Probing polymers with single fluorescent molecules

A multi-property fluorescent probe for the investigation of polymer dynamics near the glass transition

In addition to the commonly observed single molecule fluorescence intensity fluctuations due to molecular reorientation dynamics, a perylene bisimide-calixarene compound (1) shows additional on-off fluctuations due to its ability to undergo intramolecular excited state electron transfer (PET). This quenching process is turned on rather sharply when a film of poly(vinylacetate) containing 1 is heated above its glass transition temperature (T g), which indicates that the electron transfer process depends on the availability of sufficient free volume. Spatial heterogeneities cause different individual molecules to reach the electron transfer regime at different temperatures, but these heterogeneities also fluctuate in time: in the matrix above T g molecules that are mostly nonfluorescent due to PET can become fluorescent again on timescales of seconds to minutes.

The two different mechanisms for intensity fluctuation, rotation and PET, thus far only observed in compound 1, make it a unique probe for the dynamics of supercooled liquids.

Plasticization of poly(vinylpyrrolidone) thin films under ambient humidity: insight from single-molecule tracer diffusion dynamics

Studies on diffusion dynamics of single molecules (SMs) have been useful in revealing inhomogeneity of polymer thin films near and above the glass-transition temperature (T(g)). However, despite several applications of polymer thin films where exposure to solvent (or vapor) is common, the effect of absorbed solvent molecules on local morphology and rigidity of polymer matrices is yet to be explored in detail. High-T(g) hydrophilic polymers such as poly(vinylpyrrolidone) (PVP) are used as pharmaceutical coatings for drug release in aqueous medium, as they readily absorb moisture, which results in effective lowering of the T(g) and thereby leads to plasticization. The effect of moisture absorption on swelling and softening of PVP thin films was investigated by visualizing the diffusion dynamics of rhodamine 6G (Rh6G) tracer molecules at various ambient relative humidities (RH). Wide-field epifluorescence microscopy, in conjunction with high-resolution SM tracking, was used to monitor the spatiotemporal evolution of individual tracers under varied moisture contents of the matrix. In the absence of atmospheric moisture, Rh6G molecules in dry PVP films are translationally inactive, suggestive of rigid local environments. Under low moisture contents (RH 30-50%), translational mobility remains arrested but rotational motion is augmented, indicating slight swelling of the polymer network which marks the onset of plasticization. The translational mobility of Rh6G was found to be triggered only at a threshold ambient RH, beyond which a large proportion of tracers exhibit extensive diffusion dynamics. Interestingly, SM tracking data at higher moisture contents of the film (RH ≥ 60%) reveal that the distributions of dynamic parameters (such as diffusivity) are remarkably broad, spanning several orders of magnitude. Furthermore, Rh6G molecules display a wide variety of translational motion even at a fixed ambient RH, clearly pointing out the extremely inhomogeneous environment of plasticized PVP network. Intriguingly, it is observed that a majority of tracers undergo anomalous subdiffusion even under high moisture contents of the matrix. Analyses of SM trajectories using velocity autocorrelation function reveal that subdiffusive behaviors of Rh6G are likely to originate from fractional Brownian motion, a signature of tracer dynamics in viscoelastic medium.

Heterogeneity in Single-Molecule Observables in the Study of Supercooled Liquids

Bulk approaches to studying heterogeneous systems obscure important details, as they report average behavior rather than the distribution of behaviors in such environments. Small-molecule and polymeric supercooled liquids, which display heterogeneity in their dynamics without an underlying structural heterogeneity that sets those dynamics, are important constituents of this category of condensed matter systems. A variety of approaches have been devised to unravel ensemble averaging in supercooled liquids. This review focuses on the ultimate subensemble approach, single-molecule measurements, as they have been applied to the study of supercooled liquids. We detail how three key experimental observables (single-molecule probe rotation, translation, and fluorescence lifetime) have been employed to provide detail on dynamic heterogeneity in supercooled liquids. Special attention is given to the potential for, but also the challenges in, discriminating spatial and temporal heterogeneity and detailing the length scales and timescales of heterogeneity in these systems.

Single molecule probing of dynamics in supercooled polymers

Fluorescence experiments with single BODIPY molecules embedded in a poly(methyl acrylate) matrix have been performed at various temperatures in the supercooled regime. By using pulsed excitation, fluorescence lifetime and linear dichroism time trajectories were accessible at the same time. Both observables have been analyzed without data binning. While the linear dichroism solely reflects single particle dynamics, the fluorescence lifetime observable depends on the molecular environment, so that the dynamics from the polymer host surrounding a chromophore contributes to this quantity. We observe that the lifetime correlation decays slightly faster than polarization correlation, indicating the occurrence of large angular reorientations. Additionally, dichroism time trajectories have been adducted to reveal directly the geometry of rotational dynamics. We identify small but also significantly larger rotational jumps being responsible for the overall molecular reorientation.

Ortho-dichlorobenzene doped with terrylene--a highly photo-stable single-molecule system promising for photonics applications

The study of a new dye-matrix system-quickly frozen ortho-dichlorobenzene weakly doped with terrylene--via single-molecule (SM) spectroscopy is presented. The spectral and photo-physical properties, dynamics, and temperature broadening of SM spectra at low temperatures are discussed. The data reveal a broad inhomogeneous distribution, which indicates a high degree of matrix inhomogeneities, but at the same time, huge fluorescence emission rates and extraordinary SM spectral and photochemical stability with almost complete absence of blinking and bleaching. These unusual properties render the new system a promising candidate for applications in photonics, for example, for delivering single photons on demand.

Metal-enhanced fluorescence of single green fluorescent protein (GFP)

The green fluorescent protein (GFP) has emerged as a powerful reporter molecule for monitoring gene expression, protein localization, and protein-protein interaction. However, the detection of low concentrations of GFPs is limited by the weakness of the fluorescent signal and the low photostability. In this report, we observed the proximity of single GFPs to metallic silver nanoparticles increases its fluorescence intensity approximately 6-fold and decreases the decay time. Single protein molecules on the silvered surfaces emitted 10-fold more photons as compared to glass prior to photobleaching. The photostability of single GFP has increased to some extent. Accordingly, we observed longer duration time and suppressed blinking. The single-molecule lifetime histograms indicate the relatively heterogeneous distributions of protein mutants inside the structure.

Determining if a System is Heterogeneous: The Analysis of Single Molecule Rotational Correlation Functions and Their Limitations

Single molecule spectroscopy can be utilized to measure distributions of individual molecular properties that may be averaged out in the ensemble measurement. For example, complex dynamics in disordered systems can be investigated by observing single molecule rotations via fluorescence spectroscopy. The rotational time of a single transient can be calculated from the correlation function of the reduced linear dichroism signal which fluctuates over time as the molecule reorients in its surroundings. Distributions of rotational time constants can be used to characterize the heterogeneity of molecular environments in the material. This paper reviews some theoretical studies on (1) the high numerical aperture effects on the final correlation function, and how it can be related to optical anisotropy decays in a bulk measurement; (2) the statistical errors resulting from the finite observation length that will propagate into distributions of rotational times. These lead to the discussions on how to interpret correctly the distribution of properties measured from a set of single molecule data, and to determine if in fact the system is heterogeneous.

Effect of finite trajectory length on the correlation function analysis of single molecule data

The effect of finite trajectory length on single molecule rotational correlation functions has been studied by utilizing time series analysis and numerical simulations. Correlation functions obtained from the trajectories of length less than 100 times the correlation time constant (tau([script-l])) exhibit significant deviations from the true correlation function. The distributions of sample time constants (tau(F)) and stretching exponents (Beta(F)) are mapped by fitting a large number of rotational trajectories to stretched exponentials. As the trajectory length gets smaller, the distributions become broader and asymmetric and their mean values deviate from the true value predicted by pure rotational diffusion. Analysis based on higher order spherical harmonics is suggested as a method for minimizing the effect of the trajectory length. The distributions of time constants for different higher order spherical harmonics are also compared. While the focus of the paper is on rotational correlation functions, the general conclusions apply to any dynamical process that yields an exponentially decaying correlation function.

Fluorescence lifetime fluctuations of single molecules probe the local environment of oligomers around the glass transition temperature

Single molecule fluorescence experiments have been performed on a BODIPY-based dye embedded in oligo(styrene) matrices to probe the density fluctuations and the relaxation dynamics of chain segments surrounding the dye molecules. The time-dependent fluorescence lifetime of the BODIPY probe was recorded as an observable for the local density fluctuations. At room temperature, the mean fraction of holes surrounding the probes is shown to be unaffected by the molecular weight in the glassy state. In contrast, the free volume increases significantly in the supercooled regime. These observations are discussed in the framework of the entropic theories of the glass transition.

Direct measurement of the end-to-end distance of individual polyfluorene polymer chains

Wide-field imaging of individual multichromophoric molecules and successive photobleaching were used to determine, accurately, the relative position of the chromophores in such systems. First, a polyphenylene dendrimer with well-defined geometry was used to establish the accuracy in localization that can be obtained by this methodology. For a signal-to-noise ratio of 20, interchromophoric distances could be measured with 4 nm accuracy. Next, the method was used to determine the end-to-end distribution of an end-capped polyfluorene polymer. From comparison between the experimental and simulated distributions, information on the conformation of the polymer could be deduced. It was found that the polymer has a nonlinear conformation. A conjugation length of six monomer units gave the best fit of the experimental data to the proposed model.

Single-Molecule Microscopy Studies of Electric-Field Poling in Chromophore−Polymer Composite Materials

One strategy for increasing the efficiency of organic electrooptic devices based on chromophore-polymer composite materials is to improve chromophore ordering. In these materials, ordering is induced through the interaction of the chromophore dipole moment with an external electric field, applied at temperatures near the Tg of the polymer host, a process referred to as "poling". To provide insight into the molecular details of the poling process under conditions representative of device construction, the rotational dynamics of single 4-dicyano-methylene-2-methyl-6-(p-(dimethylamino)styryl)-4H-pyran (DCM) molecules in poly(methyl acrylate) at T = Tg + 11 degrees C in the presence and absence of an electric field are investigated using single-molecule confocal fluorescence microscopy. Single-molecule rotational dynamics are monitored through the time evolution of the fluorescence anisotropy. The anisotropy correlation function demonstrates nonexponential decay, with beta values derived from fits using the Kohlrausch-Williams-Watts law ranging from 0.7 to 1 with beta(KWW) = 0.83. This observation is consistent with previous studies of molecular rotation dynamics in polymer melts and reflects the dynamical heterogeneity provided by the polymer host. The rotational dynamics of DCM are weakly perturbed in the presence of a 50 V/microm electric field, typical of the field strength employed in device construction. The expected perturbation of the rotational dynamics is determined and found to be consistent with the alignment potential created by the electric field relative to the amount of thermal energy available. The relevance of these findings with respect to current models of the poling process is discussed. This work demonstrates the utility of polarization-sensitive single-molecule microscopy in elucidating the details of molecular reorientation during poling.

Matrix-Induced Intensity Fluctuations in the Fluorescence from Single Oligo(phenylenevinylene) Molecules

Single molecule spectroscopy on oligo(phenylenevinylene) (OPV) chromophores shows that the fluorescence intermittency strongly correlates to the rigidity of the environment surrounding the molecules. For OPV single molecules, environmental rigidity inhibits twisting about the vinyl linkages, the molecular motion associated with the observed "off" (nonabsorbing) state. By increasing the rigidity of a single molecule's environment, we can tune its room temperature fluorescence from rapid, sub-millisecond "blinking" fluctuations (fluid polymer environment) to completely "on" with no blinking observed (molecules adsorbed to a rigid bare glass substrate). The difference in fluorescence intermittency from environment to environment is immediately apparent and explicit in single molecule intensity trajectories under cw (continuous wave) excitation, demonstrating the sensitivity of these chromophores to their surroundings and emphasizing the importance of morphological control in real-world applications involving phenylenevinylene-based materials.