B. Crystall

Comparison of the Dl/D2/cytochrome b559 reaction centre complex of photosystem two isolated by two different methods

Photosystem 2 reaction centre complexes prepared either by solubilisation with Triton X‐100 and subsequent exchange into dodecyl maltoside or by a procedure involving a combination of dodecyl maltoside and LiC104, were characterised in terms of chlorophyll a, pheophytin a, β‐carotene and cytochrome b559 content. Time‐resolved chlorophyll fluorescence decay kinetics were measured using both types of complexes. Our data show that the isolated photosystem two reaction centre complex contain, for two pheophytin a molecules, close to six chlorophyll a, two β‐carotene and one cytochrome b559. No major differences were observed in the composition or the kinetic characteristics measured in the samples prepared by the different procedures. Time‐resolved fluorescence measurements indicate that more than 94% of the chlorophyll a in both preparations is coupled to the reaction centre complex.

Chromism and luminescence in regioregular poly(3-dodecylthiophene)

Abstract We report photoluminescence studies of poly(3-dodecylthiophene) (P3DT) in solution. In a good solvent the polymer exhibits luminescence with a high quantum efficiency and a decay time of 500 ps. In a poor solvent the emission is red shifted with a 20-fold reduction in quantum efficiency and a decay profile that is non-monoexponential, but has an average lifetime that is very similar to the good solvent environment. The data indicate a large increase in the natural radiative lifetime from approximately 1 ns in a good solvent to 20 ns in a poor solvent, which implies an emitting state that is different in the two situations. In the poor solvent the spectrum is almost identical to that of the thin film, suggesting that the polymer aggregates in the solution and the emitting species is the same in both environments. The data are consistent with the formation of an excited state that is not localized on a single chain but is delocalized over more than one chain.

Thermodynamic properties of D1/D2/cytochrome b-559 reaction centres investigated by time-resolved fluorescence measurements

Abstract Photosystem II reaction centres have been studied using time-correlated single photon counting to provide time-resolved fluorescence information. Comparative quantum yield measurements suggest that up to 94% of the chlorophyll in these preparations is coupled to the charge-separation pathway. Analysis of time-resolved fluorescence data suggests a free-energy gap between the excited chlorophyll special pair and the primary radical pair of ΔG( P 680 + Ph − - P 680∗) = −0.11 eV at 277 K. Measurements of this free-energy difference between 277 K and 77 K show that between 220 K and 77 K the contributions of entropy and enthalpy are ΔS( P 680 + Ph − - P 680∗) = +3.8 · 10 −4 eV · K −1 and ΔH( P 680 + Ph − - P 680∗) = −5.0 · 10 −3 eV , respectively. Thus the charge transfer reaction is dominated by entropy contributions between 77 K and 220 K, the ratio of entropy to enthalpy at 220 K being 17:1. The relationship between ΔG and temperature is non-linear over the range 220–270 K, which indicates that enthalpy and/or entropy are temperature dependent in this region.

Resolution of a long lived fluorescence component from D1/D2/cytochrome b-559 reaction centres

D1/D2/cytochrome b‐559 reaction centres rapidly degrade under continuous illumination. This is a problem in time‐resolved spectroscopy experiments. A long lived fluorescence component assigned to radical pair recombination has been reported previously, but contributing to less than 2% of the fluorescence. This letter reports that sample stability is significantly improved under anaerobic conditions. Fluorescence lifetime measurements described here assign 44% of the total fluorescence yield to a 36.5 ± 2.5 ns lifetime.

Luminescence efficiency and time dependence in a high electron affinity conjugated polymer

Abstract We report measurements of the efficiency and time dependence of photoluminescence in a high electron affinity cyano-substituted derivative of poly (p-phenylenevinylene). In solution the photoluminescence quantum yield is 0.52 ± 0.05, and the luminescence lifetime is 0.9 ± 0.1 ns. In solid films the luminescence quantum yield is 0.35±0.03 and the luminescence lifetime, of 5.6 ± 0.2 ns, is much longer. These results strongly suggest that the emission from the film is from an inter-chain excitation, and that inter-molecular interactions are an important factor to consider in the design of highly luminescent conjugated polymers.

Energy transfer to low energy chlorophyll species prior to trapping by P700 and subsequent electron transfer

It is found that the two singlet state lifetimes observed in medium sized isolated Photosystem One reaction centres belong to two distinct sets of particles. The nanosecond lifetime is due to PS1 particles in which P700 does not trap excitation energy, and the excitation energy is homogeneously distributed within the antennae of these particles. The spectral features of the picosecond component show that excitation energy in the antenna has become largely concentrated in one or more low energy (red) chlorophyll species within 3.5 ps. Antennae which have become decoupled from P700 also appear to be decoupled from these red “ancillary” chlorophylls, and this suggests that some substructure or level of organisation links them to P700.The rate of quenching of antenna singlet states appears to be independent of the redox state of P700 under the conditions used here, and oxidising P700 does not prevent excitation energy from reaching the red chlorophyll species in the antenna.We find no evidence in the data presented here of a chlorophyll molecule acting as a “metastable” primary acceptor (A0). The lower limit for the detection of such a species in these data is 20% of the optical density of the transient P700 bleach.

The influence of a solid/liquid interface on the fluorescence kinetics of the triphenylmethane dye malachite green

Abstract Time-resolved fluorescence measurements are reported of a triphenylmethane dye (malachite green), adsorbed at and aqueous/quartz interface, studied by evanescent wave excitation. The importance of internal conversion as the dominant deactivation pathway from the first excited singlet state of malachite green is observed to be greatly reduced for molecules at a solid/liquid interface compared to those in low viscosity solvents. Surface coverage dependent decay kinetics are observed, and at the lowest surface coverages studied, the excited singlet state lifetime of the dye is more than two orders of magnitude greater than the value observed for the dye in aqueous solution. Fluorescence decay kinetics are found to be non mono-exponential under all conditions, and the limitations of simple sum-of-exponentials fitting are discussed in light of other models currently available. It is possible to distinguish at least two populations of malachite green adsorbed at the surface.

Quenching of an intramolecular naphthalene-triethylamine exciplex by tetrabutylammonium perchlorate. Evidence for the formation of an emissive complex

Abstract The fluorescence quenching of the intramolecular exciplex formed by 1-(1-naphthyl)-2-N,N-diethylaminoethane (1N2NE) and tetrabutylammonium perchlorate (TBAP), was studied in 1,4-dioxane, tetrahydrofuran (THF) and acetonitrile (ACN). The quenching efficiency depends on the solvent polarity, the order being dioxane > THF > ACN. Using time-resolved fluorescence spectroscopy, the formation of a reversible fluorescent complex between the intramolecular exciplex of 1N2NE and TBAP in THF as solvent was observed. A model for the quenching mechanism of exciplexes by tetralkylammonium salts is discussed. The kinetics parameters for the quenching of 1N2NE by TBAP are also given.

Long-lived primary radical pair state detected by time-resolved fluorescence and absorption spectroscopy in an isolated photosystem two core

A Photosystem two (PS II) core preparation containing the chlorophyll a binding proteins CP 47, CP 43, D1 and D2, and the non-chlorophyll binding cytochrome-b559 and 33 kDA polypeptides, has been isolated from PS II-enriched membranes of peas using the non-ionic detergent heptylthioglucopyranoside and elevated ionic strengths. The primary radical pair state, P680+Pheo-, was studied by time-resolved absorption and fluorescence spectroscopy, under conditions where quinone reduction and water-splitting activities were inhibited. Charge recombination of the primary radical pair in PS II cores was found to have lifetimes of 17.5 ns measured by fluorescence and 21 ns measured by transient decay kinetics under anaerobic conditions. Transient absorption spectroscopy demonstrated that the activity of the particles, based on primary radical pair formation, was in excess of 70% (depending on the choice of kinetic model), while time-resolved fluorescence spectroscopy indicated that the particles were 91% active. These estimates of activity were further supported by steady-state measurements which quantified the amount of photoreducible pheophytin. It is concluded that the PS II core preparation we have isolated is ideal for studying primary radical pair formation and recombination as demonstrated by the correlation of our absorption and fluorescence transient data, which is the first of its kind to be reported in the literature for isolated PS II core complexes from higher plants.

Picosecond time-resolved absorption and emission studies of pyrazolotriazole azomethine dyes

The temperature- and viscosity-dependent photochemistry of three pyrazolotriazole azomethine dyes, which can undergo syn–anti isomerisation, has been studied using ps absorption, single photon counting (SPC) and steady-state emission techniques. Emission intensities are viscosity dependent, with ϕF increasing from ca. 0.005 to ca. 1 in di-n-butylphthalate as the glass-transition temperature is approached. SPC fluorescence decay kinetics are non-monoexponential for all dyes both in rigid organic glasses at 77 K and in solution. Reasonable fits to two exponentials can be obtained with a dominant (ca. 80–90%) decay process of τ≈ 4 ns and a secondary component of τ≈ 1 ns at 77 K in rigid glasses. At 297 K in 96 : 4 glycerol–ethanol the dominant fluorescence decay process (ca. 75%) has τ⩽ 10 ps, and there is a secondary component with τ≈ 60 ps. Picosecond transient absorption studies show three transient states. One is assigned to a fluorescent state because of a stimulated emission band; the lifetime is viscosity dependent with τ 60 ps in glycerol–ethanol (96 : 4) at 4 °C, but only ca. 2 ps at 297 K. A second transient is associated with strong bleaching at the dye absorption maximum, an increase in absorption in the blue spectral region, and τ≈ 6 ps; this is tentatively assigned to a twisted excited singlet state. The third state with τ≈ 3 ps is associated with a weak bleaching at the dye absorption maximum and an absorption increase above ca. 630 nm; this is tentatively assigned to a twisted ground state which relaxes to the syn- and anti-ground-state conformations. A comparison of kinetic and steady-state data suggests partitioning between a highly emitting and zero-emitting state within a few ps of excitation. This is supported by ps absorption studies which show the simultaneous formation of both the fluorescent state and the transient assigned to the twisted excited state within the laser pulse.