Structure and dynamics of single conjugated polymer chromophores by surface-enhanced raman spectroscopy

Dramatic localization of optical fields by interactions with surface plasmons on nanotextured metal surfaces allows us to record Raman spectra of individual chromophores on single chains of a high-molecularweight model conjugated polymer, poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene. The data are broadly consistent with two configurations of the chromophore that are characteristic of packed and loose conformations of the backbone. Within these types, fluctuations of the chromophore planarity are observed, and we show how they can be analyzed by studying the temporal correlation of successive spectra. Chromophores exhibit some configurational memory on the time scale of minutes. We also observe substantial spectral fluctuations and associate these mostly with thermally induced motions of the chromophore in the "hot spot". However, we also provide instances of data representing irreversible photoChemistry as well as charging and discharging of the chromophore. Finally, we show that single chromophores embedded in the polymer reorganize in response to poor solvents.