Conjugated polymers as molecular materials: how chain conformation and film morphology influence energy transfer and interchain interactions

Structural dependence of MEH-PPV chromism in solution

The chromism observed in the MEH-PPV polymer in tetrahydrofuran (THF) solution is discussed as a function of the structural morphology of the backbone chains. To evaluate this phenomenon, we carried out simulations employing a hybrid methodology using molecular dynamics and quantum mechanical approaches. Our results support the hypothesis that the morphological order-disorder transition is related to the change from red to blue phase observed experimentally. The morphological disorder is associated with total or partial twisted arrangements in the polymer backbone, which induces an electronic conjugation length more confined to shorter segments. In addition, the main band of the MEH-PPV UV-Vis spectrum at the lower wavelength is related to the blue phase, in contrast to the red phase found for the more planar backbone chains.

Bright conjugated polymer nanoparticles containing a biodegradable shell produced at high yields and with tuneable optical properties by a scalable microfluidic device

This study compares the performance of a microfluidic technique and a conventional bulk method to manufacture conjugated polymer nanoparticles (CPNs) embedded within a biodegradable poly(ethylene glycol) methyl ether-block-poly(lactide-co-glycolide) (PEG_5K-PLGA_55K) matrix. The influence of PEG_5K-PLGA_55K and conjugated polymers cyano-substituted poly(p-phenylene vinylene) (CN-PPV) and poly(9,9-dioctylfluorene-2,1,3-benzothiadiazole) (F8BT) on the physicochemical properties of the CPNs was also evaluated. Both techniques enabled CPN production with high end product yields (∼70-95%). However, while the bulk technique (solvent displacement) under optimal conditions generated small nanoparticles (∼70-100 nm) with similar optical properties (quantum yields ∼35%), the microfluidic approach produced larger CPNs (140-260 nm) with significantly superior quantum yields (49-55%) and tailored emission spectra. CPNs containing CN-PPV showed smaller size distributions and tuneable emission spectra compared to F8BT systems prepared under the same conditions. The presence of PEG_5K-PLGA_55K did not affect the size or optical properties of the CPNs and provided a neutral net electric charge as is often required for biomedical applications. The microfluidics flow-based device was successfully used for the continuous preparation of CPNs over a 24 hour period. On the basis of the results presented here, it can be concluded that the microfluidic device used in this study can be used to optimize the production of bright CPNs with tailored properties with good reproducibility.

Enhancing the photostability of poly(phenylene ethynylene) for single particle studies

Enhanced photostability of conjugated polyelectrolytes achieved by using anti-fading agents opens the way for advanced single molecule fluorescence imaging studies.

Chain conformations and phase behavior of conjugated polymers

Conjugated polymers may play an important role in various emerging optoelectronic applications because they combine the chemical versatility of organic molecules and the flexibility, stretchability and toughness of polymers with semiconducting properties. Nevertheless, in order to achieve the full potential of conjugated polymers, a clear description of how their structure, morphology, and macroscopic properties are interrelated is needed. We propose that the starting point for understanding conjugated polymers includes understanding chain conformations and phase behavior. Efforts to predict and measure the persistence length have significantly refined our intuition of the chain stiffness, and have led to predictions of nematic-to-isotropic transitions. Exploring mixing between conjugated polymers and small molecules or other polymers has demonstrated tremendous advancements in attaining the needed properties for various optoelectronic devices. Current efforts continue to refine our knowledge of chain conformations and phase behavior and the factors that influence these properties, thereby providing opportunities for the development of novel optoelectronic materials based on conjugated polymers.

Understanding the Structural Evolution of Single Conjugated Polymer Chain Conformers

Single molecule photoluminescence (PL) spectroscopy of conjugated polymers has shed new light on the complex structure–function relationships of these materials. Although extensive work has been carried out using polarization and excitation intensity modulated experiments to elucidate conformation-dependent photophysics, surprisingly little attention has been given to information contained in the PL spectral line shapes. We investigate single molecule PL spectra of the prototypical conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) which exists in at least two emissive conformers and can only be observed at dilute levels. Using a model based on the well-known “Missing Mode Effect” (MIME), we show that vibronic progression intervals for MEH-PPV conformers can be explained by relative contributions from particular skeletal vibrational modes. Here, observed progression intervals do not match any ground state Raman active vibrational frequency and instead represent a coalescence of multiple modes in the frequency domain. For example, the higher energy emitting “blue” MEH-PPV form exhibits PL maxima at ~18,200 cm⁻¹ with characteristic MIME progression intervals of ~1200–1350 cm⁻¹, whereas the lower energy emitting “red” form peaks at ~17,100 cm⁻¹ with intervals in the range of ~1350–1450 cm⁻¹. The main differences in blue and red MEH-PPV chromophores lie in the intra-chain order, or, planarity of monomers within a chromophore segment. We demonstrate that the Raman-active out-of-plane C–H wag of the MEH-PPV vinylene group (~966 cm⁻¹) has the greatest influence in determining the observed vibronic progression MIME interval. Namely, larger displacements (intensities)—indicating lower intra-chain order—lower the effective MIME interval. This simple model provides useful insights into the conformational characteristics of the heterogeneous chromophore landscape without requiring costly and time-consuming low temperature or single molecule Raman capabilities.

Suppression of Polyfluorene Photo-Oxidative Degradation via Encapsulation of Single-Walled Carbon Nanotubes

Polyfluorenes have achieved noteworthy performance in organic electronic devices but exhibit undesired green band emission under photo-oxidative conditions that have limited their broad utility in optoelectronic applications. In addition, polyfluorenes are well-known dispersants of single-walled carbon nanotubes (SWCNTs), although the influence of SWCNTs on polyfluorene photo-oxidative stability has not yet been defined. Here we quantitatively explore the photophysical properties of poly[(9,9-bis(3^'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) under photo-oxidative conditions when it is in van der Waals contact with SWCNTs. Photoluminescence spectroscopy tracks the spectral evolution of the polymer emission following ambient ultraviolet (UV) exposure, confirming that PFN exhibits green band emission. In marked contrast, PFN-wrapped SWCNTs possess high spectral stability without green band emission under the same ambient UV exposure conditions. By investigating a series of PFN thin films as a function of SWCNT content, it is shown that SWCNT loadings as low as ∼23 wt % suppress photo-oxidative degradation. These findings suggest that PFN-SWCNT composites provide an effective pathway toward utilizing polyfluorenes in organic optoelectronics.

Evolution of Nonmirror Image Fluorescence Spectra in Conjugated Polymers and Oligomers

The nonmirror image relationship between absorption and fluorescence spectra of conjugated polymers contrasts with most organic chromophores and is widely considered a signature of interchromopohore energy funneling. We apply broad-band ultrafast fluorescence spectroscopy to resolve the evolution of fluorescence spectra for dilute solutions of conjugated oligothiophenes, where no energy transfer is possible. Fluorescence spectra evolve from a mirror image of absorption, which lacks vibronic structure, toward a spectrally narrower and vibronically structured species on the hundreds of femtosecond to early picosecond time scale. Our analysis of this fluorescence spectral evolution shows that a broad distribution of torsional conformers is driven to rapidly planarize in the excited state, including in solid films, which is supported by Raman spectroscopy and quantum chemical modeling. Our data have important implications for understanding different energy-transfer regimes that are delineated by structural relaxation.

Emulsion-Based RIR-MAPLE Deposition of Conjugated Polymers: Primary Solvent Effect and Its Implications on Organic Solar Cell Performance

Emulsion-based, resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) has been demonstrated as an alternative technique to deposit conjugated polymer films for photovoltaic applications; yet, a fundamental understanding of how the emulsion target characteristics translate into film properties and solar cell performance is unclear. Such understanding is crucial to enable the rational improvement of organic solar cell (OSC) efficiency and to realize the expected advantages of emulsion-based RIR-MAPLE for OSC fabrication. In this paper, the effect of the primary solvent used in the emulsion target is studied, both experimentally and theoretically, and it is found to determine the conjugated polymer cluster size in the emulsion as well as surface roughness and internal morphology of resulting polymer films. By using a primary solvent with low solubility-in-water and low vapor pressure, the surface roughness of deposited P3HT and PCPDTBT polymer films was reduced to 10 nm, and the efficiency of P3HT:PC61BM OSCs was increased to 3.2% (∼100 times higher compared to the first MAPLE OSC demonstration [ Caricato , A. P. ; Appl. Phys. Lett. 2012 , 100 , 073306 ]). This work unveils the mechanism of polymer film formation using emulsion-based RIR-MAPLE and provides insight and direction to determine the best ways to take advantage of the emulsion target approach to control film properties for different applications.

Effect of chiral 2-ethylhexyl side chains on chiroptical properties of the narrow bandgap conjugated polymers PCPDTBT and PCDTPT

Two narrow bandgap conjugated polymers containing chiral 2-ethylhexyl side chains were synthesized: poly[(4,4-bis(2-ethylhexyl)cyclopenta-[2,1-b:3,4-b']dithiophene)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PCPDTBT*) and poly[(4,4-bis(2-ethylhexyl)cyclopenta[2,1-b:3,4-b0]dithiophene)-2,6-diyl-alt-[1,2,5]-thiadiazolo[3,4-c]pyridine] (PCDTPT*). The presence of a chiral substituent provides a handle to study the geometry of interchain aggregates and/or the secondary structure of these conjugated polymers in solution and in thin films via circular dichroism (CD) spectroscopy, provided that the asymmetry in the side-chain is translated to the optically active conjugated backbone. CD signals are observed for PCPDTBT* and PCDTPT* in poor solvent systems, which indicate the presence of chiral ordering in the aggregates. PCPDTBT* shows greater chiral order than PCDTPT* based on their relative anisotropy factors. Additionally, GIWAXS analysis reveals that PCPDTBT* films are more ordered than what is observed for the same polymer containing racemic 2-ethylhexyl chains. Upon solution deposition, the chiral ordering is found to translate to the solid-state microstructure for PCPDTBT* but not PCDTPT*. The presence of a pyridyl nitrogen on the thiadiazolo[3,4-c]pyridine ring of PCDTPT* favors a planar conformation for the backbone such that it has a higher rotational barrier compared to PCPDTBT*. This larger rotational barrier appears to limit the ability of PCDTPT* to adopt a helical structure or relevant chain distortions for achieving chiral aggregates.

Highly Emissive Excimers by 2D Compression of Conjugated Polymers

Interactions between π-conjugated polymers are known to create ground-state aggregates, excimers, and exciplexes. With few exceptions, these species exhibit decreased fluorescence quantum yields relative to the isolated polymers in liquid or solid solutions. Herein, we report a method to assemble emissive conjugated polymer excimers and demonstrate their applicability in the detection of selected solvent vapors. Specifically, poly(phenylene ethynylene)s (PPEs) with amphiphilic side chains are organized in a Langmuir monolayer at the air-water interface. Compression of the monolayer results in the reversible conversion from a face-on organization of the π-system relative to the water to what appears to be an incline-stack conformation. The incline-stack organization creates a bright yellow emissive excimeric state with increases of 28% in relative fluorescence quantum yields to the face-on monolayer conformation. Multilayers can be transferred onto the glass substrate via a Langmuir-Blodgett method with preservation of the excimer emission. These films are metastable and the fluorescence reverts to a cyan color similar to the spectra obtained in solution and spin-cast films after exposure to selected solvent vapors. This behavior has practical utility as a fluorescence-based indicator for selected volatile organic compounds.

Intramolecular and Intermolecular Interactions in Hybrid Organic-Inorganic Alucone Films Grown by Molecular Layer Deposition

Investigation of molecular interactions in polymeric films is crucial for understanding and engineering multiscale physical phenomena correlated to device function and performance, but this often involves a compromise between theoretical and experimental data, because of poor film uniformity. Here, we report the intramolecular and intermolecular interactions inside the ultrathin and conformal hybrid organic-inorganic alucone films grown by molecular layer deposition, based on sequential and self-limiting surface reactions. Varying the carbon chain length of organic precursors, which affects their molecular flexibility, caused intramolecular interactions such as double reactions by bending of the molecular backbone, resulting in formation of hole vacancies in the films. Furthermore, intermolecular interactions in alucone polymeric films are dependent on the thermal kinetics of molecules, leading to binding failures and cross-linking at low and high growth temperatures, respectively. We illustrate these key interactions and identify molecular geometries and potential energies by density functional theory calculations.

Operating organic light-emitting diodes imaged by super-resolution spectroscopy

Super-resolution stimulated emission depletion (STED) microscopy is adapted here for materials characterization that would not otherwise be possible. With the example of organic light-emitting diodes (OLEDs), spectral imaging with pixel-by-pixel wavelength discrimination allows us to resolve local-chain environment encoded in the spectral response of the semiconducting polymer, and correlate chain packing with local electroluminescence by using externally applied current as the excitation source. We observe nanoscopic defects that would be unresolvable by traditional microscopy. They are revealed in electroluminescence maps in operating OLEDs with 50 nm spatial resolution. We find that brightest emission comes from regions with more densely packed chains. Conventional microscopy of an operating OLED would lack the resolution needed to discriminate these features, while traditional methods to resolve nanoscale features generally cannot be performed when the device is operating. This points the way towards real-time analysis of materials design principles in devices as they actually operate.

There is a need to characterize devices during operation in real-time and at nanoscopic length scales. Here, King et al. perform electroluminescence-STED imaging with a polymer based light-emitting diode, revealing nanoscopic defects that would be unresolvable with traditional optical microscopy.

Effects of Processing Solvent on the Photophysics and Nanomorphology of Poly(3-butyl-thiophene) Nanowires:PCBM Blends

We report the effect of the processing solvent on the nanoscale morphology and photophysical dynamics of poly(3-butyl-thiophene) nanowires (P3BT-nw). P3BT-nw assembled in ortho-dichlorobenzene (ODCB) show higher crystallization and a longer conjugation length with increased exciton delocalization compared with those assembled in chlorobenzene (CB). It is proposed that this solvent effect is associated with the higher ordered structures formed from ODCB solution state. Charge-transfer dynamics and phase separation for P3BT-nw:PCBM blends were investigated by ultrafast fluorescence techniques. The more efficient fluorescence quenching observed in P3BT-nw:PCBM blend films processed from ODCB suggests that there is intimate contact between P3BT-nw and PCBM that facilitates charge transfer. The superior performance of organic photovoltaic devices based on P3BT-nw:PCBM bulk heterojunctions processed using ODCB is attributed to the higher crystallization of P3BT-nw, optimized phase separation, and more efficient charge transfer from P3BT-nw to PCBM.

Highly emissive 'frozen-in' conjugated polymer nanofibers

Conjugated-polymer nanofibers with a thermodynamically stable, coarsened, disordered structure in an amorphous glassy state were fabricated via a freeze-drying method using a poly(diphenylacetylene) derivative. The nanofibers were extremely emissive, with a fluorescence (FL) quantum yield of approximately 0.34, which was much higher than that of both the cast film (0.02) and the solution (0.21). Similarly, the amplitude-weighted average FL lifetime of the nanofibers was 0.74 ns, which was much longer than that of the film (0.29 ns) and the solution (0.57 ns). This unusual and enhanced FL-emission behavior was attributed to the abruptly quenched chain structure that was created by the freeze-drying process. The polymer chains in the nanofibers remained frozen-in and the side phenyl rings were retained in a relaxed state. The metastable chains did not undergo vibrational relaxation and collisional quenching to generate the radiative emission decay effectively.

Nanostructured conducting polymers for energy applications: towards a sustainable platform

Recently, there has been tremendous progress in the field of nanodimensional conducting polymers with the objective of tuning the intrinsic properties of the polymer and the potential to be efficient, biocompatible, inexpensive, and solution processable. Compared with bulk conducting polymers, conducting polymer nanostructures possess a high electrical conductivity, large surface area, short path length for ion transport and superior electrochemical activity which make them suitable for energy storage and conversion applications. The current status of polymer nanostructure fabrication and characterization is reviewed in detail. The present review includes syntheses, a deeper understanding of the principles underlying the electronic behavior of size and shape tunable polymer nanostructures, characterization tools and analysis of composites. Finally, a detailed discussion of their effectiveness and perspectives in energy storage and solar light harvesting is presented. In brief, a broad overview on the synthesis and possible applications of conducting polymer nanostructures in energy domains such as fuel cells, photocatalysis, supercapacitors and rechargeable batteries is described.

Conducting polymers as electron glasses: surface charge domains and slow relaxation

The surface potential of conducting polymers has been studied with scanning Kelvin probe microscopy. The results show that this technique can become an excellent tool to really 'see' interesting surface charge interaction effects at the nanoscale. The electron glass model, which assumes that charges are localized by the disorder and that interactions between them are relevant, is employed to understand the complex behavior of conducting polymers. At equilibrium, we find surface potential domains with a typical lateral size of 50 nm, basically uncorrelated with the topography and strongly fluctuating in time. These fluctuations are about three times larger than thermal energy. The charge dynamics is characterized by an exponentially broad time distribution. When the conducting polymers are excited with light the surface potential relaxes logarithmically with time, as usually observed in electron glasses. In addition, the relaxation for different illumination times can be scaled within the full aging model.

Functionalization-induced changes in the structural and physical properties of amorphous polyaniline: a first-principles and molecular dynamics study

In this paper, we present a first-principles and molecular dynamics study to delineate the functionalization-induced changes in the local structure and the physical properties of amorphous polyaniline. The results of radial distribution function (RDF) demonstrate that introducing -SO3(-)Na(+) groups at phenyl rings leads to the structural changes in both the intrachain and interchain ordering of polyaniline at shorter distances (≤5 Å). An unique RDF feature in 1.8-2.1 Å regions is usually observed in both the interchain and intrachain RDF profiles of the -SO3(-)Na(+) substituted polymer (i.e. Na-SPANI). Comparative studies of the atom-atom pairs, bond structures, torsion angles and three-dimensional structures show that EB-PANI has much better intrachain ordering than that of Na-SPANI. In addition, investigation of the band gap, density of states (DOS), and absorption spectra indicates that the derivatization at ring do not substantially alter the inherent electronic properties but greatly change the optical properties of polyaniline. Furthermore, the computed diffusion coefficient of water in Na-SPANI is smaller than that of EB-PANI. On the other hand, the Na-SPANI shows a larger density than that of EB-PANI. The computed RDF profiles, band gaps, absorption spectra, and diffusion coefficients are in quantitative agreement with the experimental data.

Nanoscale direct mapping of localized and induced noise sources on conducting polymer films

The localized noise-sources and those induced by external-stimuli were directly mapped by using a conducting-AFM integrated with a custom-designed noise measurement set-up. In this method, current and noise images of a poly(9,9-dioctylfluorene)-polymer-film on a conducting-substrate were recorded simultaneously, enabling the mapping of the resistivity and noise source density (NT). The polymer-films exhibited separate regions with high or low resistivities, which were attributed to the ordered or disordered phases, respectively. A larger number of noise-sources were observed in the disordered-phase-regions than in the ordered-phase regions, due to structural disordering. Increased bias-voltages on the disordered-phase-regions resulted in increased NT, which is explained by the structural deformation at high bias-voltages. On photo-illumination, the ordered-phase-regions exhibited a rather large increase in the conductivity and NT. Presumably, the illumination released carriers from deep-traps which should work as additional noise-sources. These results show that our methods provide valuable insights into noise-sources and, thus, can be powerful tools for basic research and practical applications of conducting polymer films.

Influence of the Conjugation Length on the Optical Spectra of Single Ladder-Type (p-Phenylene) Dimers and Polymers

We employ low-temperature single-molecule photoluminescence spectroscopy on a π-conjugated ladder-type (p-phenylene) dimer and the corresponding polymer methyl-substituted ladder-type poly(p-phenylene), MeLPPP, to study the impact of the conjugation length (π-electron delocalization) on their optical properties on a molecular scale. Our data show that the linear electron-phonon coupling to intramolecular vibrational modes is very sensitive to the conjugation length, a well-known behavior of organic (macro-) molecules. In particular, the photoluminescence spectra of single dimers feature a rather strong low-energy (150 cm(-1)) skeletal mode of the backbone, which does not appear in the spectra of individual chromophores on single MeLPPP chains. We attribute this finding to a strongly reduced electron-phonon coupling strength and/or vibrational energy of this mode for MeLPPP with its more delocalized π-electron system as compared to the dimer. In contrast, the line widths of the purely electronic zero-phonon lines (ZPL) in single-molecule spectra do not show differences between the dimer and MeLPPP; for both systems the ZPLs are apparently broadened by fast unresolved spectral diffusion. Finally, we demonstrate that the low-temperature ensemble photoluminescence spectrum of the dimer cannot be reproduced by the distribution of spectral positions of the ZPLs. The dimer's bulk spectrum is rather apparently broadened by electron-phonon coupling to the low-energy skeletal mode, whereas for MeLPPP the inhomogeneous bulk line shape resembles the distribution of spectral positions of the ZPLs of single chromophores.

Controlled/living polymerization towards functional poly(p-phenylene vinylene) materials

Poly(p-phenylene vinylene)s (PPVs) are an important class of highly fluorescent polymeric semiconductor materials.

Modifiable poly(p-phenylene vinylene) copolymers towards functional conjugated materials

The copolymerization of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-p-phenylene vinylene] (MDMO-PPV) with poly[2-methoxy-5-(carboxypentyloxy)-PPV] (CPM-PPV) and poly[2,5-dicyano-PPV] (CN-PPV) is studied and postpolymerization modification with water soluble substituents is demonstrated.

Red-emitting protein-coated conjugated polymer nanoparticles

Red emitting materials are desirable in biology due to the transparency of certain biological tissues at these wavelengths.

Chain conformation-dependent thermal conductivity of amorphous polymer blends: the impact of inter- and intra-chain interactions

Polymers with high thermal conductivities are of great interest for both scientific research and industrial applications.

Theranostic unimolecular micelles of highly fluorescent conjugated polymer bottlebrushes for far red/near infrared bioimaging and efficient anticancer drug delivery

Theranostic unimolecular micelles of highly fluorescent amphiphilic conjugated bottlebrushes loaded with anticancer drugs are efficient for cancer imaging and therapy.

Thin Films Formed from Conjugated Polymers with Ionic, Water-Soluble Backbones

This paper compares the morphologies of films of conjugated polymers in which the backbone (main chain) and pendant groups are varied between ionic/hydrophilic and aliphatic/hydrophobic. We observe that conjugated polymers in which the pendant groups and backbone are matched, either ionic-ionic or hydrophobic-hydrophobic, form smooth, structured, homogeneous films from water (ionic) or tetrahydrofuran (hydrophobic). Mismatched conjugated polymers, by contrast, form inhomogeneous films with rough topologies. The polymers with ionic backbone chains are conjugated polyions (conjugated polymers with closed-shell charges in the backbone), which are semiconducting materials with tunable bad-gaps, not unlike uncharged conjugated polymers.

Photoswitching and Thermoresponsive Properties of Conjugated Multi-chromophore Nanostructured Materials

Conjugated multi-chromophore organic nanostructured materials have recently emerged as a new class of functional materials for developing efficient light-harvesting, photosensitization, photocatalysis, and sensor devices because of their unique photophysical and photochemical properties. Here, we demonstrate the formation of various nanostructures (fibers and flakes) related to the molecular arrangement (H-aggregation) of quaterthiophene (QTH) molecules and their influence on the photophysical properties. XRD studies confirm that the fiber structure consists of >95% crystalline material, whereas the flake structure is almost completely amorphous and the microstrain in flake-shaped QTH is significantly higher than that of QTH in solution. The influence of the aggregation of the QTH molecules on their photoswitching and thermoresponsive photoluminescence properties is revealed. Time-resolved anisotropic studies further unveil the relaxation dynamics and restricted chromophore properties of the self-assembled nano/microstructured morphologies. Further investigations should pave the way for the future development of organic electronics, photovoltaics, and light-harvesting systems based on π-conjugated multi-chromophore organic nanostructured materials.

Exploiting Conjugated Polyelectrolyte Photophysics toward Monitoring Real-Time Lipid Membrane-Surface Interaction Dynamics at the Single-Particle Level

Herein we report the real-time observation of the interaction dynamics between cationic liposomes flowing in solution and a surface-immobilized charged scaffolding formed by the deposition of conjugated polyanion poly[5-methoxy-2-(3-sulfopropoxy)-1,4-phenylenevinylene (MPS-PPV) onto 100-nm-diameter SiO2 nanoparticles (NPs). Contact of the freely floating liposomes with the polymer-coated surfaces led to the formation of supported lipid bilayers (SLBs). The interaction of the incoming liposomes with MPS-PPV adsorbed on individual SiO2 nanoparticles promoted the deaggregation of the polymer conformation and led to large emission intensity enhancements. Single-particle total internal reflection fluorescence microscopy studies exploited this phenomenon as a way to monitor the deformation dynamics of liposomes on surface-immobilized NPs. The MPS-PPV emission enhancement (up to 25-fold) reflected on the extent of membrane contact with the surface of the NP and was correlated with the size of the incoming liposome. The time required for the MPS-PPV emission to reach a maximum (ranging from 400 to 1000 ms) revealed the dynamics of membrane deformation and was also correlated with the liposome size. Cryo-TEM experiments complemented these results by yielding a structural view of the process. Immediately following the mixing of liposomes and NPs the majority of NPs had one or more adsorbed liposomes, yet the presence of a fully formed SLB was rare. Prolonged incubation of liposomes and NPs showed completely formed SLBs on all of the NPs, confirming that the liposomes eventually ruptured to form SLBs. We foresee that the single-particle studies we report herein may be readily extended to study membrane dynamics of other lipids including cellular membranes in live cell studies and to monitor the formation of polymer-cushioned SLBs.

Solution-crystallization and related phenomena in 9,9-dialkyl-fluorene polymers. II. Influence of side-chain structure

Solution-crystallization is studied for two polyfluorene polymers possessing different side-chain structures. Thermal analysis and temperature-dependent optical spectroscopy are used to clarify the nature of the crystallization process, while X-ray diffraction and scanning electron microscopy reveal important differences in the resulting microstructures. It is shown that the planar-zigzag chain conformation termed the β-phase, which is observed for certain linear-side-chain polyfluorenes, is necessary for the formation of so-called polymer-solvent compounds for these polymers. Introduction of alternating fluorene repeat units with branched side-chains prevents formation of the β-phase conformation and results in non-solvated, i.e. melt-crystallization-type, polymer crystals. Unlike non-solvated polymer crystals, for which the chain conformation is stabilized by its incorporation into a crystalline lattice, the β-phase conformation is stabilized by complexation with solvent molecules and, therefore, its formation does not require specific inter-chain interactions. The presented results clarify the fundamental differences between the β-phase and other conformational/crystalline forms of polyfluorenes. © 2015 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1492-1506.

Optical anisotropy and strong H-aggregation of poly(3-alkylthiophene) in a surface monolayer

Slab optical waveguide absorption spectra reveal that surface segregated monolayers of a vertically oriented poly(3-buthylthiophene) derivative have large optical anisotropy, and that confinement of the polymer chains in the isolated monolayer causes strong H-aggregation.

Single and multistep energy transfer processes within doped polymer nanoparticles

Herein, we demonstrate the design of multiple fluorophores Coumarin 153 (C153) and Nile Red (NR) encapsulated in semiconducting poly[N-vinylcarbazole] (PVK) polymer nanoparticles (50-70 nm in diameter) by a simple re-precipitation technique, and elucidate their photophysical properties by steady-state and picosecond (ps) time resolved emission spectroscopy. It is interesting to note that multistep cascaded energy transfer occurs from the excited host PVK molecules to NR dye molecules through C153. The energy transfer time constants are found to be 180 ps for PVK→C153, 360 ps for PVK→NR, and 140 ps for the overall energy transfer process from PVK to NR through C153 dye molecules. The multistep energy transfer allows tuning of the wide range emission from 350 nm to 700 nm by changing the relative concentrations of the encapsulated dye molecules. Bright, stable, and white light emission of the dye doped polymer nanoparticles with a quantum yield of 14% is achieved at a particular concentration ratio of the C153 : NR dye. The generation of "cool" white emission in suspension and in the solid state film opens up new possibilities to obtain white light OLEDs based on single nanoparticles.