Artificial Light-harvesting Antenna Research Articles

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.

Dendrimers made of Ru(II) and Os(II) polypyridine subunits as artificial light-harvesting antennae

Dendrimers based on Ru(II) and Os(II) polypyridine complexes as building blocks and 2,3–dpp (2,3–dpp = 2,3–bis(2′–pyridyl)pyrazine) as bridging ligands are presented and their properties as light-harvesting antenna systems are illustrated. The dendrimers exhibit a huge absorption in the visible region and energy migration patterns whose direction and efficiency depend on the synthetically determined topography of the systems. New recent developments are also discussed, with particular regard towards ultrafast energy transfer processes and long-range electron transfer within the dendritic arrays. To cite this article: S. Serroni et al., C. R. Chimie 6 (2003).

Efficient excitation energy transfer in long meso-meso linked Zn(II) porphyrin arrays bearing a 5,15-bisphenylethynylated Zn(II) porphyrin acceptor.

Electronically coupled porphyrin arrays are suitable for artificial light harvesting antenna in light of a large absorption cross-section and fast excitation energy transfer (EET). Along this line, an artificial energy transfer model system has been synthesized, comprising of an energy donating meso-meso linked Zn(II) porphyrin array and an energy accepting 5,15-bisphenylethynylated Zn(II) porphyrin linked via a 1,4-phenylene spacer. This includes an increasing number of porphyrins in the meso-meso linked Zn(II) porphyrin array, 1, 2, 3, 6, 12, and 24 (Z1A, Z2A, Z3A, Z6A, Z12A, and Z24A). The intramolecular singlet-singlet EET processes have been examined by means of the steady-state and time-resolved spectroscopic techniques. The steady-state fluorescence comes only from the acceptor moiety in Z1A-Z12A, indicating nearly the quantitative EET. In Z24A that has a molecular length of ca. 217 A, the fluorescence comes largely from the acceptor moiety but partly from the long donor array, indicating that the intramolecular EET is not quantitative. The transient absorption spectroscopy has provided the EET rates in real time scale: (2.5 ps)(-1) for Z1A, (3.3 ps)(-1) for Z2A, (5.5 ps)(-1) for Z3A, (21 ps)(-1) for Z6A, (63 ps)(-1) for Z12A, and (108 ps)(-1) for Z24A. These results have been well explained by a revised Förster equation (Sumi formula), which takes into account an exciton extending coherently over several porphyrin pigments in the donor array, whose length is not much shorter than the average donor-acceptor distance. Advantages of such strongly coupled porphyrin arrays in light harvesting and transmission are emphasized in terms of fast EET and a large absorption cross-section for incident light.

Molecular porphyrin­fullerene architectures

This article describes the utilization of porphyrin and fullerene architectures—yielding artificial light-harvesting antenna and reaction center mimics—to tune the electronic coupling element between electron donor and electron acceptor and ultimately to affect the total reorganization energy. Most importantly, the effects that these parameters have on the rate, yield, and lifetime of the energetic charge-separated states are discussed.

Fullerene-porphyrin architectures; photosynthetic antenna and reaction center models.

Fullerenes and porphyrins are molecular architectures ideally suited for devising integrated, multicomponent model systems to transmit and process solar energy. Implementation of C60 as a 3-dimensional electron acceptor holds great expectations on account of their small reorganization energy in electron transfer reactions and has exerted a noteworthy impact on the improvement of light-induced charge-separation. This article describes how the specific compositions of porphyrin chromophores linked to C60--yielding artificial light harvesting antenna and reaction center mimics--have been elegantly utilized to tune the electronic couplings between donor and acceptor sites and the total reorganization energy. Specifically, the effects that these parameters have on the rate, yield and lifetime of the energetic charge-separated states are considered.

Adapting the Förster Theory of Energy Transfer for Modeling Dynamics in Aggregated Molecular Assemblies

The remarkable efficiencies of solar energy conversion attained by photosynthetic organisms derive partly from the designs of the light-harvesting apparatuses. The strategy employed by nature is to capture sunlight over a wide spectral and spatial cross section in chromophore arrays, then funnel the energy to a trap (reaction center). Nature's blueprint has inspired the conception of a diversity of artificial light-harvesting antenna systems for applications in solar energy conversion or photonics. Despite numerous, wide-ranging studies, truly quantitative predictions for such multichromophoric assemblies are scarce because Forster theory in its standard form often seems to fail. We report here a new framework within which energy transfer in molecular assemblies can be modeled quantitatively using a generalization of Forster's theory. Our results show that the principles involved in optimization of energy transfer in confined molecular assemblies are not revealed in a simple way by the absorption and emis...

Mimicking photosynthetic solar energy transduction.

Increased understanding of photosynthetic energy conversion and advances in chemical synthesis and instrumentation have made it possible to create artificial nanoscale devices and semibiological hybrids that carry out many of the functions of the natural process. Artificial light-harvesting antennas can be synthesized and linked to artificial reaction centers that convert excitation energy to chemical potential in the form of long-lived charge separation. Artificial reaction centers can form the basis for molecular-level optoelectronic devices. In addition, they may be incorporated into the lipid bilayer membranes of artificial vesicles, where they function as components of light-driven proton pumps that generate transmembrane proton motive force. The proton gradient may be used to synthesize adenosine triphosphate via an ATP synthase enzyme. The overall energy transduction process in the liposomal system mimics the solar energy conversion system of a photosynthetic bacterium. The results of this research illustrate the advantages of designing functional nanoscale devices based on biological paradigms.

Excitonic funneling in extended dendrimers with nonlinear and random potentials

The mean first passage time (MFPT) for photoexcitations diffusion in a funneling potential of artificial treelike light-harvesting antennas (phenylacetylene dendrimers with generation-dependent segment lengths) is computed. Effects of the nonlinearity of the realistic funneling potential and slow random solvent fluctuations considerably slow down the center-bound diffusion beyond a temperature-dependent optimal size. Diffusion on a disordered Cayley tree with a linear potential is investigated analytically. At low temperatures we predict a phase in which the MFPT is dominated by a few paths.

Artificial Light-Harvesting Antennae: Singlet Excitation Energy Transfer from Zinc Chlorin Aggregate to Bacteriochlorin in Homogeneous Hexane Solution

Abstract— Zinc chlorins possessing 31-hydroxyl and 131-carbonyl groups self-assemble in nonpolar solvents, such as hexane, in a manner similar to bacteriochlorophyll c in the chlorosomes of green photosynthetic bacteria. Visible absorption and steady-state fluorescence measurements of zinc chlorin aggregates containing a small amount of the bacteriochlorin-zinc chlorin dyad molecules showed that singlet excitation energy transfer from the zinc chlorin aggregate to the bacteriochlorin moiety of the coaggre-gated dyad occurs in the homogeneous solution. In the coaggregated dyad, the bacteriochlorin moiety plays the role of an efficient energy trap and the chlorin moiety the role of an anchor to the donor aggregate. The artificial assembly thus mimics the structure and function of natural chlorosomes and can be considered as the first in vitro supramolecular light-harvesting antenna.

Localized Electronic Excitations in Phenylacetylene Dendrimers

Electron−hole pairs created upon optical excitation of conjugated dendrimers (fractal antenna macromolecules) are shown to be localized within segments connected by benzene rings substituted at the meta-position. The absorption spectra of two families of dendrimers are analyzed using collective electronic normal modes representing the changes in charge and bond-order distributions induced by the optical field. The present approach may be used in the design of artificial light-harvesting antennae with controlled energy-funneling pathways.