Porphyrin-fullerene Dyad Research Articles

Photoinduced vectorial electron transfer in multilayered Langmuir-Blodgett films of porphyrin and phtalocyanine derivatives

A series of new phtalocyanines and phtalocyanine-fullerene dyads was synthesized. The dyads were transferred to solid substrates in the form of the Langmuir-Blodgett films. Poly(hexylthiophene) (PHT) and/or phtalocyanine (B6PH) were used as secondary electron donors in the multilayered structures, together with molecules of phtalocyanine-fullerene (B6PF) or porphyrin-fullerene (DHD6ee) dyads in a matrix of octadecylamine (ODA) molecules in a ratio of 1: 9. Directed photoinduced electron transfer in films was studied by means of the time-resolved Maxwell displacement charge method. It was established that the addition of a monolayer of molecules of the secondary B6PH donor to a monolayer of molecules of the DHD6ee/ODA (1: 9) dyad resulted in a thirty-fold increase in the sample’s photovoltaic response; it did not, however, change the recombination rate of the charges as compared to a single monolayer of the dyad molecules. In the case of bilayer samples consisting of B6PF/ODA (1: 9) and PHT/ODA (3: 2) monolayers, both quantities increased. The absorption of visible light over a wide spectral range was achieved via the use of three-component (PHT, B6PH and DHD6ee) multilayered structures. The relative sensitivities of the samples to excitation radiation were assessed and the efficiencies of their transformation of light energy to electric potential were compared. The largest sensitivity values were obtained for three-component samples prepared from PHT, B6PH, and DHD6ee monolayers in which the sensitivity value was 500 times larger than that for a separate monolayer of DHD6ee dyad molecules.

Photoinduced electron transfer in thin films of porphyrin–fullerene dyad and perylenetetracarboxidiimide

Photoinduced intra- and intermolecular electron transfer (ET) in thin films of porphyrin-fullerene dyad (P-F) and perylenetetracarboxidiimide (PTCDI) was studied by means of photoelectrical and spectroscopic methods. Films consisting of smooth 100 mol% layers of P-F and PTCDI were prepared by the Langmuir-Schäfer (LS) technique and thermal evaporation, respectively. The time-resolved Maxwell displacement charge (TRMDC) and laser flash-photolysis methods were utilized to demonstrate photoinduced ET from P-F to PTCDI regardless of which chromophore is photoexcited. Finally, the information about the electron movement in the respective thin films was used to build a layered organic solar cell, whose internal quantum yield (Φ(I)) of collected charges was 13%.

Transient states in photoinduced electron transfer reactions of porphyrin- and phthalocyanine-fullerene dyads

This paper combines the most important results on studies performed by the authors during the last decade on photoinduced electron transfer reactions of pheophytin-, phthalocyanine-, and porphyrin-fullerene dyads, in which donor and acceptor moieties are covalently linked to each other. Practically all studied molecules form an intramolecular exciplex as a transient state before the formation of the charge separation state or tight ion pair. When the center-to-center distance of the donor and acceptor pair is short (7–10 Å) both the exciplex formation and primary electron transfer are extremely fast with rate constants of 7–23 × 1012 s -1 and 40–1400 × 109 s -1, respectively. Rates become slower when the distance and orientational fluctuation increases. No systematic correlation between free energies and the rates of the formation and recombination of the exciplex and the charge separation state, respectively, were observed. The mechanism is discussed in frames of the Marcus electron transfer and the radiationless quantum transition theories.

Synthesis, Conformational Interconversion, and Photophysics of Tethered Porphyrin–Fullerene Dyads with Parachute Topology

The synthesis of a porphyrin-fullerene dyad with "parachute" topology is reported. To determine whether the dyad is "flexing" at room temperature, low-temperature NMR experiments were used. Computational modeling has shown the low-energy conformation of the dyad to be nonsymmetric. Although, (1)H NMR spectroscopy at room temperature is consistent with a molecule with C(2v) symmetry, the spectrum changes on lowering the temperature consistent with "windshield wiper"-like motion, in which the porphyrin moiety rotates from one side of the C(60) sphere to the other. Nanosecond and picosecond fluorescence lifetime experiments show two components contribute to the fluorescence decay, also consistent with the presence of more than one conformer.

Temperature Independent Ultrafast Photoinduced Charge Transfer in Donor−Acceptor Pairs Forming Exciplexes

The temperature dependence of photoinduced electron transfer reactions of phthalalocyanine- and porphyrin-fullerene dyads, in which donor and acceptor moieties are covalently linked to each other, was studied. The dyads form an intramolecular exciplex as a transient state before the formation of the charge separation state. It was observed that the formation rates of the exciplex and the charge separated states, as well as the charge recombination rate, were all independent of temperature and thus activationless processes. It seems to be a general rule that the aromatic π−π interaction between the donor and acceptor moieties is an important factor for exciplex formation as transient states in photoinduced electron transfer reactions and this causes barrierless charge separation and recombination reactions.

Effect of gold nanoparticles on intramolecular exciplex emission in organized porphyrin–fullerene dyad films

Interlayer processes of porphyrin–fullerene dyad (PF) and octanethiol-protected 3 nm-gold nanoparticles (AuNP), assembled as films by Langmuir–Schäfer method, were studied by fluorescence and photoelectrical measurements. The PF and AuNP films appear homogeneous on μm-scale and have full coverage on each other. Both the exciplex emission and the photoelectrical response of the PF dyad are altered by the AuNP film. The results show that the AuNP film has an interaction with the intramolecular exciplex of the PF dyad.

Time-Resolved EPR Characterization of a Folded Conformation of Photoinduced Charge-Separated State in Porphyrin−Fullerene Dyad Bridged by Diphenyldisilane

For development of the molecular solar-energy conversion systems, it is crucial to investigate how both the molecular geometry and electronic structure of electron donor-bridge-acceptor (D-B-A) molecules contribute to the electronic coupling for the charge-separation (CS) and charge-recombination (CR) processes. In a D-B-A system of a porphyrin-fullerene dyad (ZnP-C(60)) bridged by a diphenyldisilane spacer, we have characterized one specific folded molecular conformation in the CS state among several existing conformations using the time-resolved electron paramagnetic resonance (TREPR) method at low temperature. To determine the molecular conformation and spin-spin exchange coupling of the CS state, we have considered (1) the electron spin polarization transfer from the excited triplet state of the C(60) moiety to the CS state and (2) the sublevel-selective spin relaxations and CR in the CS state. In the CS state of this conformation, although the ZnP cation and C(60) anion radicals are in close proximity, direct overlap between their singly occupied molecular orbitals is small, resulting in detection of the long-lived CS state which has a totally different conformation from the optically detected, charge-transfer (CT) complex. It has been demonstrated that, among several folded and extended molecular conformations created by the flexibility of the -Si-Si- bridge, the EPR conformation can play a role on the prevention of the energy-wasting CR.

A glycyl-substituted porphyrin as a starting compound for the synthesis of a π–π-stacked porphyrin–fullerene dyad with a frozen geometry

A synthetic route for obtaining a porphyrin-fullerene dyad with a constrained geometry forcing the [60]fullerene sphere to lie on the porphyrin plane is reported, revealing a strong interaction in the ground state between the two chromophores.

Ab initio description of photoabsorption and electron transfer in a doubly‐linked porphyrin‐fullerene dyad

Structure, photoabsorption and excited states of two representative conformations obtained from molecular dynamics (MD) simulations of a doubly-linked porphyrin-fullerene dyad DHD6ee are studied by using both DFT and wavefunction based methods. Charge transfer from the donor (porphyrin) to the acceptor (fullerene) and the relaxation of the excited state are of special interest. The results obtained with LDA, GGA, and hybrid functionals (SVWN, PBE, and B3LYP, respectively) are analyzed with emphasis on the performance of used functionals as well as from the point of view of their comparison with wavefunction based methods (CCS, CIS(D), and CC2). Characteristics of the MD structures are retained in DFT optimization. The relative orientation of porphyrin and fullerene is significantly influencing the MO energies, the charge transfer (CT) in the ground state of the dyad and the excitation of ground state CT complex (g-CTC). At the same time, the excitation to the locally excited state of porphyrin is only little influenced by the orientation or cc distance. TD-DFT underestimates the excitation energy of the CT state, however for some cases (with relatively short donor-acceptor separations), the use of a hybrid functional like B3LYP alleviates the problem. Wavefunction based methods and CC2 in particular appear to overestimate the CT excitation energies but the inclusion of proper solvation models can significantly improve the results.

First Triazole-Linked Porphyrin−Fullerene Dyads

A general procedure for the synthesis of 1,2,3-triazole-linked porphyrin-fullerene dyads is described. Four of these compounds have been prepared and characterized.

ChemInform Abstract: Synthesis and Characterization of a New Porphyrin—Fullerene Dyad Containing a β‐Pyrrolic Linkage

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 of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Photoinduced Electron Transfer and Photocurrent in Multicomponent Organic Molecular Films Containing Oriented Porphyrin-Fullerene Dyad

Layers of poly(3-hexylthiophene), PHT, phenyl vinyl thiophene, PVT3, poly(p-phenylene-2,3′-bis(3,2′-diphenyl)-quinoxaline-7-7′-diyl), PPQ, and covalently linked porphyrin−fullerene donor−acceptor dyad, P−F, were deposited as various multilayer films, which then were used to study photoinduced electron transfer and photocurrent generation. The aim of the research was to clarify functioning of different energy and electron donating and accepting layers in charge transfer processes, which were initially created in a film consisting of parallel P−F molecules. The reactions were studied by means of time-correlated and steady-state fluorescence, time-resolved photovoltage, and electrochemical photocurrent measurements. The longest-lived charge-separated state and the highest efficiency of photocurrent generation were obtained for the multilayer structure of PHT|PVT3|porphyrin−fullerene. Porphyrin−fullerene dyads deposited parallel as the Langmuir –Blodgett film transfer electrons from porphyrin to fullerene yielding radical cation and anion moieties, respectively. The dyad on a PHT layer induces electron donation from PHT to the porphyrin cation. When PVT3 is deposited between the PHT and the P−F layers, it promotes both energy and electron transfer to the porphyrin moiety of the dyad, retards the recombination of the primary charge-separated state, and thus increases the photocurrent generation. PPQ was used as an electron acceptor from the fullerene radical anion, causing an increased lifetime of the charge separation.

Long-lived charge separated state in molecular films containing porphyrin–fullerene dyad

A photoinduced interlayer charge separation in bilayered molecular film, constructed by the Langmuir–Blodgett technique, was demonstrated. The lifetime of the primary charge separated state in a monolayer of porphyrin–fullerene dyad, P– F, was found to be in a microsecond time domain, and was increased due to the secondary electron transfer from polythiophene, PHT, to the porphyrin cation of the dyad. In the final state positive charges are located at the PHT layer and negative charges in the fullerene network, yielding the long lifetime, about 150 μs, in the final interlayer charge separated state.

Synthesis of Porphyrin–Fullerene Dyads and Their Spectroscopic Properties

AbstractPorphyrin–fullerene dyads with different linkages between the two subunits were synthesized for mimicking photosynthesis. NMR spectra and quantum mechanical calculations show special structural features that should influence light‐induced electron transfer of the photosynthetic process. Starting point of the synthetic routes is a deuterioporphyrin dimethyl ester prepared from the readily accessible blood pigment heme. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Interlayer photoinduced electron transfer in Langmuir-Blodgett films based on porphyrin and phthalocyanine derivatives

Vectorial photoinduced electron transfer was studied in Langmuir-Blodgett films. The monolayers of polythiophene and phthalocyanine were used as secondary electron donors in multilayers together with the monolayer of porphyrin-fullerene donor-acceptor dyad molecules mixed with octadecylamine (1:9). Simultaneous utilization of all the three compounds in the preparation of three-layer samples allowed us to widen the range of the visible light absorption by the structure. The relative sensitivity of the sample to the excitation laser light increased by about 350 times compared with that of a dyad monolayer.

Face‐to‐Face Pacman‐Type Porphyrin–Fullerene Dyads: Design, Synthesis, Charge‐Transfer Interactions, and Photophysical Studies

Pacman-type face-to-face zinc-porphyrin-fullerene dyads have been newly synthesized and studied. Owing to the close proximity of the donor and acceptor entities, strong pi-pi intramolecular interactions between the porphyrin and fullerene entities resulted in modulating the spectral and electrochemical properties of the dyads. New absorption and emission bands that correspond to the charge-transfer interactions were observed in the near-IR region. Time-resolved transient absorption studies revealed efficient photoinduced electron transfer from the singlet excited porphyrin to the fullerene entity. The rate constants for photoinduced electron transfer are analyzed in terms of the Marcus theory of electron transfer, which afforded a large electron coupling matrix element (V=140 cm(-1)) for the face-to-face dyads. As a consequence of the large charge-recombination driving force in the Marcus inverted region, a relatively long lifetime of the charge-separated state has been achieved.

Azobenzene-Linked Porphyrin−Fullerene Dyads

A new group of porphyrin-fullerene dyads with an azobenzene linker was synthesized, and the photochemical and photophysical properties of these materials were investigated using steady-state and time-resolved spectroscopic methods. The electrochemical properties of these compounds were also studied in detail. The synthesis involved oxidative heterocoupling of free base tris-aryl-p-aminophenyl porphyrins with a p-aminophenylacetal, followed by deprotection to give the aldehyde, and finally Prato 1,3-dipolar azomethineylide cycloaddition to C60. The corresponding Zn(II)-porphyrin (ZnP) dyads were made by treating the free base dyads with zinc acetate. The final dyads were characterized by their 1H NMR, mass, and UV-vis spectra. 3He NMR was used to determine if the products are a mixture of cis and trans stereoisomers, or a single isomer. The data are most consistent with the isolation of only a single configurational isomer, assigned to the trans (E) configuration. The ground-state UV-vis spectra are virtually a superimposition of the spectral features of the individual components, indicating there is no interaction of the fullerene (F) and porphyrin (H2P/ZnP) moieties in the ground state. This conclusion is supported by the electrochemical data. The steady-state and time-resolved fluorescence spectra indicate that the porphyrin fluorescence in the dyads is very strongly quenched at room temperature in the three solvents studied: toluene, tetrahydrofuran (THF), and benzonitrile (BzCN). The fluorescence lifetimes of the dyads in all solvents are sharply reduced compared to those of H2P and ZnP standards. In toluene, the lifetimes of the free base dyads are 600-790 ps compared to 10.1 ns for the standard, while in THF and BzCN the dyad lifetimes are less than 100 ps. For the ZnP dyads, the fluorescence lifetimes were 10-170 ps vs 2.1-2.2 ns for the ZnP references. The mechanism of the fluorescence quenching was established using time-resolved transient absorption spectroscopy. In toluene, the quenching process is singlet-singlet energy transfer (k approximately 10(11) s-1) to give C60 singlet excited states which decay with a lifetime of 1.2 ns to give very long-lived C60 triplet states. In THF and BzCN, quenching of porphyrin singlet states occurs at a similar rate, but now by electron transfer, to give charge-separated radical pair (CSRP) states, which show transient absorption spectra very similar to those reported for other H2P-C60 and ZnP-C60 dyad systems. The lifetimes of the CSRP states are in the range 145-435 ns in THF, much shorter than for related systems with amide, alkyne, silyl, and hydrogen-bonded linkers. Thus, both forward and back electron transfer is facilitated by the azobenzene linker. Nonetheless, the charge recombination is 3-4 orders of magnitude slower than charge separation, demonstrating that for these types of donor-acceptor systems back electron transfer is occurring in the Marcus inverted region.

Gold Nanoparticle Enhanced Charge Transfer in Thin Film Assemblies of Porphyrin−Fullerene Dyads

Photoinduced vectorial electron transfer in a molecularly organized porphyrin-fullerene (PF) dyad film is enhanced by the interlayer charge transfer from the porphyrin moiety of the dyad to an octanethiol protected (dcore approximately 2 nm) gold nanoparticle (AuNP) film. By using the time-resolved Maxwell displacement charge (TRMDC) method, the charge separation distance was found to increase by 5 times in a multilayer film structure where the gold nanoparticles face the porphyrin moiety of the dyad, that is, AuNP|PF, compared to the case of the PF layer alone. Films were assembled by the Langmuir-Blodgett (LB) method using octadecylamine (ODA) as the matrix compound. Atomic force microscopy (AFM) images of the monolayers revealed that AuNPs are arranged into continuous, islandlike structures and PF dyads form clusters. The porphyrin reference layer was assembled with the AuNP layer to gain insight on the interaction mechanism between porphyrin and gold nanoparticles. Interlayer electron transfer was also observed between the AuNPs and porphyrin reference, but the efficiency is lower than that in the AuNP|PF film. Fluorescence emission of the reference porphyrin is slightly quenched, and fluorescence decay becomes faster in the presence of AuNPs. The proposed mechanism for the electron transfer in the AuNP|PF film is thus the primary electron transfer from the porphyrin to the fullerene followed by a secondary hole transfer from the porphyrin to the AuNPs, resulting in an increased charge separation distance and enhanced photovoltage.

Charge separation in a conformationally-flexible porphyrin-fullerene dyad synthesised using cross-metathesis

The synthesis of a Zn (II) porphyrin-fullerene dyad in which the two chromophore units are tethered by a conformationally-flexible linker, is described. The synthesis is highlighted by the use of a cross metathesis strategy to prepare the linker between the chromophores. Photoexcitation of the Zn (II) porphyrin unit of the dyad in tetrahydrofuran leads to substantial (77%) quenching of porphyrin fluorescence. The multiple exponentials fluorescence decay kinetics observed are attributed to different rates of electron transfer from photoexcited porphyrin to fullerene in the various conformers present. A charge-separated state with a 330 ns lifetime is observed by transient spectroscopy.

Intramolecular photoinduced electron-transfer processes in buta-1,3-diynyl-benzene-linked porphyrin-fullerene dyad

Intramolecular electron-transfer process of porphyrin-fullerene dyad linked by phenyl buta-1,3-diynyl-phenyl unit ( ZnP - sp - C 60) was studied by laser flash photolysis. Picosecond fluorescence lifetime and transient absorption measurements revealed that photoinduced charge-separation takes place via the excited singlet state (1 ZnP *) with the rate constant of (1-2) × 1010 s −1. For the charge recombination, about a half of the radical-ion pair decayed quickly with 2.9 × 109 s −1 as evaluated from picosecond transient absorption measurements, whereas the remaining half was long-lived with slow decay (1.6 × 106 s −1) as estimated from nanosecond transient absorption measurements. The lifetime of the radical-ion pair of ZnP - sp - C 60 was longer than those of directly connected dyads with a buta-1,3-diynyl bridge and buta-1,3-diynyl-phenyl bridge by the insertion of an extra phenyl group in addition to the pyrrodino ring.