Unexpected lighting up: A slight rotational twist and rigidification of merocyanine dyes in a face‐to‐face π‐π‐stacked dimer aggregate resulted in an unexpected increase in the fluorescence intensity and lifetime (see graph). This result contrasts the common perception that the fluorescence of H‐aggregates is strongly quenched, but can be rationalized within the concept of exciton theory.
Unerwartetes Aufleuchten: Eine leichte Rotationsverdrillung und Versteifung von Merocyanin‐Farbstoffen in einem π‐π‐gestapelten Dimer‐Aggregat resultierte in einer unerwarteten Steigerung von Fluoreszenzintensität und ‐lebensdauer (siehe Diagramm). Diese Beobachtung widerspricht der allgemeinen Annahme, dass die Fluoreszenz von H‐Aggregaten stark gelöscht wird, kann aber durch die Exzitonentheorie erklärt werden.
Field of dreams: An unusually strong electric‐field effect influences the aggregation equilibrium of a strongly dipolar merocyanine dye. Electrooptical absorption measurements indicate that the external electric field leads to an increase of the monomer concentration at the expense of the dimer concentration (see picture).
A new FFF method is presented which combines asymmetrical flow-FFF (AF4) and electrical FFF (ElFFF) in one channel to electrical asymmetrical flow-FFF (EAF4) to overcome the restrictions of pure ElFFF. It allows for measuring electrophoretic mobility (μ) as a function of size. The method provides an absolute value and does not require calibration. Results of μ for two particle standards are in good agreement with values determined by phase analysis light scattering (PALS). There is no requirement for low ionic strength carriers with EAF4. This overcomes one of the main limitations of ElFFF, making it feasible to measure proteins under physiological conditions. EAF4 has the capability to determine μ for individual populations which are resolved into separate peaks. This is demonstrated for a mixture of three polystyrene latex particles with different sizes as well as for the monomer and dimer of BSA and an antibody. The experimental setup consists of an AF4 channel with added electrodes; one is placed beneath the frit at the bottom wall and the other covers the inside of the upper channel plate. This design minimizes contamination from the electrolysis reactions by keeping the particles distant from the electrodes. In addition the applied voltage range is low (1.5-5 V), which reduces the quantity of gaseous electrolysis products below a threshold that interferes with the laminar flow profile or detector signals. Besides measuring μ, the method can be useful to improve the separation between sample components compared to pure flow-FFF. For two proteins (BSA and a monoclonal antibody), enhanced resolution of the monomer and dimer is achieved by applying an electric field.
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