Natural Light-harvesting Complexes Research Articles

Unraveling the mechanism of tip-enhanced molecular energy transfer

Electronic Energy Transfer (EET) between chromophores is fundamental in many natural light-harvesting complexes, serving as a critical step for solar energy funneling in photosynthetic plants and bacteria. The complicated role of the environment in mediating this process in natural architectures has been addressed by recent scanning tunneling microscope experiments involving EET between two molecules supported on a solid substrate. These measurements demonstrated that EET in such conditions has peculiar features, such as a steep dependence on the donor-acceptor distance, reminiscent of a short-range mechanism more than of a Förster-like process. By using state of the art hybrid ab initio/electromagnetic modeling, here we provide a comprehensive theoretical analysis of tip-enhanced EET. In particular, we show that this process can be understood as a complex interplay of electromagnetic-based molecular plasmonic processes, whose result may effectively mimic short range effects. Therefore, the established identification of an exponential decay with Dexter-like effects does not hold for tip-enhanced EET, and accurate electromagnetic modeling is needed to identify the EET mechanism.

Symmetry-Breaking Charge Transfer in Metal-Organic Frameworks.

High quantum-yield charge carrier generation from the initially prepared excitons defines a key step in the light-harvesting and conversion scheme. Photoinduced charge transfer in molecular electron donor-acceptor assemblies is driven by a sizable ΔG0, which compromises the potential of the generated carriers. Reminiscent of the special pair at the reaction center of the natural light-harvesting complex, symmetry-breaking charge transfer (SBCT) within a pair of identical struts of metal-organic framework (MOF) will facilitate the efficient generation of long-lived charge carriers with maximized potentials without incorporating any foreign redox species. We report SBCT in pyrene-based zirconium metal-organic framework (MOF) NU-1000 that leads to efficient generation of radical ions in a polar solvent and bound CT states in a low-polar solvent. The probe unveils the role of the low-lying non-Franck-Condon excitonic states as intermediates in the formation of the SBCT state from the initially prepared Franck-Condon S1 states. Ultrafast and transient spectroscopy─probed over 200 fs-30 μs time scale─evinces a kSBCT = (110 ps)-1 in polar media (εs = 37.5) forming solvated radical ions with recombination rate kCR = (∼45 ns)-1. A slower rate with kSBCT = (203 ps)-1 was recorded in low-polar (εs = 7.0) solvent manifesting a bound [TBAPy•+ TBAPy•-] state with kCR ≈ (17 μs)-1. This discovery, along with other unique photophysical features relevant to light harvesting, should define a MOF-based platform for developing heterogeneous artificial photon energy conversion systems.

Single-photon absorption and emission from a natural photosynthetic complex

Photosynthesis is generally assumed to be initiated by a single photon1–3 from the Sun, which, as a weak light source, delivers at most a few tens of photons per nanometre squared per second within a chlorophyll absorption band1. Yet much experimental and theoretical work over the past 40 years has explored the events during photosynthesis subsequent to absorption of light from intense, ultrashort laser pulses2–15. Here, we use single photons to excite under ambient conditions the light-harvesting 2 (LH2) complex of the purple bacterium Rhodobacter sphaeroides, comprising B800 and B850 rings that contain 9 and 18 bacteriochlorophyll molecules, respectively. Excitation of the B800 ring leads to electronic energy transfer to the B850 ring in approximately 0.7 ps, followed by rapid B850-to-B850 energy transfer on an approximately 100-fs timescale and light emission at 850–875 nm (refs. 16–19). Using a heralded single-photon source20,21 along with coincidence counting, we establish time correlation functions for B800 excitation and B850 fluorescence emission and demonstrate that both events involve single photons. We also find that the probability distribution of the number of heralds per detected fluorescence photon supports the view that a single photon can upon absorption drive the subsequent energy transfer and fluorescence emission and hence, by extension, the primary charge separation of photosynthesis. An analytical stochastic model and a Monte Carlo numerical model capture the data, further confirming that absorption of single photons is correlated with emission of single photons in a natural light-harvesting complex.

Cascade Energy Transfer in Biopolymer-Templated Multi-Chromophoric Assemblies: Ratiometric Temperature Sensing.

Natural light-harvesting complexes collect energy from sunlight and transfer it to the reaction center through a cascade of energy and electron transfer steps. Artificial light-harvesting systems functioning in aqueous media mimic natural photosynthetic systems. However, their design remains a challenging task as closely packed antenna chromophores often undergo severe self-quenching. Herein, we report luminescent co-assemblies between cationic pyrene-appended imidazolium amphiphiles and two anionic biopolymeric scaffolds, heparin and DNA in aqueous media. These co-assemblies served as excellent platforms for constructing artificial light-harvesting systems as upon co-embedding of multiple external dyes, highly efficient single-step and cascade energy transfer was observed from the pyrene donors to the acceptor dyes. Most notably, the efficiency of the energy transfer process was possible to modulate by employing multiple stimuli such as pH and temperature, and this resulted in the generation of multi-color luminescent materials in solution and film states, and they were also exploited in ratiometric temperature sensing. Their stimuli-responsive luminescence in the solid state was found to be advantageous for encryption studies.

Optical Properties of Concentric Nanorings of Quantum Emitters.

A ring of sub-wavelength spaced dipole-coupled quantum emitters features extraordinary optical properties when compared to a one-dimensional chain or a random collection of emitters. One finds the emergence of extremely subradiant collective eigenmodes similar to an optical resonator, which features strong 3D sub-wavelength field confinement near the ring. Motivated by structures commonly appearing in natural light-harvesting complexes (LHCs), we extend these studies to stacked multi-ring geometries. We predict that using double rings allows us to engineer significantly darker and better confined collective excitations over a broader energy band compared to the single-ring case. These enhance weak field absorption and low-loss excitation energy transport. For the specific geometry of the three rings appearing in the natural LH2 light-harvesting antenna, we show that the coupling between the lower double-ring structure and the higher energy blue-shifted single ring is very close to a critical value for the actual size of the molecule. This creates collective excitations with contributions from all three rings, which is a vital ingredient for efficient and fast coherent inter-ring transport. This geometry thus should also prove useful for the design of sub-wavelength weak field antennae.

Manifestation of Hydrogen Bonding and Exciton Delocalization on the Absorption and Two-Dimensional Electronic Spectra of Chlorosomes.

Chlorosomes are supramolecular aggregates that contain thousands of bacteriochlorophyll molecules. They perform the most efficient ultrafast excitation energy transfer of all natural light-harvesting complexes. Their broad absorption band optimizes light capture. In this study, we identify the microscopic sources of the disorder causing the spectral width and reveal how it affects the excited state properties and the optical response of the system. We combine molecular dynamics, quantum chemical calculations, and response function calculations to achieve this goal. The predicted linear and two-dimensional electronic spectra are found to compare well with experimental data reproducing all key spectral features. Our analysis of the microscopic model reveals the interplay of static and dynamic disorder from the molecular perspective. We find that hydrogen bonding motifs are essential for a correct description of the spectral line shape. Furthermore, we find that exciton delocalization over tens to hundreds of molecules is consistent with the two-dimensional electronic spectra.

Cryogenic TEM imaging of artificial light harvesting complexes outside equilibrium

The energy transport in natural light-harvesting complexes can be explored in laboratory conditions via self-assembled supramolecular structures. One such structure arises from the amphiphilic dye C8S3 molecules, which self-assemble in an aqueous medium to a double-wall cylindrical nanotube reminiscent of natural light-harvesting complexes found in green sulphur bacteria. In this paper, we report a way to investigate the structure of inner nanotubes (NTs) alone by dissolving the outer NTs in a microfluidic setting. The resulting thermodynamically unstable system was rapidly frozen, preventing the reassembly of the outer NT from the dissolved molecules, and imaged using cryogenic transmission electron microscopy (cryo-TEM). The experimental cryo-TEM images and the molecular structure were compared by simulating high-resolution TEM images, which were based on the molecular modelling of C8S3 NTs. We found that the inner NT with outer walls removed during the flash-dilution process had a similar size to the parent double-walled NTs. Moreover, no structural inhomogeneity was observed in the inner NT after flash-dilution. This opens up exciting possibilities for functionalisation of inner NTs before the reassembly of the outer NT occurs, which can be broadly extended to modify the intra-architecture of other self-assembled nanostructures.

Correlated spectral fluctuations quantified by line shape analysis of fifth-order two-dimensional electronic spectra.

Correlated spectral fluctuations were suggested to coordinate excitation transport inside natural light harvesting complexes. We demonstrate the capacities of 2D line shapes from fifth-order coherent electronic signals (R5-2D) to report on such fluctuations in molecular aggregates and present a stochastic approach to fluctuations in correlated site and bi-exciton binding energies in the optical dynamics of Frenkel excitons. The model is applied to R5-2D line shapes of a homodimer, and we show that the peak tilt dynamics are a measure for site energy disorder, inter-site correlation, and the strength of bi-exciton binding energy fluctuations.

Energy funnelling within multichromophore architectures monitored with subnanometre resolution.

The funnelling of energy within multichromophoric assemblies is at the heart of the efficient conversion of solar energy by plants. The detailed mechanisms of this process are still actively debated as they rely on complex interactions between a large number of chromophores and their environment. Here we used luminescence induced by scanning tunnelling microscopy to probe model multichromophoric structures assembled on a surface. Mimicking strategies developed by photosynthetic systems, individual molecules were used as ancillary, passive or blocking elements to promote and direct resonant energy transfer between distant donor and acceptor units. As it relies on organic chromophores as the elementary components, this approach constitutes a powerful model to address fundamental physical processes at play in natural light-harvesting complexes.

Bioinspired Applications of Porphyrin Derivatives.

Porphyrin derivatives are ubiquitous in nature and have important biological roles, such as in light harvesting, oxygen transport, and catalysis. Owing to their intrinsic π-conjugated structure, porphyrin derivatives exhibit characteristic photophysical and electrochemical properties. In biological systems, porphyrin derivatives are associated with various protein molecules through noncovalent interactions. For example, hemoglobin, which is responsible for oxygen transport in most vertebrates, consists of four subunits of a globular protein with an iron porphyrin derivative prosthetic group. Furthermore, noncovalently arranged porphyrin derivatives are the fundamental chromophores in light-harvesting systems for photosynthesis in plants and algae. These biologically important roles originate from the functional versatility of porphyrin derivatives. Specifically, porphyrins are excellent host compounds, forming coordination complexes with various metal ions that adds functionality to the porphyrin unit, such as redox activity and additional ligand binding at the central metal ion. In addition, porphyrins are useful building blocks for functional supramolecular assemblies because of their flat and symmetrical molecular architectures, and their excellent photophysical properties are typically utilized for the fabrication of bioactive functional materials. In this Account, we summarize our endeavors over the past decade to develop functional materials based on porphyrin derivatives using bioinspired approaches. In the first section, we discuss several synthetic receptors that act as artificial allosteric host systems and can be used for the selective detection of various chemicals, such as cyanide, chloride, and amino acids. In the second section, we introduce multiporphyrin arrays as mimics of natural light-harvesting complexes. The active control of energy transfer processes by additional guest binding and the fabrication of organic photovoltaic devices using porphyrin derivatives are also introduced. In the third section, we introduce several types of porphyrin-based supramolecular assemblies. Through noncovalent interactions such as metal-ligand interaction, hydrogen bonding, and π-π interaction, porphyrin derivatives were constructed as supramolecular polymers with formation of fiber or toroidal assembly. In the last section, the application of porphyrin derivatives for biomedical nanodevice fabrication is introduced. Even though porphyrins were good candidates as photosensitizers for photodynamic therapy, they have limitations for biomedical application owing to aggregation in aqueous media. We suggested ionic dendrimer porphyrins and they showed excellent photodynamic therapy (PDT) efficacy.

Light-Harvesting “Antenna” Behavior in NU-1000

Structural or conceptual synthetic analogues of natural photosynthetic light-harvesting complexes hold promise as entities capable of both efficient collection of visible-region solar photons and r...

Improving Energy Transfer within Metal-Organic Frameworks by Aligning Linker Transition Dipoles along the Framework Axis.

Crystalline metal-organic frameworks (MOFs) can assemble chromophoric molecules into a wide range of spatial arrangements, which are controlled by the MOF topology. Like natural light-harvesting complexes (LHCs), the precise arrangement modulates interchromophoric interactions, in turn determining excitonic behavior and migration dynamics. To unveil the key factors that control efficient exciton displacements within MOFs, we first developed linkers with low electronic symmetry (as defined by large transition dipoles) and then assembled them into MOFs. These linkers possess extended conjugation along one molecular axis, engendering low optical bandgaps and improved oscillator strength for their lowest-energy transition (S0 → S1). This enhances absorption-emission spectral overlap and boosts the efficiency of Förster resonance energy transfer, which was observed experimentally by a sizable decrease in emission quantum yield (QY), accompanied by a faster population decay profile. We find that MOFs that orient these elongated linkers along their asymmetric pore channel, e.g., the hexagonal pores in an xly network, manifested >50% decrease in their emission QY with faster decay profiles relative to their corresponding solution dissolved linkers. This is due to an efficient migration of photogenerated excitons at the crystallite peripheral sites to internal sites, which was facilitated by polarized absorption-emission overlap among the parallelly aligned linkers. In contrast, symmetric MOFs, such as those with sqc-a topological net, orient elongated linkers along two perpendicular crystal axes, which hinders efficient exciton migration. The present study underscores that MOFs are promising to develop artificial LHCs, but that to achieve an efficient exciton displacement, appropriate topology-guided assembly is required to fully realize the true potential of linkers with low electronic symmetry.

Peptoid Helix Displaying Flavone and Porphyrin: Synthesis and Intramolecular Energy Transfer.

Natural light-harvesting complexes (LHCs) absorb a broad spectrum of sunlight using a collection of photosynthetic pigments whose spatial arrangement is controlled by a protein matrix and exhibit efficient energy transfer. We constructed a novel light-harvesting protein mimic, which absorbs light in the UV to visible region (280-700 nm) by displaying flavone and porphyrin on a peptoid helix. First, an efficient synthesis of 4'-derivatized 7-methoxyflavone (7-MF, 3 and 4) was developed. The flavone-porphyrin-peptoid conjugate (FPPC) was then prepared via Miyaura borylation on a resin-bound peptoid followed by Suzuki coupling between the peptoid and pigment. Circular dichroism spectroscopy indicated that the FPPC underwent helix-to-loop conversion of the peptoid scaffold upon changing the solvent conditions. A distinct intramolecular energy transfer was observed from 7-MF to porphyrin with greater efficiency in the helix than that in the loop conformation of the peptoid, whereas no clear evidence of energy transfer was obtained for unstructured FPPC. We thus demonstrate the value of the helical peptoid, which provided a controlled orientation for 7-MF and porphyrin and modulated the energy transfer efficiency via conformational switching. Our work provides a way to construct a sophisticated LHC mimic with enhanced coverage of the solar spectrum and controllable energy transfer efficiency.

Nucleosides As Organizing Architectural Moieties.

Chemistry “beyond the molecule” is epitomized in nature by a plethora of relatively weak noncovalent interactions. The three-dimensional structures of most biopolymers are controlled with noncovalent interactions, either between different parts of the same strand (as in protein α-helices) or between two separate strands (as in the DNA duplex and protein β-sheet). While nature has refined the construction of biopolymers, our mastery of the subtle noncovalent interactions as synthetic tools is in an early stage of development. Only in the past two decades people begun to develop ways of mimicking the natural light-harvesting complexes by noncovalent assembly of porphyrin units aiming to obtain favored spacing and orientation between the chromophores. The construction of multi-chromophoric assemblies has led to resurgence of interest in coordination chemistry due to formation of ordered arrays directed through molecular recognition events. As a module in the construction of supramolecular assemblies, porphyrins and metallo-porphyrins can be exploited in two different ways: porphyrins can behave as donor building blocks insofar as they comprise meso-substituents, such as pyridyl groups, which can act as ligands that can suitably coordinate to metal cations, while metalloporphyrins can act as acceptor building blocks as soon as the metal atom inside the porphyrin core has at least one axial site available for coordination. For the last years, we have focussed on cofacial bis-porphyrin tweezers for host/guest interactions and investigated the possibilty to obtain self-coordinated molecular systems with predictable spectral and redox characteristics. We report here the synthesis of a di-nucleotide bearing pendant porphyrins dedicated to adopt a pre-organized coformation with face-to-face porphyrins, and capable to self-organize in a stable sandwich type complexe with bidentate base such as DABCO.1 Using a similar strategy as the one used in antisense research, an artificial nucleotidic backbone was built from modified deoxy-uridine units linked with a more rigid linker than the phosphodiester moieties found in natural oligonucleotides. Antisense research uses modified oligonucleotides, less flexible than natural strands, to pre-organize the system toward the obtaining of stable double helices between synthesized and natural oligonucleotides. A modified oligonucleotidic backbone was here used to target a parallel conformation of the porphyrins appended to each deoxy-uridine moiety. To provide a rigid environment for the porphyrins, the uridine units were coupled in 3’-5’ stepwise fashion using ether-ester type of spacer of suitable length, and porphyrins were anchored to the uridine by means of robust carbon-carbon bonds. Earlier studies demonstrated that a peptidic linker doesn’t provide sufficient pre-organization to enhance significantly the association constant with bidentate bases such as DABCO on the contrary of some flexible linkers such as uridine or 2’-deoxyuridine. We document herein that the gain in stability for the formation of sandwich type host-guest complex with DABCO can be even greater when a dinucleotide linker is used. Such pre-organization increases the association constants by one to two orders of magnitude when compared to the association constants of the same bidentate ligands with a reference Zn(II) porphyrin. Comparison of these results with those obtained for rigid tweezers shows a better efficiency of the flexible nucleosidic dimers. We thus document the fact that the choice of rigid spacers is not the only way to pre-organize bis-porphyrins, and that some well-chosen nucleosidic linkers offer an interesting option for the synthesis of such devices. Furthermore, the chirality and enantio-purity of the nucleosidic linkers paves the way toward the selective complexation of enantio-pure bidentate guests and the resolution of racemates. Acknowledgements This work was supported by the CNRS and the French Ministry of Research. References Merkas, S. Bouatra, R. Rein, I. Piantanida, M. Zinic, N. Solladié, J. Porphyrins Phthalocyanines 2015, 19, 535-546. Figure 1

Porphyrin Boxes.

In order to fabricate efficient molecular photonic devices, it has been a long-held aspiration for chemists to understand and mimic natural light-harvesting complexes where a rapid and efficient transfer of excitation energy between chlorophyll pigments is observed. Synthetic porphyrins are attractive building blocks in this regard because of their rigid and planar geometry, high thermal and electronic stability, high molar extinction, small and tunable band gap, and tweakable optical as well as redox behavior. Owing to these fascinating properties, various types of porphyrin-based architectures have been reported utilizing both covalent and noncovalent approaches. However, it still remains a challenge to construct chemically robust, well-defined three-dimensional porphyrin cages which can be easily synthesized and yet suitable for useful applications both in solution as well as in solid state. Working on this idea, we recently synthesized box-shaped organic cages, which we called porphyrin boxes, by making use of dynamic covalent chemistry of imine condensation reaction between 4-connecting, square-shaped, tetraformylporphyrin and 3-connecting, triangular-shaped, triamine molecules. Various presynthetic, as well as postsynthetic modifications, can be carried out on porphyrin boxes including a variation of the alkyl chain length in their 3-connecting subunit, chemical functionalization, and metalation of the porphyrin core. This can remarkably tune their inherent properties, e.g., solubility, window size, volume, and polarity of the internal void. The porphyrin boxes can therefore be considered as a significant addition to the family of multiporphyrin-based architectures, and because of their chemical stability and shape persistency, the applications of porphyrin boxes expand beyond the photophysical properties of an artificial light-harvesting complex. Consequently, they have been exploited as porous organic cages, where their gas adsorption properties have been investigated. By incorporating them in a lipid bilayer membrane, an iodide selective synthetic ion channel has also been demonstrated. Further, we have explored electrocatalytic reduction of carbon dioxide using Fe(III) metalated porphyrin boxes. Additionally, the precise size and ease of metalation of porphyrin boxes allowed us to utilize them as premade building blocks for creating coordination-based hierarchical superstructures. Considering these developments, it may be worth combining the photophysical properties of porphyrin with the shape-persistent porous nature of porphyrin boxes to explore other novel applications. This Account summarizes our recent work on porphyrin boxes, starting with their design, structural features, and applications in different fields. We also try to provide scientific insight into the future opportunities that these amazing boxes have in store for exploring the still uncharted challenging domains in the field of supramolecular chemistry in a confined space.

Delocalized excitons in natural light-harvesting complexes

Natural organisms such as photosynthetic bacteria, algae, and plants employ complex molecular machinery to convert solar energy into biochemical fuel. An important common feature shared by most of these photosynthetic organisms is that they capture photons in the form of excitons typically delocalized over a few to tens of pigment molecules embedded in protein environments of light harvesting complexes (LHCs). Delocalized excitons created in such LHCs remain well protected despite being swayed by environmental fluctuations, and are delivered successfully to their destinations over hundred nanometer length scale distances in about hundred picosecond time scales. Decades of experimental and theoretical investigation have produced a large body of information offering insights into major structural, energetic, and dynamical features contributing to LHCs' extraordinary capability to harness photons using delocalized excitons. The objective of this review is (i) to provide a comprehensive account of major theoretical, computational, and spectroscopic advances that have contributed to this body of knowledge, and (ii) to clarify the issues concerning the role of delocalized excitons in achieving efficient energy transport mechanisms. The focus of this review is on three representative systems, Fenna-Matthews-Olson complex of green sulfur bacteria, light harvesting 2 complex of purple bacteria, and phycobiliproteins of cryptophyte algae. Although we offer more in-depth and detailed description of theoretical and computational aspects, major experimental results and their implications are also assessed in the context of achieving excellent light harvesting functionality. Future theoretical and experimental challenges to be addressed in gaining better understanding and utilization of delocalized excitons are also discussed.

Optical Properties of Vibronically Coupled Cy3 Dimers on DNA Scaffolds.

We examine the effect of electronic coupling on the optical properties of Cy3 dimers attached to DNA duplexes as a function of base pair (bp) separation using steady-state and time-resolved spectroscopy. For close Cy3-Cy3 separations, 0 and 1 bp between dyes, intermediate to strong electronic coupling is revealed by modulation of the absorption and fluorescence properties including spectral band shape, peak wavelength, and excited-state lifetime. Using a vibronic exciton model, we estimate coupling strengths of 150 and 266 cm-1 for the 1 and 0 bp separations, respectively, which are comparable to those found in natural light-harvesting complexes. For the strongest electronic coupling (0 bp separation), we observe that the absorption band shape is strongly affected by the base pairs that surround the dyes, where more strongly hydrogen-bonded G-C pairs produce a red-shifted absorption spectrum consistent with a J-type dimer. This effect is studied theoretically using molecular dynamics simulation, which predicts an in-line dye configuration that is consistent with the experimental J-type spectrum. When the Cy3 dimers are in a standard aqueous buffer, the presence of relatively strong electronic coupling is accompanied by decreased fluorescence lifetime, suggesting that it promotes nonradiative relaxation in cyanine dyes. However, we show that the use of a viscous solvent can suppress this nonradiative recombination and thereby restore the dimer fluorescent emission. Ultrafast transient absorption measurements of Cy3 dimers in both standard aqueous buffer and viscous glycerol buffer suggest that sufficiently strong electronic coupling increases the probability of excited-state relaxation through a dark state that is related to Cy3 torsional motion.

Strong Plasmonic Enhancement of a Single Peridinin-Chlorophyll a-Protein Complex on DNA Origami-Based Optical Antennas.

In this contribution, we fabricate hybrid constructs based on a natural light-harvesting complex, peridinin-chlorophyll a-protein, coupled to dimer optical antennas self-assembled with the help of the DNA origami technique. This approach enables controlled positioning of individual complexes at the hotspot of the optical antennas based on large, colloidal gold and silver nanoparticles. Our approach allows us to selectively excite the different pigments present in the harvesting complex, reaching a fluorescence enhancement of 500-fold. This work expands the range of self-assembled functional hybrid constructs for harvesting sunlight and can be further developed for other pigment-proteins and proteins.

Nucleosides As Organizing Architectural Moieties

Chemistry “beyond the molecule” is epitomized in nature by a plethora of relatively weak noncovalent interactions. The three-dimensional structures of most biopolymers are controlled with noncovalent interactions, either between different parts of the same strand (as in protein α-helices) or between two separate strands (as in the DNA duplex and protein β-sheet). While nature has refined the construction of biopolymers, our mastery of the subtle noncovalent interactions as synthetic tools is in an early stage of development. Only in the past two decades people begun to develop ways of mimicking the natural light-harvesting complexes by noncovalent assembly of porphyrin units aiming to obtain favored spacing and orientation between the chromophores. The construction of multi-chromophoric assemblies has led to resurgence of interest in coordination chemistry due to formation of ordered arrays directed through molecular recognition events. As a module in the construction of supramolecular assemblies, porphyrins and metallo-porphyrins can be exploited in two different ways: porphyrins can behave as donor building blocks insofar as they comprise meso-substituents, such as pyridyl groups, which can act as ligands that can suitably coordinate to metal cations, while metalloporphyrins can act as acceptor building blocks as soon as the metal atom inside the porphyrin core has at least one axial site available for coordination. For the last years, we have focussed on cofacial bis-porphyrin tweezers for host/guest interactions and investigated the possibilty to obtain self-coordinated molecular systems with predictable spectral and redox characteristics. We report here the synthesis of a di-nucleotide bearing pendant porphyrins dedicated to adopt a pre-organized coformation with face-to-face porphyrins, and capable to self-organize in a stable sandwich type complexe with bidentate base such as DABCO.1 Using a similar strategy as the one used in antisense research, an artificial nucleotidic backbone was built from modified deoxy-uridine units linked with a more rigid linker than the phosphodiester moieties found in natural oligonucleotides. Antisense research uses modified oligonucleotides, less flexible than natural strands, to pre-organize the system toward the obtaining of stable double helices between synthesized and natural oligonucleotides. A modified oligonucleotidic backbone was here used to target a parallel conformation of the porphyrins appended to each deoxy-uridine moiety. To provide a rigid environment for the porphyrins, the uridine units were coupled in 3’-5’ stepwise fashion using ether-ester type of spacer of suitable length, and porphyrins were anchored to the uridine by means of robust carbon-carbon bonds. Earlier studies demonstrated that a peptidic linker doesn’t provide sufficient pre-organization to enhance significantly the association constant with bidentate bases such as DABCO on the contrary of some flexible linkers such as uridine or 2’-deoxyuridine. We document herein that the gain in stability for the formation of sandwich type host-guest complex with DABCO can be even greater when a dinucleotide linker is used. Such pre-organization increases the association constants by one to two orders of magnitude when compared to the association constants of the same bidentate ligands with a reference Zn(II) porphyrin. Comparison of these results with those obtained for rigid tweezers shows a better efficiency of the flexible nucleosidic dimers. We thus document the fact that the choice of rigid spacers is not the only way to pre-organize bis-porphyrins, and that some well-chosen nucleosidic linkers offer an interesting option for the synthesis of such devices. Furthermore, the chirality and enantio-purity of the nucleosidic linkers paves the way toward the selective complexation of enantio-pure bidentate guests and the resolution of racemates. Acknowledgements This work was supported by the CNRS and the French Ministry of Research. References S. Merkas, S. Bouatra, R. Rein, I. Piantanida, M. Zinic, N. Solladié, J. Porphyrins Phthalocyanines 2015, 19, 535-546. Figure 1

Self-assembly of biomimetic light-harvesting complexes capable of hydrogen evolution

Biomimetics provides us a new perspective to understand complex biological process and strategy to fabricate functional materials. However, a great challenge still remains to design and fabricate biomimetic materials using a facile but effective method. Here, we develop a biomimetic light harvesting architecture based on one-step co-assembly of amphiphilic amino acid and porphyrin. Amphiphilic amino acid can self-assemble into nanofibers via π-stacking and hydrogen binding interactions. Negatively charged porphyrin adsorbs on the surface of the assembled nanofibers through electrostatic force, and the nanofibers further organize into porous urchin-like microspheres induced presumably by hydrophobic interaction. The assembled amphiphilic amino acid nanofibers work as a template to tune the organization of porphyrin with an architecture principle analogous to natural light harvesting complex. The co-assembled microspheres exhibit enhanced light capture due to the light reflection in the porous structure. Reaction center (platinum nanoparticles) can be effectively coupled with the light harvesting microspheres via photoreduction. After visible light illumination, hydrogen evolution occurs on the hybrid microspheres.