Artificial Photosynthetic Models Research Articles

Assembling Artificial Photosynthetic Models in Water Using β-Cyclodextrin-Conjugated Phthalocyanines as Building Blocks.

Two water-soluble zinc(II) phthalocyanines substituted with two or four permethylated β-cyclodextrin (β-CD) moieties at the α positions have been utilized as building blocks for the construction of artificial photosynthetic models in water. The hydrophilic and bulky β-CD moieties not only can increase the water solubility of the phthalocyanine core and prevent its stacking in water but can also bind with a tetrasulfonated zinc(II) porphyrin (ZnTPPS) and/or sodium 2-anthraquinonesulfonate (AQ) in water through host-guest interactions. The binding interactions of these species have been studied spectroscopically, while the photoinduced processes of the resulting complexes have been investigated using steady-state and time-resolved spectroscopic methods. In the ternary complexes, the ZnTPPS units serve as light-harvesting antennas to capture the light energy and transfer it to the phthalocyanine core via efficient excitation energy transfer. The excited phthalocyanine is subsequently quenched by the electron-deficient AQ units through electron transfer. Femtosecond transient absorption spectroscopy provides clear evidence for the singlet-singlet energy transfer from the photo-excited ZnTPPS to the phthalocyanine core with a rate constant (kENT ) in the order of 109 s-1 . The population of phthalocyanine radical cations indicates the occurrence of electron transfer from the excited phthalocyanine to the AQ moieties, forming a charge-separated state.

Frontispiece: Bioinspired Artificial Photosynthetic Systems

Inspired by the natural photosynthetic apparatus, research efforts are directed towards the development of artificial photosynthetic model systems. This perspective introduces recent advances in developing photobioelectrochemical cells based on native photosystems, the synthesis of DNA-based artificial photosynthetic modules, and the assembly of dynamic networks mimicking photosynthesis. For more information, see the Review by I. Willner et al. on page ff28.

A Brief History of Photoactive Interlocked Systems Assembled by Transition Metal Template Synthesis

More than three decades of research efforts have yielded powerful methodologies based on transition metal template-directed syntheses for the assembly of a huge number of interlocked systems, molecular knots, machines and synthesizers. Such template techniques have been applied in the preparation of mechanically linked electron donor–acceptor artificial photosynthetic models. Consequently, synthetic challenging photoactive rotaxanes and catenanes have been reported, in which the chromophores are not covalently linked but are still associated with undergoing sequential energy (EnT) and electron transfer (ET) processes upon photoexcitation. Many interlocked photosynthetic models produce highly energetic, but still long-living charge separated states (CSS). The present work describes in a historical perspective some key advances in the field of photoactive interlocked systems assembled by transition metal template techniques, which illustrate the usefulness of rotaxanes and catenanes as molecular scaffolds to organize electron donor–acceptor groups. The effects of molecular dynamics, molecular topology, as well as the role of the transition metal ion used as template species, on the thermodynamic and kinetic parameters of the photoinduced energy and electron transfer processes in the interlocked systems are also discussed.

Gated Dissipative Dynamic Artificial Photosynthetic Model Systems.

Gated dissipative artificial photosynthetic systems modeling dynamically modulated environmental effects on the photosynthetic apparatus are presented. Two photochemical systems composed of a supramolecular duplex scaffold, a photosensitizer-functionalized strand (photosensitizer is Zn(II)protoporphyrin IX, Zn(II)PPIX, or pyrene), an electron acceptor bipyridinium (V2+)-modified strand, and a nicking enzyme (Nt.BbvCI) act as functional assemblies driving transient photosynthetic-like processes. In the presence of a fuel strand, the transient electron transfer quenching of the photosensitizers, in each of the photochemical systems, is activated. In the presence of a sacrificial electron donor (mercaptoethanol) and continuous irradiation, the resulting electron transfer process in the Zn(II)PPIX/V2+ photochemical module leads to the transient accumulation and depletion of the bipyridinium radical-cation (V·+) product, and in the presence of ferredoxin-NADP+ reductase and NADP+, to the kinetically modulated photosynthesis of NADPH. By subjecting the mixture of two photochemical modules to one of two inhibitors, the gated transient photoinduced electron transfer in the two modules is demonstrated. Such gated dissipative process highlights its potential as an important pathway to protect artificial photosynthetic module against overdose of irradiance and to minimize photodamage.

Synergy of Electrostatic and π-π Interactions in the Realization of Nanoscale Artificial Photosynthetic Model Systems.

In the scientific race to build up photoactive electron donor‐acceptor systems with increasing efficiencies, little is known about the interplay of their building blocks when integrated into supramolecular nanoscale arrays, particularly in aqueous environments. Here, we describe an aqueous donor‐acceptor ensemble whose emergence as a nanoscale material renders it remarkably stable and efficient. We have focused on a tetracationic zinc phthalocyanine (ZnPc) featuring pyrenes, which shows an unprecedented mode of aggregation, driven by subtle cooperation between electrostatic and π–π interactions. Our studies demonstrate monocrystalline growth in solution and a symmetry‐breaking intermolecular charge transfer between adjacent ZnPcs upon photoexcitation. Immobilizing a negatively charged fullerene (C60) as electron acceptor onto the monocrystalline ZnPc assemblies was found to enhance the overall stability, and to suppress the energy‐wasting charge recombination found in the absence of C60. Overall, the resulting artificial photosynthetic model system exhibits a high degree of preorganization, which facilitates efficient charge separation and subsequent charge transport.

(Invited) Bodipy-Nanocarbon Hybrids for Mimicking Early Events of Natural Photosynthesis

Mimicking the early events of natural photosynthesis, and utilizing that information to build energy harvesting devices is considered one of the avenues to tap into the vast amount of solar energy. Research performed on this topic by scientists across the globe has been highly promising and has strengthened our fundamental understanding of natural and artificial photosynthesis. In the process of mimicking the early events of photosynthesis, excitation transfer and electron transfer in a sequential fashion, similar to what happens in the antenna and reaction centre of natural photosynthesis, are deemed to be of paramount interest. Accordingly, supramolecular donor-acceptor systems capable of performing these photo processes are considered very important. Although several donor-acceptor systems are known to undergo either energy transfer or electron transfer or simultaneous occurrence of these two photo processes in competition upon photoexcitation, there are not many systems where energy transfer and electron transfer in a sequential route have been reported. Nanocarbons such as fullerenes, single wall carbon nanotubes, graphene oxide, and graphene have been widely used in the construction of such multi-modular donor-acceptor systems. Porphyrins and phthalocyanines are class of widely used photosensitizers. In recent years, application BF2-chelated dipyrromethene (BODIPY) and BF2-chealted azadipyrromethene (azaBODIPYs), in building artificial photosynthetic models have witnessed a rapid growth. This is primarily due to the synthetic versatility of these sensitizers to couple with nanocarbons to modulate their photo- and redox properties to promote efficient photoinduced energy and electron transfer processes. In few instances, accomplishing the combined ‘antenna-reaction center’ functionality of photosynthesis has been possible. The present contribution summarizes recent progress made in our laboratory in this area of research.

Enhancement of photon absorption in the light-harvesting complex of isolated chloroplast in the presence of plasmonic gold nanosol—a nanobionic approach towards photosynthesis and plant primary growth augmentation

A strong enhancement of electromagnetic field generated through resonating surface polaritons on metal films and particles has stimulated considerable interest in diverse research fields such as photobiology as, for example, in photosynthesis. This surface plasmon resonance (SPR) of metal nanoparticles (NPs) can augment photosynthetic chemical energy production in an artificial photosynthetic model system. In this paper, the effect of three water-dispersible gold NPs (GNPs) of nearly similar size but with different surface moieties (aspartate, bovine serum albumin, and citrate capping) is experimentally shown on the photosynthetic efficiency of isolated chloroplasts of Vigna radiata. It was found that GNPs can augment photon absorption in the light-harvesting molecular complexes (LHMCs) and can cause increased distinctive optical and electronic properties. Photosynthetic light reactions like electron transport rate, photophosphorylation, and oxygen evolution rate were increased in the presence of GNPs, indicating higher photosynthetic activity. The rate of increase was dependent on the surface capping of GNPs, and it was maximum in case of citrate-capped GNP. Moreover, it was also found that these GNP-induced enhancements had positive correlation with the primary growth parameters of V. radiata.

An artificial photosynthetic model based on a molecular triad of boron dipyrromethene and phthalocyanine.

A boron dipyrromethene (BDP) unit and its monostyryl derivative (MSBDP) were introduced at the axial positions of a silicon(iv) phthalocyanine (SiPc) core. The absorption spectrum of this compound virtually covered the entire visible region (300-700 nm) and could be interpreted as a superposition of the spectra of individual components. The intramolecular photoinduced energy and charge transfer processes of this triad were studied using steady-state and time-resolved spectroscopic methods in polar and nonpolar solvents. Upon BDP-part excitation, a fast and highly efficient excitation energy transfer (EET) occurred resulting in strong quenching of its fluorescence and the formation of the first excited singlet state of SiPc or MSBDP. It was found that both EET and charge transfer (CT) processes competed with each other in the depopulation of the first excited singlet state of the MSBDP moiety. The former strongly superseded CT in nonpolar toluene, whereas the latter was dominant in a polar environment. Direct or indirect (via EET) excitation of the SiPc-part of the triad was followed by CT yielding the charge-separated (CS) species BDP-SiPc(˙-)-MSBDP(˙+). The energy gap between the CS state and the S1-state of the SiPc moiety was found to be only 0.06 eV in toluene, which facilitated the back CT process and resulted in the appearance of thermally activated delayed fluorescence. With increasing solvent polarity, the energy of the CS state reduced resulting in the disappearance of the delayed fluorescence in CHCl3, tetrahydrofuran or N,N-dimethylformamide. The charge recombination rate, k(CR), was very fast in polar DMF (3.3 × 10(10) s(-1)), whereas this process was two-orders of magnitude slower in nonpolar toluene (k(CR) = 4.0 × 10(8) s(-1)).

Towards multielectron photocatalysis: a porphyrin array for lateral hole transfer and capture on a metal oxide surface.

Current molecular water-oxidation photoelectrocatalytic cells have substantial kinetic limitations under normal solar photon flux where electron-hole recombination processes may outcompete charge buildup on the catalytic centers. One method of overcoming these limitations is to design a system where multiple light-harvesting dyes work cooperatively with a single catalyst. We report a porphyrin monomer/dyad array for analysis of lateral hole transfer on a SnO2 surface consisting of a free-base porphyrin that functions to absorb light and initiate charge injection into the conduction band of SnO2, which leaves a positive charge on the organic moiety, and a free-base porphyrin/Zn-porphyrin dyad molecule that functions as a thermodynamic trap for the photoinduced holes. By using transient absorption spectroscopy, we have determined that the holes on the surface-bound free-base porphyrins are highly mobile via electron self-exchange between close-packed neighbors. The lateral charge-transfer processes were modelled by treating the system statistically with a random-walk method that utilizes experimentally derived kinetic parameters. The results of the modelling indicate that each self-exchange (hop) occurs within 25 ns and that the holes are efficiently transferred to the Zn-porphyrin. This hole-harvesting scheme provides a framework for enhancing the efficiency of multielectron photoelectrocatalytic reactions such as the four-electron oxidation of water.

Synthesis and Emission Properties of meso-Substituted Porphyrins

Discovering of various organic dyes including porphyrins have attracted considerable attention to mimic the energy and electron transfer process for the artificial photosynthetic model systems which can be applied in developing optoelectronic devices. The accessibility on tailoring porphyrin properties makes them a good candidate to, be developed as the red light emitting materials for these applications. Thus, symmetrical and unsymmetrical molecular models of porphyrins with appended fluorenyl components and extended π electrons conjugated system were synthesized towards increasing the efficiency of energy and electron transfer. In the photophysical studies, the emission spectra proposed the evidence of energy transfer of appended fluorenyl arms into the porphyrin macrocyclic whereas the extension of conjugating system in porphyrins exhibits lower absorption energy and intensified the red fluorescent properties.

Photosynthetic Antenna–Reaction Center Mimicry by Using Boron Dipyrromethene Sensitizers

Various molecular and supramolecular systems have been synthesized and characterized recently to mimic the functions of photosynthesis, in which solar energy conversion is achieved. Artificial photosynthesis consists of light-harvesting and charge-separation processes together with catalytic units of water oxidation and reduction. Among the organic molecules, derivatives of BF2-chelated dipyrromethene (BODIPY), "porphyrin's little sister", have been widely used in constructing these artificial photosynthetic models due to their unique properties. In these photosynthetic models, BODIPYs act as not only excellent antenna molecules, but also as electron-donor and -acceptor molecules in both the covalently linked molecular and supramolecular systems formed by axial coordination, hydrogen bonding, or crown ether complexation. The relationships between the structures and photochemical reactivities of these novel molecular and supramolecular systems are discussed in relation to the efficiency of charge separation and charge recombination. Femto- and nanosecond transient absorption and photoelectrochemical techniques have been employed in these studies to give clear evidence for the occurrence of energy- and electron-transfer reactions and to determine their rates and efficiencies.

A boron dipyrromethene–phthalocyanine pentad as an artificial photosynthetic model

A silicon(IV) phthalocyanine with two axial p-phenylene-linked boron dipyrromethene and monostyryl boron dipyrromethene moieties has been prepared. The resulting pentad absorbs strongly in most of the UV-visible region and serves as an artificial photosynthetic antenna-reaction centre model.

Intramolecular hydrogen bonding as a synthetic tool to induce chemical selectivity in acid catalyzed porphyrin synthesis

A straightforward procedure based on the formation of intramolecular hydrogen bonds to impart selectivity in the preparation of multi-functionalized porphyrins has been developed. To illustrate the concept, the synthesis of a biomimetic artificial photosynthetic model able to undergo electron and proton transfer reactions upon irradiation is reported.

ChemInform Abstract: Artificial Photosynthetic Model Systems Using Light-Induced Electron Transfer Reactions in Catalytic and Biocatalytic Assemblies

Artificial photosynthetic devices provide a means for the use of solar light in generating fuel materials and valuable chemicals and for the removal of environmental pollutants. Control of photosensitized electron transfer reactions and development of catalysts for utilizations of the intermediate electron transfer products are essential aspects in designing artificial photosynthetic systems. Homogeneous and heterogeneous catalysts as well as biocatalysts (enzymes and cofactors) can be coupled to photochemically induced electron transfer reactions and effect photosynthetic transformations such as hydrogen evolution, CO2-fixation, hydrogenation, and hydroformylation processes. The progress in tailoring artificial photosynthetic devices in the context of thermodynamic and kinetic limitations of such systems is described. Integrated systems, where catalytic performance and control of electron transfer reactions which occur in organized assemblies are specifically emphasized.

Tetra(β-phenothiazinyl) zinc phthalocyanine: An easily prepared D 4–A system for efficient photoinduced electron transfer

Four identical electron donor (D) moieties, phenothiazines (PTZs), were covalently attached onto the same acceptor (A), zinc phthalocyanine (ZnPc), to form ZnPc(β-PTZ) 4, i.e. D 4–A. The symmetrical D 4–A was synthesized by the condensation method to examine intra-molecular photoinduced electron transfer (PET). The common electron donor-spacer–acceptor (D–A) photosynthetic models, which involve the asymmetrical synthesis of a mono-substituted porphyrin or its analogs, suffer from a low yield and arduous isolation, since D–A is only one of several products (D n –A or A n –D, n = 0, 2–4). The D 4–A preparation, however, can be carried out without the problem. The steady state and time-resolved fluorescence of the D 4–A were measured and compared with that of ZnPc(β-R) 4 (R = H, OPh). The result showed that the excited singlet state of phthalocyanine moiety in the D 4–A molecule was efficiently quenched by phenothiazine units owing to intra-molecular PET. The rate constant of PET ( k et) was calculated and the value is much higher than the rate constant of fluorescence emission, intersystem crossing and internal conversion of ZnPc moiety. The laser flash photolysis study revealed the presence of a long-lived charge-separated state due to PET. The results suggest that a D 4–A system can be a more efficient artificial photosynthetic model than D–A towards the practical commercial use.

Multiple‐Charge Separation in Nanoscale Artificial Photosynthetic Models

Strong attraction: Hundreds of electron donors can be covalently attached to a single-walled carbon nanotube (SWCNT) to form a Dn A artificial photosynthetic model (see figure). Multiple electrons are simultaneously or sequentially transferred to the nanotube upon light absorption and multiple-charge separation states (D+.)k Ak .− are formed.

The progress on design and synthesis of photoactive porphyrins-based dyads, triads and polymers

Porphyrins are the most frequently employed building blocks as electron donors and sensitizers in artificial photosynthetic models for solar energy conversion. Recently, we have reported a series of covalently linked, donor-acceptor dyads, triads and copolymers containing porphyrin analogs aiming to improve light-harvesting capacity and charge-separation efficiency with a potential application in solar cells. In this review, we would like to summarize our recent studies on these photoactive, porphyrin-containing composite systems focusing on the designs and properties of these systems based on intermolecular electro- and energy transfer.

Creation of Fullerene-Based Artificial Photosynthetic Systems

Abstract Various porphyrin–fullerene-linked systems have been prepared to elucidate the intrinsic electron-transfer properties of fullerenes. Photodynamical studies on porphyrin–fullerene-linked systems showed that spherical fullerenes accelerated photoinduced electron transfer and charge-shift, but slowed down charge recombination, which is in sharp contrast with those of conventional planar acceptors such as quinones and imides. For the first time, it was shown that the unique electron-transfer properties result from the small reorganization energies of fullerenes arising from the delocalized π system on the sphere together with the rigid structure. The small reorganization energies make it possible to produce a long-lived charge-separated state with a high quantum yield in donor–fullerene systems. The finding also will allow us to construct light energy conversion systems as well as artificial photosynthetic models. Highly efficient photoinduced energy and electron transfer were achieved on gold and ITO electrodes modified with self-assembled monolayers of the porphyrin–fullerene-linked systems. We also developed dye-sensitized bulk heterojunction solar cell possessing both characteristics of dye-sensitized and bulk heterojunction solar cells. These results showed the advantages of fullerenes as electron acceptors in artificial photosynthetic model, photonic molecular devices and machines, and organic molecular electronics including solar cells.

Synthesis and Photophysical Properties of Electron-Rich Perylenediimide-Fullerene Dyad

An electron-rich perylenediimide-C60 dyad has been prepared to explore a new type of donor-acceptor system. Time-resolved absorption measurements in benzonitrile revealed unambiguous evidence for the formation of a charge-separated state consisting of perylene diimide radical cation and C60 radical anion via photoinduced electron transfer, showing a new class of artificial photosynthetic models in terms of charge separation.

Diels–Alder cycloaddition as an efficient tool for linking π-donors onto fullerene C60

Abstract Diels–Alder reaction of endocyclic, acyclic 1,3-dienes or (hetero) o-quinodimethanes with the dienophilic fullerene C60 is presented as an efficient tool for linking electroactive units giving rise to systems where both the donor and C60 partners take up well defined volumes and orientations with respect to one another. While porphyrin is the most frequently used electron donor in artificial photosynthetic models, C60-based tetrathiafulvalene (TTF) or π-conjugated oligomer adducts remain interesting candidates for the preparation of photovoltaic devices. In this account, we focus on the use of the Diels–Alder cycloaddition and its use in the synthesis of TTF–C60 dyads, (TTF)n–C60 polyads and C60–TTF–C60 dumbbells as part of ongoing research into materials which display efficient photo-induced electron transfer. To cite this article: P. Hudhomme, C. R. Chimie 9 (2006).