The UV-visible absorption cross-sections of IONO₂

International audienceThe UV-visible absorption spectrum of gaseous IONO2 has been measured over the wavelength range 245--415 nm using the technique of laser photolysis with time-resolved UV-visible absorption spectroscopy. IONO2 was produced in situ in the gas phase by laser flash photolysis of NO2/CF3I/N2 mixtures. Post-flash spectra were deconvolved to remove contributions to the observed absorption from other reactant and product species. The resulting spectrum attributed to IONO2 consists of several overlapping broad absorption bands. Assuming a quantum yield of unity for IONO2 photolysis, model calculations show that during sunlit hours at noon, 53° N, the first order solar photolysis rate coefficient (J value) for IONO2 is 4.6 x 10-2 s-1

Anisotropic organization and microscopic manipulation of self-assembling synthetic porphyrin microrods that mimic chlorosomes: Bacterial light-harvesting systems

Being able to control in time and space the positioning, orientation, movement, and sense of rotation of nano- to microscale objects is currently an active research area in nanoscience, having diverse nanotechnological applications. In this paper, we demonstrate unprecedented control and maneuvering of rod-shaped or tubular nanostructures with high aspect ratios which are formed by self-assembling synthetic porphyrins. The self-assembly algorithm, encoded by appended chemical-recognition groups on the periphery of these porphyrins, is the same as the one operating for chlorosomal bacteriochlorophylls (BChl's). Chlorosomes, rod-shaped organelles with relatively long-range molecular order, are the most efficient naturally occurring light-harvesting systems.(1, 2) They are used by green photosynthetic bacteria to trap visible and infrared light of minute intensities even at great depths, e.g., 100 m below water surface or in volcanic vents in the absence of solar radiation. In contrast to most other natural light-harvesting systems, the chlorosomal antennae are devoid of a protein scaffold to orient the BChl's; thus, they are an attractive goal for mimicry by synthetic chemists, who are able to engineer more robust chromophores to self-assemble. Functional devices with environmentally friendly chromophores-which should be able to act as photosensitizers within hybrid solar cells, leading to high photon-to-current conversion efficiencies even under low illumination conditions-have yet to be fabricated. The orderly manner in which the BChl's and their synthetic counterparts self-assemble imparts strong diamagnetic and optical anisotropies and flow/shear characteristics to their nanostructured assemblies, allowing them to be manipulated by electrical, magnetic, or tribomechanical forces. © 2011 American Chemical Society

Fluctuating exciton localization in giant π-conjugated spoked-wheel macrocycles

Conjugated polymers offer potential for many diverse applications, but we still lack a fundamental microscopic understanding of their electronic structure. Elementary photoexcitations (excitons) span only a few nanometres of a molecule, which itself can extend over microns, and how their behaviour is affected by molecular dimensions is not immediately obvious. For example, where is the exciton formed within a conjugated segment and is it always situated on the same repeat units? Here, we introduce structurally rigid molecular spoked wheels, 6 nm in diameter, as a model of extended π conjugation. Single-molecule fluorescence reveals random exciton localization, which leads to temporally varying emission polarization. Initially, this random localization arises after every photon absorption event because of temperature-independent spontaneous symmetry breaking. These fast fluctuations are slowed to millisecond timescales after prolonged illumination. Intramolecular heterogeneity is revealed in cryogenic spectroscopy by jumps in transition energy, but emission polarization can also switch without a spectral jump occurring, which implies long-range homogeneity in the local dielectric environment