ABSTRACT
Single chains of conjugated polymers e.g. MEH-PPV (poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene) have become interesting objects for single molecule spectroscopy (SMS) studies. However, most of the experiments so far were performed without full awareness of the isolation status of the polymer chains in host matrices. We used steady-state and time-resolved fluorescence methods and 2D polarization single molecule imaging technique to unravel the isolation/aggregation status of MEH-PPV in spin-coated films prepared at different conditions. It turned out that a sample showing isolated bright spots in fluorescence images could be obtained in a very broad concentration range of MEH-PPV when toluene was used as a solvent and PMMA as a matrix. If the MEH-PPV concentration was not sufficiently low, a substantial fraction of the fluorescence spots should be assigned to individual nano-aggregates rather than truly isolated chains of the polymer. Contrary to single aggregates, truly isolated MEH-PPV chains showed blue-shifted emission spectra, mono-exponential fluorescence decay dynamics with relatively long lifetimes (0.4-1.2 ns), and high polarization anisotropy. We argue that insufficient control of the concentration in the published SMS studies of MEH-PPV resulted in incorrect assigning of some spectroscopic properties of single aggregates to isolated MEH-PPV chains. We believe this to be the main origin of discrepancies among the published data in this field.
ABSTRACT
The amphiphilic dye 3,3'-bis(2-sulfopropyl)-5,5',6,6'-tetrachloro-1,1'-dioctylbenzimidacarbocyanine (C8S3) self-aggregates in aqueous solution to form tubular J-aggregates with a diameter of 17.0 +/- 0.5 nm, a wall thickness of approximately 4 nm, and a length exceeding several hundred nanometers. The absorption spectrum shows the typical features expected for tubular J-aggregates with several sharp and red-shifted absorption bands. Morphological investigations using cryo-transmission electron microscopy (cryo-TEM) and spectroscopic investigations reveal a high stability of the tubular morphology but a tendency of the aggregates to assemble into ropelike bundles after several weeks of storage. It is found that aggregation in solutions containing additives such as alcohols or surfactants results in the formation of new types of aggregates. A second type of tubular aggregate with a diameter of 13.0 +/- 0.5 nm is observed when the solutions contain more than 10 wt % MeOH. On the time scale of days these tubular aggregates transform into ribbonlike structures characterized by a new absorption spectrum, and they convert after several weeks into giant tubes with diameters of up to 500 nm.
