Control of Energy Transfer to CdTe Nanowires via Conjugated Polymer Orientation

Nanoscale organic-inorganic hybrid materials were prepared from two highly fluorescent building blocks: CdTe nanowires (NWs) as an energy acceptor, and a rigid rod-like and water soluble poly(p-phenyleneethynylene) (PPE) as a donor. The PPE was covalently bonded to the CdTe by traditional amide chemistry. Two different orientations, parallel and perpendicular, of PPE to CdTe NW were achieved by manipulating the density of PPE at the NW surface through controlling the molar ratio of PPE and the NW during the chemical tethering. Strong dependence of fluorescence resonance energy transfer (FRET) on the orientation was observed. While efficient FRET was observed when the orientation of the donor PPE and the acceptor CdTe NW is parallel, essentially no FRET was observed from the perpendicularly oriented PPE-tethered CdTe NW system. Optical and transmission electron microscopy experiments as well as Monte Carlo simulation consistently suggest that entropy change during the tethering is a crucial factor for the determination of the orientation of PPE at the NW surface.