Highly Anisotropic Conjugated Polymer Aggregates: Preparation and Quantification of Physical and Optical Anisotropy

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

Highly Anisotropic Conjugated Polymer Aggregates: Preparation and Quantification of Physical and Optical Anisotropy

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

Structure-Dependent Electronic Interactions in Ethyne-Bridged Porphyrin Arrays Investigated by Single-Molecule Fluorescence Spectroscopy

By using single-molecule fluorescence spectroscopy, we have investigated the electronic interaction of ethyne-bridged porphyrin arrays (ZNE) depending on their structure. The fluorescence dynamics of ZNE show a large amount of one-step photobleaching behaviors, indicating the high degree of π-conjugation. The ratio of one-step photobleaching behavior decreased as the number of porphyrin units increased. This behavior indicates that the linear and shortest Z2E shows a strong electronic coupling between constituent porphyrin moieties. Structural properties and orientation of ZNE were also measured by wide-field excitation fluorescence spectroscopy (ExPFS) and defocused wide-field imaging (DWFI). The ExPFS and DWFI show that the structure of absorbing and emitting units of Z2E and Z3E are linear. On the other hand, star-shaped pentamer with five porphyrins acts as an absorbing unit, but unidirectional trimer moiety acts as an emitting unit in the Z5E molecule. Collectively, these studies provide further information on the electronic interaction depending on their structure and length

Reconstruction of the Molecular Structure of a Multichromophoric System Using Single-Molecule Defocused Wide-Field Imaging

Single-molecule defocused wide-field imaging (DWFI) has been demonstrated to be useful to determine molecular structure parameters, such as the orientations of transition dipole moments and the angular relationships between chromophores in multichromophoric molecular systems. For a series of acetylene-linked perylene bisimide (PBI) macrocycles with different ring size comprising three to six PBI dyes, we reconstructed the molecular structure of the multichromophoric system using DWFI method. Furthermore, we revealed that the structural heterogeneities and distortions depend on the ring size. Our findings illustrate the use of DWFI to gain deeper insight into the structure–property relationships of artificial light-harvesting molecular systems

Conformation-Dependent Photostability among and within Single Conjugated Polymers

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 (<i>M</i>) measurements. Upon partial photobleaching, <i>M</i> distributions of collapsed, highly ordered MEH-PPV molecules shifted toward lower values. Conversely, <i>M</i> 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 <i>M</i> value. Decreases in <i>M</i> values seen experimentally in the population of MEH-PPV molecules with high <i>M</i> 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

Segmental Dynamics of an Isolated Component Polymer Chain in Polymer Blends Near the Glass Transition

The segmental dynamics of a component chain isolated in its blending partner chains is examined using the reorientation of polymer-tethered fluorescent probes near the glass transition. It is found that the temperature dependence of the dynamics of an isolated component follows that of the other component, with a horizontal shift corresponding to the glass transition temperature modification, which may result from a local composition of ≈10% isolated component. On the contrary, the dynamic heterogeneity, another key dynamic feature near the glass transition, shows that the local dynamic environment of an isolated component becomes either as heterogeneous as a more inherently heterogeneous component or more heterogeneous than either. These observations emphasize that not only the chain connectivity but also the dynamic modulation of a component by the other component needs to be addressed in order to understand the segmental dynamics of an isolated component in polymer blends

Facile Ligand Exchange of Ionic Ligand-Capped Amphiphilic Ag₂S Nanocrystals for High Conductive Thin Films

A surface ligand modification of colloidal nanocrystals (NCs) is one of the crucial issues for their practical applications because of the highly insulating nature of native long-chain ligands. Herein, we present straightforward methods for phase transfer and ligand exchange of amphiphilic Ag2S NCs and the fabrication of highly conductive films. S-terminated Ag2S (S–Ag2S) NCs are capped with ionic octylammonium (OctAH+) ligands to compensate for surface anionic charge, S2–, of the NC core. An injection of polar solvent, formamide (FA), into S–Ag2S NCs dispersed in toluene leads to an additional envelopment of the charged S–Ag2S NC core by FA due to electrostatic stabilization, which allows its amphiphilic nature and results in a rapid and effective phase transfer without any ligand addition. Because the solvation by FA involves a dissociation equilibrium of the ionic OctAH+ ligands, controlling a concentration of OctAH+ enables this phase transfer to show reversibility. This underlying chemistry allows S–Ag2S NCs in FA to exhibit a complete ligand exchange to Na+ ligands. The S–Ag2S NCs with Na+ ligands show a close interparticle distance and compatibility for uniformly deposited thin films by a simple spin-coating method. In photoelectrochemical measurements with stacked Ag2S NCs on ITO electrodes, a 3-fold enhanced current response was observed for the ligand passivation of Na+ compared to OctAH+, indicating a significantly enhanced charge transport in the Ag2S NC film by a drastically reduced interparticle distance due to the Na+ ligands

Inhomogeneity in the Excited-State Torsional Disorder of a Conjugated Macrocycle

The photophysics of conjugated polymers has generally been explained based on the interactions between the component conjugated chromophores in a tangled chain. However, conjugated chromophores are entities with static and dynamic structural disorder, which directly affects the conjugated polymer photophysics. Here we demonstrate the impact of chain structure torsional disorder on the spectral characteristics for a macrocyclic oligothiophene <b>1</b>, which is obscured in conventional linear conjugated chromophores by diverse structural disorders such as those in chromophore size and shape. We used simultaneous multiple fluorescence parameter measurement for a single molecule and quantum-mechanical calculations to show that within the fixed conjugation length across the entire ring an inhomogeneity from torsional disorder in the structure of <b>1</b> plays a crucial role in causing its energetic disorder, which affords the spectral broadening of ∼220 meV. The torsional disorder in <b>1</b> fluctuated on the time scale of hundreds of milliseconds, typically accompanied by spectral drifts on the order of ∼10 meV. The fluctuations could generate torsional defects and change the electronic structure of <b>1</b> associated with the ring symmetry. These findings disclose the fundamental nature of conjugated chromophore that is the most elementary spectroscopic unit in conjugated polymers and suggest the importance of engineering structural disorder to optimize polymer-based device photophysics. Additionally, we combined defocused wide-field fluorescence microscopy and linear dichroism obtained from the simultaneous measurements to show that <b>1</b> emits polarized light with a changing polarization direction based on the torsional disorder fluctuations