Surface-grafted multiporphyrin arrays as light-harvesting antennae to amplify photocurrent generation

Organized multiporphyrin arrays were developed on the conductive surface by a novel coordination-directed molecular architecture aiming at efficient photoelectric conversion. The basic strategy employs the mutual coordination of two imidazolylporphyrinatozinc(II) units to form a cofacial dimer. Thus, meso,meso-linked bis(imidazolylporphyrinatozinc) (Zn2(ImP)2) was organized onto imidazolylporphyrinatozinc on the gold substrate as a self-assembled monolayer. The organized Zn2(ImP)2 bearing allyl side chains was covalently linked by ring-closing olefin metathesis catalyzed with Grubbs catalyst. Alternating coordination/metathesis reactions allow the stepwise accumulation of multiporphyrin arrays on the gold electrode. A successive increase in absorption over a wide wavelength range occurred after each accumulation step of Zn2(ImP)2 on the gold electrode, and cathodic photocurrent generation was enhanced in the aqueous electrolyte system, containing viologen as an electron carrier. The significant increase of the photocurrent indicates that the multiporphyrin array works as a "light-harvesting antenna" on the gold electrode.     

Bioinspired molecular design of light-harvesting multiporphyrin arrays

Recent progress in fundamental studies on multiporphyrin arrays has provided structural parameters for the molecular design of artificial light-harvesting antennae which mimic the wheel-like antenna complexes of photosynthetic purple bacteria. Covalent and noncovalent approaches have been employed for the construction of artificial light-harvesting multiporphyrin arrays. Such arrays are categorized into ring-shaped, windmill-shaped, star-shaped, and dendritic architectures. In particular, dendritic multiporphyrin arrays have been proven to be promising candidates for both providing a large absorption cross-section and enabling the vectorial transfer of energy over a long distance to a designated point. Such molecular and supramolecular systems are also expected to be potent components for molecular electronics and photonic devices.     

Excitation energy transfer in multiporphyrin arrays with cyclic architectures: towards artificial light-harvesting antenna complexes

Since highly symmetric cyclic architecture of light-harvesting antenna complex LH2 in purple bacteria was revealed in 1995, there has been a renaissance in developing cyclic porphyrin arrays to duplicate natural systems in terms of high efficiency, in particular, in transferring excitation energy. This tutorial review highlights the mechanisms and rates of excitation energy transfer (EET) in a variety of synthetic cyclic porphyrin arrays on the basis of time-resolved spectroscopic measurements performed at both ensemble and single-molecule levels. Subtle change in structural parameters such as connectivity, distance, and orientation between neighboring porphyrin moieties exquisitely modulates not only the nature of interchromophoric interactions but also the rates and efficiencies of EET. The relationship between the structure and EET dynamics described here should assist a rational design of novel cyclic porphyrin arrays, more contiguous to real applications in artificial photosynthesis.     

Dendritic multiporphyrin arrays as light-harvesting antennae: effects of generation number and morphology on intramolecular energy transfer

A series of star- and cone-shaped dendritic multiporphyrin arrays, (nPZn)4PFB and (nPZn)1PFB, respectively, that contain energy-donating dendritic zinc porphyrin (PZn) wedges of different numbers (n = 1, 3, and 7) of the PZn units, attached to an energy-accepting free-base porphyrin (PFB) core, were synthesized by a convergent growth approach. For the cone-shaped series ((nPZn)1PFB), the efficiency of energy transfer (phi ENT) from the photoexcited PZn units to the focal PFB core, as evaluated from the fluorescence lifetimes of the PZn units, considerably decreased as the generation number increased: (1PZn)1PFB (86%), (3PZn)1PFB (66%), and (7PZn)1PFB (19%). In sharp contrast, the star-shaped series ((nPZn)4PFB) all showed high phi ENT values: (1PZn)4PFB (87%), (3PZn)4PFB (80%), and (7PZn)4PFB (71%). Energy transfer efficiencies of (3PZn)4-ester-PFB, (1PZn)4-ester-PFB, and (3PZn)1-ester-PFB, whose dendritic PZn wedges are connected by an ester linkage to the PFB core, were almost comparable to those of the corresponding ether-linked versions. Fluorescence depolarization (P) studies showed much lower P values for star-shaped (7PZn)4PFB and (3PZn)4PFB than cone-shaped (7PZn)1PFB and (3PZn)1PFB, respectively, indicating a highly efficient energy migration among the PZn units in the star-shaped series. Such a morphology-assisted photochemical event is probably responsible for the excellent light-harvesting activity of large (7PZn)4PFB molecules.     

Layer-by-layer assembly of metal-mediated multiporphyrin arrays

Two types of multiporphyrin arrays, mediated by PdCl4(2-) complex ions at the air-water interface, were alternately transferred onto solid supports to form three-dimensional organized multilayers by a layer-by-layer method.     

Construction of multiporphyrin arrays via selective cross-metathesis

Selective cross-metathesis of type I and type II meso-functionalized porphyrin olefins afforded alkenyl-coupled dimeric and trimeric porphyrin systems in good yield with excellent E/Z selectivity. The synthetic utility of the method is demonstrated through the preparation of mixed metalated (M = 2H, Zn) porphyrin dimer and trimer. [reaction: see text]     

Energy transfer, excited-state deactivation, and exciplex formation in artificial caroteno-phthalocyanine light-harvesting antennas

We present results from transient absorption spectroscopy on a series of artificial light-harvesting dyads made up of a zinc phthalocyanine (Pc) covalently linked to carotenoids with 9, 10, or 11 conjugated carbon-carbon double bonds, referred to as dyads 1, 2, and 3, respectively. We assessed the energy transfer and excited-state deactivation pathways following excitation of the strongly allowed carotenoid S2 state as a function of the conjugation length. The S2 state rapidly relaxes to the S* and S1 states. In all systems we detected a new pathway of energy deactivation within the carotenoid manifold in which the S* state acts as an intermediate state in the S2-->S1 internal conversion pathway on a sub-picosecond time scale. In dyad 3, a novel type of collective carotenoid-Pc electronic state is observed that may correspond to a carotenoid excited state(s)-Pc Q exciplex. The exciplex is only observed upon direct carotenoid excitation and is nonfluorescent. In dyad 1, two carotenoid singlet excited states, S2 and S1, contribute to singlet-singlet energy transfer to Pc, making the process very efficient (>90%) while for dyads 2 and 3 the S1 energy transfer channel is precluded and only S2 is capable of transferring energy to Pc. In the latter two systems, the lifetime of the first singlet excited state of Pc is dramatically shortened compared to the 9 double-bond dyad and model Pc, indicating that the carotenoid acts as a strong quencher of the phthalocyanine excited-state energy.     

Construction of multiporphyrin arrays using ruthenium and rhodium coordination to phosphines

The synthesis of linear multiporphyrin arrays with mono- and bisphosphine-substituted porphyrins as ligand donors and ruthenium(II) or rhodium(III) porphyrins as ligand acceptors is described. With appropriate amounts of the building blocks mixed, linear dimeric and trimeric arrays have been synthesized and analyzed by (1)H NMR and (31)P NMR spectroscopy. The Ru/Rh acceptor porphyrins can be located either at the periphery or in the center of the array. Likewise, the monophosphine porphyrins can be positioned at the periphery, thus allowing a high degree of freedom in the overall composition of the arrays. This way, both donor and acceptor porphyrins can act as chain extenders or terminators. One of the trimeric complexes with two nickel and one ruthenium porphyrin has also been analyzed by X-ray crystallography. Attempts have also been made to synthesize higher order arrays by mixing appropriate amounts of the porphyrins; however, from the NMR data it cannot be concluded if monodisperse five, seven, or nine porphyrin arrays are present or if the solutions are composed of a statistical mixture of smaller and larger arrays.     

DNA-directed artificial light-harvesting antenna

Designing and constructing multichromophoric, artificial light-harvesting antennas with controlled interchromophore distances, orientations, and defined donor-acceptor ratios to facilitate efficient unidirectional energy transfer is extremely challenging. Here, we demonstrate the assembly of a series of structurally well-defined artificial light-harvesting triads based on the principles of structural DNA nanotechnology. DNA nanotechnology offers addressable scaffolds for the organization of various functional molecules with nanometer scale spatial resolution. The triads are organized by a self-assembled seven-helix DNA bundle (7HB) into cyclic arrays of three distinct chromophores, reminiscent of natural photosynthetic systems. The scaffold accommodates a primary donor array (Py), secondary donor array (Cy3) and an acceptor (AF) with defined interchromophore distances. Steady-state fluorescence analyses of the triads revealed an efficient, stepwise funneling of the excitation energy from the primary donor array to the acceptor core through the intermediate donor. The efficiency of excitation energy transfer and the light-harvesting ability (antenna effect) of the triads was greatly affected by the relative ratio of the primary to the intermediate donors, as well as on the interchromophore distance. Time-resolved fluorescence analyses by time-correlated single-photon counting (TCSPC) and streak camera techniques further confirmed the cascading energy transfer processes on the picosecond time scale. Our results clearly show that DNA nanoscaffolds are promising templates for the design of artificial photonic antennas with structural characteristics that are ideal for the efficient harvesting and transport of energy.     

Zn-containing porphyrin as a biomimetic light-harvesting molecule for biocatalyzed artificial photosynthesis

Among the porphyrin molecules with different metal insertion sites and functional ligands, Zn-porphyrin most efficiently regenerates NADH through photo-induced electron transfer in the presence of [Cp*Rh(bpy)H(2)O](2+), a rhodium-based electron mediator. Photochemical regeneration of NADH by Zn-porphyrin is successfully coupled with redox enzymatic synthesis under dark state conditions.     

Giant porphyrin wheels with large electronic coupling as models of light-harvesting photosynthetic antenna

Starting from a 1,3-phenylene-linked diporphyrin zinc(II) complex 2ZA, repeated stepwise Ag(I)-promoted coupling reactions provided linear oligomers 4ZA, 6ZA, 8ZA, and 12ZA. The intramolecular cyclization reaction of 12ZA under dilute conditions (1x10(-6) M) gave porphyrin ring C12ZA with a diameter of approximately 35 A in 60% yield. This synthetic strategy has been applied to a 1,3-phenylene-linked tetraporphyrin 4ZB to provide 8ZB, 12ZB, 16ZB, 24ZB, and 32ZB. The intramolecular coupling reaction of 24ZB gave a larger 24-mer porphyrin ring C24ZB with a diameter of approximately 70 A in 34% yield. These two large porphyrin rings were characterized by means of 1H NMR spectroscopy, matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectroscopy, UV-visible spectroscopy, gel permeation chromatography (GPC) analysis, and scanning tunneling microscopy (STM) techniques. The STM images of C12ZA reveal largely circular structures, whereas those of C24ZB exhibit mostly ellipsoidal shapes, indicating more conformational flexibility of C24ZB. Similar to the case of C12ZA, the efficient excitation energy transfer along the ring has been confirmed for C24ZB by using the time-correlated single-photon counting (TCSPC) and picosecond transient absorption anisotropy (TAA) measurements, and occurs with a rate of (35 ps)(-1) for energy hops between neighboring tetraporphyrin subunits. Collectively, the present work provides an important step for the construction of large cyclic-arranged porphyrin arrays with ample electronic interactions as a model of light-harvesting antenna.     

Magneto-chiral dichroism of artificial light-harvesting antenna

We have demonstrated the presence of magneto-chiral dichroism (MChD) of chiral J-aggregates of zinc chlorins. To the best of our knowledge, this is the first observation of MChD in artificial light-harvesting antennas.     

Regiospecifically alpha-13C-labeled porphyrins for studies of ground-state hole transfer in multiporphyrin arrays

Insight into the electronic communication between the individual constituents of multicomponent molecular architectures is essential for the rational design of molecular electronic and/or photonic devices. To clock the ground-state hole/electron-transfer process in oxidized multiporphyrin architectures, a p-diphenylethyne-linked zinc porphyrin dyad was prepared wherein one porphyrin bears two (13)C atoms and the other porphyrin is unlabeled. The (13)C atoms are located at the 1- and 9-positions (alpha-carbons symmetrically disposed to the position of linker attachment), which are sites of electron/spin density in the a(1u) HOMO of the porphyrin. The (13)C labels were introduced by reaction of KS(13)CN with allyl bromide to give the allyl isothiocyanate, which upon Trofimov pyrrole synthesis followed by methylation gave 2-(methylthio)pyrrole-2-(13)C. Reaction of the latter with paraformaldehyde followed by hydrodesulfurization gave dipyrromethane-1,9-(13)C, which upon condensation with a dipyrromethane-1,9-dicarbinol bearing three pentafluorophenyl groups gave the tris(pentafluorophenyl)porphyrin bearing (13)C labels at the 1,9-positions and an unsubstituted meso (5-) position. Zinc insertion, bromination at the 5-position, and Suzuki coupling with an unlabeled porphyrin bearing a suitably functionalized diphenylethyne linker gave the regiospecifically labeled zinc porphyrin dyad. Examination of the monocation of the isotopically labeled dyad via electron paramagnetic resonance (EPR) spectroscopy (and comparison with the monocations of benchmark monomers, where hole transfer cannot occur) showed that the hole transfer between porphyrin constituents of the dyad is slow (<10(6) s(-1)) on the EPR time scale at room temperature. The slow rate stems from the a(1u) HOMO of the electron-deficient porphyrins, which has a node at the site of linker connection. In contrast, analogous dyads of electron-rich porphyrins (wherein the HOMO is a(2u) and has a lobe at the site of linker connection) studied previously exhibit rates of hole transfer that are fast (>5 x 10(7) s(-1)) on the EPR time scale at room temperature.     

Theoretical solar-to-electrical energy-conversion efficiencies of perylene-porphyrin light-harvesting arrays

The efficiencies of organic solar cells that incorporate light-harvesting arrays of organic pigments were calculated under 1 sun of air mass 1.5 solar irradiation. In one set of calculations, photocurrent efficiencies were evaluated for porphyrin, phthalocyanine, chlorin, bacteriochlorin, and porphyrin-bis(perylene) pigment arrays of different length and packing densities under the assumption that each solar photon absorbed quantitatively yielded one electron in the external circuit. In another more realistic set of calculations, solar conversion efficiencies were evaluated for arrays comprising porphyrins or porphyrin-(perylene)2 units taking into account competitive excited-state relaxation pathways. A system of coupled differential equations for all reactions in the arrays was solved on the basis of previously published rate constants for (1) energy transfer between the perylene and porphyrin pigments, (2) excited-state relaxation of the perylene and porphyrin pigments, and (3) excited-state electron injection into the semiconductor. This formal analysis enables determination of the optimal number of pigments in an array for solar-to-electrical energy conversion. The optimal number of pigments depends on the molar absorption coefficient and the density at which the arrays can be packed on an electrode surface. Taken together, the ability to employ fundamental photophysical, kinetic, and structural parameters of modular molecular architectures in assessments of the efficiency of solar-to-electrical energy conversion should facilitate the design of molecular-based solar cells.     

Meso-13C-labeled porphyrins for studies of ground-state hole transfer in multiporphyrin arrays

Understanding electronic communication among interacting chromophores provides the foundation for a variety of applications. The ground-state electronic communication in diphenylethyne-linked zinc-porphyrin dyads has been investigated by a novel molecular design strategy that entails introduction of a 13C-atom (*) at specific sites of the porphyrins where there is substantial electron density in the relevant frontier (highest occupied) molecular orbital. The site of 13C substitution is at a meso-position, either the site of attachment of the linker (proximal, "P") or the site trans to the linker (distal, "D"). The substituents (R) at the non-linking meso-positions are mesityl, tridec-7-yl ("swallowtail"), or p-tolyl groups. Altogether five isotopically labeled porphyrin dyads have been prepared. The hole/electron-transfer properties of one-electron oxidized dyads have been examined by electron paramagnetic resonance (EPR) spectroscopy. The introduction of the meso-13C label provides a "clock" (via the hyperfine interactions) that allows investigation of a time scale for hole transfer that is 3-4 times shorter than that provided by the natural abundance 14N nuclei of the pyrrole nitrogen atoms. The EPR studies indicate that the hole transfer, which has been previously shown to be fast on the time scale of the 14N hyperfine clock ( approximately 220 ns), remains fast on the time scale of the 13C hyperfine clock ( approximately 50 ns).     

Carotenoid photoprotection in artificial photosynthetic antennas

A series of phthalocyanine-carotenoid dyads in which a phenylamino group links a phthalocyanine to carotenoids having 8-11 backbone double bonds were examined by visible and near-infrared femtosecond pump-probe spectroscopy combined with global fitting analysis. The series of molecules has permitted investigation of the role of carotenoids in the quenching of excited states of cyclic tetrapyrroles. The transient behavior varied dramatically with the length of the carotenoid and the solvent environment. Clear spectroscopic signatures of radical species revealed photoinduced electron transfer as the main quenching mechanism for all dyads dissolved in a polar solvent (THF), and the quenching rate was almost independent of carotenoid length. However, in a nonpolar solvent (toluene), quenching rates displayed a strong dependence on the conjugation length of the carotenoid and the mechanism did not include charge separation. The lack of any rise time components of a carotenoid S(1) signature in all experiments in toluene suggests that an excitonic coupling between the carotenoid S(1) state and phthalocyanine Q state, rather than a conventional energy transfer process, is the major mechanism of quenching. A pronounced inhomogeneity of the system was observed and attributed to the presence of a phenyl-amino linker between phthalocyanine and carotenoids. On the basis of accumulated work on various caroteno-phthalocyanine dyads and triads, we have now identified three mechanisms of tetrapyrrole singlet excited state quenching by carotenoids in artificial systems: (i) Car-Pc electron transfer and recombination; (ii)(1) Pc to Car S(1) energy transfer and fast internal conversion to the Car ground state; (iii) excitonic coupling between (1)Pc and Car S(1) and ensuing internal conversion to the ground state of the carotenoid. The dominant mechanism depends upon the exact molecular architecture and solvent environment. These synthetic systems are providing a deeper understanding of structural and environmental effects on the interactions between carotenoids and tetrapyrroles and thereby better defining their role in controlling natural photosynthetic systems.     

Formation of porphyrins in the presence of acid-labile metalloporphyrins: a new route to mixed-metal multiporphyrin arrays

The ability to incorporate distinct metalloporphyrins at designated sites in multiporphyrin arrays is essential for diverse applications in materials and biomimetic chemistry. The synthesis of such mixed-metal arrays via acid catalyzed reactions has largely been restricted to metalloporphyrins of stability class II (e.g., Cu, Co, Ni) or I. We describe routes for the rational synthesis of mixed-metal arrays via acid-catalyzed condensations that are compatible with metalloporphyrins of stability class III (e.g., Zn) and IV (e.g., Mg). The routes are demonstrated for p-phenylene-linked arrays. The key finding is that several mild Lewis acids [InCl(3), Sc(OTf)(3), Yb(OTf)(3), and Dy(OTf)(3)], which are known to catalyze the dipyrromethane + dipyrromethane-dicarbinol condensation in CH(2)Cl(2) at room temperature without acidolysis, do not demetalate zinc or magnesium porphyrins under the same conditions. Rational routes to porphyrin dyads and triads employ reaction of a (porphyrin)-dipyrromethane and a (porphyrin)-dipyrromethane-dicarbinol. The porphyrin-forming reactions (six examples) proceed in yields of 18-28%. The metalation states of the arrays prepared in this manner include Zn-free base (ZnFb), MgFb, ZnFbMg, ZnFbZn, and ZnFbFb. Studies of the catalysis process indicate that the dipyrromethane + dipyrromethane-dicarbinol condensation is catalyzed by both the Lewis acid and a Br?nsted acid derived in situ from the Lewis acid. Taken together, the ability to employ otherwise "acid-labile" metalloporphyrins as precursors in condensation procedures should broaden the scope of accessible mixed-metal multiporphyrin arrays and motivate further studies of the application of mild Lewis acid catalysts in porphyrin chemistry.     

Porphyrin light-harvesting arrays constructed in the recombinant tobacco mosaic virus scaffold

We have demonstrated the construction of multiple porphyrin arrays in the tobacco mosaic virus (TMV) supramolecular structures by self-assembly of recombinant TMV coat protein (TMVCP) monomers, in which Zn-coordinated porphyrin (ZnP) and free-base porphyrin (FbP) were site-selectively incorporated. The photophysical properties of porphyrin moieties incorporated in the TMV assemblies were also characterized. TMV-porphyrin conjugates employed as building blocks self-assembled into unique disk and rod structures under the proper conditions as similar to native TMV assemblies. The mixture of a ZnP donor and an FbP acceptor was packed in the TMV assembly and showed energy transfer and light-harvesting activity. The detailed photophysical properties of the arrayed porphyrins in the TMV assemblies were examined by time-resolved fluorescence spectroscopy, and the energy transfer rates were determined to be 3.1-6.4x10(9) s(-1). The results indicate that the porphyrins are placed at the expected positions in the TMV assemblies.