PUB - Publikationen an der Universität Bielefeld

Molecular photonics is a new emerging field of research around the premise that it is possible to develop optical devices using single molecules as building blocks. Currently used waveguides, applied for example in telecommunication, rely on the classical physics of bulk materials: Maxwell's equations allow propagating modes in the far field, and the wavelength of light imposes a fundamental lower limit on device size. However, nature has evolved several examples of photonic nanostructures to guide light over much smaller length scales for "light harvesting" in plants and photosynthetic bacteria. This fundamentally quantum mechanical solution is most often based on near-field dipole-dipole interactions, i.e. fluorescence resonance energy transfer (FRET). As a consequence, light-harvesting complexes have inspired researchers to engineer molecular optical devices, such as molecular photoswitches or molecular photonic wires. A molecular photonic wire is distinguished from an electronic wire by supporting excited-state energy transfer rather than electron- (or hole-) transfer processes and could find application in, for example, optical computing as short-range interconnects in dense optical circuits. In this work an alternative access to molecular photonic wires was elaborated. This approach was based on (I) the use of conventional, single molecule compatible chromophores, (II) an energy cascade as the driving force for the excited-state energy, and (III) an arrangement of chromophores such that strong electronic interactions promoting fluorescence quenching are prevented. The main requirement the chromophores have to fulfil is their compatibility with single molecule spectroscopy, i.e. photostability and photophysics. To achieve a very regular arrangement of chromophores, DNA was used as rigid scaffold. Well-developed labelling and post-labelling strategies of DNA were exploited to introduce a variety of different chromophores in a modular conception. Best results could be obtained when dye-labelled oligonucleotides were hybridised against a long DNA-strand already carrying a primary donor chromophore and a biotin for specific immobilization. The distance between subsequent chromophores was adjusted to 3.4 nm, i.e. 10 bases, which ensured efficient FRET and prevented direct orbital interaction. Photonic wires were synthesised carrying up to 5 chromophores and covering a spectral range from 488 nm to 750 nm. In ensemble experiments the maximum overall transfer efficiency was determined to be 21 percent, but as indicated by steady state and time-resolved measurements, a broad heterogeneity within the samples was suspected. To disentangle the complexity of the photophysics of so-built photonic wires, a confocal fluorescence microscope, single molecule sensitive on four spectrally separated detectors, was developed. For the first time, a quadruple jump of energy transfer along a single photonic wire was demonstrated with an overall transfer efficiency of about 90 percent. Confirmation that the energy is transferred stepwise comes from prolonged excitation of single molecules, which results in sequential photobleaching and a shift in the emission from the red back towards the blue. Fluorescence lifetime information revealed further aspects of energy transfer, and complemented spectral data in order to identify fluorophores involved in particular energy transfer steps. Leakages in energy transfer, created by photodestruction of a fluorophore inside the chain, were revealed. Polarization modulation of the excitation light in combination with fluorescence lifetime gave insight into the rotational mobility of the fluorophore serving as input unit, i.e. Rhodamine Green. After the accomplishment of the photonic wire, a further goal was the development of a molecular photoswitch. Hitherto, only one demonstration of chemically synthesized photoswitching of single molecules at room temperature had been reported. In the context of this work, it was shown that commercially available unmodified carbocyanine dyes such as Cy5 and Alexa647 could be used as efficient reversible single-molecule optical switch, whose fluorescent state after apparent photobleaching can be restored at room temperature upon irradiation in the range of 488 - 532 nm. In oxygen-free environment and in the presence of 100 mM [beta]-mercaptoethanol (MEA) as triplet quencher, more than 20 switching cycles could be achieved for single Cy5 molecules with a reliability of more than 90 percent. Single pair FRET experiments with high time resolution revealed the existence of three intermediates prior to fluorescence restoration. In addition to the importance of such single-molecule photoswitches e.g. for optical data storage, the results presented in this work imply limitations for the use of carbocyanine dyes in sp-FRET experiments.