Quantum Efficiency Of Photochemistry Research Articles

Methyl jasmonate is a more effective senescence-promoting factor in Cucurbita pepo (zucchini) cotyledons when compared with darkness at the early stage of senescence

The effects of short-term darkening and methyl jasmonate (MeJA) on cotyledon senescence were studied 24h after transfer of intact 7-day-old Cucurbita pepo (zucchini) seedlings to darkness or spraying with 100 microM MeJA. The jasmonate inhibitory effect on chlorophyll content and chloroplast transcriptional activity was stronger compared with darkness. Further, MeJA reduced the photosynthetic rate whereas darkness did not affect photosynthesis. Neither stress factor affected the photochemical quantum efficiency of photosystem II (PSII) estimated by the variable fluorescence (F(v))/maximal fluorescence (F(m)) ratio, suggesting the existence of mechanisms protecting the functional activity of PSII at earlier stages of senescence, thus making this parameter more stable compared to others used to quantify senescence. Both stress factors caused a decrease in the content of physiologically active cytokinins, especially trans-zeatin (Z), with the jasmonate effect being much more pronounced when compared to darkness. Our results indicate that MeJA is a more potent inducer of senescence in zucchini cotyledons, at least within the relatively short period of the 24h treatment. This is likely due to its stronger down-regulatory effect on the levels of physiologically active cytokinins.

Physiological state of phytoplankton communities in the Southwest Atlantic sector of the Southern Ocean, as measured by fast repetition rate fluorometry

The majority of the Southern Ocean is a high-nutrient low-chlorophyll (HNLC) ecosystem. Localized increases in chlorophyll concentration measured in the wake of bathymetric features near South Georgia demonstrate variations in the factors governing the HNLC condition. We explore the possibility that the contrast between these areas of high-chlorophyll and surrounding HNLC areas is associated with variations in phytoplankton photophysiology. Total dissolvable iron concentrations, phytoplankton photophysiology and community structure were investigated in late April 2003 on a transect along the North Scotia Ridge (53–54°S) between the Falkland Islands and South Georgia (58–33°W). Total dissolvable iron concentrations suggested a benthic source of iron near South Georgia. Bulk community measurements of dark-adapted photochemical quantum efficiency (F v/F m) exhibited a sharp increase to the east of 46°W coincident with a decrease in the functional absorption cross-section (σPSII). Phytoplankton populations east of 46°W thus displayed no physiological symptoms of iron or nitrate stress. Contrasting low F v/F m west of 46°W could not be explained by variations in the macronutrients nitrate and silicic acid and may be the result of taxon specific variability in photophysiology or iron stress. We hypothesize that increased F v/F m resulted from local relief from iron-stress near South Georgia, east of Aurora Bank, an area previously speculated to be a “pulse point” source of iron. Our measurements provide one of the first direct physiological confirmations that iron stress is alleviated in phytoplankton populations near South Georgia.

Photosynthetic responses of tropical tree species from different successional groups under contrasting irradiance conditions

This study evaluated the photosynthetic responses of seven tropical trees of different successional groups under contrasting irradiance conditions, taking into account changes in gas exchange and chlorophyll a fluorescence. Although early successional species have shown higher values of CO2 assimilation (A) and transpiration (E), there was not a defined pattern of the daily gas exchange responses to high irradiance (FSL) among evaluated species. Cariniana legalis (Mart.) Kuntze (late secondary) and Astronium graveolens Jacq. (early secondary) exhibited larger reductions in daily-integrated CO2 assimilation (DIA) when transferred from medium light (ML) to FSL. On the other hand, the pioneer species Guazuma ulmifolia Lam. had significant DIA increase when exposed to FSL. The pioneers Croton spp. trended to show a DIA decrease around 19%, while Cytharexyllum myrianthum Cham. (pioneer) and Rhamnidium elaeocarpum Reiss. (early secondary) trended to increase DIA when transferred to FSL. Under this condition, all species showed dynamic photoinhibition, except for C. legalis that presented chronic photoinhibition of photosynthesis. Considering daily photosynthetic processes, our results supported the hypothesis of more flexible responses of early successional species (pioneer and early secondary species). The principal component analysis indicated that the photochemical parameters effective quantum efficiency of photosystem II and apparent electron transport rate were more suitable to separate the successional groups under ML condition, whereas A and E play a major role to this task under FSL condition.

Dynamics of trace metals during the subarctic Pacific iron experiment for ecosystem dynamics study (SEEDS2001)

During the first iron-enrichment experiment in the northwest subarctic Pacific (SEEDS2001), the dynamics of dissolved and acid dissolvable trace metals were studied. Seawater samples were collected from the upper water column (5–70 m) of In- and Out-patch stations. Immediately after the collection, an aliquot of seawater for dissolved species was filtered through an 0.2 μm filter and acidified to pH 2.2. An aliquot for acid dissolvable species was acidified without filtration and stored for 3 years at an ambient temperature. Before the Fe enrichment, the dissolved Fe concentration in the surface mixed layer was <0.13 nM (the detection limit) and the acid dissolvable Fe concentration was 4.7 nM. The difference was a particulate fraction. Since the photochemical quantum efficiency of algal photosystem II was low, Fe in this fraction was not easily available to phytoplankton. After the Fe enrichment, acid dissolvable Fe in the patch decreased from 9.6 nM on day 2 to 4.6 nM on day 13, which was still ∼3 nM higher than that at the Out-patch station. Dissolved Fe was 1.4 nM on day 2 and decreased exponentially to <0.13 nM on day 11. The concentration ratio of the acid dissolvable fraction to the dissolved fraction for the other trace metals was lower than that for Fe. The acid dissolvable concentrations for Mn, Co, Ni, Cu, Zn and Cd did not show significant change during the observation. The dissolved concentrations for Co, Ni, Cu, Zn and Cd in the surface layer on day 2 were 0.039, 5.0, 1.7, 2.2 and 0.27 nM, respectively. They decreased exponentially to 0.013, 4.3, 1.2, 0.87 and 0.21 nM on day 13, respectively. The mole ratio of the concentration difference between days 2 and 13 was similar to the elemental ratio reported for phytoplankton. These are the first data showing that mesoscale iron fertilization alters the dynamics of dissolved Co, Ni, Cu and Zn.

Basin-scale variability of phytoplankton bio-optical characteristics in relation to bloom state and community structure in the Northeast Atlantic

Phytoplankton physiological data collected throughout the Iceland Basin and Rockall Trough during the North Atlantic spring bloom from May to June 2001 are presented. Physiological parameters including the maximum photochemical quantum efficiency ( F v /F m) and the functional absorption cross section of photosystem II ( σ PSII) were measured using fast repetition rate fluorometry. Information on the taxonomic and pigment characteristics of the phytoplankton populations was also collected, with pigment data being used to reconstruct absorption spectra. Significant changes in the physiological properties of PSII were found to be associated with the progression of the spring bloom from diatom to flagellate domination. Changes in both community composition and physiology were in turn correlated with environmental parameters. Lower F v /F m, higher σ PSII and corresponding decreases in cell size were associated with the observed decrease of nutrients that accompanied increasing stratification. Differences in σ PSII were primarily associated with the changing pigment composition of the phytoplankton populations, with the largest changes appearing to be governed by the amount of absorption by photosynthetic carotenoids. The physiological state of PSII was thus found to be an indicator of bloom status and community structure in this productive temperate region principally as a result of taxon specific variability.

Physiological responses of three species of marine pico-phytoplankton to ammonium, phosphate, iron and light limitation

Experiments were conducted with three species of marine pico-phytoplankton: Synechococcus sp. (CCMP 839), Pelagomonas calceolata (CCMP 1756) and Prasinomonas capsulatus (CCMP 1617) in order to collect physiological parameters for pico-phytoplankton to be utilised in Ocean Biogeochemical Climate Models. The main parameters to follow the effects of ammonium, phosphate, iron and light limitation were cell growth rates (μ), half saturation constants for growth (K m), N, P and Fe quota (per cell or per mol C), and photochemical quantum efficiency (F v/F m). The nitrate and phosphate limitation experiments demonstrated that the small phytoplankton species could grow at low N and P concentrations. K m values were in the micro-molar (NH 4 +) and sub-micro-molar (PO 4 3−) range. N and P quota were in the femto-molar range per cell and varied from nutrient-deplete to nutrient-replete conditions. F v/F m values were only adversely affected at the lowest N and P concentrations in these experiments. In the Fe limitation experiments, it was shown that all three species were adversely affected only at extremely low Fe concentrations. Iron chelating agents had to be added to force the species in Fe limitation till ultimately growth stopped. K m values with respect to dissolved Fe were in the femto-molar range. Fe quota were in the low zepto-molar (10 −21 M) range per cell, and varied considerably from Fe limiting to Fe replete growth conditions. F v/F m values diminished only at the lowest iron concentrations. In the light limitation experiments, growth rates and photochemical quantum efficiencies were adversely affected only at irradiance levels below 10 μmol photons m −2 s −1. These results indicate that the pico-phytoplankton species will hardly ever be completely stopped in their growth by NH 4 +, PO 4 3−, Fe or light (separately) under natural conditions.

Quantitative Photochemistry of Cp‘Pt(CH3)3 (Cp‘ = η5-C5H4CH3) in Solution: A Highly Efficient Organometallic Photoinitiator for Hydrosilylation

The quantitative photochemistry of the platinum organometallic complex, Cp'Pt(CH(3))(3) (Cp' = eta(5)-C(5)H(4)CH(3)), has been investigated in methylcyclohexane and n-pentane solutions at 293 K following UV irradiation into the lowest lying absorption bands. Absolute photochemical quantum efficiency (phi(cr)) results obtained for Cp'Pt(CH(3))(3) at 313 and 366 nm reveal that the system very effectively photoreacts with phi(cr) = 0.34-0.41 in these solvents and 0.79-0.85 when 53 mM Et(3)SiH is incorporated. These photoefficiencies indicate that the quantitative photochemistry is 2 orders of magnitude higher than previously recognized. The application of Cp'Pt(CH(3))(3) as an effective photoinitiator for hydrosilylation reactions involving vinyl/hydride silicone mixtures is demonstrated.

Variations in leaf pigment content and photosynthetic activity of Phragmites australis in healthy and die-back reed stands of Lake Fertõ/Neusiedlersee

The photosynthetic capacity of the common reed (Phragmites australis /Cav./ Trin. ex Steudel) was studied in various reed stands in the littoral zone of Lake Fert6. Measurements were performed in three healthy and two die­ back reed stands in the summer of 1997. In the leaves of declining reeds, the chlorophyll content was lower than in the vigorous sites. In the former sites, there was a significant rise in the total carotenoid pool (320- 480 mmol mol-1chl (a + b)) as compared to that of the vigorous sites (250- 350 mmol mol- 1 chi (a + b). The size of the xanthophyll cycle pool and the ,B-carotene content of leaves significantly increased in the die-back sites. In early summer, the potential photochemical quantum efficiency of Photosystem II (Fv/Fm) did not differ considerably (0. 79-0.81) from site to site, yet by August it significantly decreased (0.74-0.77) in the die-back sites as compared to the vigorous sites. The maximum C02 assimilation rate measured on the 3rd and 4th leaves ranged from 11 to 17 C02f..i.mol m-2 s-1 and from 9 to 12 C02f..i.mol m-2 s-1 in the vigorous sites and the die-back sites, respectively. The stomatal conductance was also lower in the die-back sites (200-350 mmol H20 m-2 s-1) than in the vigorous reed stands (380-510 mmol H20 m-2s- 1) which might result in the functional impairment of the gas ventilation system of the declining reeds, and consequently in oxygen deficiency and damage to the rhizome.

Co-limitation by iron and light of Chaetoceros brevis, C. dichaeta and C. calcitrans (Bacillariophyceae)

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 217:287-297 (2001) - doi:10.3354/meps217287 Co-limitation by iron and light of Chaetoceros brevis, C. dichaeta and C. calcitrans (Bacillariophyceae) Klaas R. Timmermans1,*, Margaret S. Davey2, Bas van der Wagt1, Josje Snoek1, Richard J. Geider2,**, Marcel J. W. Veldhuis1, Loes J. A. Gerringa1, Hein J. W. de Baar1 1Netherlands Institute for Sea Research, Department of Marine Chemistry and Geology, PO Box 59, 1790 AB Den Burg, Texel, The Netherlands 2Marine Biological Association, The Laboratory, Citadel Hill, Plymouth PL1 2PB, United Kingdom *E-mail: klaas@nioz.nl **Present address: Department of Biological Sciences, University of Essex, Colchester CO4 3SQ, United Kingdom ABSTRACT: The interaction between iron and light limitation was investigated in 3 marine diatom species: 2 Antarctic diatom isolates, Chaetoceros brevis and C. dichaeta, and an isolate from temperate waters, C. calcitrans. In C. calcitrans and C. brevis, grown in the laboratory using trace metal buffered medium, both iron and light limitation affected growth rates, cellular chlorophyll a fluorescence, spinal index (i.e., the number and size of spines) and cell size. Growth rates and cell size declined at lower iron concentrations and lower light intensities. Cellular chlorophyll a fluorescence increased with lower light, but decreased due to iron limitation. The spinal index, based on the ratio of side scatter to forward scatter, increased in iron- and light-limited cells. The large diatom C. dichaeta, grown in natural Southern Ocean water (without ethelenediaminetetraacetic acid [EDTA]) showed qualitatively similar responses to co-limitation by iron and light. C. dichaeta only grew under long-day light conditions. This response was further modified by the availability of iron. Addition of iron resulted in higher growth rates. In contrast, ambient iron concentrations did not limit the growth rate of the small Antarctic diatom C. brevis. However, iron limitation could be induced by addition of the natural iron binding ligand desferrioxamine B (DFOB). Addition of iron to Fe-depleted cultures of C. dichaeta and C. brevis reversed the effects of DFOB, as evidenced by rapid increases (within 24 h) in photochemical quantum efficiency (Fv/Fm) and decreases in effective absorption of the cross-section of photosystem II (σPSII), the turnover time of the photosynthetic unit (τ) and the electron transfer rate (1/ τ), followed by an increase in growth rates after 48 h. The interactions between iron and light in Antarctic diatoms are sufficient to explain the observation that in the Southern Ocean some species (i.e., the small C. brevis) thrive under low iron and low light conditions, whereas other species (i.e., the large C. dichaeta) can bloom only under conditions of relatively high iron concentrations and favorable high light intensities or a long-day light period. These differences in physiological responses will have consequences for primary production, the carbon cycle and biogeochemical cycles. KEY WORDS: Marine diatoms · Iron · Light · Co-limitation · Growth rates · Fluorescence · Cell size · Chaetoceros calcitrans · C. brevis · C. dichaeta Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 217. Online publication date: July 31, 2001 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2001 Inter-Research.

Quantitative wavelength-dependent photochemistry of the [CpFe(eta 6-ipb)]PF6 (ipb = isopropylbenzene) photoinitiator.

The photochemically induced arene dissociation reaction of the widely used cationic photoinitiator complex [CpFe-(eta 6-isopropylbenzene)]PF6 has quantitatively been investigated in several different solvents at 293 K as a function of excitation wavelength at 355, 458, 488, 514, 633, and 683 nm. The complex was excited into the lowest-lying singlet ligand field manifold (355-514 nm) and directly into the corresponding lowest-lying triplet ligand field state (633, 683 nm). Absolute photochemical quantum efficiency (phi cr) results reveal that the system exhibits a strong excitation wavelength dependence in each investigated solvent and that the reaction is extremely efficient in the UV and visible regions. The wavelength dependence also reveals that the photochemistry does not occur solely from the lowest-lying ligand field triplet excited state. New insights in terms of both photophysical and mechanistic aspects of this system are obtained from the quantitative photochemical results.

Primary productivity off the coast of East Antarctica (80–150°E): January to March 1996

During February and March 1996, an interdisciplinary research project was undertaken off the coast of East Antarctica, south and southwest of Australia, from approximately 63 to 66.25°S and 80 to 150°E. Coastal, continental shelf and open ocean waters were sampled, encompassing the Antarctic Slope Front (ASF) and the Antarctic Divergence (AD). Sea ice coverage during the previous winter exhibited a gradual decline in northwards extent from 80°E, where its edge was located at 57°S, to 150°E, where it reached 62–63°S. Productivity versus irradiance ( P– I) experiments were conducted to calculate primary productivity, and measurements of the photochemical quantum efficiency of photosynthesis ( F v/ F m) were made using a fast repetition rate (FRR) fluorometer. Phytoplankton pigment (HPLC) and nutrient (nitrate, phosphate and silicate) concentrations also were obtained. Mean chlorophyll a concentrations in the mixed layer were highest (>1 μg l −1) on the continental shelf along 93.5°E and in the shelf and shelf-break waters of a meridional transect along 120°E. Integrated production over the mixed layer ranged from greater than 700 mg C m −2 d −1 slightly north of the AD along 80°E, to less than 50 mg C m −2 d −1 in open ocean water at the northeastern corner of the survey area. The elevated biomass and productivity west of approximately 120°E was attributed to macro- and micro-nutrient release and upper water column stratification following the spring/summer ice melt that preceded and coincided with the sampling period. Macronutrient concentrations were not found to be limiting, and in the upper 75 m of the water column ranged between 20.8 and 32.7 μM for nitrate, from 0.75 to 2.3 μM for phosphate, and from 22.6 to 74.9 μM for silicate. Nutrient uptake rates, inferred from depletion in the mixed layer, indicated that silicate uptake was uncoupled from that of nitrate and phosphate in the northeastern region of the survey area. The mixed layer deepened slightly and critical depths shoaled by a factor of three as the survey moved eastwards from 80 to 150°E, a pattern most likely due to the onset of winter. Daily irradiance also decreased between January and March, and manipulations of the light field used in the production modelling of the eastern transects resulted in productivity estimates similar to the western transects. These observations, together with the analysis of the FRR data, suggest that the phytoplankton community east of approximately 120°E may have been co-limited by iron concentrations and incident irradiance. The lower biomass and productivity in the east also coincided with greater southwards extension of warmer oceanic waters, and a shift from a krill/diatom to salp/picoplankton biological community.

Biological response to iron fertilization in the eastern equatorial Pacific (IronEx II). I. Microplankton community abundances and biomass

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 201:27-42 (2000) - doi:10.3354/meps201027 Biological response to iron fertilization in the eastern equatorial Pacific (IronEx II). I. Microplankton community abundances and biomass M. R. Landry1,*, M. E. Ondrusek2, S. J. Tanner3, S. L. Brown1, J. Constantinou1, R. R. Bidigare1,4, K. H. Coale3, S. Fitzwater5 1Department of Oceanography, University of Hawai¹i at Manoa, Honolulu, Hawai¹i 96822, USA 2DOC/NOAA/NESDIS, E/RA30, 5200 Auth Rd. Camp Springs, Maryland 20746, USA 3Moss Landing Marine Laboratories, PO Box 450, Moss Landing, California 96039, USA 4Hawai¹i Institute of Marine Biology, PO Box 1346, Kaneohe, Hawai¹i 96744, USA 5MBARI, PO Box 628, Moss Landing, California 95039, USA *E-mail: landry@soest.hawaii.edu ABSTRACT: During the IronEx II experiment in the eastern equatorial Pacific (May to June 1995), the response of the microplankton community to mesoscale iron fertilization was followed using a combination of marker-pigment, microscopical and flow cytometric techniques. Phytoplankton standing stock bloomed dramatically over a period of 6 d following 3 iron additions of 2 and 1 nM, respectively. Carbon biomass in the patch increased by a factor of 4, chlorophyll a by about a factor of 16 and diatoms by >70 fold relative to contemporaneous levels in the ambient community. The bloom then plateaued sharply and remained at a more or less constant level for 4 d, despite the addition of more iron (1 nM) and physiological indices (low C:chl a ratio and elevated photochemical quantum efficiency) suggesting that the cells were healthy and growing rapidly. Relatively large pennate diatoms (Nitzschia spp., median length 20 to 24 µm) dominated the patch bloom, with smaller pennate species and centric diatoms declining in relative importance. Heterotrophic bacteria increased at a slow rate (0.08 d-1) for >10 d during the experiment, as did heterotrophic nanoflagellates. There were also indications of enhanced cell size, cellular pigment content and possibly growth rates of small phytoplankton. Nonetheless, little difference was observed between the ambient community and the peak patch bloom with respect to the size composition of auto- and heterotrophic populations <10 µm in cell size. The relative constancy of the smaller size fractions, the sharp curtailment of net growth of the bloom after 6 d, and >3-fold increase in large heterotrophic dinoflagellates and ciliates suggest that protistan grazers may have played an active role in controlling the phytoplankton response to increased iron availability. KEY WORDS: Diatom · Bloom · Phytoplankton pigments · Carbon:chlorophyll · Size structure · Heterotrophic protists Full article in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 201. Online publication date: August 09, 2000 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2000 Inter-Research.

Overexpression of Arabidopsis hexokinase in tomato plants inhibits growth, reduces photosynthesis, and induces rapid senescence.

Sugars are key regulatory molecules that affect diverse processes in higher plants. Hexokinase is the first enzyme in hexose metabolism and may be a sugar sensor that mediates sugar regulation. We present evidence that hexokinase is involved in sensing endogenous levels of sugars in photosynthetic tissues and that it participates in the regulation of senescence, photosynthesis, and growth in seedlings as well as in mature plants. Transgenic tomato plants overexpressing the Arabidopsis hexokinase-encoding gene AtHXK1 were produced. Independent transgenic plants carrying single copies of AtHXK1 were characterized by growth inhibition, the degree of which was found to correlate directly to the expression and activity of AtHXK1. Reciprocal grafting experiments suggested that the inhibitory effect occurred when AtHXK1 was expressed in photosynthetic tissues. Accordingly, plants with increased AtHXK1 activity had reduced chlorophyll content in their leaves, reduced photosynthesis rates, and reduced photochemical quantum efficiency of photosystem II reaction centers compared with plants without increased AtHXK1 activity. In addition, the transgenic plants underwent rapid senescence, suggesting that hexokinase is also involved in senescence regulation. Fruit weight, starch content in young fruits, and total soluble solids in mature fruits were also reduced in the transgenic plants. The results indicate that endogenous hexokinase activity is not rate limiting for growth; rather, they support the role of hexokinase as a regulatory enzyme in photosynthetic tissues, in which it regulates photosynthesis, growth, and senescence.

Importance of initial nucleation step on low temperature photoassisted MOVPE growth of ZnSe

In this work, low temperature photoassisted metal-organic vapor phase epitaxy (MOVPE) growth of ZnSe on GaAs was carried out using dimethylzinc (DMZn) as the Zn precursor and dimethylselnium (DMSe) or diethylselnium (DESe) as the Se precursor at atmospheric pressure under H2 carrier gas, using the 458 nm wavelength from an Ar+ laser. In situ laser interferometry showed that the growth rate for thin films is non-linear and was explained in terms of the efficiency of coupling of the photo-irradiation with the growing film. The growth rate (GR) can be described by GR = P η F (1 − exp (−α d)) where η is the photo-chemical quantum efficiency, d is the film thickness, F is a structural factor, P is the laser intensity and α is the absorption coefficient. In situ laser interferometry has been essential in making these measurements to observe the initial non-linearity and ensure that characteristic growth rates are taken from steady state growth. Growth using DMSe as the Se precursor was very photosensitive due to its higher photochemical quantum efficiency arising from the additional source of CH3. radicals. These layers were mostly rough due to the slight variation in laser intensity across a region giving different degree of enhancement which is magnified by the growth rate non-linearity leading to non-uniform, and roughened growth. In the case of growth using DESe, it was possible to grow an initial nucleation layer of ZnSe pyrolyticaly prior to photo-assisted growth which ensured that there was sufficient thickness, d, of ZnSe such that the exponential term in the above equation became negligible and the growth rate was dependent only on the linear terms. The slight variation in laser intensity across the surface does not then cause a very significant variation in growth rate, thus maintaining smooth morphology and uniform growth rate.

Iron(III)-doped Q-sized TiO 2 coatings in a fiber-optic cable photochemical reactor

The photochemical quantum efficiencies and oxidation performance of iron(III)-doped quantum-sized TiO 2 (Fe/Q-TiO 2), Degussa P25 (P25), and hybrid Fe/Q-TiO 2/P25 photocatalytic coatings are investigated using an optical fiber bundled array reactor. Fe/Q-TiO 2 coatings made from hydrosols of varying Fe/Q-TiO 2 content, 5–20 wt.%, a 13 wt.% P25 coating, and a hybrid, layered Fe/Q-TiO 2/P25 (5/13 wt.%) coating are tested. The light absorption efficiencies of the Fe/Q-TiO 2 coatings are inferior to the P25 coating, absorbing a maximum of only 80% of the input light compared with greater than 95% for P25 and 90% for the hybrid coating. The Fe/Q-TiO 2 coatings are found to increase the linear light transmission in a single optical fiber relative to P25 coated fibers by a factor of two owing to a reduced interfacial surface coverage of the photocatalyst particles on the quartz fiber. The hybrid coating does not significantly enhance linear light transmission. Slurry-phase photoefficiencies for the photooxidation of 4-chlorophenol for the Fe/Q-TiO 2 photocatalyst are found to be significantly lower than those measured for P25, φ Fe/Q-TiO2 = 0.002 vs. φ P25 = 0.012. In addition, the length of the coated fiber-bundle used in our reactor is insufficient to capitalize on the increased light transmission for the Fe/Q-TiO 2 coating. Thus, we are unable to investigate the effect of increased light transmission on the photoefficiency of the system. Initial reaction rates for the photooxidation of 4-chlorophenol range from 2.0 to 4.5 μM h −1 generally increasing with increasing hydrosol Fe/Q-TiO 2 content and an average relative quantum efficiency of φ Fe/Q-TiO2 = 0.004 ± 0.001 is observed. These values are significantly low compared with initial rates and relative quantum efficiencies of 18.0 μM h −1, and φ Fe/Q-TiO2/P25 = 0.011 and 20.4 μM h −1 and φ P25 = 0.012 for the hybrid and P25 coatings, respectively.

Radical Cation Probes for Photoinduced Intramolecular Electron Transfer in Metal−Organic Complexes

Two transition metal complexes of the type fac-(bpy)ReI(CO)3(DA)+ (where bpy = 2,2‘-bipyridine and DA is a pyridine ligand that is substituted with a 1,2-diamine electron donor) have been prepared. The 1,2-diamine serves as a “reactive donor ligand” owing to its propensity to undergo rapid C−C bond fragmentation when activated by single electron transfer oxidation. Photoexcitation of the diamine complexes affords a ligand-to-ligand charge transfer (LLCT) state via intramolecular electron transfer quenching of a metal-to-ligand charge transfer (MLCT) state, [(bpy)ReI(CO)3(DA)]+ + hν → [(bpy•-)ReII(CO)3(DA)]+*(MLCT) → [(bpy•-)ReI(CO)3(DA•+)]+*(LLCT). Photochemical product and quantum efficiency studies indicate that the diamine reactive donor ligand undergoes photoinduced C−C bond fragmentation with high efficiency, presumably via the radical cation (DA•+) which is present in the LLCT excited state. Laser flash photolysis allows direct detection of the metal complex based radicals that are formed by C−C bon...

A Photochemical Procedure for Determining Reaction Quantum Efficiencies in Systems with Multicomponent Inner Filter Absorbances

A kinetic procedure for measuring the photochemical quantum efficiencies of reactions in which several components have overlapping absorbances is fully described. This method has been found to be particularly suitable for photochemical determinations where inner filter effects are substantial, such as those typically observed in the ligand substitution and intermolecular C-H/Si-H bond activation mechanisms of metal complexes. The quantitative measurement of the Si-H bond activation photochemistry of (η(5)-C(5)H(5))Rh(CO)(2) in triethylsilane solution is demonstrated in detail, although the procedure has widespread application in photochemistry.

Photochemical C−H Bond Activation Reactivity of (HBPz‘3)Rh(CO)2 (Pz‘ = 3,5-Dimethylpyrazolyl) in Alkane Solutions

The intermolecular C−H bond activation photochemistry of (HBPz‘3)Rh(CO)2 (Pz‘ = 3,5-dimethylpyrazolyl) has been investigated in various alkane solutions at 293 K. Excitations have been performed at 313−458 nm into the lowest energy absorption band of this complex, and the reactions have been monitored throughout photolysis by in situ UV−visible and FTIR spectroscopy. The spectral results reveal that the photochemistry is exceptionally clean and that the reagent complex can be completely converted to the corresponding (HBPz‘3)Rh(CO)(R)H photoproduct at each of the irradiation wavelengths. Absolute photochemical quantum efficiencies (φCH) for these reactions have been determined and illustrate that the intermolecular C−H bond activation process is strongly wavelength dependent. Very effective conversion (φCH = 0.31−0.34) is attained upon near-UV excitation at 313 or 366 nm. In contrast, inefficient conversion (φCH = 0.010−0.011) is observed on visible excitation at 458 nm. The differences in reactivities are interpreted on a photophysical scheme in which the irradiations produce two ligand field (LF) electronically excited states with different primary photoproducts. The thermal chemistry of (HBPz‘3)Rh(CO)2 in room-temperature solution is characterized as involving facile η3 ↔ η2 ligand interconversions. Accordingly, the complex is readily protonated to form [{η2-HBPz‘2(Pz‘H)}Rh(CO)2]BF4, and when PPh3 is added to a solution of (HBPz‘3)Rh(CO)2, the ligand substitution product, (HBPz‘3)Rh(CO)PPh3, is formed immediately. In contrast, there is no evidence of thermal ligand scavenging of (HBPz‘3)Rh(CO)2 by pyridine and upon irradiation in pyridine solutions the photochemistry is dominated by the C−H activation reaction of the hydrocarbon solvent. In triethylsilane solutions the observed spectral changes reveal that intermolecular Si−H bond activation takes place readily on excitation of (HBPz‘3)Rh(CO)2 at 366 nm. The thermal chemistry of the analogous square planar complex (H2BPz2)Rh(CO)2 has been investigated, and the complex has been found to be reactive toward PPh3 and not R−H in room-temperature solution under dark conditions. These experimental observations imply that the long-wavelength (458 nm) photochemistry and thermal chemistry of (HBPz‘3)Rh(CO)2 are associated with a (η2-HBPz‘3)Rh(CO)2 intermediate that is unable to facilitate hydrocarbon C−H bond activation. Importantly, the observed results also suggest that the short-wavelength (313, 366 nm) photochemistry proceeds via an extremely short-lived monocarbonyl (HBPz‘3)Rh(CO) complex that undergoes efficient C−H activation.

Photochemical mechanisms in intermolecular C-H bond activation reactions of organometallic complexes

This article describes observations obtained from detailed investigations of the photochemistry of (η 5-C 5R 5)Rh(CO) 2 (R = H, Me) and (HBPz * 3)Rh(CO) 2 (Pz * = 3,5-dimethylpyrazolyl) complexes in hydrocarbon solutions. Photochemical quantum efficiency data have been recorded for ligand photosubstitution and intermolecular Si-H/C-H bond activation processes following excitation at various wavelengths in the visible and near-UV regions. The results obtained, taken in conjunction with prior photochemical measurements (viz. matrix isolation, flash photolysis and low temperature studies in liquefied noble gases) lead to a proposed mechanistic scheme that explains both the ligand photosubstitution and C-H/Si-H bond activation reactions.

Excited state dynamics in chlorophyll-based antennae: the role of transfer equilibrium

We present computer simulations of excited state dynamics in models of PS I and PS II which are based upon known structural and spectral properties of the antennae. In particular, these models constrain the pigment binding sites to three-dimensional volumes determined from molecular properties of the antenna complexes. The simulations demonstrate that within a 10-30 ps after light absorption, rapid energy transfer among coupled antenna chlorophylls leads to a quasiequilibrium state in which the fraction of the excited state on any antenna chlorophyll, normalized to the total excited state remaining on the model, remains constant with time. We describe this quasiequilibrium state as a "transfer equilibrium" (TE) state because of its dependence on the rates of processes that couple excited state motion and quenching in the antenna as well as on the individual antenna site energies and temperature. The TE state is not a true equilibrium in that loss of the excited state primarily due to photochemistry (but also due to fluorescence, thermal emission, and intersystem crossing) continues once TE is established. Depending on the dynamics of the system, the normalized distribution of excited state at TE may differ substantially from the Boltzmann distribution (the state of the model at infinite time in the absence of any avenues for decay of excited state). The models predict lifetimes, equilibration times, and photochemical yields that are in agreement with experimental data and affirm trap-limited dynamics in both photosystems. The rapid occurrence of TE states implies that any excited state dynamics that depends on antenna structure and excitation wavelength must occur before the TE state is established. We demonstrate that the excited state distribution of the TE state is central to determining the excited state lifetime and quantum efficiency of photochemistry.