Fluorescence lifetimes (τf) of bacteriochlorophyll a (BChl a) have been measured by the method of time‐correlated single‐photon counting on dilute (1 μM) solutions of the pigment in 15 solvents. There is a pronounced dependence of τf on the nature of the solvent. Specifically, τf, is longer when the central magnesium is hexacoordinated than when pentacoordinated and shorter when the macrocycle is hydrogen‐bonded than when it is not, but the latter effect is more pronounced. Both trends were confirmed by parallel studies on bacteriopheophytin a (BPheo a). Because of the short lifetimes (˜ 2.2–3.6 ns), quenching of fluorescence by molecular oxygen is not a significant factor in aerated solutions of the bacterial pigments. However, reabsorption artifacts are non‐negligible, which necessitates studies on dilute solutions. Fluorescence quantum yields (øf) have been estimated for BChl a in 13 solvents by comparing the observed fluorescence lifetimes with the radiative lifetimes calculated from the integrated absorption spectra.
Fluorescence lifetimes (TO of chlorophyll a (Chi a ) have been measured by the single-photoncounting technique over a wide range of concentrations ( -10-7--10-4 M ) in deoxygenated pyridine, diethyl ether. toluene and methanol. At pigment concentrations > 1 pcM, reabsorption of fluorescence induces significant artifacts on measured values of T~ which are dependent on detection wavelength and the specific geometry of the experiment. There is a clear dependence of T( on the nature and degree of solvation, including both coordination of the central magnesium and hydrogen-bonding of the solvent (vir. alcohols) to the macrocycle. Quenching of the excited singlet state by molecular oxygen was measured quantitatively in ether. and a bimolecular rate constant markedly slower than the diffusioncontrolled limit was obtained.
Abstract— Reaction center (RC) complexes isolated from the photosynthetic bacterium Rhodopseudomonas sphaeroides R‐26 were dried as a film onto platinum and semiconductor (SnO2) electrodes. The light‐induced primary charge separation which occurs across the biological complex couples electrically with the SnO2 but not with the metal electrode on the time scale of observation. As the working electrode in a two‐electrode photoelectrochemical cell, RC‐coated SnO2 generated photovoltages as high as 80 mV and photocurrents as high as 0.5µA·cm2 when exposed to light of λ >600nm. The number of quinone molecules per RC strongly influences the photovoltage and photocurrent observed. Photo‐effects generated by RC electrodes persist after several days of storage; however, the kinetics and polarity of the effects are subject to change. The potential use of RC electrodes lies more as a new probe of photosynthetic electron transport rather than as a solar energy conversion device because modification to the RCs and their environment affect the electrical properties of the cell. An energy‐level model is proposed to explain how the photoelectrochemical cell functions.
A photoelectric effect has been observed in monolayer assemblies of chlorophyll and acceptor molecules deposited on a semitransparent aluminum electrode. The counterelectrode was mercury. With acceptors containing saturated side chains an open-circuit photovoltage of 150 mV and a short-circuit photocurrent of 34 nA cm-2 were detected across a load resistor. The quantum yield of electrons per photon was -2 X The current-voltage behavior was rectifying with a dark conductivity of -1.5 X 9-' cm-l. If acceptors with unsaturated side chains were used such as ubiquinone or plastoquinone ( I O mol % in stearic acid), the conductivity improved by a factor of IO. The open-circuit photovoltage now was 270 mV and the short-circuit photocurrent detected was 140 nA cm-2. The quantum yield was -2 X The reduction of the quinone group left the photoresponse unchanged. Similarly, the use of the polyisopropene squalene instead of plastoquinone left the photoresponse unchanged. The suggestion is made that the polyisopropene chain may be acting as "nature's molecular wire" making possible the tunnelling of electrons through lipid membranes and the maximum power conversion efficiency was -4 X and the maximum power conversion efficiency was -4 X
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