High molecular weight hyperbranched polyarylenes were synthesized in high yields by one-pot copolycyclotrimerizations of 2,5-diethynylthiophene (1), 4,4' -biphenyldiyne (2) and 2,7-diethynylfluorenes (3) with l-he...High molecular weight hyperbranched polyarylenes were synthesized in high yields by one-pot copolycyclotrimerizations of 2,5-diethynylthiophene (1), 4,4' -biphenyldiyne (2) and 2,7-diethynylfluorenes (3) with l-heptyne (4) and 1-dodecyne (5) using TaCl5·Ph4Sn catalysts in toluene. The structures and properties of the polymers were characterized and evaluated by 1R, NMR, TGA, UV, fluorescence and optical limiting analyses. All the polymers possess high thermal stability and emit strong blue light upon UV irradiation, whose intensities are higher than that from poly( 1 -phenyl-1 -octyne) ( PPO) , a well-known highly luminescent disubstitut-ed polyacetylene. Little aggregation-induced red shift in the photoluminescence is observed in the thin films of the polymers.展开更多
Recent observations support an emerging paradigm that climate variability dominates nutrient enrichment in costal eco-systems, which can explain seasonal and inter-annual variability of phytoplankton community composi...Recent observations support an emerging paradigm that climate variability dominates nutrient enrichment in costal eco-systems, which can explain seasonal and inter-annual variability of phytoplankton community composition, biomass (Chl-a), and primary production (PP). In this paper, we combined observation and modeling to investigate the regulation of phytoplankton dynamics in Chesapeake Bay. The year we chose is 1996 that has high river runoff and is usually called a 'wet year'. A 3-D physical-biogeochemical model based on ROMS was developed to simulate the seasonal cycle and the regional distributions of phytoplankton biomass and primary production in Chesapeake Bay. Based on the model results, NO3 presents a strong contrast to the river nitrate load during spring and the highest concentration in the bay reaches around 80 mmol Nm-3 . Compared with the normal year, phytoplankton bloom in spring of 1996 appears in lower latitudes with a higher concentration. Quantitative comparison between the modeled and observed seasonal averaged dissolved inorganic nitrogen concentrations shows that the model produces reliable results. The correlation coefficient r2 for all quantities exceeds 0.95, and the skill parameter for the four seasons is all above 0.95.展开更多
The syntheses of three nonlinear reverse saturation absorption compounds-Indanthrone and its two derivatives are discussed. The properties of nonlinear reverse saturable absorption of the compounds were studied hy usi...The syntheses of three nonlinear reverse saturation absorption compounds-Indanthrone and its two derivatives are discussed. The properties of nonlinear reverse saturable absorption of the compounds were studied hy using the Z-scanning technique , and the influences of its conjugated structure on the absorption threshold value and the absorbable light density were discussed based on the reverse saturation absorption principle. The results shows that when the structure＇s conjugation property of Indanthrone and its derivatives becomes more poweoful , its absorption threshold reduees, the light lowest transmittance increases.展开更多
Aims a key idea in plant community ecology is that the identity of the limiting resource shifts from soil nutrients in low productivity sites to light in high productivity sites.This idea,and its implications for plan...Aims a key idea in plant community ecology is that the identity of the limiting resource shifts from soil nutrients in low productivity sites to light in high productivity sites.This idea,and its implications for plant community structure,has been tested many times in artificial productivity gradients(fertilization studies),but whether it applies to natural productivity gradients is unclear.Methods To test whether seedling light limitation would increase across a natural productivity gradient,I conducted a cross-site field experi-ment in southwest michigan,USA.A each of six old fields natu-rally varying in productivity,I exposed transplanted seedlings of big bluestem(Andropogon gerardii)to a light addition(tie-back)treat-ment that increased light availability and measured their biomass after one and two growing seasons.Important Findingsseedlings responded positively to the tie-back treatment,but positive responses did not increase across the natural productivity gradient.These results suggest that although light does limit seedling establishment,the strength of light limitation does not depend on variation in productivity in natural systems.Instead,light limitation interacted with a variety of site differences to determine establishment.although the general principle that light limitation increases with productivity is well established,these results indicate that it may not always occur in natural systems.展开更多
基金Jointly funded by the Hongkong Research Grants Council (HKUST6149/97P,and 6187/99P) and Natural Science Foundation of Hubei Province(2000J024)
文摘High molecular weight hyperbranched polyarylenes were synthesized in high yields by one-pot copolycyclotrimerizations of 2,5-diethynylthiophene (1), 4,4' -biphenyldiyne (2) and 2,7-diethynylfluorenes (3) with l-heptyne (4) and 1-dodecyne (5) using TaCl5·Ph4Sn catalysts in toluene. The structures and properties of the polymers were characterized and evaluated by 1R, NMR, TGA, UV, fluorescence and optical limiting analyses. All the polymers possess high thermal stability and emit strong blue light upon UV irradiation, whose intensities are higher than that from poly( 1 -phenyl-1 -octyne) ( PPO) , a well-known highly luminescent disubstitut-ed polyacetylene. Little aggregation-induced red shift in the photoluminescence is observed in the thin films of the polymers.
基金supported by the National Science Foundation project of M. Li (OCE-082543)
文摘Recent observations support an emerging paradigm that climate variability dominates nutrient enrichment in costal eco-systems, which can explain seasonal and inter-annual variability of phytoplankton community composition, biomass (Chl-a), and primary production (PP). In this paper, we combined observation and modeling to investigate the regulation of phytoplankton dynamics in Chesapeake Bay. The year we chose is 1996 that has high river runoff and is usually called a 'wet year'. A 3-D physical-biogeochemical model based on ROMS was developed to simulate the seasonal cycle and the regional distributions of phytoplankton biomass and primary production in Chesapeake Bay. Based on the model results, NO3 presents a strong contrast to the river nitrate load during spring and the highest concentration in the bay reaches around 80 mmol Nm-3 . Compared with the normal year, phytoplankton bloom in spring of 1996 appears in lower latitudes with a higher concentration. Quantitative comparison between the modeled and observed seasonal averaged dissolved inorganic nitrogen concentrations shows that the model produces reliable results. The correlation coefficient r2 for all quantities exceeds 0.95, and the skill parameter for the four seasons is all above 0.95.
文摘The syntheses of three nonlinear reverse saturation absorption compounds-Indanthrone and its two derivatives are discussed. The properties of nonlinear reverse saturable absorption of the compounds were studied hy using the Z-scanning technique , and the influences of its conjugated structure on the absorption threshold value and the absorbable light density were discussed based on the reverse saturation absorption principle. The results shows that when the structure＇s conjugation property of Indanthrone and its derivatives becomes more poweoful , its absorption threshold reduees, the light lowest transmittance increases.
基金National Science Foundation[0909942]Lauff and Porter awards from the Kellogg Biological Station+2 种基金an A.L.Rogers Endowed Research Scholarship from Michigan State Universitythe NSF Long-Term Ecological Research Program at KBSMSU AgBioResearch.This is KBS contribution 1705.
文摘Aims a key idea in plant community ecology is that the identity of the limiting resource shifts from soil nutrients in low productivity sites to light in high productivity sites.This idea,and its implications for plant community structure,has been tested many times in artificial productivity gradients(fertilization studies),but whether it applies to natural productivity gradients is unclear.Methods To test whether seedling light limitation would increase across a natural productivity gradient,I conducted a cross-site field experi-ment in southwest michigan,USA.A each of six old fields natu-rally varying in productivity,I exposed transplanted seedlings of big bluestem(Andropogon gerardii)to a light addition(tie-back)treat-ment that increased light availability and measured their biomass after one and two growing seasons.Important Findingsseedlings responded positively to the tie-back treatment,but positive responses did not increase across the natural productivity gradient.These results suggest that although light does limit seedling establishment,the strength of light limitation does not depend on variation in productivity in natural systems.Instead,light limitation interacted with a variety of site differences to determine establishment.although the general principle that light limitation increases with productivity is well established,these results indicate that it may not always occur in natural systems.
