Abstract

A novel perylene imide and oligo-pentaphenyl bisfluorene containing molecule is shown to undergo electron transfer to form an emissive charge transfer state in di-benzyl ether and THF. At the single molecule level in a PMMA film, fluorescence spectra characteristic of both emissive states (locally excited and charge transfer) are observed with 44% of the molecules studied showing switching between the two states. These results demonstrate that charge transfer fluorescence from single molecules can be used to report on the properties and dynamics of a molecule’s immediate surroundings or nano-environment.

Echtzeitmessungen zur Katalyse- und Substratkinetik einer Hydrolyse durch ein einzelnes Enzymmolekül werden mithilfe von konfokaler Fluoreszenzmikroskopie (CFM) möglich (siehe Bild, CFM-Strahl=grün). Eine einzige Lipase kann eine Vielzahl von Konformationen einnehmen, die alle zur Gesamtaktivität des Enzyms beitragen. Dieser Sachverhalt wird bei Messungen an Ensembles häufig übersehen.

Real-time measurement of the catalysis and substrate kinetics of a single-enzyme hydrolysis reaction is demonstrated with confocal fluorescence microscopy (CFM; see picture, green=CFM beam). A single lipase is shown to have a broad range of conformations; each conformation contributes to the overall enzymatic activity, an observation that is often masked by ensemble measurements.

The dynamics of reversible photoinduced electron transfer in individual perylenediimide–triphenylamine-based donor–acceptor dendrimers immobilized in polystyrene were investigated by single-molecule spectroscopy. By changing the local concentration of oxygen, it was possible to influence the transition of the charge-separation state (CSS) either to the locally excited state (LES) or to the ground state (GS, see graphic).

Die Dynamik des photoinduzierten reversiblen Elektronentransfers in individuellen, in Polystyrol immobilisierten Perylendiimid-Triphenylamin-Donor-Acceptor-Dendrimeren wurde durch Einzelmolekülspektroskopie untersucht. Über die lokale Sauerstoffkonzentration lässt sich der Übergang des ladungsgetrennten Zustands (CSS) zum lokal angeregten Zustand (LES) oder zum Grundzustand (GS) steuern (siehe Bild).

The cover picture shows two different guest chromophores in a dendritic host. In this system the thiacarbocyanine dye A can act as an energy donor for the thiadicarbocyanine dye B. This system has been studied at the ensemble as well as at the single-molecule level. Because of the many free parameters in the system the results at the ensemble level are too complex to analyze in detail. At the single-molecule level a wide variety of energy transfer efficiencies was observed. Moreover, single entities showed fluctuations of the energy transfer efficiency as a function of time. In the background of the cover picture a smoothed surface of a 3D histogram of donor decay time versus fluorescence intensity is plotted for a single entity. Clearly two domains of energy transfer efficiency are observed. The single-molecule results permit a more in-depth understanding of the photophysics of this complex system. Find out more in the article by Hofkens and De Schryver et al. on pages 1005–1013.

Abstract

We report on single-molecule experiments that were performed on two biological systems, the recently cloned red fluorescent protein of a coral of the Discosoma genus (DsRed) and the enhanced green fluorescent protein, both of which were immobilized in polyvinylalcohol. Fluorescence intensity trajectories were recorded and compared to those of a reference compound, the organic dye hexaphenylbenzene perylenemonoimide. Evidence was found for the presence of DsRed as an oligomer even at subnanomolar concentrations. Both proteins show triplet blinking in the millisecond time range. Collective effects could be observed for the Dsred oligomers.

Collective on/off behavior and different spectral behavior has been identified for individual molecules of different isomers (see for example the fluorescence spectra for two isomers as a function of time) with three perylenedicarboximide chromophores. In 8 % of the dendrimer molecules, the chromophores interact to form an excimer-like structure which possess a higher rate of intersystem crossing than the rest of the molecules. This is the first time that this phenomenon has been proven experimentally at the single-molecule level.

Sowohl kollektives An-Aus-Verhalten als auch unterschiedliche spektrale Eigenschaften wurden für individuelle Moleküle verschiedener Isomere eines Dendrimers mit drei Perylendicarboximid-Chromophoren gefunden (siehe z. B. die zeitabhängigen Fluoreszenzspektren zweier Isomere). Bei 8 % der Dendrimermoleküle wechselwirken die Chromophore miteinander und bilden eine Excimer-artige Struktur, die schneller interkombiniert als die restlichen Moleküle. Dieses Phänomen konnte zum ersten Mal für ein Einzelmolekülsystem nachgewiesen werden.