SMD homepage

1. Dendrimers.                2. Polymers.                3. Cellbiology.          4. Fluorescent proteins.

5. Cooperations.            6. Labpictures.            7. Alumni.                  8. Positions.

Revealing competitive Förster-type resonance energy-transfer pathways in single bichromophoric molecules. PDF

(Johan Hofkens, Mircea Cotlet, Tom Vosch, Sven Jordens, Gerd Schweitzer, Frans De Schryver)

The efficiency of competing Förster-type energy-transfer pathways in single bichromophoric  systems was demonstrated by monitoring simultaneously the fluorescence intensity, fluorescence lifetime, and the number of independent emitters with time. Peryleneimide end-capped fluorene trimers, hexamers, and polymers with interchromophore distances of 3.4, 5.9, and on average 42 nm, respectively, served as bichromophoric systems. Because of different energy-transfer efficiencies, variations in the interchromophore distance enable the switching between homoenergy transfer (energy hopping), singlet-singlet annihilation, and singlet-triplet annihilation. The data suggest that similar energytransfer pathways have to be considered in the analysis of singlemolecule trajectories of donor acceptor pairs as well as in natural and synthetic multichromophoric systems such as light-harvesting antennas, oligomeric fluorescent proteins, and dendrimers. Here we report selectively visualization of different energy-transfer pathways taking place between identical fluorophores in individual bichromophoric molecules.

(1)(2)

(1) Structure of the bichromophoric compounds used in this study.

(2) Optimized structures [force-field calculations in SPARTAN (Wavefunction, Irvine, CA)] of the PI end-capped fluorene trimer, hexamer, and polymer. The size of the molecules is scaled with the R0 values of the different ET processes (red, hopping; brown, S1–S1 annihilation; yellow, S1–T1 annihilation).

(3)

(3) Typical intensity traces and interphoton arrival-time distributions recorded for the hexameric compound (46 molecules analyzed) and the polymer (80 molecules analyzed) dispersed in a polymethyl-metacrylate polymer matrix. Only those traces that have two distinct intensity levels for which the decay time in both levels is similar to the decay time in solution (4 ns) are analyzed. This is to exclude from the analysis dimers or oligomers resulting from the tendency of the fluorene backbone to aggregate. (A) Data recorded for the hexamer. The intensity trace shows two levels as expected for a bichromophoric molecule and consecutive bleaching. Interphoton arrivaltime distributions taken from the first intensity level (B) and the second intensity level of the trace (D, after 1 s) show a vanishing, small central peak, although high-excitation powers were used. This directly demonstrates that efficient S-S annihilation takes place in this individual hexamer. After 0.72 s, a collective on off jump can be observed in the high-intensity level, as can be seen in C. This is explained by the generation of a triplet and subsequent opening of the S-T ET pathway. After the disappearance of the triplet, the molecule switches again to the S-S annihilation pathway. The original long off period (duration of 300 ms) occurring at 1.1 s is not understood at the moment. (E) Intensity trace recorded for a PI end-capped polymer. The appearance of the interphoton arrival-time histograms, however, is completely different, although identical excitation conditions are used. In the high-intensity level, clearly coincident photons are detected as reflected in the ratio of the lateral to the central peak of 0.5 (F). This shows that S-S annihilation is not an efficient ET pathway for this molecule. (G) The only efficient ET pathway that is present temporarily in the molecule is S-T annihilation, which is reflected in the collective on off jumps after 6.7 and 7.1 s in the intensity trace. (H) The interphoton arrival-times distribution of the second intensity level, corresponding to a situation in which one chromophore is bleached, clearly shows the pattern one expects for a single emitter.