A series of π-extended cyclic thiophene oligomers of 12, 18, 24, and 30 repeat units have been studied using methods of ultrafast time-resolved absorption, fluorescence upconversion, and three-pulse photon echo. These measurements were conducted in order to examine the structure-function relationships that may affect the coherence between chromophores within the organic macrocycles. Our results indicate that an initial delocalized state can be seen upon excitation of the cyclic thiophenes. Anisotropy measurements show that this delocalized state decays on an ultrafast time scale and is followed by the presence of incoherent hopping. From the use of a phenomenological model, we conclude that our ultrafast anisotropy decay measurements suggest that the system does not reside in the Förster regime and coherence within the system must be considered. Three-pulse photon echo peak shift experiments reveal a clear dependence of initial peak shift with ring size, indicating a weaker coupling to the bath (and stronger intramolecular interactions) as the ring size is increased. Our results suggest that the initial delocalized state increases with ring size to distances (and number of chromophores) comparable to the natural light-harvesting system.

Single-molecule imaging pushes fluorescence microscopy beyond the diffraction limit of traditional microscopy. Such super-resolution imaging, which relies on the detection of bright, stable fluorescent probes to achieve nanometer-scale resolution, is often hindered in biological systems by dim, blinking fluorescent proteins (FPs). Here, we use gold nanorods and single-molecule fluorescence detection to achieve plasmonenhanced emission from intrinsically fluorescent proteins. We measure a doubled photon emission rate from the red FP mCherry and detect three times more photons before photobleaching from the photoactivatable FP PAmCherry. We further explore the effect of near-field nanorod interactions on the yellow FP mCitrine, for which the observed emission enhancements cannot overcome measurable quenching. Overall, our work indicates that plasmonic particles improve both the brightness and photostability of FPs and extends the applications of plasmon-enhanced fluorescence to the arena of biological imaging. Furthermore, because gold nanorods are nontoxic, they are promising extracellular imaging substrates for enhancing emission from FP-labeled membrane-bound proteins in live cells.

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