Cationic Cyclizations and Rearrangements Promoted by a Heterogeneous Gold Catalyst

A heterogeneous gold catalyst with remarkable activity for promoting the electrophilic reactions of aryl vinyl ketones and aryl dienyl ketones is described. The catalyst is easy to prepare, is robust, and can be recycled. Low loadings are effective for different types of cationic reactions, including Nazarov cyclizations, lactonizations, and [1,2] shifts

Chain Length and Substituent Effects on the Formation of Excimer-Like States in Nanoaggregates of CN-PPV Model Oligomers

The effects of aggregate formation on the photophysical properties of alkoxy and cyano-substituted polyphenylene phenylene vinylene oligomers (CN-PPVs) were studied in bulk solution to better understand the consequences of aggregation for the emission properties of the polymer. Nanoaggregates of oligomers from 5 to 13 repeat units in length were formed using a solvent reprecipitation method. The propensity for these aggregates to exhibit excimer-like emission in solution was found to be a strong function of oligomer chain length and the solvents used in the reprecipitation process. Short-chain oligomers produced nanoaggregates with absorption and fluorescence spectra and emission lifetimes essentially identical to those of the monomer. The aggregates of long-chain oligomers have broad and red-shifted emission spectra and relatively long emission lifetimes, both of which are characteristic of excimer states. However their absorption spectra are also perturbed suggesting that the oligomer chains in these aggregates interact strongly in their electronic ground states as well. For intermediate chain lengths, dual monomer-like (green) and excimer-like (red) emission is observed. Single aggregate dispersed emission spectra from aggregates deposited onto glass coverslips demonstrate that, in the absence of solvent, the predominant emitters are monomer-like rather than excimer-like. Moreover, the monomer-like emitters are found to be far more photostable than the analogous non-CN substituted aggregates, whereas the photostability of the excimer-like emitters is exceptionally poor under the illumination conditions used for microscopy. Comparisons between the properties of these nanoaggregates and the corresponding CN-substituted polymer are drawn

Aggregation Effects on the Emission Spectra and Dynamics of Model Oligomers of MEH-PPV

The effects of aggregate formation on the photophysical properties of oligomers of MEH-PPV were studied in bulk solution to better understand the effects of aggregation on the emission properties of the polymer. Nanoaggregates of oligomers from 3 to 17 repeat units in length were formed using a solvent reprecipitation method. The spectra are not readily modeled using the classical dipole−dipole coupling picture of interchain interactions. A strong dependence of the photophysics on the oligomer chain length is also observed. Short-chain oligomers produce nanoaggregates with absorption and emission spectra essentially identical to those of the monomer. Long-chain oligomers form aggregates having more strongly perturbed absorption and fluorescence spectra and decreased emission yields. In these aggregates, the size of the 0−0 band relative to that of the vibronic replicates is a sensitive function of aggregate size and solvent precipitation conditions. Their fluorescence lifetimes are also strongly wavelength dependent. These trends are explained in terms of a core−shell model that postulates the existence of “single-chain-like” and “aggregate-like” emitters within a single aggregate

Tree cover response to climate change in the forest-tundra of north-central Canada: fire-driven decline, not northward advance

Climate-vegetation models predict rapid northward advance of the subarctic forest-tundra in the coming century, although modelled responses may not be congruent with field data. This study aimed to determine how forest-tundra vegetation has responded to climate change in north-central Canada. Vegetation cover and gradients were mapped and compared to changes in climate parameters between 1955 and 2006. Increased aridity and annual and July warming corresponded to spatial isotherm shifts of one-half the width of the forest-tundra transition. Over the 51-year period, the areal extent of live trees decreased 26% (5227 km2) while the areal extent of recently-burned trees increased 16-fold (7768 km2). Changes in the areal extent of treeless wetland, tall shrubs, and upland tundra were non-significant. There was significant forest loss in the southern forest-tundra and modest forest gain in the northern forest-tundra. Overall, forest loss outpaced forest gain. The forest-tundra increased in areal extent by ~6% via an overall broadening of the transition region. Contrary to model predictions, no appreciable northward migration of the forest-tundra was detected over the 51-year period despite significant climate change. Increased wildfire activity and moisture stress may limit the potential of tree vegetation to expand northward under a warming climate.</p