Multiple Energy Transfer Dynamics in Blended Conjugated Polymer Nanoparticles

Energy transfer dynamics in blended conjugated polymer nanoparticles (CPNs) were investigated in order to further our understanding of photoswitching and anomalous saturation behavior we previously observed, and as a way to probe the complex energy transport processes occurring in similar systems of interest such as nanostructured bulk heterojunction photovoltaic devices. We prepared blended poly­[(9,9-dioctylfluorenyl-2,7-diyl)-<i>co</i>-(1,4-benzo-{2,1′,3}-thiadiazole)] (PFBT)/poly­[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) nanoparticles with varying blending ratios. Efficient energy transfer from PFBT to MEH-PPV was observed, yielding bright, red-shifted emission. The donor exhibited complex decay kinetics consistent with energy transfer in complex, nanoscale, multichromophoric systems. The fluorescence decay kinetics and steady-state quenching efficiencies are compared to a multiple energy transfer model and prior results for dye-doped nanoparticles. The analysis indicates that the high energy transfer efficiency is largely due to multistep energy transfer (i.e., exciton diffusion), while the lifetime heterogeneity appears to be strongly influenced by acceptor polymer polydispersity as well as nanoscale inhomogeneity. The emerging picture could inform efforts to optimize CPNs for advanced imaging applications, and to optimize energy transport in bulk heterojunction photovoltaic devices

Photoactivation and Saturated Emission in Blended Conjugated Polymer Nanoparticles

Blended poly­[(9,9-dioctylfluorenyl-2,7-diyl)-<i>co</i>-(1,4-benzo-{2,1′,3}-thiadiazole)] (PFBT)/poly­[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) conjugated polymer nanoparticles were prepared and characterized by conventional and single-particle fluorescence spectroscopy. The particles exhibit red emission and improved quantum efficiency resulting from highly efficient energy transfer from donor PFBT to acceptor MEH-PPV as well as suppression of MEH-PPV aggregation. Photobleaching results indicate better photostability in the blended sample compared to undoped MEH-PPV nanoparticles and photoactivation of donor emission, which could be useful for single-molecule localization-based super-resolution microscopy. Single blended nanoparticles exhibit bright fluorescence as well as saturation behavior at very low excitation intensities. These and other properties of the blended conjugated polymer nanoparticles could provide substantial improvements in resolution when employed in super-resolution microscopy

Conjugated Polymer Nanoparticles Incorporating Antifade Additives for Improved Brightness and Photostability

Conjugated polymer nanoparticles with incorporated antifade agents were prepared, and ensemble and single particle measurements showed that incorporation of antifade agents effectively improves the fluorescence quantum yield and photostability of the conjugated polymer nanoparticles, likely by a combination of triplet quenching and suppression of processes involved in photogeneration of hole polarons (cations), which act as fluorescence quenchers. The photostability of conjugated polymer nanoparticles and CdSe quantum dots was compared, at both the ensemble and single particle level. The results provide confirmation of the hypothesis that quenching by photogenerated hole polarons is a key factor limiting the fluorescence quantum yield and maximum emission rate in conjugated polymer nanoparticles. Additionally, the results indicate the involvement of oxygen in photogeneration of hole polarons. The results also provide insight into the origin of quenching processes that could limit the performance of conjugated polymer devices