Quantum Efficiency Of Photochemistry Research Articles

Solution photochemistry of (.eta.5-C5R5)Rh(CO)2 (R = H, Me) complexes: pathways for photosubstitution and C-H/Si-H bond activation reactions

The detailed ligand photosubstitution chemistry of CpRh(CO){sub 2} and Cp *Rh(CO){sub 2} (Cp = {eta}{sup 5}-C{sub 5}H{sub 5}; Cp* = {eta}{sup 5}-C{sub 5}R{sub 5}) has been investigated in deoxygenated decalin solutions containing a series of phosphine and phosphite (PR{sub 3}) ligands. The solution photoreactions have been monitored by diode-array UV-visible and FTIR spectroscopy following excitation at 313 at 458 nm. Spectral sequences recorded over the time of photolysis reveal that these photoreactions involve clean and complete conversions to the corresponding CpRh(CO)PR{sub 3} and Cp*Rh(CO)PR{sub 3} products. The reactions are attributed to occur from ligand field (LF) excited states. Photochemical quantum efficiencies have been determined and in each system they exhibit a linear relationship with the entering PR{sub 3} ligand photosubstitution reactions proceed via an associative mechanism. A comparison of the quantum efficiency results obtained for the Cp and Cp*Rh(CO){sub 2} has also been studied in triethylsilane (Et{sub 3}SiH) solution to determine the nature of the Si-H bond activation reaction. In these cases the observed quantum efficiency values are independent of Et{sub 3}SiH concentration, illustrating that these reactions proceed via a CO dissociative process. Ligand photosubstitution quantum efficiencies obtained following 313-nm excitation provide further evidence for the participation of the dissociativemore » mechanism arising from a higher energy LF excited state. The experimental results lead to postulated mechanistic scheme that represents both the ligand photosubstitution and photochemical C-H/Si-H bond activation pathways and to a description of the reaction intermediates involved. 33 refs., 8 figs., 1 tab.« less

Tolerance of Poplar ( Populus sp.) to Environmental Stresses II. Photosynthetic Characteristics of Poplar Clones Grown at Low and High Light Intensities

Summary Four poplar clones ( Populus sp., Beaupre, Raspalje, Fritzi Pauley and Robusta) were grown at low and high photon flux densities (PFD) and the photosynthetic characteristics of the leaves were examined and compared using photoacoustic and fluorometric techniques. The growth of all the clones (shoot length and leaf area) was markedly reduced under low PFD conditions. Photoacoustic measurements indicated that growth at low PFD induced a substantial increase in the maximal quantum yield for oxygen evolution. This increased quantum yield was apparently due to an increase in the size of the photosynthetic unit (number of chlorophyll molecules associated with each reaction center) and in the quantum efficiency of photochemistry in PS II, as indicated by measurements of chlorophyll fluorescence induction in the presence of DCMU. The maximal oxygen evolution rate as well as the light saturation level (minimal PFD for quantum yield = 0) were reduced in low-light-grown poplar cuttings. The photochemical quenching of chlorophyll fluorescence (q Q ) was also decreased in shade-adapted plants- an effect which was particularly pronounced at high PFD above 1000 µmol m -2 s -1 . On the other hand, energy-dependent fluorescence quenching q E was much less affected: a noticeable increase in q E was, however, observed at moderate PFD (400–1000 µmol m −2 s −1 ). The same trends were observed in all the clones, although the amplitude of the changes described above varied from one clone to the other, with the Beaupre clone being apparently the least responsive to changes in the light environment. In contrast to leaves of poplar cuttings grown at high PFD which were not able to perform significant state 1-state 2 transitions, state transitions were clearly seen by modulated chlorophyll fluorescence changes in low-light-grown plants, indicating a better regulation and optimalization of the light distribution between the two photosystems in these latter plants.

Estimation of algal standing stock and growth parameters usin in vivo fluorescence

This paper is addressed to the following questions: 'How accurately does in vivo chlorophyll fluorescence of samples treated with Diuron (DCMU), notated FM estimate autotrophic standing stock?' and 'How accurately does the Diuron response, ΔF (= FM - FV), estimate primary production?'. Data are drawn from several cruises in the western Tasman Sea, ranging between latitudes 25�S. and 53�S. The prediction of chlorophyll (�g chl u I-1) from FM (standard chl a fluorescence units) is given by Chl= 2 × 10-3 FM. The standard error is about 6 × 10-4 FM so the 'true' value will usually be within �60% of the estimated mean value. The production parameter, PA(�g Cl-1), as measured by 14C uptake under saturating light conditions, may be predicted for the euphotic zone other than the surface layer by PA = 3 × 10-2ΔF. If mean ΔF values are used the 'true' value will usually lie within �25% of the estimate. A strong diel effect was found in the parameter Φ( =ΔFFM-1), the 'photochemical quantum efficiency', an estimator of relative growth rate. The diel cycle has the form of a major midday minimum, a minor midnight minimum, and pre-dawn and post-sunset maxima. This cycle must be removed before comparisons of Φ of samples taken at different times can be made.

Effects of dehydration on reaction centers from Rhodopseudomonas sphaeroides

Air-dried films of reaction centers from Rhodopseudomonas sphaeroides were found to resemble aqueous suspensions of these reaction centers in their optical and photochemical properties, except that the long wave absorption band of bacteriochlorophyll was shifted from about 860 to 845 nm. The quantum efficiency of the photochemical reaction that produces oxidized bacteriochlorophyll and reduced ubiquinone was > 0.75 in the films, using 800-nm actinic light. Dehydration of the films caused further blue shifts of all the absorption bands between 500 and 900 nm, and some loss (about 20%) of intensity of the long wave band, coupled to a gain of absorbance centered at 795 nm. The latter changes were not accompanied by an increase at 1250 nm, ruling out the oxidation of bacteriochlorophyll as a cause of these changes. Dehydration caused about 50% of the reaction centers to become photochemically inactive. For the component that remained active, the photochemical quantum efficiency was about 0.5 at room temperature, rising to 0.7 or more as the temperature was brought below 250 K. The yield of 900 nm fluorescence was approximately the same in air-dried films as in aqueous suspensions; dehydration of a film raised the fluorescence yield about 3-fold. The yield of this fluorescence in a dehydrated film was independent of temperature (±10%) between 300 and 70 K. The back reaction after illumination, return of electrons from reduced ubiquinone to oxidized bacteriochlorophyll, has kinetics in which a most rapid first-order component is mixed with slower components. The fastest component, identified with the return of electrons from ‘primary’ ubiquinone to oxidized bacteriochlorophyll, was predominant in the materials tested here. Its half time in an aqueous suspension of reaction centers fell from 86 ms at 300 K to 33 ms at 200 K and declined more gradually to 15 ms at 50 K. An air-dried film showed similar behavior. The dramatic change of half-time between 300 and 200 K can be ascribed to one or more phase transitions involving water; in a dehydrated film the half-time of the fastest decay component was about 22 ± 5 ms, independent of the temperature between 300 and 70 K. The original properties of an air-dried film were restored fully, after dehydration, by exposure to either H 2O or 2H 2O vapor. There was no significant difference, in these measurements, between H 2O-restored and 2H 2O-restored films.

Pigment content and molar extinction coefficients of photochemical reaction centers from Rhodopseudomonas spheroides

Reaction center particles isolated from carotenoidless mutant Rhodopseudomonas spheroides were studied with the aim of determining the pigment composition and the molar extinction coefficients. Two independent sets of measurements using a variety of methods show that a sample with A 800 nm = 1.00 contains 20.8 ± 0.8 μM tetrapyrrole and that the ratio of bacteriochlorophyll to bacteriopheophytin is 2:1. Measurements were made of the absorption changes attending the oxidation of cytochrome c coupled to reduction of the photooxidized primary electron donor in reaction centers, using laser flash excitation. The ratio of the absorption change at 865 nm (due to the bleaching of P870) to that at 550 nm (oxidation of cytochrome) was found to be 5.77. These results, combined with other data, yield a pigment composition of 4 bacteriochlorophyll and 2 bacteriopheophytin molecules in a reaction center. Based on this choice, extinction coefficients are determined for the 802- and 865-nm bands: ε 802 nm = 288 (± 14) mM −1 · cm −1 and ε 865 nm = 128 (± 6) mM −1 · cm −1. For reversible bleaching of the 865-nm band, Δε red - ox 865nm = 112 (± 6) mM −1 · cm −1 (referred to the molarity of reaction centers). Earlier reported values of photochemical quantum efficiency are recomputed, and the revised values are shown to be compatible with those obtained from measurements of fluorescence transients.