Single-molecule spectroscopy reveals the conformational heterogeneity of conducting polymers encapsulated within hollow silica spheres

Poly-2,6-naphthylidenevinylene (PNV) has been synthesized inside microporous hollow silica spheres of a uniform diameter of 600 nm. The spheres obtained by acid hydrolysis followed by basic condensation of phenyltrimethoxysilane and final aerobic calcination at 550 d̀C have a specific surface area (Sbet) of 482 m2 g-1 have an average pore size of 6.8 Å, and are nonfluorescent. Adsorption of 2,6-bis(bromomethyl)naphthalene as PNV precursor and subsequent room-temperature treatment with potassium tert-butoxide renders PNV inside the hollow spheres. Use of small monomer amounts allows preparation of silica spheres with only one or very few encapsulated polymers, as revealed by single-molecule fluorescence spectroscopy. Fluorescence intensity and polarization experiments on individual PNV molecules within the spheres uncover different spectroscopic behaviors arising from the heterogeneous distribution of polymer chain conformations. Tight-coiled PNV molecules fluoresce from a small number of emitting sites due to efficient excited-state energy funneling, whereas multiple emitter behavior is found for extended polymer chains. Coexistence of both extended and coiled segments in individual polymers accounts for the intermediate fluorescing properties encountered for a significant portion of PNV molecules within the particles. © 2008 American Chemical Society.