Porphyrin Fullerene Research Articles

Driving Force and Electronic Coupling Effects on Photoinduced Electron Transfer in a Fullerene-based Molecular Triad¶

Tuning thermodynamic driving force and electronic coupling through structural modifications of a carotene (C) porphyrin (P) fullerene (C60) molecular triad has permitted control of five electron and energy transfer rate constants and two excited state lifetimes in order to prepare a high-energy charge-separated state by photoinduced electron transfer with a quantum yield of essentially unity (≥96%). Excitation of the porphyrin moiety of C–P–C60 is followed by a combination of photoinduced electron transfer to give C–P·+–C60·− and singlet–singlet energy transfer to yield C–P–1C60. The fullerene excited state accepts an electron from the porphyrin to also generate C–P·+–C60·−. Overall, this initial state is formed with a quantum yield of 0.97. Charge shift from the carotenoid to yield C·+–P–C60·− is at least 60 times faster than recombination of C–P·+–C60·−, leading to the overall quantum yield near unity for the final state. Formation of a similar charge-separated species from the zinc analog of the triad with a yield of 40% is also observed. Charge recombination of C·+–P–C60·− in 2-methyltetrahydrofuran yields the carotenoid triplet state, rather than the ground state. Comparison of the results for this triad with those for related triads with different structural features provides information concerning the effects of driving force and electronic coupling on each of the electron transfer steps.

Supramolecular Nanostructuring of Silver Surfaces via Self‐Assembly of [60]Fullerene and Porphyrin Modules

AbstractRecent achievements in our laboratory toward the “bottom‐up” fabrication of addressable multicomponent molecular entities obtained by self‐assembly of C60 and porphyrins on Ag(100) and Ag(111) surfaces are described. Scanning tunneling microscopy (STM) studies on ad‐layers constituting monomeric and triply linked porphyrin modules showed that the molecules self‐organize into ordered supramolecular assemblies, the ordering of which is controlled by the porphyrin chemical structure, the metal substrate, and the surface coverage. Specifically, the successful preparation of unprecedented two‐dimensional porphyrin‐based assemblies featuring regular pores on Ag(111) surfaces has been achieved. Subsequent co‐deposition of C60 molecules on top of the porphyrin monolayers results in selective self‐organization into ordered molecular hybrid bilayers, the organization of which is driven by both fullerene coverage and porphyrin structure. In all‐ordered fullerene–porphyrin assemblies, the C60 guests organize, unusually, into long chains and/or two‐dimensional arrays. Furthermore, sublimation of C60 on top of the porous porphyrin network reveals the selective long‐range inclusion of the fullerene guests within the hosting cavities. The observed mode of the C60 self‐assembly originates from a delicate equilibrium between substrate–molecule and molecule–molecule interactions involving charge‐transfer processes and conformational reorganizations as a consequence of the structural adaptation of the fullerene–porphyrin bilayer.

Synthesis and characterization of dendritic poly( l-lysine) containing porphyrin–fullerene moieties

Amphiphilic l-lysine dendrons containing porphyrin and fullerene bearing amino acids with the ratio of 2:0, 2:1 and 1:1 were synthesized and characterized using UV–visible absorption spectroscopy, fluorescence emission spectroscopy and transmission electron microscopy. Photoluminescence spectra of these dendrimers showed photoluminescence quenching due to photoinduced electron transfer from porphyrin to fullerene moieties. Transmission electron microscopic observation evidenced the self-association of these dendrimers in water. Controllable and reversible dissociation and re-aggregation of some of the water soluble dendrimers have been investigated using electronic absorption and fluorescence spectra upon the addition of 2-hydroxypropyl-β-cyclodextrin and 1-adamantanecarboxylic acid in their aqueous dispersion. Enhanced ellipticity of dendritic porphyrin–fullerene conjugate compared to that only dendritic porphyrin indicates intramolecular porphyrin–fullerene interactions.

Distributed decay kinetics of charge separated state in solid film

Photoinduced electron transfer in solid films of porphyrin–fullerene dyads was studied using femtosecond pump–probe method. The relaxation of the main photo-product, intramolecular exciplex, was found to be essentially non-exponential. To analyze the decays a model accounting for a distribution of the free or reorganization energies is proposed. The model allows to fit the experimental data using three parameters: non-disturbed time constant, τ 0, width of the energy distribution, Δ E, and sensitivity parameter, a = −(Δ G 0 + λ 0)/2 λ 0, where Δ G 0 and λ 0 are the average free and reorganization energies, respectively. The parameters were found to be τ 0 = 50 ps, Δ E = 0.09 eV and a = 2 for the exciplex relaxation.

Organic solar cells. Supramolecular composites of porphyrins and fullerenes organized by polypeptide structures as light harvesters

We have constructed supramolecular solar cells composed of a series of porphyrin–peptide oligomers [porphyrin functionalized α-polypeptides, P(H2P)n or P(ZnP)n (n = 1, 2, 4, 8, 16)], and fullerenes assembled on a nanostructured SnO2 electrode using an electrophoretic deposition method. Remarkable enhancement in the photoelectrochemical performance as well as the broader photoresponse in the visible and near-infrared regions is seen with increasing the number of porphyrin units in α-polypeptide structures. Formation of supramolecular clusters of porphyrins and fullerenes prepared in acetonitrile–toluene = 3 : 1 has been confirmed by transmission electron micrographs (TEM) and the absorption spectra. The highly colored composite clusters of porphyrin–peptide oligomers and fullerenes have been assembled as three-dimensional arrays onto nanostructured SnO2 films using an electrophoretic deposition method. A high power conversion efficiency (η) of ∼1.6% and the maximum incident photon-to-photocurrent efficiency (IPCE = 56%) were attained using composite clusters of free base and zinc porphyrin–peptide hexadecamers [P(H2P)16 and P(ZnP)16] with fullerenes, respectively. Femtosecond transient absorption and fluorescence measurements of porphyrin–fullerene composite films confirm improved electron-transfer properties with increasing number of porphyrins in a polypeptide unit. The formation of molecular assemblies between porphyrins and fullerenes with a polypeptide structure controls the electron-transfer efficiency in the supramolecular complexes, meeting the criteria required for efficient light energy conversion.

Metal ion dependent fluorescence quenching in a crown ether bridged porphyrin—fullerene dyad

The fluorescence decay kinetics from a benzonitrile solution of a dibenzo-18-crown-6 ether bridged porphyrin-fullerene dyad has been studied in the presence of a range of metal ions. Dual-exponential fluorescence decay behaviour has been attributed to conformational flexibility of the molecule influencing quenching interactions between the photo-excited porphyrin and fullerene. Additions of sodium, potassium and lithium ions significantly modulate the observed fluorescence decay processes while the larger tetrabutylammonium ion has only a minor affect. The results are discussed in terms of ion inclusion within the crown ether affecting both the bridge conformational properties and donor-acceptor electronic interactions.

The Remarkable Ability of B3LYP/3‐21G(*) Calculations to Describe Geometry, Spectral and Electrochemical Properties of Molecular and Supramolecular Porphyrin—Fullerene Conjugates

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

IR reflection–absorption spectroscopic study of Langmuir–Blodgett films of selected porphyrins and their dyads to fullerene on gold substrates

The paper deals with IR absorption and reflection–absorption examinations of Langmuir–Blodgett layers of zinc porphyrins covalently linked to fullerene C 60. The spectra were recorded with unpolarized and polarized light (p-polarization and s-polarization) at various incident angles to receive data on interactions of the systems with the solid substrates and on orientation of the systems with respect to the solid substrate. A comparison of the calculated and experimental IR absorption data were discussed in terms of the normal modes assigned for the porphyrins. All alternations in the shape or localization of the absorption bands represented normal modes after porphyrin oligomer or porphyrin–fullerene dyad formation reflect the changes of the charge density distribution and configuration of the bonded molecular moieties. We have shown that the polarized reflection–absorption spectra of thin films are strongly dependent on the light polarization. However, the reflection–absorption spectra of LB films of the porphyrin, porphyrin oligomer and their dyads with fullerene recorded with unpolarized light approximate the absorption spectra of adequate compounds dispersed in KBr matrix. A similarity between the p-polarized and unpolarized spectra suggests that both porphyrins and dyads are nearly perpendicularly oriented to the surface of the gold substrate.

Synthesis and Spectroscopic Properties of a Covalently Linked Porphyrin—Fullerene C60 Dyad.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Photoinduced Electron Transfer of Dialkynyldisilane-Linked Zinc Porphyrin–[60]Fullerene Dyad

Abstract A zinc porphyrin–fullerene dyad with a disilane as a σ-conjugated linker has been newly synthesized to evaluate the electron transfer ability of the oligosilane chain. Its photoinduced processes have been studied using the time-resolved fluorescence and absorption measurements. Photoexcitation of the dyad causes the energy and/or electron transfer from the excited singlet state of the ZnP to C60 moiety in polar solvents. The charge separation takes place as a final step in the excited-state process to yield the radical-ion pair with a radical cation on the zinc porphyrin and a radical anion on the fullerene, similar to other porphyrin–fullerene dyads. Its lifetime has been estimated to be 0.43–0.52 μs on the basis of the decay rate of the fullerene radical anion in polar solvents.

Photoinduced electron transfer of double-bridged phthalocyanine–fullerene dyads

Three new double-bridged phthalocyanine–fullerene dyads were studied with spectroscopic and electrochemical methods. The two-linker strategy was applied for the first time to phthalocyanine–fullerene dyads. Photoinduced electron transfer (ET) of the dyads was found to occur from the excited state of phthalocyanine both in polar and non-polar environment. Examination of ET energetics revealed that the phthalocyanine–fullerene dyads exist mainly in an extended conformation as opposed to a face-to-face orientation common for the similar double-bridged porphyrin–fullerene dyads.

Synthesis and Spectroscopic Properties of a Covalently Linked Porphyrin–Fullerene C60 Dyad

A covalently linked porphyrin–fullerene C60 dyad 6 was conveniently synthesized by 1,3‐dipolar cycloaddition using 5‐(4‐carbonylphenyl)‐10,15,20‐tris(4‐methoxylphenyl)porphyrin 5, N‐methylglycine and fullerene C60. Spectroscopic studies show that dyad 6 is a promising architecture with potential application as photoactive organic material.

Computational analysis of the conformations of a doubly linked porphyrin–fullerene dyad

A doubly linked porphyrin–fullerene dyad is studied by molecular dynamics (MD) simulations in polar and non-polar solvent, as well as in vacuum. Two conformations representing the most probable vacuum structures are optimised with semi-empirical and density functional methods (DFT) and the structural differences are assessed. The distances between the centres of the porphyrin and fullerene appear very similar in MD simulations regardless of the environment. Optimization with DFT calculations produces centre-to-centre distances that are within 0.07–5% of the corresponding MD values of realistic environments. The results validate the electronic structure calculations relevant to describing photoinduced electron-transfer and related properties in the dyad.

Synthesis and Photophysics of Porphyrin—Fullerene Donor—Acceptor Dyads with Conformationally Flexible Linkers.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Facile Fabrication and Photolectrochemical Properties of Porphyrin–Fullerene Assemblies by Self-Assembly and Surface Sol–Gel Processes

Ultrathin photoelectric conversion films consisting of a porphyrin–fullerene photoredox pair were fabricated by the combined use of room-temperature covalent-bonding and surface sol–gel processes. First, cysteamine was self-assembled on an indium–tin-oxide (ITO) electrode. The cysteamine-modified electrode was then immersed in C60 solution, giving immobilization of C60 via bond formation between the amino group of cysteamine and C60. Next, the C60-modified electrode was dipped in 2-ethanolamine solution to implant the hydroxy group to the immobilized C60 via the bond formation between C60 and the amino group; thus, the hydroxy group was exposed as the outermost layer. Then, Ti(OBu)4 and tetracarboxyporphyrin (TCPP) were alternately assembled on the C60 layer by the surface sol–gel process, to give an assembly of TCPP, titanium oxide species [Ti(O)], and C60 on the ITO electrode. The double layering of TCPP–Ti(O) was possible. The spectral characterization of the films was carried out. In the presence of sacrificial reagents, anodic photocurrents were generated from these modified electrodes. The incorporation of the C60 layer resulted in the substantial enhancement of the photocurrents as compared with that of the TCPP layer alone, suggesting effective electron-transfer reactions between TCPP and C60 that contribute to the photocurrent increase. The photocurrents increased by the double layering of the TCPP and Ti(O) layers.

Modification of electronic structure in supramolecular fullerene–porphyrin systems studied by fluorescence, photoacoustic and photothermal spectroscopy

Three fullerene–porphyrin hybrids (fullerene–porphyrin, fullerene–porphyrin dyad, two fullerenes–porphyrin dyad) in dimethylsulfoxide and chloroform were investigated. Strong interaction in the porphyrins covalently linked to the fullerene C 60 was studied with the electronic absorption and fluorescence examinations as well as with the steady-state photoacoustic and time-resolved optoacoustic spectroscopy (LIOAS). Analysis of LIOAS waveforms was done with Marti et al . approach and Rudzki-Small et al . deconvolution procedure. The basic spectral parameters of the fullerene–porphyrin systems were evaluated: locations of absorption/fluorescence bands, fluorescence quantum yields, thermal deactivation parameters, yields of triplet state population, thermal relaxation of triplet states of porphyrin moieties and their alteration upon the presence of covalent linkage to fullerene. Formation of an excimer with participation of the porphyrin second excited singlet state was shown. The changes in triplet state behaviour were discussed in terms of changes in fullerene–porphyrin hybrids electronic structure due to the strong interaction of porphyrin and fullerene subunits. It was also shown that LIOAS experiment can be used not only in establishing the basis spectral features of molecular triplet states, but it is an excellent tool for investigation of the changes in the electronic molecular structure in porphyrin species in the presence of fullerene covalent linkage.

Synthesis and characterization of porphyrin–ferrocene–fullerene triads

Novel porphyrin–fullerene systems linked by ferrocene and related model compounds were successfully synthesized and characterized. Conformationally flexible 1,1′-disubstituted ferrocene functioned as effective modulator of the conformation between porphyrin and fullerene, as 1H NMR spectra indicated, the porphyrin and C 60 moieties in the triads showed gauche type conformation. The electrochemical and photophysical studies showed that there were considerable interactions between porphyrin and fullerene in the ground state due to intramolecular π-stacking of the these two chromophores, assisted by the ferrocence linker. Fluorescence lifetime measurements indicated there might be two different quenching processes occurring simultaneously (intersystem crossing and electron transfer).

Redox Active Two-Component Films of Palladium and Covalently Linked Zinc Porphyrin–Fullerene Dyad

Redox active films have been generated electrochemically by the reduction of dyads consisting of fullerene C60 covalently linked to zinc meso-tetraphenyloporphyrin, ZnPC60, and palladium acetate. The films are believed to consist of a polymeric network formed via covalent bonds between the palladium atoms and the fullerene moieties. In these films, the zinc porphyrin moiety is covalently linked to the polymeric chains through the pyrrolidine ring of the fullerene. The ZnPC60/Pt films are electrochemically active in both positive and negative potential excursions. At positive potentials, two oxidation steps for the zinc porphyrin are observed. In the negative potential range, electron transfer processes involving the zinc porphyrin and the fullerene entities are observed. Film formation is also accompanied by palladium deposition on the electrode surface. The presence of a metallic phase in the film influences its morphology, structure and electrochemical properties.

The remarkable ability of B3LYP/3-21G(*) calculations to describe geometry, spectral and electrochemical properties of molecular and supramolecular porphyrin–fullerene conjugates

Abstract In recent years, application of calculated density functional theory Kohn–Sham (DFT-KS) orbitals and eigenvalues have gained increased popularity due to their ability to display and predict physico-chemical properties of large systems. Our interest in this area has been to employ the exchange-correlation hybrid functional B3LYP with a small 3-21G (*) basis to determine the geometry of large donor–acceptor assemblies followed by comparison of the computed KS orbital energies with measured electrochemical and spectral properties. The surprisingly localized orbitals allow prediction of the site of electron transfer during electrochemical oxidation and reduction of covalently bonded or self assembled porphyrin–fullerene donor–acceptor systems. The highest occupied orbitals (HOMOs) track oxidation potentials while the lowest unoccupied orbitals (LUMOs) track the reduction potentials of these compounds. Such studies are important to determine the position of energy levels of these donor–acceptor systems for understanding the pathways of photo initiated electron or energy transfer processes. The geometry and association energy of supramolecular assemblies (including loosely bound complexes self-assembled by H-bonding, electrostatics, or by π–π porphyrin–fullerene interactions) are also well represented, but the association energy must be scaled to agree with experimental values. The unusual success of the B3LYP/3-21G (*) method may be attributable, in part, to fortuitous shadowing of the interaction energy by significant basis set superposition effects. Results of several molecular and supramolecular systems at this computational level, comprised of porphyrin–fullerene donor–acceptor entities, developed in our laboratory, are reviewed. To cite this article: M.E. Zandler, F. D'Souza, C. R. Chimie 9 (2006) .

Porphyrin–fullerene photosynthetic model systems with rotaxane and catenane architectures

Abstract 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) .