Two methods for amplifying the optical nonlinearity of a conjugated porphyrin polymer: Transmetallation and self-assembly

A new soluble conjugated metalloporphyrin polymer has been synthesised as its complex with zinc(II), lead(II) and copper(II) and as the free-base. The zinc complex aggregates in non-coordinating solvents, due to coordination of the amide side-chains to the zinc centres. These aggregates dissociate on addition of pyridine to give single-strand complexes; the dissociation process displays an amazingly high positive cooperativity (Hill coefficient: n H = 3.2 at 50% sat., rising to 11.5 at 95% sat.). The zinc polymer binds 4,4′-bipyridyl to form a double-strand ladder complex; this self-assembly process holds neighbouring porphyrins coplanar and increases the π-conjugation, resulting in a red-shift in the electronic absorption. Degenerate four wave mixing, at 1064 nm, was used to characterise the optical nonlinearity of these polymers. Both the real and imaginary parts of the third-order susceptibility χ(3) are strongly amplified in the lead(II) complex and in the double-strand assembly. We estimate that the two-photon absorption cross-section of the ladder complex is 5 x 104 GM per macrocycle at 1064 nm, which is an order of magnitude higher than the highest values reported for other chromophores, suggesting that these polymers may be relevant to a variety of applications including photodynamic therapy

Degenerate four-wave mixing studies of butadiyne-linked conjugated porphyrin oligomers

The third-order electronic susceptibilities have been measured in solution for a series of conjugated porphyrin oligomers, and for a conjugated polymer of this type. At 1064 nm, the non-resonant susceptibilities of the oligomers and the polymer are large (χ (3)≈10 -17-10 -16 m 2 V -2), have comparable real and imaginary parts, and the real part is negative in sign. The molecular susceptibility per ring chromophore, (γ/N), shows a linear dependence upon chain length, N. We propose a general mechanism for the enhancement of the third-order susceptibilities in conjugated porphyrin based materials due to two-photon resonance with charge transfer states

Changes in forest area along stream networks in an agricultural catchment of the Great Barrier Reef Lagoon

Scenes from the series of multispectral sensors on the Landsat satellites were used to map recent changes (between 1972 and 2004) in forest cover within and adjacent to stream networks of an intensively farmed region of the southern Great Barrier Reef catchment (Australia). Unsupervised ISODATA classifications of Tasseled-Cap transformed data (at 57 m ground resolution) mapped forest and cleared areas within 150 m of Pisoneer catchment waterways with 72.2% overall accuracy (K = 0.469), when adjusted for the size of each class. Although the user's accuracy was higher for the forest class (82.1 ± 8.4% at α = 0.05), large errors of commission (34.2 ± 8.3%) substantially affected map accuracy for the cleared class. The main reasons for misclassification include: (1) failure to discriminate narrowly vegetated riparian strips; (2) misregistration of scenes; and (3) spectral similarity of ground cover. Error matrix probabilities were used to adjust the mapped area of classes, resulting in a decline of forest cover by 12.3% and increase of clearing by 18.5% (22.4 km change; 95% confidence interval: 14.3-29.6 km ) between 1972 and 2004. Despite the mapping errors, Landsat data were able to identify broad patterns of land cover change that were verified from aerial photography. Most of the forest losses occurred in open forest to woodland habitat dominated by Eucalyptus, Corymbia, and Lophostemon species, which were largely replaced by sugarcane cropping. Melaleuca communities were similarly affected, though they have a much smaller distribution in the catchment