The light generating mechanism of a series of light emitting diodes with electron donor-bridge-acceptor systems (D-b-A) as the emitting species was examined by constructing model diodes based on small organic molecules (OLEDs) as well as on molecularly doped electroactive (poly-N-vinylcarbazole, PVK) and insulating (polystyrene, PS) polymers (PLEDs). The direct electrogeneration of an intramolecular charge-transfer (CT) fluorescence of the donor-bridge-acceptor systems occurred readily in OLED devices with a D-b-A system as the emissive layer. In diodes with PS as the host matrix, hole-injection and electron-injection occurred directly in the D-b-A molecules residing close to the anode and the cathode, respectively. In the PVK diodes, hole-injection occurred primarily into PVK and the positive charge carrier was subsequently trapped on the D-b-A molecule, whereas electron-injection at the cathode side occurred directly into the D-b-A molecules. Charge-hopping between neighboring molecules then occurred until a hole and electron resided on the same molecule, which is equivalent to the formation of the CT excited state, and which finally relaxed by intramolecular charge recombination under the emission of CT fluorescence.
/ In 1991 and 1992, temperature, salinity, and fluorescence were measured by automatic continuous registration using instrumentation on a ferry crossing the southern North Sea daily along a transect between Zeebrugge, Belgium, and Hull, United Kingdom. The temperature ranged between 4 and 21 degrees C off the Belgian coast and between 6 and 17 degrees C in the middle of the transect. Salinity varied between 34 and 35.5 PSU in the offshore part of the transect, but showed much larger variation along the Belgian and UK estuarine coasts. Fluorescence, which was used as a measure of phytoplankton biomass, was highest at the continental coast and lowest near the English coast. Spring blooms of phytoplankton were found along the continental coast and in the channel-influenced water; in 1991 the blooms were denser than in 1992. Some summer blooms were also recorded. Water masses could be distinguished on the basis of salinity and fluorescence patterns. The general patterns in the data are compared with the literature and discussed in relation to river discharge, light penetration, and wind speed and direction. Finally, the value of automatic and frequent measurements of fluorescence for monitoring phytoplankton is compared with less frequent observations at selected stations. It is concluded that accurate information about phytoplankton biomass can only be established from the high-frequency data. KEY WORDS: Monitoring; Sampling; North Sea; Ferry; Fluorescence; Salinity; Temperature
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