Alisdair N. Macpherson

PREPARATION AND PHOTOPHYSICAL STUDIES OF PORPHYRIN‐C60 DYADS

Abstract Porphyrin‐C60 dyads in which the two chromophores are linked by a bicyclic bridge have been synthesized using the Diels‐Alder reaction. The porphyin singlet lifetimes of both the zinc (Pzn‐C60) and free base (P‐C60) dyads, determined by time‐resolved fluorescence measurements, are ≦17 ps in toluene. This substantial quenching is due to singlet‐singlet energy transfer to C60 The lifetime of Pzn‐1C60 is ‐5 ps in toluene, whereas the singlet lifetime of an appropriate C60 model compound is 1.2 ns. This quenching is attributed to electron transfer to yield Pznbull;+‐C60bull;‐. In toluene, P‐1C60 is unquenched; the lack of electron transfer is due to unfavorable thermodynamics. In this solvent, a transient state with an absorption maximum at 700 ran and a lifetime of‐10 μs was detected using transient absorption methods. This state was quenched by oxygen, and is assigned to the C60 triplet. In the more polar benzonitrile, P‐1C60 underoes photoinduced electron transfer to give P•+‐C60bull;‐. The electron transfer rate constant is −2 × 1011 s−1.

Efficient Energy Transfer from the Carotenoid S2 State in a Photosynthetic Light-Harvesting Complex

Previously, the spatial arrangement of the carotenoid and bacteriochlorophyll molecules in the peripheral light-harvesting (LH2) complex from Rhodopseudomonas acidophila strain 10050 has been determined at high resolution. Here, we have time resolved the energy transfer steps that occur between the carotenoids initial excited state and the lowest energy group of bacteriochlorophyll molecules in LH2. These kinetic data, together with the existing structural information, lay the foundation for understanding the detailed mechanisms of energy transfer involved in this fundamental, early reaction in photosynthesis. Remarkably, energy transfer from the rhodopin glucoside S(2) state, which has an intrinsic lifetime of approximately 120 fs, is by far the dominant pathway, with only a minor contribution from the longer-lived S(1) state.

Light-Harvesting Systems in Chlorophyll c-Containing Algae

Our knowledge of the diverse peripheral light-harvesting complexes (LHCs) found in species of algae which contain chlorophyll (Chi) c as an accessory pigment is reviewed. Sequencing of genes encoding intrinsic LHCs with three putative transmembrane helices is proceeding rapidly in all groups, but basic biochemistry, particularly of the LHC components attached to PS I-enriched complexes, is currently neglected. All LHCs appear to have two different environments for Chi c, and the longer wavelength forms (> 640 nm) have their Qy transition in the plane of the membrane, or the long axis of the particle. A limited amount of time-resolved spectroscopic data has been interpreted as excluding Chi c as an intermediate in energy transfer between either phycobilins or carotenoids and Chi a, but this may be premature. All the light-harvesting proteins seem to be encoded by multigene families, and for diatoms and brown algae, many genes have been sequenced. This will allow the role of individual gene products in determining the adaptive responses to environmental variation to be appraised.

Bioinspired energy conversion

Artificial photosynthetic antenna systems have been synthesized based on carotenoid polyenes and polymer-polyenes covalently attached to tetrapyrroles. Absorption of light in the blue/green region of the spectra excites the polyenes to their S2 state, and ultrafast singlet energy transfer to the tetrapyrroles occurs when the chromophores are in partial conjugation. The additional participation of other excited states of the polyene in the energy-transfer process is a requirement for perfect antenna function. Analogs of photosynthetic reaction centers consisting of tetrapyrrole chromophores covalently linked to electron acceptors and donors have been prepared. Excitation of these constructs results in a cascade of energy transfer/electron transfer which, in selected cases, forms a final charge-separated state characterized by a giant dipole moment (>150 D), a quantum yield approaching unity, a significant fraction of the photon energy stored as chemical potential, and a lifetime sufficient for reaction with secondary electron donors and acceptors. A new antenna-reaction center complex is described in which a carotenoid moiety is located in partial conjugation with the tetrapyrrole π-system allowing fast energy transfer (<100 fs) between the chromophores. In this assembly, the energy transduction process can be initiated by light absorbed by the polyene.

High-efficiency Energy Transfer from Carotenoids to a Phthalocyanine in an Artificial Photosynthetic Antenna¶

Abstract Two carotenoid pigments have been linked as axial ligands to the central silicon atom of a phthalocyanine derivative, forming molecular triad 1. Laser flash studies on the femtosecond and picosecond time scales show that both the carotenoid S1 and S2 excited states act as donor states in 1, resulting in highly efficient singlet energy transfer from the carotenoids to the phthalocyanine. Triplet energy transfer in the opposite direction was also observed. In polar solvents efficient electron transfer from a carotenoid to the phthalocyanine excited singlet state yields a charge-separated state that recombines to the ground state of 1.

Enhanced intersystem crossing in donor/acceptor systems based on zinc/iron or free-base/iron porphyrins.

The deactivation pathways of the singlet excited state of a series of zinc or free-base donor porphyrins covalently linked by a bridge to a paramagnetic iron(III) chloride porphyrin acceptor have been studied. These donor-bridge-acceptor systems all share a similar geometry (25 A donor-acceptor center-to-center distance), but the bridges vary in electronic structure. In previously reported investigations of zinc/iron porphyrin systems, the fluorescence quenching of the donor has predominantly been assigned to electron transfer. However, for the porphyrin systems studied in this paper, we show that the dominant deactivation channels are enhanced intersystem crossing and singlet energy transfer. In both series, the intersystem crossing rate (S1-->T1) of the donor moiety is almost doubled in the presence of a paramagnetic high-spin metal-porphyrin acceptor. The significant spectral overlap of the donor fluorescence and acceptor absorption in both series allows for efficient singlet energy transfer (Forster mechanism). Furthermore, the bridging chromophores mediate energy transfer and the enhancement is inversely dependent upon the energy gap between the donor and bridge excited states. Although Marcus theory predicts thermodynamically favorable electron transfer to occur in the systems investigated, the quenching rate constants were found to be independent of solvent polarity, and no charge-separated state could be detected, indicating very small electronic coupling for electron transfer.

A new porphyrin derivative for use as a diene in the Diels-Alder reaction

Abstract The first representative of a new class of porphyrins featuring a tetramethine bridge joining the 2 and 20 positions of the macrocycle has been prepared and found to undergo the Diels-Alder reaction, yielding structures in which functional groups are linked to the porphyrin macrocycle through a rigid, bicyclic bridge.

Kinetics of multistep photoinitiated electron transfer reactions in a molecular triad

Abstract Time-resolved fluorescence and subpicosecond transient absorption experiments have been carried out on a triad molecular device (CPQ) comprising a porphyrin (P) covalently linked to an electron-donating carotenoid pigment (C) and a quinone (Q), which acts as an electron acceptor. Rate constants governing the intramolecular electron transfer processes originating from the excited singlet state of the porphyrin have been determined. Excitation of the porphyrin moiety of the triad in benzonitrile solution results in photoinduced electron transfer to give CP .+ Q .− ( k 1 =2.5 × 10 9 s −1 ) followed by rapid non-photochemical electron transfer to yield C .+  PQ .− ( k 3 ≈ 1 × 10 11 s −1 ). The intermediate CP .+ Q .− , also decays by charge recombination to the ground state with a rate constant k 2 of about 6 × 10 11 s −1 . Nanosecond laser flash experiments were used to measure a quantum yield of 0.13 for the long-lived (370 ns) charge-separated species C .+ PQ .− . Excitation of the carotenoid moiety of the triad results in singlet energy transfer to the attached porphyrin with a quantum yield of about 0.07.

Photoinduced charge separation in a carotenoid-porphyrin-diquinone tetrad: enhancement of quantum yields via control of electronic coupling

Abstract A molecular tetrad consisting of a free-base porphyrin (P) linked to a carotenoid polyene (C) and a diquinone moiety (Q A –Q B ) has been synthesized, and its photochemistry has been investigated using time-resolved techniques. Excitation of the porphyrin moiety of the tetrad in dichloromethane solution is followed by photoinduced electron transfer to yield an initial C-P + -Q − A –Q B state, which is formed with arate constant of 2.3×10 9 s −1 and a quantum yield of 0.87. In chloroform, the rate is 4.1×10 9 s −1 and the quantum yield is 0.94. Transient absorption studies show that this state evolves by subsequent electron transfer pathways to a final C + -P-Q A –Q − B charge-separated state whose lifetime is 7.4 μs in dichloromethane and 740 ns in chloroform. The quantum yield of the final state is 0.49 in dichloromethane and 0.57 in chloroform. The yield of the final state is substantially higher than that in a related, previously-reported tetrad in spite of the fact that the quantum yield of the initial C-P + -Q − A –Q B species is lower. This fact is interpreted in terms of the rates of charge-separation reactions relative to those of charge recombination. It is shown that yields of charge separation in multicomponent molecules may be altered in a predictable fashion using the basic tenets of electron transfer theory.

Ultrafast Energy Transfer from Rhodopin Glucoside in the Light Harvesting Complexes of RPS. Acidophila.

The light-harvesting antenna of purple bacteria are ideal complexes in which to study the energy transfer function of carotenoids. Light-harvesting complexes which accumulate a single type of carotenoid can be readily isolated and the carotenoid can be selectively excited in the blue-green spectral region. Furthermore, nature assembles a variety of purple bacteria antenna complexes, with a range of energy transfer efficiencies, by packing bacteriochlorophylls into spectroscopically distinct ring-like structures in close proximity to a range of carotenoids with different conjugation lengths (1).