Single Molecule Nanoparticles of the Conjugated Polymer MEH−PPV, Preparation and Characterization by Near-Field Scanning Optical Microscopy

Swelling-controlled polymer phase and fluorescence properties of polyfluorene nanoparticles

Highly fluorescent nanoparticles of the conjugated polymer poly(9,9-dioctylfluorene) (PFO) with distinct phases were prepared, and their photophysical properties were studied by steady state and time-resolved fluorescence spectroscopy. An aqueous suspension of PFO nanoparticles prepared by a reprecipitation method was observed to exhibit spectroscopic characteristics consistent with the glassy phase of the polymer. We demonstrate that controlled addition of organic solvent leads to partial transformation of the disordered polymer chains into the planarized conformation (beta-phase), with the fractions of each component phase dependent on the amount of solvent added. Fluorescence spectroscopy of the PFO nanoparticles containing beta-phase indicates efficient energy transfer from the glassy-phase regions of the nanoparticles to the beta-phase regions. Salient features of the nanoparticles containing beta-phase include narrow, red-shifted fluorescence and increased fluorescence quantum yield as compared to the glassy-phase nanoparticles. Fluorescence lifetime measurements indicate that the increased quantum yield of the beta-phase PFO originates from a decrease in the nonradiative decay rate, with little change in the radiative rate. This decrease is likely due to exciton trapping by the beta-phase, which leads to a reduction in the energy transfer efficiency to quencher species present within the nanoparticle.

Energy Transfer in a Nanoscale Multichromophoric System: Fluorescent Dye-Doped Conjugated Polymer Nanoparticles

We report on the fluorescence properties and the combined effects of energy diffusion and energy transfer in polyfluorene nanoparticles doped with a variety of fluorescent dyes. As the doping host, polyfluorene possesses extraordinary "light harvesting" ability, resulting in higher per-particle brightness as compared to dye-loaded silica nanoparticles of similar dimensions. Both the steady-state fluorescence spectra and time-resolved fluorescence measurements indicate highly efficient energy transfer from the host polymer to the acceptor dye molecules. A model that takes into account the combined effects of energy diffusion, Förster transfer, and particle size was developed. Comparisons of experimental data to the model results elucidate the importance of particle size and energy diffusion within the polymer in determining the optical properties of the doped conjugated polymer nanoparticles. Fluorescence quantum yields of ~40% and peak extinction coefficients of 1.5 × 10(9) M(-1)cm(-1) were determined for aqueous suspensions of ~30 nm diameter polymer nanoparticles doped with perylene or coumarin 6 (2 wt %). Photobleaching experiments indicate that energy transfer phenomena strongly influence the photostability of these dye-doped nanoparticles. Significant features of these nanoparticles include the high brightness, highly red-shifted emission spectrum, and excellent photostability, which are promising for biological labeling and sensing applications. In addition, the nanoparticles are a useful model system for studying energy transfer in dense, nanostructured, multichromophoric systems.

Energy Transfer in a Nanoscale Multichromophoric System: Fluorescent Dye-Doped Conjugated Polymer Nanoparticles

We report on the fluorescence properties and the combined effects of energy diffusion and energy transfer in polyfluorene nanoparticles doped with a variety of fluorescent dyes. As the doping host, polyfluorene possesses extraordinary "light harvesting" ability, resulting in higher per-particle brightness as compared to dye-loaded silica nanoparticles of similar dimensions. Both the steady-state fluorescence spectra and time-resolved fluorescence measurements indicate highly efficient energy transfer from the host polymer to the acceptor dye molecules. A model that takes into account the combined effects of energy diffusion, Förster transfer, and particle size was developed. Comparisons of experimental data to the model results elucidate the importance of particle size and energy diffusion within the polymer in determining the optical properties of the doped conjugated polymer nanoparticles. Fluorescence quantum yields of ~40% and peak extinction coefficients of 1.5 × 10(9) M(-1)cm(-1) were determined for aqueous suspensions of ~30 nm diameter polymer nanoparticles doped with perylene or coumarin 6 (2 wt %). Photobleaching experiments indicate that energy transfer phenomena strongly influence the photostability of these dye-doped nanoparticles. Significant features of these nanoparticles include the high brightness, highly red-shifted emission spectrum, and excellent photostability, which are promising for biological labeling and sensing applications. In addition, the nanoparticles are a useful model system for studying energy transfer in dense, nanostructured, multichromophoric systems.

Energy transfer mediated fluorescence from blended conjugated polymer nanoparticles

Nanoparticles consisting of a derivative of the blue-emitting conjugated polymer polyfluorene doped with green-, yellow-, and red-emitting conjugated polymers were prepared by a reprecipitation method. The nanoparticles can be described as a system of densely packed chromophores that exhibit efficient energy transfer from the host to the dopant polymers. Fluorescence quenching analysis of the host polymer as a function of the dopant concentration indicates that one energy acceptor molecule can effectively quench 90% of the fluorescence of a nanoparticle consisting of 100-200 host conjugated polymer molecules. A nanoparticle energy transfer model was developed that successfully describes the quenching behavior of a small number of highly efficient energy acceptors per nanoparticle. The fluorescence brightness of the blended polymer nanoparticles was determined to be much higher than that of inorganic quantum dots and dye-loaded silica particles of similar dimensions. The combination of high fluorescence brightness and tunable fluorescence of these blended nanoparticles is promising for ultrasensitive fluorescence-based assays.

Charging and discharging of single conjugated-polymer nanoparticles

Despite intense, long-term interest in organic semiconductors from both an applied and fundamental perspective, key aspects of the electronic properties of these materials remain poorly defined. A particularly challenging problem is the molecular nature of positive charge carriers, that is, holes or oxidized species in organics. Here, the unique ability of single-molecule spectroelectrochemistry (SMS-EC) to unravel complex electrochemical process in heterogeneous media is used to study the oxidation of nanoparticles of the conjugated polymer poly(9,9-dioctylfluorene-co-benzothiadiazole). A reversible hole-injection charging process has been observed that occurs primarily by initial injection of shallow (untrapped) holes, but soon after the injection, a small fraction of the holes becomes deeply trapped. Good agreement between experimental data and simulations strongly supports the presence of deep traps in the studied nanoparticles and highlights the ability of SMS-EC to study energetics and dynamics of deep traps in organic materials at the nanoscale.

Enhanced resonance energy transfer from PVK to MEH-PPV in nanoparticles

Organic semiconductor nanoparticles were prepared from poly(N-vinylcarbazole) (PVK), poly(2-methoxy-5-(2(')-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV) and their blend solution via a reprecipitation method, respectively. A wide photoluminescence band centred at 430 nm has been found in the PVK nanoparticles, which is obviously red-shifted by comparison with the PVK film. The red-shifted emission from the PVK nanoparticles has a good spectral superposition with the absorption of the MEH-PPV nanoparticles. However, the spectral superposition is very poor between the two polymers in the composite films. The markedly enhanced emission from MEH-PPV is experimentally observed in the composite polymer nanoparticles and attributed to Förster energy transfer from PVK to MEH-PPV for excitation at the absorption maximum of PVK.

Single molecule spectroscopy of poly 3-octyl-thiophene (P3OT)

We report on the spectroscopy of isolated chains of P3OT, in a highly dilute solution in the inert polymer host poly(methyl-methacrylate) (PMMA). This environment permits a detailed analysis of emission transitions in the 1.9-2.2 eV range by suppressing the formation of the lowest emitting-energy aggregated form of P3OT. Herein it is observed that the 1.9-2.2 eV band is in fact split into low (red) and high (blue) energy forms in a highly analogous situation to that found for the conjugated polymer MEH-PPV. Another focus of this work is an investigation of the interaction of singlet and triplet excitons in P3OT. The results indicate that, like in MEH-PPV, triplet excitons are highly efficient fluorescence quenchers for P3OT, strongly quenching the fluorescence of P3OT under even relatively low excitation intensities.

Highly luminescent Eu3+ chelate nanoparticles prepared by a reprecipitation-encapsulation method

Aqueous suspensions of highly luminescent Eu3+ chelate nanoparticles are prepared by a novel reprecipitation-encapsulation method. An alkyl alkoxysilane encapsulation agent is included during the nanoparticle formation process, forming a nanoparticle encapsulation layer that inhibits aggregation as evidenced by UV-vis spectroscopy and atomic force microscopy. In addition, the encapsulated nanoparticles exhibit a small size (10 nm), intense luminescence, and excellent photostability. We estimate that the molar extinction coefficients of the approximately 10 nm particles are approximately 5.0x10(7) M-1 cm-1 with a luminescence quantum yield of 6%, indicating a luminescence brightness many times larger than that of conventional fluorescent dyes and comparable to that of colloidal semiconductor quantum dots. The small size, high brightness, highly red-shifted luminescence, and long luminescence lifetimes of the nanoparticles are of interest for luminescence labeling and sensing applications.

Size-Dependent Spectroscopic Properties of Conjugated Polymer Nanoparticles

This paper is focused on how the spectroscopic properties of conjugated polymers evolve in the size range between single polymer chains and the bulk material. The measurements used single-particle spectroscopy techniques and include both static and dynamic measurements. The main observation of this work is that the spectroscopic properties of MEH-PPV evolve rapidly as a function of nanoparticle size and achieve bulk-like properties for nanoparticles greater than 10 nm in size. Nanoparticles were assembled by a reprecipitation technique and characterized by fluorescence emission spectroscopy. The physical origin of the size-dependent spectroscopic properties is assigned to the distance dependence of four main processes: electronic energy transfer between blue and red sites, triplet-triplet annihilation, singlet exciton quenching by triplets, and singlet exciton quenching by hole polarons.

Toward Efficient Photomodulation of Conjugated Polymer Emission: Optimizing Differential Energy Transfer in Azobenzene-Substituted PPV Derivatives

We present fluorescence studies of quenching behavior in photoaddressable azobenzene-substituted derivatives of the fluorescent conjugated polymer poly(p-phenylenevinylene) (PPV). The azobenzene side chains partially quench the PPV fluorescence, and we have shown previously that the quenching efficiency is greater when the azobenzene side chains are cis than when they are trans. This effect provides a photoaddressable means of modulating the fluorescence intensity of PPV derivatives. To optimize the efficiency of photoinduced intensity modulation, it is important to understand the molecular nature of quenching by both trans- and cis-azobenzene side chains. Here we investigate the photophysical origins of quenching by the two isomers using steady-state and time-resolved fluorescence spectroscopy. We present results from the azobenzene-modified PPV derivative poly(2-methoxy-5-((10-(4-(phenylazo)phenoxy)decyl)oxy)-1,4-phenylenevinylene) (MPA-10-PPV) and two new related polymers, a copolymer lacking half of the azobenzene side chains and an analogue of MPA-10-PPV with a tert-butyl-substituted azobenzene. These studies reveal that steric interactions influence the extent of PPV emission quenching by trans-azobenzene but do not affect the efficient quenching by cis-azobenzene. The difference in dynamic quenching efficiencies between trans- and cis-azobenzene isomers is consistent with fluorescence resonance energy transfer. These results show that it is possible to control the efficiency of photoswitchable fluorescence modulation through specific structural variations designed to encourage or block quenching by trans-azobenzene. This is a promising approach to providing useful general guidelines for designing photomodulated PPV derivatives.

Preparation and encapsulation of highly fluorescent conjugated polymer nanoparticles

A facile method has been developed to prepare aqueous dispersions of encapsulated conjugated polymer nanoparticles exhibiting high fluorescence brightness. Salient features of the nanoparticles include their small diameter and spherical morphology. Encapsulation of the nanoparticles with a silica shell reduces the rate of photooxidation and allows facile attachment of functional groups for subsequent bioconjugation and nanoparticle assembly. Functionalization of the nanoparticle with amine groups followed by the addition of Au nanoparticles resulted in the formation of nanoparticle assemblies, as evidenced by the efficient quenching of the conjugated polymer fluorescence by the Au nanoparticles.