted in light of the Marcus theory of outer-sphere electron transfer to afford the reorganization of PCET (λ = 2.74 eV). The comparison of the k(obs) values of acid-promoted sulfoxidation of thioanisoles by [(N4Py)Fe(IV)(O)](2+) with the k(et) values of PCET from one-electron reductants to [(N4Py)Fe(IV)(O)](2+) at the same PCET driving force reveals that the acid-promoted sulfoxidation proceeds by one-step oxygen atom transfer from [(N4Py)Fe(IV)(O)](2+) to thioanisoles rather than outer-sphere PCET.