Investigating the photochemistry and photophysics of conjugated polymers by single molecule spectroscopy

We have investigated the single molecule spectroscopy of dyes and conjugated polymers, including 1,1′-didodecyl-3,3,3′ ,3′-tetramethylindocarbocyanine perchlorate (Dil), sulforhodamine G, dPPV-PPyV, poly[2-methoxy, 5-(2′-ethyl-hexyloxy)- p-phenylene-vinylene] (MEH-PPV) and a poly(phenyleneethynylene) derivative. The spectral integrated fluorescence intensity, the fluorescence spectrum, and excitation spectrum dynamics of these systems have been analyzed by statistical methods. They show highly diverse photochemical and photophysical behaviors. DiI single molecules show fluorescence intensity “on/off blinking” due to triplet bottlenecks. Sulforhodamine G's intensity fluctuation results from spectral diffusion due to environmental fluctuations, which is confirmed by two-color excitation spectral kinetics. The multichromophoric conjugated polymer dPPV-PPyV reveals surprising single step photobleaching and acute jumps in fluorescence intensity. These jumps are shown not to result from spectral diffusion and are attributed to efficient intramolecular energy transfer along the polymer chain to localized photo-generated fluorescence quenching polymer defects. The emission from individual MEH-PPV molecules exhibits large-amplitude, discrete intensity fluctuations, which are also assigned to excited state quenching by fluorescence quenchers. The observed intramolecular electronic energy transfer in MEH-PPV is extremely efficient and rapid, involving an exciton migration volume on the order of a single polymer molecule. The extraordinarily fast diffusion at the molecular level suggests that energy migration involves an energy “funnel” created by the distribution of “chromophoric site energies” in the polymer. The emission spectrum and the emission-excitation spectrum reveal a surprising degree of polarization anisotropy, indicating that MEH-PPV molecules adapt to a considerably more ordered conformation than expected based on the canonical Gausian random coil model. These findings have implications for the use of conjugated polymers in light-emitting diode displays and sensors.