A series of molecular pentads, each consisting of a porphyrin dyad (P-P) covalently linked to a carotenoid polyene (C) and a diquinone moiety (Q(A)-Q(B)), have been prepared, and the photochemical properties of these molecules have been studied using steady-state and transient absorption and emission spectroscopies. Each of the pentads undergoes photoinduced electron transfer from the C-P-1P-Q(A)-Q(B) singlet state to yield the charge-separated state C-P-P.+-Q(A).--Q(B). Competing with charge recombination of this species are additional electron-transfer reactions operating in series and in parallel which converge on a final C.+-P-P-Q(A)-Q(B).- state. The electron-transfer rate constants and the quantum yields of the various charge-separated species are sensitive functions of the state energies and the electronic coupling between the porphyrin and diquinone moieties. One of the pentads undergoes photoinduced electron transfer to produce the final C.+-P-P-Q(A)-Q(B).- state with a quantum yield of 0.83 and a lifetime of 55 mus. This example of an artificial photosynthetic reaction center preserves about half of the initial excited singlet state energy as chemical potential. Other pentads have charge-separation lifetimes of several hundred microseconds.

The energy transfer and charge separation kinetics of a photosystem I (PS I) core particle of an antenna size of 100 chlorophyll/P700 has been studied by combined fluorescence and transient absorption kinetics with picosecond resolution. This is the first combined picosecond study of transient absorption and fluorescence carried out on a PS I particle and the results are consistent with each other. The data were analyzed by both global lifetime and global target analysis procedures. In fluorescence major lifetime components were found to be 12 and 36 ps. The shorter-lived one shows a negative amplitude at long wavelengths and is attributed to an energy transfer process between pigments in the main antenna Chl pool and a small long-wavelength Chl pool emitting around 720 nm whereas the longer-lived component is assigned to the overall charge separation lifetime. The lifetimes resolved in transient absorption are 7-8 ps, 33 ps, and [unk]1 ns. The shortest-lived one is assigned to energy transfer between the same pigment pools as observed also in fluorescence kinetics, the middle component of 33 ps to the overall charge separation, and the long-lived component to the lifetime of the oxidized primary donor P700(+). The transient absorption data indicate an even faster, but kinetically unresolved energy transfer component in the main Chl pool with a lifetime <3 ps. Several kinetic models were tested on both the fluorescence and the picosecond absorption data by global target analysis procedures. A model where the long-wave pigments are spatially and kinetically connected with the reaction center P700 is favored over a model where P700 is connected more closely with the main Chl pool. Our data show that the charge separation kinetics in these PS I particles is essentially trap limited. The relevance of our data with respect to other time-resolved studies on PS I core particles is discussed, in particular with respect to the nature and function of the long-wave pigments. From the transient absorption data we do not see any evidence for the occurrence of a reduced Chl primary electron acceptor, but we also can not exclude that possibility, provided that reoxidation of that acceptor should occur within a time <40 ps.

Time-resolved and steady-state fluorescence spectra have been determined in intact cells of wild type and selected Photosystem II mutants of the cyanobacterium Synechocystis sp. PCC 6803. Upon excitation of phycobilisome components, in wild type fluorescence lifetimes and spectra are observed that are compatible with energy transfer within the phycobilisome, between the phycobilisome and Photosystem II, and within Photosystem II (PS II). In a mutant carrying a spinach/Synechocystis CP47 hybrid protein (one of the chlorophyll-binding antennae within PS II), energy transfer between phycobilisomes and PS II. In a mutant containing some CP43 (another chlorophyll-binding antenna protein in PS II) but lacking all other major PS II proteins, chlorophyll can still be bound to CP43, but significant long-lived fluorescence from this chlorophyll could not be observed. In a mutant lacking all major PS II components, phycobilisomes displayed a fluorescence yield larger than in wild type, but considerably smaller than observed in isolated systems. The conformation of one or more phycobilins in or near the anchor protein most likely is important in determining the fluorescence yield of phycobilisomes.

A variety of molecular triads and dyads consisting of covalently linked carotenoid (C) and/or porphyrin (P) moieties have been prepared and studied with transient absorption and time-resolved fluorescence techniques. Diporphyrins of the type P(A)-P(B) and C-P(A)-P(B) triads demonstrate interporphyrin singlet-singlet energy transfer with rate constants ranging from 8.1 x 10(8) to 2.3 x 10(10) s-1. The energy-transfer rates are not in accord with those predicted by the Forster dipole-dipole theory, and it is suggested that energy transfer involves a contribution from an electron-exchange mechanism. Interporphyrin photoinitiated electron transfer is observed in molecules possessing sufficient thermodynamic driving force to produce P(A).+-P(B).- and C-P(A).+-P(B).- charge-separated states. The electron-transfer rate constant increases with increasing reaction free energy change for the molecules studied, and rate constants up to 3.5 x 10(9) s-1 and quantum yields up to 0.68 were measured. The carotenodiporphyrin triad systems undergo a subsequent electron-transfer step to give final C.+-P(A)-P(B).- states. These states are rather long lived (tau-almost-equal-to 250 ns), and the overall quantum yields range up to 0.32.

Picosecond time-resolved fluorescence emission spectra were recorded for cells of the cyanobacterium Synechococcus 6301. Fluorescence decay was measured by single-photon timing and kinetic components were resolved by global data analysis. Time-resolved fluoresence decay components were assigned to PS I, PS II and the phycobilisome terminal emitter by comparing spectra for cells with open PS II centres (fluorescence at Fo) with those for cells with closed PS II centres (fluorescence at Fm) for excitation wavelengths of 620 nm and 670 nm. Time-resolved spectra were recorded under these conditions for cells adapted to state 1 or to state 2. The state 2 transition reduced the amplitude of the PS II fluorescence emission by about 60%, with a complementary increase in the apparent amplitude of the emission from the phycobilisome terminal emitter. Similar changes in amplitude were observed for cells at Fo and at Fm. State transitions had no significant effect on the lifetime of PS II fluorescence decay. These results indicate that state transitions alter the extent of energy transfer from the phycobilisome to PS II.

The excited state kinetics of three different allophycocyanin (AP) complexes has been studied by picosecond fluorescence spectroscopy. Both the fluorescence kinetics and the decay-associated fluorescence spectra of the different complexes can be understood on the basis of a structural model for AP which uses (a) an analogy to the known x-ray determined structure of C-phycocyanin, (b) the biochemical analogies of AP and C-phycocyanin, and (c) the biochemical composition of AP-B (AP-681). A model is developed that describes the excited state kinetics as a mixture of internal conversion processes within a coupled exciton pair and energy transfer processes between exciton pairs. We found excited state relaxation times in the range of 13 ps (AP with linker peptide) up to 66 ps (AP-B). The trimeric aggregates AP 660 and AP 665 show one fast relaxation component each, as was expected on the basis of their symmetry properties. The lower symmetry of AP-B (AP-681) gives rise to two fast lifetime components (tau(1) = 23 ps and tau(2) = 66 ps) which are attributed to internal conversion and/or energy transfer between excitonic states formed by the coupling of symmetrically and spectrally nonequivalent chromophores. It is proposed that the internal conversion between exciton states of strongly coupled chromophores fulfills the requirements of the small energy gap limit. Thus, internal conversion rates in the order of tens of picoseconds are feasible. The influence of the interaction of the linker peptide on the properties of the AP trimer are manifested in the fluorescence kinetics. Lack of the linker peptide in AP 660 gives rise to a heterogeneity in the chromophore conformations and chromophore-chromophore interactions.

Photoinhibition and photobleaching as a function of time at high fluence rates were studied using picosecond time-resolved emission spectra (TRES) and fluorescence induction in the cyanobacterium Anabaena variabilis. Photoinhibition and photobleaching can be distinguished using these techniques as phenomena which operate on different parts of the photosynthetic apparatus with very different induction periods. Photoinhibition is a short-term irradiation effect (time scale of hours) under our irradiation conditions and operates on photosystem II reaction centres in agreement with earlier studies. On photoinhibition the minimum fluorescence intensity F0 is increased and the maximum fluorescence Fmax is decreased. Both effects lead to a decrease in the variable fluorescence yield. In contrast, photobleaching is a long-term effect (time scale of days) and operates on the phycobilisome antenna. This finding is supported by experiments with sodium azide, a protective agent against photobleaching. Although sodium azide cannot fully prevent the formation of long-lived fluorescence components (detached phycobiliproteins), large negative amplitudes measured in the sodium-azide-protected samples indicate an efficient energy transfer to the reaction centre which is absent in the control.

A synthetic five-part molecular device has been prepared that uses a multistep electron transfer strategy similar to that of photosynthetic organisms to capture light energy and convert it to chemical potential in the form of long-lived charge separation. It consists of two covalently linked porphyrin moieties, one containing a zinc ion (P Zn ) and the other present as the free base (P). The metallated porphyrin bears a carotenoid polyene (C) and the other a diquinone species (Q A -Q B ). Excitation of the free-base porphyrin in a chloroform solution of the pentad yields an initial charge-separated state, C-P Zn -P ⋅+ -Q A ⋅- -Q B , with a quantum yield of 0.85. Subsequent electron transfer steps lead to a final charge-separated state, C ⋅+ -P Zn -P-Q A -Q B ⋅- , which is formed with an overall quantum yield of 0.83 and has a lifetime of 55 microseconds. Irradiation of the free-base form of the pentad, C-P-P-Q A -Q B , gives a similar charge-separated state with a lower quantum yield (0.15 in dichloromethane), although the lifetime is increased to ∼340 microseconds. The artificial photosynthetic system preserves a significant fraction (∼1.0 electron volt) of the initial excitation energy (1.9 electron volts) in the long-lived, charge-separated state.

Abstract— Tetraarylporphyrins substituted with nitro groups at fipyrrolic positions are potential candidates for electron‐accepting pigments in model systems for photosynthesis. The photophysics of 2‐nitro‐5,10,15,20‐tetra‐p‐tolylporphyrin and its zinc analog have been studied in order to evaluate this potential. The ground state absorption spectrum, the triplet‐triplet absorption spectrum, the fluorescence emission spectrum, and associated photophysical parameters have been determined. The molecules have short singlet lifetimes and anomalous temperature‐ and solvent‐dependent emission spectra which are consistent with the formation of an intramolecular charge transfer state of the type P+ ‐NO2; in which the nitro group is twisted about its bond to the porphyrin, relative to the ground state conformation.

The antenna of photosystem (PS) I in higher plants, green algae, and cyanobacteria contains various chlorophyll (Chl) pigment proteins each with characteristic absorbance and corresponding fluorescence spectra. A pigment called C695 has been held responsible for the low and room temperature fluorescence emission F720 while a pigment C705 is attributed to the low temperature emission band F730. The bulk of the PS I Chl a antenna absorbs at ∼ 678 nm and generally emits with a maximum around 684 nm and sometimes up to 695 nm at room temperature (F685).

Over the past years a vast amount of information has been gathered on the structure of the reaction center and antenna system of the purple bacterium R. viridis. (1,2). However, because of its far red shifted emission maximum (approximately 1030 nm) until recently it was not possible to measure its fluorescence kinetics on a picosecond time scale. We have investigated the energy transfer kinetics of whole cells, chromatophore membranes, photoreceptor units (quantasomes) and reaction centers in a dark adapted state and in the presence of different redox agents, using single photon timing fluorescence techniques.

Abstract— Time resolved emission spectra have been measured of Anabaena variabilis cells which were grown under different light conditions. The spectra of algae photoinhibited with strong white light for 6 h as well as of algae irradiated with blue light are similar to those of the control (weak white light). Cells that were photobleached with strong white light or red light (5 days each) show dramatic changes in their time resolved emission spectra. The contributions of long‐lived components to the time resolved emission spectra are large in photobleached cells. In both the reference sample and in photoinhibited cells the short‐lived components with lifetimes in the picosecond range prevail which indicates efficient energy transfer within the antenna pigments. The results upon photobleaching are discussed in terms of a functional decoupling of the phycobilisome rods from the core while photoinhibition does not influence the pigment composition and the molecular organization of the antenna pigments.

Decay-associated emission spectra of synchronized cultures of Scenedesmusobliquus have been studied at two stages of their life cycle corresponding to the maximum and minimum of photosynthetic capacity. These decay-associated spectra comprise three kinetic components. The two components which are assigned to photosystem II show variations in their relative amplitudes depending on the life cycle of the cells. From the correlations observed in the decay-associated fluorescence spectra on the one hand and the fluorescence induction parameters on the other hand we obtained further evidence that the two photosystem II fluorescence components are directly related to the two fluorescence induction phases. This correlation supports our previous assignment of the two photosystem II fluorescence decay components of about 0.3 ns and about 0.6 ns lifetimes at the F0 level (open photosystem II reaction centres to photosystem II α units and photosystem II β units respectively. The most pronounced difference between cells at the 8th hour of the life cycle and those at the 16th hour consists in the size of the photosystem II β units which are about 30% larger for the latter. In agreement with previous studies it was found that at these two stages the photosystem I units do not differ in size.