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

Measurement of Exciton Transport in Conjugated Polymer Nanoparticles

A novel approach is proposed for determining exciton transport parameters in conjugated polymers. Exciton dynamics of conjugated polymer nanoparticles doped with dyes were investigated by time-resolved fluorescence spectroscopy. Highly efficient energy transfer from the polymer PFBT to the dye perylene red was evident in the fluorescence spectra and excited state kinetics. Exciton transport parameters were obtained by fitting to a model that included the effects of nanoparticle size, exciton diffusion, energy transfer, and quenching by defects. The results indicate substantial quenching by defects, owing primarily to exciton diffusion, which can greatly increase the effective quenching volume of defects. We estimated the amount of quenching by defects, and included quenching by defects in our model, yielding an estimated exciton diffusion length of 12 nm and diffusion constant of 8.0 × 10^–9 m² s^–1 for nanoparticles of PFBT. The results indicate that quenching by defects can lead to substantial error in determined exciton transport parameters, unless such quenching is properly accounted for in the model

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

Effect of Swelling on Multiple Energy Transfer in Conjugated Polymer Nanoparticles

Many key processes in conjugated polymers are strongly influenced by multiple energy transfer (i.e., exciton diffusion). We investigated the effect of solvent-induced swelling on the kinetics of multiple energy transfer in nanoparticles of the conjugated polymers PFBT and MEH-PPV. Multiple energy transfer between equivalent chromophores results in an increased rate of quenching by defects due to a cascading or funneling effect. The effects of swelling on energy transfer between polymer chromophores and the resulting exciton dynamics were modeled using a random walk on a lattice of chromophores. The simulation results show good agreement with experimental fluorescence quantum yield, and decay kinetics results at low to moderate THF concentrations. We found that the time scale for energy transfer between chromophores (∼5 ps for MEH-PPV nanoparticles and ∼100 ps for PFBT nanoparticles) is highly sensitive to swelling, slowing by an order of magnitude or more for swelled particles. The results support quenching by defects or polarons, amplified by multiple energy transfer or a cascade effect, as a likely explanation for the typically low fluorescence quantum yield of conjugated polymer particles as compared to the free polymer in solution as well as similar effects observed in thin films

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

Bioconjugation of Ultrabright Semiconducting Polymer Dots for Specific Cellular Targeting

Semiconducting polymer dots (Pdots) represent a new class of ultrabright fluorescent probes for biological imaging. They exhibit several important characteristics for experimentally demanding in vitro and in vivo fluorescence studies, such as their high brightness, fast emission rate, excellent photostability, nonblinking, and nontoxic feature. However, controlling the surface chemistry and bioconjugation of Pdots has been a challenging problem that prevented their widespread applications in biological studies. Here, we report a facile yet powerful conjugation method that overcomes this challenge. Our strategy for Pdot functionalization is based on entrapping heterogeneous polymer chains into a single dot, driven by hydrophobic interactions during nanoparticle formation. A small amount of amphiphilic polymer bearing functional groups is co-condensed with the majority of semiconducting polymers to modify and functionalize the nanoparticle surface for subsequent covalent conjugation to biomolecules, such as streptavidin and immunoglobulin G (IgG). The Pdot bioconjugates can effectively and specifically label cellular targets, such as cell surface marker in human breast cancer cells, without any detectable nonspecific binding. Single-particle imaging, cellular imaging, and flow cytometry experiments indicate a much higher fluorescence brightness of Pdots compared to those of Alexa dye and quantum dot probes. The successful bioconjugation of these ultrabright nanoparticles presents a novel opportunity to apply versatile semiconducting polymers to various fluorescence measurements in modern biology and biomedicine