Self-assembled pi-nanotapes as donor scaffolds for selective and thermally gated fluorescence resonance energy transfer (FRET).

Self-assembled nanotapes of a few tailor-made oligo(p-phenylenevinylene)s (OPVs) have been prepared and used as supramolecular donor scaffold to study the fluorescence resonance energy transfer (FRET) to a suitable acceptor. In nonpolar solvents, FRET occurs with nearly 63-81% efficiency, exclusively from the self-assembled OPVs to entrapped Rhodamine B, resulting in the quenching of the donor emission with concomitant formation of the acceptor emission at 625 nm. The efficiency of FRET is considerably influenced by the ability of the OPVs to form the self-assembled aggregates and hence could be controlled by structural variation of the molecules, and polarity of the solvent. Most importantly, FRET could be controlled by temperature as a result of the thermally reversible self-assembly process. The FRET efficiency was significantly enhanced (ca. 90%) in a xerogel film of the OPV1 which is dispersed with relatively less amount of the acceptor (33 mol %), when compared to that of the aggregates in dodecane gel. FRET is not efficient in polar solvents due to weak self-organization of the chromophores. These results indicate that energy transfer occurs exclusively from the self-assembled donor and not directly from the individual donor molecules. The present study illustrates that the self-assembly of chromophores facilitates temperature and solvent controlled FRET within pi-conjugated nanostructures.