Poly [2-methoxy, 5-(2′-ethyl-hexyloxy)-p-phenylene-vinylene](MEH-PPV) is an important electroluminescent polymer that is used extensively in light-emitting diodes and other photonic applications because of its excellent processability and favorable electronic, and spectroscopic properties. 1-3 Commercially available MEH-PPV has a mass-averaged molecular weight of 996,000, with a ratio of mass to number average molecular weights of 2.2. A typical MEH-PPV molecule is comprised of

1700 monomer units. The optical states of MEH-PPV and their dynamics and dependence on the morphology of MEH-PPV are under active investigation. 1-4 It is generally accepted that the optical coherence length for MEH-PPV is∼ 10-15 monomer units, 5, 6 and that absorption spectrum is due to the overlapping singlet single absorption of about a hundred weakly coupled, localized quasi chromorphores. Correspondingly, the excitations (excitons) are believed to be localized on the order of tens of monomer units, but capable of undergoing energy migration during the singlet excited state lifetime (∼ 200 ps). 7, 8 This paper describes single-molecule spectroscopy9 on MEHPPV molecules that were isolated (at about 0.2 molecules per μm2) in a polystyrene spin-coated polymer thin (50 nm-1000 nm) film. Single-molecule fluorescence images, spectra, and intensity kinetic traces (transients) were obtained with a home-built, sample-scanning fluorescence confocal microscope. 10 Molecules were continuously irradiated at 488 nm, which is close to the peak of the absorption spectrum. This should generate excitons at random locations along the polymer chain. Typical singlemolecule fluorescence intensity vs irradiation time “transients” of MEH-PPV are shown in Figure 1a, c. The transients exhibit large-amplitude, discrete intensity fluctuations. Analogous transients were observed for MEH-PPV, for thin or thick polymer films, for MEH-PPV in a polystyrene or poly (methyl methacrylate) host, and even for MEH-PPV molecules directly on a glass substrate without a polymer host. Many of the transients exhibit a small number (typically 2 or 3) of quasi discrete intensity levels. This is demonstrated by parts b and d of Figure 1, which are intensity histograms of the transients displayed in parts a and c of this figure, respectively, and by parts e and f of Figure 1, which show simulated data for an emitting molecule with two discrete intensity levels. Analogous intensity jumps have been reported for the conjugated polymer dPPV-dPPyV. 11 An individual intensity jump occurs within one counting period (typically 10 ms) and appears to be due to a single photochemical event. The time interval between jumps is on the millisecond to tens of seconds time scale. The decrease in fluorescence signal is attributed to quenching of singlet excitons of MEH-PPV by reversibly formed, long-lived, photogenerated fluorescence “quencher defect”. The transition from a higher intensity level to a lower one is assigned to the photochemical generation of the quencher defect and from a lower to a higher level to the thermal repair of the quencher defect. Candidates for the quencher defect include the dipolaron, ie, a separated radical cation/radical anion pair, and certain photooxidation intermediates for conjugated polymers, which have been reported to be efficient singlet excited state quenchers. 12 The quenching efficiency of a single quencher defect was estimated by the fluorescence intensity drop that occurred on the transition from a higher to lower intensity level. The intensity drops were typically∼ 20% or greater. On the basis of a typical quenching efficiency of 20% or greater for MEH-PPV, the …