The concept of dynamic heterogeneity and the picture of the supercooled liquid as a mosaic of environments with distinct dynamics that interchange in time have been invoked to explain the nonexponential relaxations measured in these systems. The spatial extent and temporal persistence of these regions of distinct dynamics have remained challenging to identify. Here, singlemolecule fluorescence measurements using a probe similar in size and mobility to the host o-terphenyl unambiguously reveal exponential relaxations distributed in time and space and directly demonstrate ergodicity of the system down to the glass transition temperature. In the temperature range probed, at least 200 times the structural relaxation time of the host is required to recover ensemble-averaged relaxation at every spatial region in the system.espite decades of intensive study, a full theory of the glass transition is lacking; so too is a full understanding of the causal relationships between the unusual phenomena displayed by glass-forming liquids in the supercooled regime and the glass transition. One such phenomenon is the onset of nonexponential relaxations in the supercooled regime. Consistent with such relaxations, a variety of experiments have suggested the presence of dynamic heterogeneity, where-over a given time-molecular mobility in a given region may differ by orders of magnitude from that in another region, potentially just nanometers away (1, 2). While some experiments have sought to quantify the size of these regions, others have sought to quantify their persistence in time, as supercooled liquids are assumed to be ergodic, requiring that over long times, all dynamic environments are sampled (3-5). Precise description of dynamic heterogeneity in glass formers remains challenging due to the ensemble, subensemble, and/or time averaging inherent in most experimental techniques. However, such description remains of significant interest given the poorly understood causal relationship between dynamic heterogeneity and the glass transition as well as the need for experimental observations that may distinguish between various theories of the glass transition (6, 7).Typically, putative dynamic heterogeneity has been recognized in experiments through the shape of the ensemble relaxation, which is well described by a stretched exponential (exp[−(t/τ fit ) β ]) where the deviation of β below 1 describes the degree of stretching and has been interpreted as a barometer of dynamic heterogeneity. Multiple scenarios are consistent with an ensemble stretched exponential relaxation, and two limiting cases can be straightforwardly described: The ensemble stretched exponential emerges from (i) a superposition of exponentials with different relaxation times or (ii) identical stretched exponentials with the same relaxation time. The former limit describes a system with variation of time scales distributed in space but not time, while the latter represents the opposite extreme. The former case describes a system that is not ergodic over times acce...

Controlling morphological order of conjugated polymers over mesoscopic and microscopic scales could yield critical improvements in the performance of organic electronics.Here, we utilize a multimodal apparatus allowing for controlled solvent vapor annealing and simultaneous wide-field epifluorescence microscopy to demonstrate bottom-up growth of morphologically ordered anisotropic aggregates prepared from single poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) chains, with length scales controllable from tens of nanometers to several micrometers. Preparation of micrometer-scale fiber aggregates that interconnect to form spanning networks is also demonstrated. We quantify aggregate physical and optical anisotropy, degree of quenching, and exciton diffusion characteristics as a function of aggregate size. The demonstration of controlled preparation of highly anisotropic aggregates provides a path for controlled postprocessing of organic thin films at length scales relevant to the operation of devices.

To thoroughly elucidate how molecular conformation and photophysical properties of conjugated polymers (CPs) are related requires simultaneous probing of both. Previous efforts used fluorescence imaging with one nanometer accuracy (FIONA) to image CPs, which allowed simultaneous estimation of molecular conformation and probing of fluorescence intensity decay. We show that calculating the molecular radius of gyration for putative folded and unfolded poly(2-methoxy-5-(2'-ethylhexyloxy)1,4-phenylenevinylene) (MEH-PPV) molecules using FIONA underestimates molecular extension by averaging over emitters during localization. In contrast, employing algorithms based on single molecule high resolution imaging with photobleaching (SHRImP), including an approach we term all-frames SHRImP, allows localization of individual emitters. SHRImP processing corroborates that compact MEH-PPV molecules have distinct photophysical properties from extended ones. Estimated radii of gyration for isolated 168 kDa MEH-PPV molecules immobilized in polystyrene and exhibiting either stepwise or continuous intensity decays are found to be 12.6 and 25.3 nm, respectively, while the distance between exciton recombination sites is estimated to be ∼10 nm independent of molecular conformation.

The relationship between photostability and conformation of 2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPV) conjugated polymers was studied via excitation polarization modulation depth (M) measurements. Upon partial photobleaching, M distributions of collapsed, highly ordered MEH-PPV molecules shifted toward lower values. Conversely, M distributions of MEH-PPV molecules with random coil conformations moved toward higher values after partial photobleaching. Monte Carlo simulations of randomly distributed dipole moments along polymer chains subjected to partial photobleaching revealed that a statistical effect leads to an increase in peak M value. Decreases in M values seen experimentally in the population of MEH-PPV molecules with high M values, however, are due to conformation-dependent photostability within single MEH-PPV polymers. We show that, while folded MEH-PPV molecules are relatively more photostable than extended MEH-PPV molecules in an ensemble, extended portions of particular molecules are more photostable than folded domains within single MEH-PPV molecules.

We have probed photoinduced charge separation dynamics in organic semiconductor thin films and at their interfaces by femtosecond time-resolved second harmonic generation (TR-SHG), using the model systems of fullerene (C 70 ) thin films and copper phthalocyanine (CuPc)/C 70 interfaces. In neat C 70 thin films, the formation of an internal electric field on a ∼10-ps time scale following photoexcitation is attributed to the photo-Dember effect, namely, charge separation resulting from a gradient in excitation density and the differential electron/hole mobility. When an ultrathin film of the electron donor CuPc was deposited on top of the C 70 film, an additional interfacial charge separation channel occurring on the ultrafast time scale of ∼0.1 ps was observed. We discuss how SHG fields from different origins can interfere to give the overall transient response.

Monitoring single molecule probe rotations is an increasingly common approach to studying dynamics of complex systems, including supercooled liquids. Even with advances in fluorophore design and detector sensitivity, such measurements typically exhibit low signal to noise and signal to background ratios. Here, we simulated and analyzed orthogonally decomposed fluorescence signals of single molecules undergoing rotational diffusion in a manner that mimics experimentally collected data of probes in small molecule supercooled liquids. The effects of noise, background, and trajectory length were explicitly considered, as were the effects of data processing approaches that may limit the impact of noise and background on assessment of environmental dynamics. In many cases, data treatment that attempts to remove noise and background were found to be deleterious. However, for short trajectories below a critical signal to background threshold, a thresholding approach that successfully removed data points associated with noise and spared those associated with signal allowed for assessment of environmental dynamics that was as accurate and precise as would be achieved in the absence of noise.

How charge carriers are distributed in a bulk heterojunction (BHJ) under illumination is central to the understanding of organic photovoltics and photodetectors. Here, we apply nonlinear optical microscopy to quantitatively map the spatial distributions of electric fields in two lateral organic BHJs: poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (PCBM) and poly(4,4-dioctyldithieno(3,2-b:2′,3′-d)silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl) and PCBM. For the former, we observe the development with time of a depletion region adjacent to the electron-collecting electrode. In the latter, the device is stable and characterized by a nearly linear potential drop. We discuss the origins of field distributions and space charge accumulation in organic BHJs.

Understanding the mechanisms that contribute to conjugated polymer aggregate formation and growth may yield enhanced control of aggregate morphology and functional properties on the mesoscopic scale. In situ optical imaging of the growth of MEH-PPV aggregates in real time in controlled swollen films shows that growth occurs through multiple mechanisms and is more complex than previously described. Direct evidence is provided for both Ostwald ripening and aggregate coalescence as operative modes of aggregate growth in solvent swollen films. These growth mechanisms have a distinct and strong impact on the evolution of morphological order of growing aggregates: while Ostwald ripening allows preservation of highly ordered morphology, aggregate coalescence occurs with no preferential orientation, leading to attenuation in degree of ordering.